CHEMISTRY FORM TWO STUDY NOTES TOPIC 3: WATER & TOPIC 4: FUEL AND ENERGY

August 13, 2017

700

TOPIC 3: WATER

Occurrence and Nature of Water

The Occurrence and Nature of Water

Describe the occurrence and nature of water

Water
is the most abundant liquid in nature. It is a compound of hydrogen and
oxygen. It occurs on land as seas, oceans, rivers, springs, wells, etc.
It also occurs in the atmosphere as rain, water vapour, clouds, etc.
Water is the essential constituent of animal and plant life. Without
water, no life could exist on earth. All living things need water to
survive. About 60% of the human body by mass is made of water. A human
being needs to drink about 2 litres of water per day to replace the
water lost from the body via sweat, urine, breath, faeces, etc. If you
did not replace this by eating and drinking, you would die in a matter
of days.

Water
is more important than food. A human being can survive without food for
many weeks, but will die in a few days without water. So without water,
no life can be sustained.

Water
is the main constituent of the earth’s surface. 70% of the earth’s
surface is covered by water. The remaining 30% is covered by land.

Types of water

There
are four kinds of natural water namely, rain water, spring and well
water, river water, and lake and sea water. Natural water is never pure.
Water from difference natural sources contains substances dissolved in
it.

Rain water

This
is naturally distilled water. It is almost pure and it contains only
gases and dust dissolved from the air. If the dissolved gases are
acidic, e.g. sulphur dioxide, carbon dioxide or nitrogen dioxide, they
may form “acid rain”. In heavily industrialized countries where emission
of these gases is very great, acid rains have been experienced. Rain
water in non-industrial areas is fairly pure. It is safe to drink though
it is tasteless. The taste in water is due to dissolved substances in
it.

Spring and well water

When
the rain falls, some water sinks into the ground to form ground water.
This water percolates down the earth until it meets layers of impervious
or impermeable (non-porous) rocks, which stop it from percolating or
seeping any further. The ground water may reach the earth’s surface as a
spring. When a whole deep enough is dug to reach the ground water, a
well results. Spring or well water is supposed to be clean, although it
contains dissolved substances. As water passes through the earth, it is
naturally filtered.

River water

River
water contains dissolved and suspended solid materials. The water in
some rivers is very muddy or sandy depending on the nature of the land
from which the river originates and on which it flows. Most of the water
we drink or use at home and industries is from rivers. To make the
river water fit for use, all the substances dissolved and suspended in
it must be removed or filtered.

Lake and sea water

Lakes
and seas receive water from rivers. River water contains dissolved
salts. As it flows through the land, some of its water evaporates into
the air. When it reaches the sea or lake, more water still evaporates.
As a result, sea and lake water will necessarily contain vast quantities
of dissolved substances. Sea water contains about 3.6% by mass of the
dissolved solids. Most of the dissolved solids compose largely of sodium
chloride that can be obtained from sea water in large quantities. Three
quarters of the ocean salts is sodium chloride (common salt).

The Water Cycle

Describe the water cycle

Water
is always on move, travelling a never-ending, cyclical journey between
the earth and the sky. This journey is referred to as the water cycle or hydrological cycle.
The water cycle describes the continuous movement of water on, above
and below the surface of the earth. During its movement, water is
continuously reused and recycled. It also changes its physical state or
form (liquid, vapour, and ice) at various stages in the water cycle.
Figure 3.1 is a diagrammatic representation of the water cycle. It shows
how the water moves around the earth’s environment, changing its form
through the process of evaporation, transpiration (loss of water from
plants), condensation and precipitation (rainfall, snow, hail, fog,
smog, etc.) Stages of the water cycle are described below:

Heat
from the sun causes water to evaporate from exposed water bodies such
as oceans, seas, lakes, rivers dams, etc. This causes huge amounts of
water vapour to float (laden) in the air. The vapour rises up. In the
cooler upper parts of the atmosphere, the vapour cools and condenses to
form tiny water droplets. The droplets form clouds.
The clouds
are drifted by wind. They cool further, and the droplets join to form
larger drops of water which fall down as rain due to gravitation pull.
On the other hand, if the air is very cold, they fall as hail, sleet or
snow. The whole process is called precipitation.
Some rain water
soaks, and reappears as springs. Some flows over the ground as streams.
The springs and streams feed rivers. The rivers flow to the ocean, sea
or lake. The whole cycle starts again.

The water cycle

Water Cycle and Environmental Conservation

Relate water cycle to environmental conservation

Everyone
understands why it is so important to keep our water clean. The fresh
water that is available for use by people, plants and animals must be
clean and safe.

Sometimes
human carelessness pollutes the water system, loading harmful and
unhealthy substances into the system at a rate that exceeds its natural
restorative capabilities. When harmful substances are discarded
(disposed off; dumped) into the environment, they may very well end up
as part of the water cycle. An example of these acts may happen when
untreated municipal and industrial wastes are directed into the water
bodies such as rivers, lakes and seas. These substances are toxic and
may harm human, marine, animal and plant life.

When
chemicals are released into the air, they might well return to the
earth with rain and snow or by simply settling. For example in
industrial areas, sulphur dioxide dissolves in water from the clouds and
with oxygen from the atmosphere to form sulphuric acid.

Sulphur dioxide + water + oxygen gives sulphuric acid = “acid rain”

This
then falls as “acid rain”. The acid rain washes salts from the top
soil. Acidic water and metal salts run into the lakes or rivers. The
introduction of these new substances consequently increases the acidity
and concentration of metal salts in the lake, river or stream. As a
result, fish and other marine life die.

Nitrogen oxides, NO_x,
can also cause acid rain. When nitrogen dioxide gas reacts with water
and oxygen in the atmosphere, the result is a weak solution of nitric
acid.

Carbon dioxide also reacts with water in the atmosphere to form a weak carbonic acid (rain water).

Pure
water has a pH of 7.0. Normal rain is slightly acidic because of the
carbon dioxide gas dissolved into it. It has a pH of about 5.5.

It has been confirmed that carbon dioxide (CO₂), sulphur dioxide (SO₂) and nitrogen oxides (NO_x) are the primary causes of acid rain.

When
harmful substances are dumped on land or buried in the ground, they
might well find their way into ground water or surface water. These
substances contaminate the water, which may be someone’s or some
community’s drinking water.

Water
plays an important role in the conservation of the environment and in
determining human settlement and development. It also governs plant and
animal distribution. Animals and plants, as components of the
environment, are mainly concentrated in water or in areas where water is
found.

Plant
roots bind the soil particles together, making the soil compact and
less susceptible to erosion. However, vegetation will only grow and
flourish on land that receives sufficient rainfall. This is possible
only if the water cycle is properly maintained by conserving natural
forests and planting more trees to attract rainfall. So it is obvious
that there is a strong relationship between rainfall (as a crucial stage
of the water cycle) and the vegetation and soil (as components of the
environment).

We
use water from the lakes, rivers, wells or springs to irrigate crop and
non-crop plants. So, when we distort the water cycle in some way or the
other we may not have enough rainfall to fill up rivers or springs from
which we obtain the water we use to conserve our environment
(vegetation).

Properly
watered soils support more plants. We all know that plants absorb
carbon dioxide from the atmosphere, therefore, helping to purify the air
naturally. In addition, plants produce oxygen gas, which is needed by
all living organisms. If there is not enough rainfall, most plants will
die, hence resulting to excessive accumulation of carbon dioxide, which
may rise to toxic levels.

Excessive carbon dioxide in the atmosphere leads to intense heating of the earth’s surface, a phenomenon described as global warming.
The consequence of global warming include encroachment and extension of
desert and arid lands, prolonged droughts, changes in rainfall
patterns, etc.

These
few facts show that there is a strong relationship and correlation
between environmental conservation and the water cycle. Environmental
degradation can lead to serious and irreparable aftermath to the water
cycle.

Properties of Water

Simple Experiments on Physical and Chemical Properties of Water

Perfom simple experiments on physical and chemical properties of water

Activity 1

Perfom simple experiments on physical and chemical properties of water.

Properties of Water

Explain properties of water

Physical properties

Includes

Extremely
pure water is colourless, odourless and tasteless. The colour, taste or
odours in water are due to dissolved impurities of organic and
inorganic nature.
Pure water is a very poor conductor of heat
and electricity. However, water containing some dissolved inorganic
impurities may conduct appreciably.
Pure water freezes at 0ºC.
Pure water boils at 100ºC at a pressure of 760 mmHg;
and pure water will boil away completely with no change in temperature.
Its melting point and boiling point are abnormally high due to hydrogen
bonding.
It is the only substance that occurs naturally in all the three states of matter – solid, liquid and gas.
Water,
as compared to other liquids, dissolves almost all substances, though
in varying degrees of solubility. For this reason, water is usually
called the universal solvent.
It has a high surface tension than other liquids.
It has a high specific heat index, which means that it can absorb a lot of heat before getting hot.
It is miscible with many liquids, for example ethanol.
The maximum density of pure water is 1 g cm^-3
at 4ºC. When water is cooled gradually, it reaches its maximum density
at 4 centigrade. The actual change from water to ice takes place at 0ºC.
Pure water is neutral to litmus and has a pH of 7.0.
Water
expands when it freezes. Most substances contract when they change from
liquid to solid state. Water is one of the very few substances that
expand when they freeze. This behaviour is called anomalous expansion of water.
Ice is therefore much less dense than water. The water molecules in the
ice crystals are further apart from each other than in liquid water.

Chemical properties

Action of heat

Water
is extremely stable to heat. A stable compound does not decompose
easily by heating. It requires a very high temperature to decompose
water. Water decomposes lightly at 2500ºC. It approaches complete
decomposition at 5000ºC

Reaction with metals

The state in which water reacts with metals depends on the position of a metal in the electrochemical series as shown below:

It
can be seen that water attacks metals differently depending on the
metal’s position in the activity series. This is called the chemical
activity series of metals.

Potassium

Potassium
is vigorously attacked with cold water, producing hydrogen gas. The
reaction of water with potassium is very violent and the hydrogen
produced catches fire spontaneously with a lilac flame. The colour is
due to the burning of small quantities of potassium vapour.

Sodium

The
reaction of sodium with water is vigorous but the hydrogen liberated
does not catch fire. Sodium reacts with cold water to produce hydrogen
gas, which is detected by effervescence as the gas is liberated. If a
flame is applied, it burns with a yellow flame (the yellow colour is
from sodium).

Calcium

Calcium
reacts with water relatively slowly compared to sodium and potassium.
The gas (hydrogen) given off explodes if mixed with air, and if a flame
applied.

Magnesium

Magnesium reacts with steam to liberate hydrogen and magnesium oxide.

Zinc

If zinc is heated to redness in a current of steam, hydrogen is liberated.

Iron

Iron does not react with cold water, but readily reacts with excess steam at red heat.

The above reaction can be made to proceed in the reverse direction by passing excess of hydrogen over heated triiron tetraoxide.

Reaction with non-metals

Carbon

Red-hot carbon reacts with steam at 1000ºC to give a mixture of carbon monoxide and hydrogen, known as water gas.

Red-hot carbon reacts with steam at 1000ºC to give a mixture of carbon monoxide and hydrogen, known as water gas.

Chlorine reacts with water to form a mixture of two acids.

Reaction with oxides

1. Water reacts with the oxides of most reactive metals to form hydroxides:

2. Water reacts with the oxides of some non–metals to form acids:

Formation of hydrates

Water
combines with many salts to form hydrates. Different salt hydrates have
different number of molecules of water of crystallization. The
following are some examples:

Synthesis of water

Water is a compound of hydrogen and oxygen. Its formula is H₂O. You could make it in the laboratory by burning a jet of hydrogen in air. The reaction is fast and dangerous:

Hence,
in the synthesis of water, hydrogen has to be prepared and then reacted
with the oxygen of the air to give water. The apparatus used for water
synthesis is as shown in figure 3.2 Hydrogen is produced by the
reaction between zinc and cold dilute sulphuric acid

But
the hydrogen to be used for synthesis of water has to be absolutely
dry. This is achieved by passing it through anhydrous calcium chloride.
It is then allowed to pass through the jet. When the hydrogen has
displaced all the air in the apparatus, the gas is lit at the jet. The
water forms as a gas. The gas condenses to liquid on an ice-cold tube.
The burning hydrogen reacts with the oxygen of the air as given by the
equation below:

Physical tests for water

1. Water can be recognized by its action of turning white anhydrous copper (II) sulphate to blue.

The
test, however, confirms the presence of water and not the absence of
everything else except water. For example, a dilute sulphuric acid would
turn anhydrous copper (II) sulphate from white to blue. That is why
this test is called a physical test as opposed to a chemical test for
water.

2.
The presence of water can also be shown by the use of cobalt chloride
paper. This is a filter paper impregnated with cobalt (II) chloride. The
paper is blue in colour. The blue paper turns pink when in contact with
water.

A chemical test for water

The
two tests above only confirm the presence of water but do not indicate
the purity of the water. Now, how can we test if an unknown colourless
liquid contains water or if it is pure water? The presence of water will
do the following:

Will turn anhydrous copper (II) sulphate from white to blue.
Will turn anhydrous cobalt (II) chloride from blue to pink.

To
find out if a liquid is pure water, its boiling point or its freezing
point must be measured. Pure water boils at exactly 100ºC and freezes
at 0ºC at pressure of one atmosphere (760 mmHg). This is a chemical test for water.

Treatment and Purification of Water

Processes of Domestic Water Treatment and Purification

Perform processes of domestic water treatment and purification

Water
for domestic use is chiefly obtained from rivers, springs and wells;
and sometimes from lakes and seas. However, lake and sea waters may be
to too salty for drinking or washing and hence not normally used for
such purposes. But for some countries, the sea is a major source of
drinking water. However, this water must be desalinized (have its salt
removed) and purified before being used for drinking. The process is
very expensive. It involves an expenditure of big sums of money. It is
only practised in developed countries.

River
and spring water must be boiled and filtered before drinking. At homes,
water is normally boiled in big pans, cooled down, and then filtered by
using a white, sterile and clean piece of cloth. The cloth is tied
around the mouth of the container as shown in figure 3.3(a). As water is
poured through the cloth, the particles in it are filtered off. The
clean water is then poured in clay pots or plastic buckets and placed in
a cool place, or put in a refrigerator to cool down ready for drinking.

Alternatively,
boiled water can be filtered using a funnel as shown in figure (b)
bellow, but you must ensure the gravel, sand and cotton wool used are
thoroughly sterile. Sterilization can be achieved by soaking the gravel
and sand in hot boiled water for quite some time.

The
gravel traps any large floating substances. The coarse sand prevents
small particles from passing through. The fine sand ensures even the
small suspended particles do not pass through, while the cotton wool
filters the very tiny particles.

(a): Filtering water using a piece of cloth

(b): Filtering water using a funnel

At
home, water can also be purified with chemical purifiers. These
chemicals are in liquid or tablet form. To purify water, a recommended
amount of the purifier is added to a specific amount of water in a
container. The water is shaken or stirred well. Then it is left to
settle for at least 20 minutes before it can be safe for drinking and
other domestic uses. To get the clearest water, it is advisable to
filter the water thoroughly before adding the purifier. The commonest
and most widely used purifiers are the waterguard and aquaguard.

In
developed countries, commercial filters may be used to purify water at
home. These filters contain charcoal or ceramic element that purifies
the water as it passes through the filter.

The Processes of Urban Water Treatment

Describe the processes of urban water treatment

We obtain our water supply from surface water (for example, rivers, lakes and reservoirs) and ground water
(for example, underground aquifers and lakes). Water from these sources
is never completely pure, particularly if it is drawn from a river. The
water may contain:

bacteria – most are harmless, but some can cause diseases.
dissolved substances – for example, calcium and magnesium compounds dissolved from rocks; and gases from the air.
solid substances and debris – particles of mud, sand, grit, twigs, dead plants and perhaps tins and rags that people have dumped.

All
these impurities are gathered by water as it passes through different
parts of land as rivers or streams. Before water is safe to drink, the
bacteria and solid substances must be removed.

Different
towns and regions of the world apply different methods of water
treatment. The more sophisticated and expensive methods are used by rich
nations such as the UK and USA. Some steps in water treatment, however,
are basic and used by all. They include the following:

Urban water treatment and purification

1. After water has been pumped through the screen to get rid of the larger bits of rubbish, it is pumped through a coarse filter which traps larger particles of solid. The filter could be beds of gravel and fine sand or anthracite.

2. In older purification plants, it may go to a sedimentation tank where
chemicals are added to make smaller particles stick together. Then they
sink to the bottom of the tank. Many chemicals could be used but the
basic ones are the following:

(i) copper sulphate to remove algae;

(ii) sodium carbonate for softening; and

(iii) Aluminium sulphate in the form of potash alum, K₂SO₄.AL₂(SO₄)₃.24H₂O and slaked lime,Ca(OH)₂are
added for coagulating and precipitating all the suspended earthy
material (clay matter). Bacteria and other microorganisms are captured
by the coagulated mud, and precipitated. Sometimes instead of potash
alum, iron (III) alum, (NH₄)₂.Fe(SO₄)_3.24H₂Ocan be used.

The two chemicals (potash alum and slaked lime) react to form aluminium hydroxide and calcium sulphate:

The
aluminium hydroxide is bulky and sticky. Therefore, bacteria,
microorganisms and small particles can stick to it and get precipitated.
The calcium sulphate is by far denser than water. Both the solid
products (aluminium hydroxide, plus organic and inorganic particles
stuck on it, and calcium sulphate) sink to the bottom of the tank. The
whole process is called sedimentation.

3. Water from the sedimentation tank is passed through a fine filter.
The filter could be made of layers of sand, gravel or carbon granules
with thousands of tiny pores. The carbon removes coloured matter, odours
(tastes) and noxious smells from the water. Filtration beds are
expensive to install and require considerable labour to maintain.

4. After filtering, the water is chlorinated and may be aerated.
Chlorine is added to kill harmful bacteria. Chlorine is such a useful
disinfectant that it is used in swimming pools to kill bacteria. In
aeration, water is pumped through fountains and sprout into the air.
Aeration kills many dangerous aquatic bacteria. In some countries and
regions, water is fluorinated by adding sodium fluoride to the water
supply to help prevent tooth decay. Finally, the water is pumped to
storage tanks, and then to homes and factories.

Importance of water treatment and purification

It
is very important that community water supply be well treated and
purified. There are several reasons for this practice. The following are
some of the reasons:

To kill harmful and disease-causing microorganisms such as bacteria, fungi, actinomycetes, amoeba, salmonella, etc.
To remove toxic substances dissolved in water
To remove solid substances and debris from the water such as tins, lags, plant remains, sand, algae, spirogyra, etc.
To remove suspended earthy material (clay matter)
To remove odour and unpleasant smells caused by different contaminants dissolved in water.
To
remove water hardness – sodium carbonate is added in water to remove
both temporary and permanent hardness in water to make the water soft.
Soft water forms lather easily with soap as compared to hard water which
forms scum instead. This means that soft water requires less soap to
form enough lather than hard water does. Therefore, soft water saves
soap and hence money that could have been spent to purchase extra soap
for washing.
The sodium fluoride added to water in some areas helps to fight tooth decay.

Uses of Water

Uses of Water

State uses of water

Water
is one of the most vital natural resources for all life on earth. The
availability and quantity of water have always played an important part
in determining not only where people can live, but also their quality of
life. Even though there always has been plenty of fresh water on earth,
water has not always been available when and where it is needed, nor is
it always suitable for all uses. Water must be considered as a finite
resource that has limits and boundaries to its availability and
sustainability for use.

Where
water supply is limited, conflicts may result between and among the
various uses. The balance between supply and demand for water is a
delicate one. The availability of usable water has and will continue to
dictate where and to what extent development will occur. Water must be
in sufficient supply for an area to develop, and an area cannot continue
to develop if water demand far exceeds supply.

Water has numerous uses in life. The following are some of the uses of water:

Biological use:
Water is essential to life. Most of the reactions in animals and plants
take place in solutions in water. Plants absorb minerals from the soil
in solution form. Animals and plants are found near or in areas where
water can be found.
Domestic use:
Domestic water use is probably the most important daily use of water for
most people. It includes water that is used in the home every day
including water for normal household purposes such as washing clothes
and dishes, drinking, bathing, food preparation, flushing toilets, and
watering lawns and gardens, etc.
Industrial use:
Water is a valuable resource to the nation’s industries for such
purposes as processing, cleaning, transportation, dilution, and cooling
in manufacturing industries. Major water-using industries include cloth,
steel, chemical, paper, and petroleum refining. Industries often reuse
the same water repeatedly for more than one purpose. Water is used as a
solvent in many industrial processes. It is also used for cooling
certain parts of machines.
Irrigation:
Water is artificially applied to farm, orchard pasture, and
horticultural crops, as well as leaching of salts from the crop root
zone in sodic soils. Non-agricultural activities include self-supplied
water to irrigate public and private flower gardens, loans, football
pitches, etc. Crop production in areas that receive little rainfall per
year can be achieved through the practice of irrigation. Water for
irrigation purposes can be drawn from rivers, lakes, swamps and even
from seas.
Water as a solvent: Water
is regarded as a universal solvent. It dissolves almost all substances.
For this reason, it is used for dissolution of chemicals ranging from
poisonous chemicals used in agriculture to non-poisonous chemicals used
in hospitals, laboratories, research stations and for other general
purposes.
Cooling and heating: Due to
its high specific heat capacity, water is used as a coolant for cooling
automobile engines and other machines. Hot water is used during winter
for heating homes in temperate countries. In higher plants, evaporation
causes a cooling effect and therefore helps to cool plant organs. During
hot weather, some animals tend to wallow in water in order to cool
their bodies either through evaporation or by water itself.
Habitat: Water is a habitat for fish and all aquatic animals and plants.
Livestock use:
This includes water for stock animals, feedlots, dairies, fish farms
and other non-farm animals. In arid regions of Tanzania, the Government
has constructed dams to supply water to cattle, and for some domestic
uses.
Mining: Water is used in mines
for extraction of naturally occurring minerals: solids, such as coal and
ores; liquids, such as crude petroleum; and gases, such as natural gas.
This includes quarrying, milling (such as crushing, screening, washing,
and flotation), and other operations as part of mining activity.
Generation of electricity:
Hydroelectric power is generated by river water. Fast-moving river
water (especially in waterfalls and cataracts) is used to turn turbines
to generate hydroelectricity that is supplied to homes, industries,
towns, etc. Most of the electricity we use at home is generated by this
means. Only a small portion is generated through other means.
Navigation and recreation:
People, goods and services can be transported via water bodies like
rivers, lakes and oceans by using vessels such as boats, dhows, canoes
and ships. Water is also used for sports such as swimming, canoeing,
fishing, yachting, water skiing, and many other sports carried out on,
in and under the water.

The Solubility of Different Substances in Water and Organic Solvents

Compare the solubility of different substances in water and organic solvents

Water
is a very good solvent for many ionic substances. There are few
substances, which do not dissolve in water to some extent. Even when you
drink a glass of water, you are also drinking a little of the glass as
well. The amount is very small indeed, but for certain experiments
ordinary glass vessels cannot be used as containers for water because of
this solvent effect. Water is the commonest solvent in use, but other
liquids, are also important. The other solvents are generally organic
liquids such as ethanol, propanone, trichloroethane, etc. These organic
solvents are also important because they will often dissolve substances
that do not dissolve in water. The following table shows an example of
substances that dissolve in water.

Substances soluble and insoluble in water

Soluble compounds	Insoluble compounds
1 All common sodium, potassium and ammonium salts
2. All common nitrates of metals
3. All common chlorides except…………………..	silver, mercury (I) and lead chloride
4. All common sulphates except…………………..	lead, barium and calcium sulphates
5. Sodium, potassium, and ammonium carbonates…	but other common carbonates are insoluble
6. Sodium, potassium and ammonium hydroxides…	but other common hydroxides are insoluble.

When salt is added to water and the mixture stirred, the salt dissolves. The product formed is termed as a solution. The solid that dissolves is known as a solute and the liquid (water) in which a solute dissolves is a solvent.

We
can continue to add more salt and stir until no more salt dissolves. At
this point, the water has dissolved the maximum amount of salt
possible. The amount of salt dissolved denotes the maximum amount of
salt which can normally be held in solution.

Adding a salt to water

The solution made is called saturated solution. The amount of the salt that has dissolved is called the solubility of the salt in water. The solubility
of a substance is usually expressed as the mass of the substance
dissolved in 100g of water. Solubility is sometimes expressed in moles
of solute per dm³ of solution at that temperature.

To
give a quantitative meaning to solubility, it is necessary to fix the
amount of the solvent used and to state the temperature at which
dissolution occurs. The amount of solvent is usually fixed at 100g. For
example, the solubility of sugar (sucrose) at 20ºC is 240g in 100g of
water. What is the maximum weight of sugar that will dissolve at 20ºC in
a cup containing 350g of water? A saturated solution of a solute at a particular temperature is the one which will not dissolve any more of the solute at that temperature.

The solubility
of a solute in water at a given temperature is the maximum amount of it
that will dissolve in 100g of water at that temperature.

Dissolving a solid in water

Generally,
the solubility of a solute increases with increase in temperature.
However, there are a few exceptions e.g. the solubility of calcium
hydroxide decreases with increase in temperature. Sugar dissolves very
slowly in water at room temperature (20ºC). Stirring helps to make sugar
dissolve more quickly. But if you keep on adding sugar to the water
even with continuous stirring, eventually no more sugar will dissolve.
Extra sugar sinks to the bottom. The solution is saturated.

Dissolving a solid in water at room temperature

Now
let us look at what happens when you heat the sugar solution. If you
heat the solution up to 20ºC there is still undissolved sugar at the
bottom of the beaker. Increasing the temperature to 50ºC makes some
sugar dissolve but there is still some left. But if the temperature is
raised up to 80ºC all the sugar dissolves. You might even be able to
dissolve more sugar!

Dissolving a solid in water at higher temperatures

Therefore,
sugar is more soluble in hot than in cold water. In fact, this is
usually the case with soluble solids. If a solid is soluble in a liquid,
it usually gets more soluble as the temperature rises.

Solubility of different substances in different solvents

The solubility of a substance depends on the following factors:

1. The type of solvent used:
Iodine is slightly soluble in water. Only 0.3g will dissolve in 100g of
water at 20ºC. However, it is much more soluble in cyclohexane (organic
solvent). 2.8g of iodine dissolve in 100g of cyclohexane at 20ºC

2. The particles in it: Let us consider the dissolution of sodium chloride in water. When dissolved in water, the salt dissolves to form Na⁺ and Cl^–ions. If sodium chloride is added to water, the Na⁺ ions will be attracted to the slightly negatively charged oxygen atoms of the water molecules whereas Cl^–ions will be attracted to the slightly positively charged hydrogen atoms of the water.

3. The temperature of the solvent:
As we saw early, the temperature affects the solubility of substances,
particularly solids. The higher the temperature the higher is the
solubility.

If
you shake some cyclohexane with a solution of iodine in water, almost
all iodine leaves the water and moves into cyclohexane layer. So,
cyclohexane is much better than water at separating iodine particles
from each other. The iodine particles are more attracted to cyclohexane
than they are to water. So, the solubility of each substance is
different. Look at these examples:

Compound	Mass (g) dissolving in 100g of water at 25ºC
Silver nitrate	241.3
Calcium nitrate	102.1
Magnesium chloride	53.0
Potassium nitrate	37.9
Potassium sulphate	12.0
Calcium hydroxide	0.113
Calcium carbonate	0.0013
Silver chloride	0.0002

As
you can see, one compound of a metal may be slightly soluble while
another is almost soluble (compare silver nitrate and silver chloride).
It depends on particles.

Measuring the solubility of a solid in water

Let us take potassium sulphate as our example. This is what to do:

Put a weighed amount (say 2g) of potassium sulphate in a test tube. Add a little water from a measuring cylinder.
Heat the test tube gently until the water is hot but not boiling. Add more water if necessary until the solid is just dissolved.
Let the solution cool while stirring it with a thermometer. Note the temperature at which the first crystals form.

Measuring the solubility of a solid in water

Now look again at step 3.
If you add a little more water, heat the solution again to make sure
all the crystals have dissolved, and then let it cool, you will be able
to find the solubility at a lower temperature. You can repeat this for a
range of temperatures.

Calculating solubility

Since
you know the mass of solute and the volume of water you used, you can
work out the solubility as shown in the calculation below:

Example 1

2 grams of potassium sulphate were dissolved in 12.5 cm³of water. On cooling, the first crystals appeared at 60ºC. What is the solubility of potassium sulphate in water at 60ºC?

Solution

12.5 cm³
of water weighs 12.5g. Also, remember that solubility is measured by
100g of water. If 2g of the salt dissolved in 12.5g of water, then the
amount of the salt in 100g of water.

Therefore, the solubility of potassium sulphate in water at 60ºC is 16 grams.

Solubility of gases

Solid
solutes usually get more soluble in water as the temperature rises. The
opposite is true for gases. Table 3.3 shows the solubility of different
gases in water at different temperatures.

Solubility of different gases in water

Gas	Solubility (cm³ per 100cm³of water) at…..
Gas	0ºC	20ºC	40ºC	60ºC
Oxygen Carbon dioxide Sulphur dioxide Hydrogen chloride	4.8171798050500	3.392.3425047400	2.556.6217044500	1.936.0-42000

Look
at carbon dioxide. It is quite soluble in water at room temperature
(20ºC). But when it is pumped into soft drinks under pressure, a lot
more dissolves. Then when you open the bottle, it fizzes out of
solution.

Look at hydrogen chloride. At room temperature, it is over 14000 times more soluble than oxygen.

Generally,
the solubility of gases changes with temperature and pressure. It
decreases with temperature and increases with pressure.

Solubility curves

The
solubility of a particular solid in water can be measured over a range
of temperatures up to 100ºC. The maximum mass of solid that will
dissolve in 100g of water is found at each temperature. The values at
each temperature can then be plotted to give a solubility curve. A curve
that shows how the solubility of a substance changes with temperature
is what we call a solubility curve.

Table bellow shows the solubility of some salts in water at different temperatures.

Solubility of some salts in water

Temperature in ºC	Solubility in g of salt per 100g of water
	Sodium chloride	Copper (II) sulphate	Potassium nitrate
10	38	18	20
20	38	20	30
30	38	24	44
40	38.5	28	60
50	38.5	34	80
60	39	42	104
70	39	50	152

For
most substances, solubility in water increases with increase in
temperature. Table above shows the solubility of some salts in water at
different temperatures.

When the values for each salt shown on the table are represented on a graph paper, different solubility curves result.

Look
at the values in table above again. On a graph paper, use the same set
of axes to plot solubility (vertical axis) against temperature
(horizontal axis). Draw a smooth best-fit curve for each salt.

Which of the salts is the most soluble at 15ºC?
Which of the three salts is the most soluble at 55ºC?
At which temperature do sodium chloride and potassium nitrate have the same solubility?

The
curves in figure bellow show how the solubility of different salts
changes with temperature. You can see that the solubility of most solids
increases with increase in temperature. The increase for sodium
chloride is very small and almost negligible. The increase for the other
salts is as shown in the graph.

Solubility curves for three solids in water (solubility measured in grams of solid per 100g of water)

For
gases, the solubility decreases with increase in temperature. This
means that decreasing the temperature will increase the solubility of
gases. Figure 3.10 shows the solubility curves for some common gases.
Compare these curves with those for solids in figure above.

The solubility of three gases from the air in water (solubility measured in grams of gas per 100g of water)

Using solubility curves

Data can be obtained from the solubility curves in various ways. For example, look at figure above.

(a) What mass of potassium nitrate dissolves in 100g of water at

40ºC and
50ºC?

From the graph:

At 50ºC, 137.5g of potassium nitrate dissolve in 100g of water.
At 40ºC, 62.5g of potassium nitrate dissolve in 100g of water.

(b) What mass of potassium nitrate will crystallize out when a saturated solution in 100g of water is cooled from 50ºC to 40ºC?

TOPIC 4: FUEL AND ENERGY

A
fuel is a substance that can be combusted or burnt to release energy as
a byproduct. The energy can be in the form of heat, light, electricity,
sound etc. This energy can be harnessed to power machines or used for
other purposes such as heating or lighting. Combustion is the burning of
fuel with energy released as a byproduct. Fuel is a very important
substance for the existence of a modern man. Examples of fuels include
petroleum products (petrol, diesel, fuel oil, kerosene, spirits, etc),
natural gas, coal, wood, charcoal, producer gas, water gas, etc.

Fuel Sources

Different Sources of Fuels

Identify different sources of fuels

There
are many types of substances that are used as fuels. The fuels exist as
solids, liquids or gases. The most common substances that are used as
fuels in Tanzania include wood, wood charcoal, coal, petroleum products
and natural gas. These fuels are obtained from different sources as
analysed below:

Wood:
wood is obtained from logs or poles of trees. The wood used as fuel in
Tanzania is obtained from natural and artificial forests. Wood fuel is
mainly used in rural areas where there are no alternative fuels. Wood is
also a major source of fuel used by government institutions such as
schools, colleges, hospitals, and military institutions.
Charcoal:
This fuel is made by heating certain substances such as wood and bones
in a limited supply of air. Wood charcoal is the main source of fuel in
urban areas and in some townships.
Coal: coal
used in Tanzania is mined at Kiwira coal mines. It is used indirectly
for generating electricity or directly for powering machines in
processing and manufacturing industries and factories. The electricity
generated from coal is used in such industries as Tanga cement and
several other industries in Dar es Salaam.
Natural gas:
This gaseous fuel is mined at Songosongo in Kilwa (Lindi region),
located in southern Tanzania. The gas is used as a fuel at homes and in
small industries. It is also used to generate electricity that is used
in various manufacturing and processing industries. The electricity
generated from this gas is also sold to Tanzania Electricity Supply
Company (TANESCO) who distributes the energy to its various clients.
Petroleum products (kerosene, diesel, petrol, fuel oil, fuel gas, etc.)
These petroleum fractions are obtained from crude oil by the process of
fractional distillation of crude oil (petroleum). Diesel, petrol and
oil are used in vehicles and other machines. Kerosene is used in
kerosene lamps and stoves for heating at homes and for other general
purposes.

Methods of Obtaining Fuels from Locally Available Materials

Describe methods of obtaining fuels from locally available materials

Methods of making charcoal

When
we heat certain organic matter in a limited supply of air, we obtain a
black, solid residue called charcoal. The organic matter can be from
plant or animal sources for example, wood or animal bones. Heating a
substance in limited supply of air is called destructive distillation.

Wood
or bone charcoal is made by the process of destructive distillation of
wood or bones respectively. Charcoal is largely pure carbon. The entry
of air during carbonization (destructive distillation) process is
controlled so that the organic material does not burn down to ash as in
conventional fire, but instead decompose to form charcoal.

Procedure for making wood charcoal

Cut wood into small pieces.
Arrange the wood pieces into a pile of wood on the ground.
Cover the pieces of wood with soil, leaving one open space for setting fire.
Set fire to the wood and then cover the open space with soil. Make sure that the wood is burning.
After the wood is burned, uncover the soil and pull out the black solid substance underneath. This is the charcoal.

Coal formation

Coal
is formed from the remains of lush vegetation that once grew in warm
shallow coastal swamps. The following are the stages in the process of
coal formation:

The dead
vegetation collects in the bottom of the swamp. It may start to decay.
But decay soon stops, because the microbes that cause it need oxygen,
and the oxygen dissolved in the stagnant, warm water is quickly
depleted.
The vegetation is buried under debris.
Over
hundreds of thousands of years, the environment changes. Seas flood the
swamps. Heavy layers of sediment pile up on the dead vegetation,
squeezing out gas and water and turning it into peat.
As the peat is buried deeper, the increasing heat and pressure compress it progressively to form different types of coal.
As
the process continues, the coal gets harder and more compact. Its
carbon content also increases, giving different types of coal. Table
bellow shows a summary of the stages in the process:

Stages of formation of different types of coal

	Name of coal	Carbon content
	Peat	60%
Pressure and Heat	Lignite	70%	Hardness
	Bituminous coal	80%
	Anthracite	95%

As
carbon content increases so does energy given out per unit weight. But
hard coal tends to have higher sulphur content,hence likely to cause
environmental pollution. When burnt, the sulphur in the coal produces
sulphur dioxide gas that is released into the atmosphere, causing air
pollution.S_(s)+O_2(g)->S0_2(g)

Categories of Fuels

Fuels
can be classified into three groups according to the physical state of
the fuel. A fuel can be in any of the three states of matter namely,
solid, liquid or gaseous state.

Fuels According to their States

Classify fuels according to their states

Solid fuels

Solid
fuels include wood, charcoal, peat, lignite, coal, coke, etc. The
immediate use of all these fuels is for heating and lighting. However,
these fuels have a long history of industrial use. Coal was the fuel for
the industrial revolution, from firing furnaces to running steam
locomotives and trains. Wood was extensively used to run locomotives.
Coal is still used for generation of power until now. For example, in
Tanzania the coal mined at Kiwira is used for generation of electricity.
Also Tanga Cement Company uses coal as a source of power to run
machines for production of cement.

Wood
is used as a solid fuel for cooking, heating or, occasionally, as a
source of power in steam engines. The use of wood as a fuel source for
home heating is as old as civilization itself. Wood fuel is still common
throughout much of the world. It is the main source of energy in rural
areas.

Wood charcoal
yields a large amount of heat in proportion to its quantity than is
obtained from a corresponding quantity of wood, and has a further
advantage of being smokeless. Wood charcoal is often used for cooking
and heating, in blacksmithing, etc.

Animal charcoal
is used for sugar refining, water purification, purification of factory
air and for removing colouring matter from solutions and from brown
sugar. Animal charcoal is made by destructive distillation of animal
bones.

Coke
is a fuel of great industrial use. Coke is obtained by destructive
distillation of coal. Most of the coke produced in industry is used as a
reducing agent in the production of metals such as pig iron. A
substantial amount of coke is also used for making industrial gases such
as water gas and producer gas.

Coke
is a better fuel than coal because when it is burning, it produces a
clean and smokeless flame. When coal is used as a fuel, it produces many
toxic gases during burning. Coke has high heat content and leaves very
little ash.

Coal
is a complex mixture of substances, and its composition varies from one
place to another. It depends on coal’s age and condition under which it
was formed. Anthracite is a very hard black coal and it is the oldest
of all types of coal.

When
coal is heated in a limited supply of air, it decomposes. This thermal
decomposition is called destructive distillation of coal. The products
are coke, coal tar, ammoniacal liquor and coal gas.

Liquid fuels

Liquid
fuels include petrol (gasoline) diesel, alcohol (spirit), kerosene
(paraffin), liquid hydrogen, etc. Liquid fuels have advantage over solid
fuels because they produce no solid ashes, and can be regulated by
automatic devices. They are relatively more convenient to handle, store
and transport than solid fuels.

Most
liquid fuels in wide use are derived from fossils. Fossil fuels include
coal, natural gas and petroleum. These fuels are formed from remains of
sea plants and animals which lived millions of years ago. The remains
became buried under layers of sediment. Immense heat and pressure
resulted in the formation of coal gas and oil.

Energy
produced when petroleum products (diesel, petrol, kerosene, natural gas
etc) are burned, originated from the sun. This energy was transferred
to animals through their consumption of plants or plant products. When
the animals died, got buried, and compressed by heat and pressure, they
produced oil which gives off that energy when burnt.

Petroleum fuels
are used in cars and in various other machines. Fuels used in cars and
lories (petrol and diesel), kerosene (for jet aircraft) and fuel oil
(for ships), all came from crude oil. Some oil fuel is also used for
electricity generation.

Ethanol
burns with a clean, non-smoky flame, giving out quite a lot of heat. On
a small scale, ethanol can be used as methylated spirit (ethanol mixed
with methanol or other compounds) in spirit lamps and stoves. However,
ethanol is such a useful fuel that some countries have developed it as a
fuel for cars. In countries where ethanol can be produced cheaply, cars
have been adapted to use a mixture of petrol and ethanol as fuel.

Brazil
has a climate suitable for growing sugarcane. Ethanol produced by
fermentation of sugarcane has been used as an alternative fuel to
gasoline (petrol), or mixed with gasoline to produce “gasohol”.
Currently, about half of Brazil’s cars run on ethanol or “gasohol”.
“Gasohol” now accounts for 10% of the gasoline sales in the U.S.A.

The
idea about the use of biofuel for fuelling automobiles and other
machines has been borrowed by other countries including Tanzania.
However, the programme has raised a bitter concern among different
activists. Their doubt is that emphasis on growing crops for biofuel
production may take up land that could otherwise be used for growing
food crops. This, therefore, would mean that there would not be enough
land to grow enough food to feed the ever-increasing human population.
Hence, hunger will prevail. Notwithstanding all these shouting, biofuel
crop production is there to stay!

Gaseous fuels

The
use of gaseous fuels for domestic heating is common in urban areas.
Compressed gas that is delivered to our homes in steel cylinders is
liquefied propane, butane, or mixture of the two. When the valve is
opened, the liquid gas vapourizes quickly into gas and passes through a
pipe to the stove. Gaseous fuels are the most convenient fuels to
handle, transport and store.

The following is a list of types of gaseous fuels:

Fuel naturally found in nature: -natural gas -methane from coal mine
Fuel
gas from solid fuels or materials: -gas derived from coal (water gas
and producer gas) -gas derived from wastes and biomass (biogas)
Fuel gas made from petroleum.

Gaseous fuels used in industry

Producer gas and water gas are important industrial fuels.

Producer gas

Producer
gas is produced by burning a solid carbonaceous fuel, such as coke, in a
limited supply of air in a producer furnace. The reaction is exothermic
and this makes coke to get hotter. Carbonaceous fuels are fuels that
contain a high proportion of carbon. The producer gas is a mixture of
carbon monoxide and nitrogen.

When
air, mixed with a little steam, is passed through the inlet in the
lower part of the furnace, the coke (carbon) combines with oxygen (from
air) to form carbon dioxide:

As the carbon dioxide formed rises up through the red-hot coke, it is reduced to carbon monoxide:

Since
more heat (406 kJ) is produced in the lower part than is absorbed in
the upper part of the furnace (163 kJ), some excess heat is obtained in
the long run. This heat keeps the coke hot. The nitrogen gas in the
air is not affected at all during the process. Hence, the overall
reaction equation may be represented as follows:

As a fuel, producer gas burns to give out carbon dioxide.

Because
a good deal of producer gas contains nitrogen, a gas that does not
support combustion, it has a lower calorific value compared to water
gas. See table 4.2 for comparison.

Water gas

Water
gas is produced by passing steam over white-hot coke at 1000°C. The gas
is a mixture of hydrogen and carbon monoxide. The reaction is
endothermic, causing the coke to cool.

Water gas burns as a fuel to give carbon dioxide and steam.

However,
carbon monoxide is a very poisonous gas. The gas made from petroleum or
coal contains some carbon monoxide, which makes it poisonous. Natural
gas is safer and efficient, as it contains no carbon monoxide.

Characteristics of a good fuel

A
good fuel burns easily to produce a large amount of energy. Fuels
differ greatly in quality. There are certain characteristics, which make
a good fuel. After all, there is no fuel among the different fuels
known that posses all the virtues that a good fuel should have.
Generally, a good fuel has the following

characteristics:

It
should be environmentally friendly (not harm the environment) in the
course of its production and use, that is, it should not produce harmful
or toxic products such as much smoke, carbon dioxide, carbon monoxide,
sulphur dioxides, etc, which pollutes the air.
It must be affordable to most people i.e. it must be cheap.
It should not emit or produce dangerous by-products such as poisonous fumes, vapour or gases.
It
should have high calorific value i.e. it must burn easily and produce a
tremendous quantity of heat energy per unit mass of the fuel.
It should be easy and safe to transport, store, handle and use.
It should be readily available in large quantities and easily accessible.
It
should have high pyrometric burning effect (highest temperature that
can be reached by a burning fuel). Normally gaseous fuels have the
highest pyrometric effect as compared to liquid and solid fuels.
It should have a moderate velocity of combustion (the rate at which it burns) to ensure a steady and continuous supply of heat.
A
good fuel should have an average ignition point (temperature to which
the fuel must be heated before it starts burning). A low ignition point
is not good because it makes the fuel catch fire easily, which is
hazardous, while high ignition point makes it difficult to start a fire
with the fuel.
A good fuel should have a low content of
non-combustible material, which is left as ash or soot when the fuel
burns. A high content of no-combustible material tends to lower the heat
value of the fuel.

Calorific values of fuels

The
heating value or calorific value of a substance, usually a fuel or
food, is the amount of heat released during the combustion of a specific
amount of it. The calorific value is a characteristic of each
substance. It is measured in units of energy per unit of substance,
usually mass, such as Kcal/Kg, J/g, KJ/Kg, KJ/Mol, MJ/m³, etc. Heating value is commonly determined by use of an instrument called bomb calorimeter.

By
custom, the basic calorific value for solid and liquid fuels is the
gross calorific value at constant volume, and for gaseous fuels, it is
the gross calorific value at constant pressure.

Calorific values of solid, liquid and gaseous fuels

Solid and liquid fuels	Calorific value (MJ/kg)
Alcohols
Ethanol	30
Methanol	23
Coal and coal products
Anthracite (4% water)	36
Coal tar fuels	36 – 41
General purpose coal (5-10% water)	32 – 42
High volatile coking coals (4% water)	35
Low temperature coke (15% water)	26
Medium-volatile coking coal (1% water)	37
Steam coal (1% water)	36
Peat
Peat (20% water)	16
Petroleum and petroleum products
Diesel fuel	46
Gas oil	46
Heavy fuel oil	43
Kerosene	47
Light distillate	48
Light fuel oil	44
Medium fuel oil	43
Petrol	44.80 – 46.9
Wood
Wood (15% water)	16
Gaseous fuels at 15ºC, 101.325 kPa, dry	Calorific value (MJ/m³)
Coal gas coke oven (debenzolized)	20
Coal gas low temperature	34
Commercial butane	118
Commercial propane	94
North sea gas, natural	39
Producer gas coal	6
Producer gas coke	5
Water gas carburetted	19
Water gas blue	11

Measuring the heat given out by fuels

We
burn fuels to provide us with heat energy. The more heat a fuel gives
out the better. The amount of heat given out when one mole of fuel burns
is called heat of combustion. This is often written as

This
value can be measured in the laboratory indirectly by burning the fuel
to heat water. Simple apparatus is shown in figure bellow. The basic
idea is: Heat gained by the

Heat gained by the water = heat given out by the fuel.

Method

These are the steps:

Pour a measured volume of water into the tin. Since you know its volume you also know its mass (1 cm³ of water has a mass of 1g).
Weigh the fuel and its container.
Measure the temperature of the water.
Light the fuel and let it burn for a few minutes.
Measure the water temperature again, to find the increase.
Reweigh the fuel and container to find how much fuel was burned.

Measuring the energy value of a fuel

Calculations

It
takes 4.2J of energy to raise the temperature of 1g of water by 1ºC.
This constant value is called specific heat capacity of water, usually
represented as 4.2J_g^-1C^-1 (4.2 joules per gram per centigrade). So, you can calculate the energy given out when the fuel burns by using this equation:

Energy given out = 4.2_g-1C^-1 mass of water (g) its rise in temperature (ºC).

Then since you know what mass of fuel you burned you can work out the energy that would be given out by burning one mole of it.

Example 1

The experiment gave these results for ethanol and butane. Make sure you understand the calculations:

Experimental results for heat determination

Ethanol (burned in a spirit lamp)	Butane (burned in a butane cigarette lighter)
Results	Results
Mass of ethanol used: 0.9g	Mass of butane: 0.32g
Mass of water used: 200g	Mass of water used: 200g
Temperature rise: 20ºC	Temperature rise: 12ºC
Calculations	Calculations
Heat given out = or 16.8KJ	Heat given out = = 10080J or10.08KJ
The formula mass of ethanol is 46. 0.9g gives out 16.8KJ of energy. So, 46g gives out of energy	The formula mass of butane is 58. 0.32 gives out 10.08KJ of energy. So, 58g gives out KJ of energy
So, H _combustion for ethanol is -859KJ/mol	So, H _combustion for butane is –1827 KJ/mol

Example 2

Determination of energy (calorific) value of ethanol

The energy/heating/calorific value of a fuel refers to the amount of heat given out when a specific amount of fuel is burned.

Experiment

Aim: To find out the energy value of ethanol.

Materials: water, beaker, thermometer, weighing balance, spirit lamp and ethanol.

Procedure:

Pour a known volume of water into a beaker.
Measure the temperature of the water.
Fill the spirit lamp with enough ethanol.
Weight the mass of both the ethanol and the lamp.
Light the lamp and let it continue burning for a few minutes before putting it off.
Measure the water temperature again, to find the increase.
Reweigh the ethanol and its container to find how much ethanol was burned.

Record the following:

Mass of spirit lamp + ethanol (initially)
Mass of spirit lamp + ethanol (finally)
Mass of ethanol burned
Final temperature of water
Initial temperature of water
Rise in temperature of water
Mass of water

The amount of heat (q) released by ethanol is given by:

Specimen calculation:

Mass of lamp and ethanol initially = 50g

Mass of lamp and ethanol finally = 49.5g

Mass of ethanol burned = 50.0 — 49.5 = 0.5g

Mass of water = 100g

Final temperature of water = 42ºC

Initial temperature of water = 20ºC

Rise in temperature = 42ºC – 20ºC = 22ºC

Specific heat capacity of water = 4.2 Jg^-1C^-1

Heat given out = Mass of water Xspecific heat capacity Xtemperature rise

Repeat
similar procedures with kerosene, charcoal, coal, firewood etc. and
compare your results. Which fuel has more energy per gram? That is the
most efficient fuel.

How reliable is the experiment?

The following table compares the experimental results with values from data book.

Fuel	Heat of combustion in KJ/mol
	From the experiment	From a data book
Ethanol	-859	-1367
Butane	-1827	-2877

Note the
big difference! The experimental results are almost 40% lower for both
fuels. Why do you think there is such a big difference? There are two
reasons for this:

Heat loss:
Not all the heat from the burning fuel is transferred to the water.
Some is lost to the air, and some to the container that holds the fuel.
Incomplete combustion: In case of a complete combustion, all the carbon in a fuel is converted to carbon dioxide. But here combustion is incomplete.
Some carbon is deposited as soot on the bottom of the lamp and some
converted to carbon monoxide. For example, when butane burns, a mixture
of all these reactions may take place:

The less oxygen there is, the more carbon monoxide and carbon will form.

Uses of Fuels

Uses of Fuels

List uses of fuels

You
have already learned different types of fuels and their energy values.
Fuels can be put into several uses. The use of a given kind of a fuel
for a particular function depends on the economic value of that use.
Generally, the uses of fuels include the following:

1. Source of mechanical power:
Vehicles, machines and several other devices are powered by fuels such
as diesel, petrol, oil, etc as a source of mechanical power. In some
countries, vehicles have been modified to use natural gas as a source of
power. In Tanzania for example, plans are underway to modify car fuel
systems so that a natural gas obtained from Songosongo in Kilwa could
power cars. This will help a great deal to reduce the cost of running
cars on liquid fuels whose price in the world market is continuously
escalating. Hydrogen may become an important fuel for cars and homes in
the future, as we run out of oil and gas. It has two big advantages:

Its reaction with oxygen produces just water. No pollution to the environment!
It
is a ‘renewable’ resource. It can be made by electrolysis of acidified
water. As cheaper sources of electricity for electrolysis are developed,
this may become an attractive option.

2. Cooking and heating:
Fuels like wood, liquefied gas (propane or butane or a mixture of the
two), charcoal and kerosene are burned to provide energy for cooking and
heating. When burned, these substances provide enough heat to cook food
and even heat different substance at home. Inhabitants of cold
countries in temperate regions of the world burn different kinds of
fuels to produce heat for heating homes and water.

3. Generation of electricity:
The machines and devices responsible for electricity production and
supply are fuelled by heavy liquid fuels such as diesel, fuel oil, etc.
Most generators use liquid fuels such as petrol and diesel to generate
electricity. So, fuels play an important role in electricity production.
In Tanzania, coal from Kiwira mines is used for generation of
electricity used in Tanga Cement Factory and some industries in Dar es
Salaam. This is why escalation of crude oil in the world market results
to increased cost of electricity supplied to homes and industries. In
developed countries, uranium is used as a fuel to generate electricity
which is used at homes and in industries.

4. Lighting:
Kerosene is used in paraffin lamps, tin lamps and hurricane lamps by
the rural communities to light homes. The use of paraffin is important
in rural areas of Tanzania where 90% of the total population stay and
earn their living. It is estimated that only 10% of the population have
access to electricity. So, you can see how crucial this fuel is to the
majority of the people.

5. Industrial uses:
Industrial operations such as welding and metal fabrication make use of
oxyacetylene flame which produces extremely high heat to melt and cut
metals.

6. Other alternative uses: manufacture of different kinds of products such as petroleum jelly, nylon and plastic.

The Environmental Effects on Using Charcoal and Firewood as Source of Fuels

Assess the environmental effects on using charcoal and firewood as source of fuels

Trees
are the most common source of fuels in developing countries like
Tanzania. Fuels from trees are mainly used for domestic purposes. People
cut down trees for firewood and for burning charcoal that is mainly
supplied to urban areas to be used as fuel.

Because
of the rapidly growing human population, the demand for trees as a
source of fuel has ever increased to the extent that this resource is no
longer sustainable. The act of cutting down trees for firewood,
charcoal, timber, and for obtaining logs that are shipped to overseas
has made this resource to be depleted. This leads to environmental
destruction, a result that causes many problems to the human society and
other organisms as well.

Trees
have several advantages apart from providing us with fuels. Trees help
in the attraction of rainfall and conservation of water sources in
various areas. Trees also help in removing bad gases from air such as
carbon dioxide that is emitted to the atmosphere due to various human
activities. In so doing, trees help to maintain the balance of gases in
the atmosphere.

Trees
and other vegetation provide habitats and shelters for wild animals and
birds of the air. Presence of trees also help to maintain the survival
of microorganisms found in the soil, which are important for the balance
of nature. Trees can make our country look beautiful and hence attract
local and foreign eco-tourists, a fact which can contribute to our
country’s revenue, and economic growth.

Deforestation
results to scarcity of rainfall as we are experiencing these years.
This is because trees attract rainfall. Scarce rainfall leads to
drought. Prolonged drought causes famine. Therefore, people will suffer
from famine if they continue to use firewood or charcoal as their
sources of fuels.

The
other effect is soil erosion, which leads to loss of soil fertility.
Trees act as a soil cover, which makes the soil resist the impact of
raindrops. Deforestation means removal of the soil cover and hence
making the soil bare. It is obvious that tree cutting for firewood or
charcoal will expose the soil to agents of soil erosion such as wind,
water and animals. This will make the soil more prone to erosion. So
long as plants depend on the top soil (which contains more plant
nutrients) for survival and existence, an eroded soil will consequently
support very few or no vegetation at all. The aftermath of this is soil
aridity.

As
noted early, trees help absorb excess carbon dioxide produced by
respiring living organisms. Cutting down trees will lead to excessive
accumulation of carbon dioxide in air. Carbon dioxide, among other
gases, is responsible for excessive heating of the earth, a phenomenon
called global warming. This is because the gas forms a layer in
the atmosphere that acts as a blanket. The layer of carbon dioxide gas
so formed prevents heat emitted by the heated earth from escaping to the
upper atmosphere. This causes extreme heating of the earth’s surface.
Consequences of global warming are many, the worst being drought that
could ultimately lead to extinction of plant and animal species.

Vegetation that has dried up due to prolonged drought

In brief, cutting down trees for charcoal and firewood can lead to the following environmental problems:

prolonged drought spells and hence famine;
drastic change in rainfall patterns;
global warming and climate change;
increased soil erosion and rapid depletion of soil nutrients;
increased aridity and desertification;
loss of valuable species of economic or medicinal value;
broken food chain and reduced ecosystem stability;
destruction of animal habitats and shelters;
extinction of animal, microbial and plant species; and
loss of biodiversity.

Therefore,
it is important to plant more trees and to reduce our dependence on
trees for fuels in order to improve our environment. Tree planting
campaign should be a regular practice and the trees that have already
been planted should be cared for. Natural forests should be conserved.
Local Governments should be encouraged to make and enforce the bylaws
against those people cutting down trees carelessly for charcoal burning.
At the same time, the central Government must look for the alternative
energy sources for her citizens urgently.

Continued
use of trees for fuels will end up our life on earth. Let us take
actions to conserve our environment so that we continue living a healthy
life

Conservation of Energy

What is energy?

Energy
is defined as the ability to do work or bring about change. Energy
makes changes; it does things for us. It moves cars along the road, and
boats over the water. It bakes cakes in the oven and keeps ice frozen in
the freezer. It plays our favourite songs on the radio and lights our
homes. Energy makes our bodies grow and allow our minds to think. People
have learned how to change energy from one form to another so that we
can do work more easily and live more comfortably. The source of all
energy on earth is the sun.

Forms of energy

Energy
exists in many different forms such as heat, light, sound, electrical,
etc. The amount of energy can be measured in joules, kilojoules,
megajoules, calories, etc. There are many forms of energy, but they can
all be put in two categories: Kinetic and Potential.

Forms of energy

KINETIC ENERGY	POTENTIAL ENERGY
Kinetic energy is energy in motion of waves, electrons, atoms, molecules, substances, and objects.	Potential energy is stored energy and the energy of position – gravitational energy.
Electrical energy is the movement of electrical charges. Everything is made of tiny particles called atoms. Atoms are made of even smaller particles called electrons, protons and neutrons. Applying a force can make some of the electrons move. Electrical charges moving through a wire is called electricity. Lightning is another example of electrical energy.	Chemical energy is energy stored in the bonds of atoms and molecules. This energy holds these particles together. Biomass, petroleum, natural gas, and propane are examples of stored chemical energy.
Radiant energy is electromagnetic energy that travels in transverse waves. Radiant energy includes visible light, x-rays, gamma rays and radio waves. Light is one type of radiant energy. Solar energy is an example of radiant energy.	Stored mechanical energy is energy stored in objects by the application of a force. Compressed springs and stretched rubber bands are examples of mechanical energy.
Thermal energy, or heat energy, is the internal energy in substances caused by the vibration and movement of the atoms and molecules within substances. Geothermal energy is an example of thermal energy.	Nuclear energy is energy stored in the nucleus of an atom – the energy that holds the nucleus together. The energy can be released when the nuclei are combined or when a nucleus splits apart (disintegrates). Nuclear power plants split the nuclei of uranium atoms in a process called fission. The sun combines the nuclei of hydrogen atoms in a process called fusion. Scientists are working creating fusion energy on earth, so that someday there might be fusion power plants.
Motion energy is the energy which enables movement of objects and substances from one place to another. Objects and substances move when a force is applied according to Newton’s laws of motion. Wind is an example of motion energy.	Gravitational energy is the energy of position or place. A rock resting at the top of a hill contains gravitational potential energy. Hydropower, such as water in reservoir behind a dam, is an example of gravitational potential energy.
Sound energy is the movement of energy through substances in longitudinal (compression/rarefaction) waves. Sound is produced when a force causes an object or substance to vibrate – the energy is transferred through the substance in a wave

Kinetic
energy is energy in motion. Its existence can be shown by winds, ocean
currents, running water, moving machines or a falling body.

Potential
energy is energy at rest. It is found stored in different forms, e.g.
in coal, petroleum and natural gas, batteries and muscles. Such energy
does not work so long as it is stored. It is capable of doing work when
it is converted to other forms of energy such as heat, light or
radiation.

The Law of Conservation of Energy

Explain the law of conservation of energy

Energy conversion (Energy changes)

Can
energy be created or destroyed? When wood or charcoal is burned, it
appears as if energy is destroyed and wasted. In fact, the energy in
these kinds of fuels is not destroyed when the fuels are burned. It is
simply converted to other forms of energy such as heat and light.

When
you are seated on a desk in class, you are possessing potential energy.
When you stand up and walk away from the classroom, you are
transforming the potential (chemical) energy in your muscles to kinetic
energy.

The Law of Conservation of Energy
states that energy can neither be created nor destroyed but it can only
be changed from one form to another. When we use energy, it does not
disappear. We simply convert it from one form to another.

Potential
(chemical) energy in a dry cell is converted to electrical energy which
is finally converted to sound energy in radio speakers. In a tape
record player, the same chemical energy is ultimately converted to
kinetic energy to drive the cassettes. When the potential energy is all
used up, the batteries are dead. In the case of rechargeable batteries,
their potential energy is restored through recharging.

The
chemical energy in your mobile phone battery can be converted into
sound, light, text, etc. The main energy changes that occur in a
variety of simple situations are:

Battery chemical to electrical, sound or light;
Car engine chemical to mechanical and then kinetic;
Light bulb electrical to light and heat;
Parachutist potential to kinetic;
Solar heat to electrical and kinetic;
Wind mill kinetic to electrical;
Running water kinetic to electrical;
Muscles chemical to kinetic, etc.

What other situations of energy changes do you know? Mention them

All
energy changes that occur during chemical and physical changes must
conform to the Law of Conservation of Energy, that is, energy can only
be changed from one form into its equivalent of another form with no
total loss or gain.

The
most common form of energy in chemistry is the heat change. A chemical
reaction must involve some change in energy. As the reaction occurs,
chemical bonds of reactant molecules are broken while those of the
product molecules are formed. Energy is given out when a chemical bond
forms and it is consumed when a bond is broken.

Take an example of combustion (respiration) of glucose in living cells:

During
respiration process, the bonds of glucose and oxygen are broken down
while those of carbon dioxide and water are formed. Heat is absorbed
when chemical bonds are broken and it is released when the bonds are
formed. The total amount of heat absorbed by the reactants is equal that
released by the products. Heat absorbed is given a positive sign (+ve)
while heat given out is assigned a negative sign (-ve). So the total
energy change is equal to zero. This means that no energy has been
created or destroyed.

Experiments on the Conservation of Energy from One Form to Another

Carry out experiments on the conservation of energy from one form to another

Activity 1

Carry out experiments on the conservation of energy from one form to another

Renewable Energy Biogas

Renewable
energy sources include biomass, geothermal energy, hydroelectric power,
solar energy, wind energy, and chemical energy from wood and charcoal.
These are called renewable energy sources because they are replenished
within a short time. Day after day, the sun shines, wind blows, river
flows and trees are planted. We use renewable energy sources mainly to
generate electricity.

In
Tanzania most of the energy comes from non-renewable sources. Coal,
petroleum, natural gas, propane and uranium are examples of
non-renewable energy sources. These fuels are used to generate
electricity, heat our homes, move our cars and manufacture many kinds of
products. These resources are called non-renewable because they cannot
be replenished within a short time. They run out eventually. Once, for
example, coal or petroleum is depleted, it may take millions of years to
be replaced. So, these are non-renewable energy sources.

BIOGAS

Biogas
is a gaseous fuel produced by the decomposition of organic matter
(biomass). Under anaerobic conditions, bacteria feed on waste organic
products, such as animal manure and straw, and make them decay. The
product formed from this decay is called biogas, which consists mainly
of methane, though other gases such as carbon dioxide, ammonia, etc, may
also be produced in very small quantities. The biogas produced can be
used as a fuel for cooking, heating, etc.

Raw
materials for biogas production may be obtained from a variety of
sources, which include livestock and poultry wastes, crop residues, food
processing and paper wastes, and materials such as aquatic weeds, water
hyacinth, filamentous algae, and seaweeds.

The Working Mechanism of Biogas Plant

Explain the working mechanism of biogas plant

The
organic waste products are fed in a biogas plant. Prior to feeding the
material into the plant, the raw material (domestic poultry wastes and
manure) to water ratio should be adjusted to 1:1 i.e. 100 kg of excreta
to 100 kg of water. Then adequate population of both the acid-forming
and methanogenic bacteria are added.

The
bacteria anaerobically feed on the liquid slurry in the digester. The
major product of this microbial decomposition is biogas, which largely
contain methane gas. The gas so produced is collected in the gas holder
and then taped off. The gas is used as a fuel for cooking, heating and
other general purposes.

The biological and chemical conditions necessary for biogas production

Domestic
sewage and animal and poultry wastes are examples of the nitrogen-rich
materials that provide nutrients for the growth and multiplication of
the anaerobic organisms. On the other hand, nitrogen-poor materials like
green grass, maize stovers, etc are rich in carbohydrates that are
essential for gas production. However, excess availability of nitrogen
leads to the formation of ammonia gas, the concentration of which
inhibits further microbial growth. This can be corrected by dilution or
adding just enough of the nitrogen-rich materials at the beginning.

In
practice it is important to maintain, by weight, a C:N close to 30:1
for achieving an optimum rate of digestion. The C:N can be manipulated
by combining materials low in carbon with those that are high in
nitrogen, and vice versa.

A
pH range for substantial anaerobic digestion is 6.0 – 8.0. Efficient
digestion occurs at a pH near to neutral (pH 7.0). Low pH may be
corrected by dilution or by addition of lime.

To
ensure maximum digestion, stirring of the fermentation material is
necessary. Agitation (stirring) can be done either mechanically with a
plunger or by means of rotational spraying of fresh organic wastes.
Agitation ensures exposure of new surfaces to bacterial action. It also
promotes uniform dispersion of the organic materials throughout the
fermentation liquor, thereby accelerating digestion.

A Model of Biogas Plant

Construct a model of biogas plant

The
biogas plant consists of two components: the digester (or fermentation
tank) and a gas holder. The digester is a cube-shaped or cylindrical
waterproof container with an inlet into which the fermentable mixture is
introduced in the form of liquid slurry. The gas holder is normally an
airproof steel container that floats on the fermentation mix. By
floating like a ball on the fermentation mix, the gas holder cuts off
air to the digester (anaerobiosis) and collects the gas generated. As a
safety measure, it is common to bury the digester in the ground or to
use a green house covering.

Structure of the biogas plant

The Use of Biogas in Environmental Conservation

Explain the use of biogas in environmental conservation

Environmental
conservation is a major concern in life. We need to live in a clean and
health environment so as to enjoy our lives better. The use of biogas
as an alternative source of energy is essential in environmental
conservation due to a number of reasons. These are some of the reasons:

Biogas does not produce much smoke or ash, which could otherwise
pollute the atmosphere or land. When the gas is burned it produces very
little smoke and no ash as compared to other sources of fuel such as
wood.
The use of biogas for cooking and heating prevents the
cutting down of trees to harvest firewood, or burn charcoal for fuel, a
practice that could result to soil erosion, drought, etc. Hence, using
the biogas as fuel helps to conserve the environment as no more cutting
of trees may be done.
Using cow dung, poultry manure and other
excreta for biogas production helps keep the environment clean because
these materials are put into alternative use instead of just being
dumped on land, a fact that could lead to pollution of the environment.
Some
biomass employed in biogas production is toxic and harmful. By letting
these materials be digested by bacteria, they may be turned into
non-toxic materials that are harmless to humans, plants, animals and
soil.
The excreta used for production of biogas produce foul
smell if not properly disposed of. Using this excrete to generate biogas
means no more bad smell in air.
Health hazards are associated
with the use of sludge from untreated human excreta as fertilizer. In
general, a digestion time of 14 days at 35ºC is effective in killing the
enteric bacterial pathogens and the enteric group of viruses. In this
context, therefore, biogas production would provide a public health
benefit beyond that of any other treatment in managing the rural health
and environment of developing countries.

CHEMISTRY FORM TWO ALL TOPICS.
CHEMISTRY FORM TWO TOPIC 1 & 2.
CHEMISTRY FORM TWO TOPIC 3 & 4.
CHEMISTRY FORM TWO TOPIC 5 & 6.
CHEMISTRY FORM TWO TOPIC 7.

O’LEVEL CHEMISTRY
CHEMISTRY STUDY NOTES, FORM FOUR.
CHEMISTRY STUDY NOTES, FORM THREE.
CHEMISTRY STUDY NOTES, FORM TWO.
CHEMISTRY STUDY NOTES FORM ONE.

CHEMISTRY FORM TWO STUDY NOTES TOPIC 3: WATER & TOPIC 4: FUEL AND ENERGY

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