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CHEMISTRY FORM THREE STUDY NOTES TOPIC 3: ACIDS, BASES, AND SALTS & TOPIC 4: THE MOLE CONCEPT AND RELATED CALCULATIONS.

 TOPIC 3: ACIDS, BASES, AND SALTS
Acids and Bases
The Natural Sources of Acids and Bases
Investigate the natural sources of acids and bases
In
everyday life, we deal with many substances that chemists classify as
acids. For example, orange juice and grapefruit juice contain citric
acid. These juices, and others of the like, contain ascorbic acid, a
substance more commonly known as vitamin C. Examples of natural sources
of acids and the type of acids they contain are shown in table below.
Some natural sources acids
Source Type of acid present
Mineral acids (HCl, H2SO4, HNO3, etc.) Minerals
Tobacco Salicylic acid
Tea Tannic acid
Coffee Chlorogenic acid
Sugar beet Glutaric and adipic acids
Blackberry Isocitric acid
Spinach, tomato Oxalic acid
Sour (fermented milk) Lactic acid
Bee, ant and nettle stings Methanoic acid (formic acid)
Grapes, bananas, tamarinds Tartaric acid
Citrus fruits Citric
acid ( lemons and limes have particularly high concentrations of citric
acid; it can constitute as much as 8% of the dry weight of these
fruits)
Acids
have a sour taste. Vinegar, lemon juice, grapefruit juice and spoilt or
fermented milk are all sour tasting because of the presence of acids.
The acids present in animal and plant materials are known as organic acids.
Salads
are often flavoured with vinegar, which contains dilute acetic acid.
Boric acid is a substance that is sometimes used to wash the eyes.
In any chemistry laboratory, we find acids such as hydrochloric acid (HCl), sulphuric acid (H2SO4), and nitric acid (HNO3).
These acids are called mineral acids because they can be prepared from
naturally occurring compounds called minerals. Mineral acids are
generally stronger and should be handled with great care, especially the
concentrated acids, for they are very corrosive. They can eat away
metals, skin and clothing. Nevertheless, some acids are not corrosive
even when they are concentrated. They are called weak acids. Ethanoic
acid is one example. It is found in vinegar. In general, organic acids
are weaker than natural acids.
You
can tell if a substance is acid or not by its effect on litmus. Litmus
is a purple dye. It can be used as a solution, or on paper, called
litmus paper.Litmus solution is purple. Litmus paper for testing acids
is blue while that for testing bases is red in colour. Acids will turn
litmus solution red. They will also turn blue litmus paper red.
Bases
do not usually occur naturally. So they are not normally obtained from
natural sources. However, they are prepared in the laboratory or in
industry. Bases can be classified into oxides, hydroxides or carbonates.
Therefore, bases can be defined as the oxides, hydroxides or carbonates
of metals. Bases taste bitter. A bitter taste is a characteristic of
all bases.
Most bases are insoluble in water. The bases which dissolve in water are known as alkalis. The most common alkalis are potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium hydroxide, Ca(OH)2, and ammonium hydroxide (NH4OH), also known as ammonia solution.
Alkalis turn litmus solution blue and red litmus paper blue.A substance, such as litmus, which changes from one colour to another when mixed with an acid or base, is called an indicator.
Table 3.2 shows how acids and bases (alkalis) affect the colours of
different indicators. We can use this clue of colour changes to tell
whether an unknown substance is an acid or base (alkali).
Some common indicator colour changes
Indicator Colour in acid Colour in alkali (base)
Methyl orange orange yellow
Phenolphthalein colourless Pink
Litmus red blue
Bromothymol blue yellow blue
The Reactions of Acids with Various Materials
Determine the reactions of acids with various materials
Acids
react with different substances to produce different products. These
reactions are best carried out by using dilute acid solutions. The
following are some reactions of dilute acids with various substances.
Reaction with metals
Acids react with quite reactive metals (not very reactive ones) to produce salt and liberate a hydrogen gas.
metal + acid → salt + hydrogen
It is unsafe to try this reaction with very
reactive metals such as sodium or calcium. The reaction with such
metals is so violent. Metals less reactive than lead, such as silver and
gold have no reaction with dilute acids. Even with lead, it is
difficult to see any reaction in a short time.
The salt produced when a dilute acid reacts with a metal depends on the acid and a metal used:
  • Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)
  • Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Reaction with carbonates
Acids
react with carbonates to give salt, water and carbon dioxide. In
general, all carbonates give off carbon dioxide when they react with
acids.
acid + metal carbonate → salt + water + carbon dioxide
The
normal methods of preparing carbon dioxide in the laboratory are based
on this reaction. Dilute hydrochloric acid is reacted with marble chips
(calcium carbonates):
2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g)
Reaction with oxides and hydroxides (alkalis)
Hydroxides: acids react with alkalis, forming salt and water:
NaOH(aq) + HNO3(aq) → CaCl(aq) + H2O(l)
Oxides: They also react with metals oxides, forming salt and water:
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
The
bases (oxides, hydroxides) all react in the same way with acids, and in
the process, salts are formed. This type of reaction is known as neutralization reaction. It can be summarized up in a general equation:
acid + base → salt + water
Reaction with hydrogencarbonates (bicarbonates)
Acids react with hydrogencarbonates, forming salt, water and liberating carbon dioxide gas:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(g)
The Reactions of Alkalis with Various Materials
Determine the reactions of alkalis with various materials
Alkalis
react with acids to produce salt and water. All alkalis, except ammonia
solution, will react with ammonium compounds liberating ammonia gas.
Aqueous solutions of alkalis will precipitate the insoluble hydroxides
of other metals from the solutions of metal salts. Caustic alkalis
attack aluminium, zinc and lead to form salts. They react with carbon
dioxide to form carbonates.
The Reactions of Bases with Various Substances
Determine the reactions of bases with various substances
Characteristic reactions of bases
Dissolution in water
Most bases are insoluble in water. Some are soluble in water. Soluble bases are known as alkalis. The commonest alkalis are sodium hydroxide, (NaOH), calcium hydroxide, Ca(OH)2, potassium hydroxide, (KOH), and ammonium hydroxide or ammonia solution, (NH4OH). All alkaline solutions contain hydroxyl ions, OH-. In sodium hydroxide solution, the ions are produced like this:
NaOH(aq)→ Na+(aq)+ OH-(aq)
Like acids, alkalis can also be classified as strong or weak. Ammonia solution is a weak base because it ionizes just partially:
The rest of the bases are strong bases because they ionize fully into ions in solution.
Reaction with acids
Bases react with acids to produce salt and water. Refer to the reactions of acids with oxides and hydroxides discussed early.
Reaction with ammonium compounds
Alkalis turn litmus solution blue and red litmus paper blue.A substance, such as litmus, which changes from one colour to another when mixed with an acid or base, is called an indicator.
Table 3.2 shows how acids and bases (alkalis) affect the colours of
different indicators. We can use this clue of colour changes to tell
whether an unknown substance is an acid or base (alkali).
Reaction with aqueous salts of metals
Aqueous
solutions of alkalis will precipitate the insoluble hydroxides of other
metals from the solutions of metal salts. Only NH4OH, KOH and NaOH are soluble enough in water.
All other hydroxides are insoluble and can be precipitated from aqueous solution by these three alkalis.
  • When
    sodium hydroxide solution is added to copper (II) sulphate solution, a
    pale blue precipitate of copper (II) hydroxide is formed. CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2 SO4(aq)
  • Another example is the reaction between potassium hydroxide and iron (II) chloride, which precipitates iron (II) hydroxide. FeCl3(aq) + 3KOH(aq) → Fe(OH)3(s) + 3KCl(aq)
Reaction with metals
Caustic
alkalis attack very few metals. The metals known to be attacked by the
alkalis are aluminum, zinc and lead, where the aluminate, zincate and
plumbate (II) are formed respectively. The aluminum will react thus:2Al(s) + 6NaOH(aq) + 6H2O(l) → 2Na3Al(OH)6(aq) + 3H2(g)(sodium aluminate)
Reaction with carbon dioxide
When carbon dioxide gas is bubbled through aqueous solutions of the caustic alkalis, the carbonates are formed.
2NaOH(aq) +CO2(g) → Na2CO3(aq) + H2O(l). With excess of the gas, the hydrogencarbonates are formed.Na2CO3(aq) + H2O(l) + CO2(g) → 2NaHCO3(aq)
Reaction with chlorine
Chlorine reacts with excess of cold dilute caustic alkalis to form the hypochlorite, (NaClO or KClO).2NaOH(aq) + Cl2(g) → NaCl(aq) + NaClO(aq) + H2O(l).2KOH(aq) + Cl2(g) → KCl(aq) + KClO(aq) + H2O(l)
If excess chlorine is bubbled through hot concentrated solutions of caustic alkalis, the chlorates are formed, (NaClO3 or KClO3).
6NaOH(aq) + 3Cl2(g) → 5NaCl(aq) + NaClO3(aq) + 3H2O(l).6KOH(aq) + 3Cl2(g) → 5KCl(aq) + KClO3(aq) + 3H2O(l)
Applications of acid-base neutralization in everyday life
Applications of acid-base neutralization in everyday life
Acid-base neutralization has many applications in everyday life. The following are some of these applications:
Indigestion and pain relief
The
dilute hydrochloric acid produced in your stomach is used for digestion
and killing bacteria that might have been swallowed together with food
or taken with water. However, excess acid causes indigestion, which can
be painful. To ease the pain, we take an anti-acid treatment. Anti-acids
are a broad group of compounds with no toxic effects on the body. They
are used to neutralize the effects of acid indigestion.
Some of these anti-acids such as milk of magnesia [insoluble magnesium hydroxide, Mg(OH)2]
help to neutralize and hence counteract the excess acid in the stomach.
This treatment, therefore, prevents indigestion and pains. The
neutralization reaction equation is:
Mg(OH)2(aq) + 2HCl(aq)→ MgCl2(aq) + 2H2O(l)
Other
anti-acids such as “Alka-Seltzer” contain soluble materials, including
sodium hydrogencarbonate. These tablets also contain some citric acid (a
solid acid). On adding water, the acid and some of the sodium
hydrogencarbonate react, producing carbon dioxide gas. This helps to
spread and dissolve the other less soluble material. When taken, more
sodium hydrogencarbonate neutralizes the excess hydrochloric acid in the
stomach, thus easing digestion.
Some
anti-acid tablets also contain painkiller to relieve pain. “Soluble
aspirin” tablets dissolve and work in a similar way to “Alker-Seltzer”
tablets. Vitamin C (ascorbic acid) can be added to the tablets. Note
that it is important to add water to start the action of the acid.
Descaling kettles
The limescale (CaCO3)
is formed inside boilers, kettles and water heaters when hard water is
boiled. The limescale can be removed by treatment with an acid that is
strong enough to react with CaCO3, but not strong enough to
damage the metal. Vinegar can be used to discale kettles. Commercial
“discalers” use other acid solutions such as methanoic acid
Prevention of tooth decay
Food
remnants sticking onto teeth (plaque), after eating especially sugary
food is acted upon by bacteria in your mouth. The pH of a sugar solution
is 7. However, bacteria in your mouth break down the sugar in plaque to
form acids, for example lactic acid. These acids lower the pH. Tooth
decay begins when the pH falls below 5.8. The acid attacks the tooth
enamel.
To
help prevent tooth decay many types of toothpaste contain basic
substances to neutralize the acids produced by these bacteria in your
mouth. The pH of these basic substances is alkaline (higher than 7). The
pH of saliva is slightly alkaline (pH 7.4), so it can also help to
counteract the acid, particularly after a meal. After eating a sweat,
for example, it takes about 15 minutes for saliva to raise the pH above
5.8, and stop further decay.
Soil treatment
Most
plants grow best when pH of the soil is close to 7. They prefer the pH
of between 6.5 and 7.0. If the soil pH is below 6.0, the soil is too
acidic. Above the pH of 8.0, the soil is too alkaline. If the soil is
too acidic or too alkaline, the plants grow poorly or not at all.
Chemicals
can be added to the soil to adjust its pH. Most often, if the soil is
too acidic, it is usually treated by liming. In this context, liming
means addition of quicklime (calcium oxide), slaked lime (calcium
hydroxide) or powdered chalk or limestone (calcium carbonate) to an
acidic soil. These compounds (bases) have the effect of neutralizing the
acidity of the soil.
If
the soil is too alkaline, acids such as sulphuric acid, nitric acid or
hydrochloric acid may be added to the soil to neutralize excessive
alkalinity. However, these compounds are very expensive and hence
uneconomical to apply on large-scale basis.
Insect stings treatment
When
a bee stings someone, it injects an acid liquid into the skin. The bee
sting, which is acidic in nature, can be neutralized by rubbing on calamine solution,
which contains zinc carbonate or baking soda, which is sodium
hydrogencarbonate. These compounds are basic in nature and so have the
effect of neutralizing the acid in the sting.
Wasp
stings are alkaline in nature, and can be neutralized with vinegar,
which contains ethanoic acid. Ant and nettle stings contain methanoic
acid. These may be neutralized by rubbing an extract squeezed from
crushed onion leaves (which contain basic compounds) on the affected
skin. The acid in the sting can also be neutralized by applying weak
alkalis such as ammonia solution, ash extract, baking powder, etc.
Factory wastes treatment
Liquid
wastes from factories often contain acid. If it reaches a river, lake
or ocean, the acid will kill fish and other aquatic life. This can be
prevented by adding slaked lime (calcium hydroxide) to the waste, to
neutralize the acid before being dumped into water bodies.

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