CHEMISTRY FORM THREE STUDY NOTES TOPIC 8: COMPOUNDS OF METALS

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Metals
combine with other substances to form compounds. There is a diversity
of metal compounds known. One can make an endless list of the compounds
of the metals. In this chapter, we are going to concentrate our efforts
on the following compounds of the metals: oxides, hydroxides, carbonates
and hydrogen carbonates, nitrates, chlorides and sulphates.
Preparation of Oxides of Some Metals by Direct and Indirect Methods
Prepare oxides of some metals by direct and indirect methods
All
elements except helium, neon and argon form compounds with oxygen. This
is because oxygen is quite reactive. Binary compounds of oxygen are
known as oxides. Therefore, a metal oxide is a binary compound of oxygen
and a metal.
Metal Oxides
Classify metal oxides
Chemical properties of metal oxides
  1. Reaction with water:The
    oxides of potassium, sodium and calcium are very soluble in water. They
    will react vigorously with cold water to produce the corresponding
    hydroxides. The oxides of metals below calcium in the reactivity series
    are all insoluble in water.
  2. Reaction with acids:The oxides of metals above hydrogen in the reactivity series react with dilute acids to produce salt and water.
  3. Reaction with alkalis:Some oxides react with alkalis to produce salt and water. The oxides of this nature include ZnO, Al2O3, PbO and SnO.
The Reactions of Metal Oxides with Water and Dilute Acids
Demonstrate the reactions of metal oxides with water and dilute acids
Preparation of metal oxides
Metal oxides can be prepared by:
  1. direct methods. This involves heating metals directly in air.
  2. indirect
    methods. This involves such methods like heating carbonates and
    hydrogencarbonates of certain metals in air, and reacting certain metals
    with certain acids.
Preparation of metal oxides by direct combination(Direct method)
In
this method, oxides can be prepared by direct combination of metals
with oxygen. This involves heating a metal in air. When some metals are
burned in the air, they react with oxygen of the air to form metal
oxides. However, this method is not intensively used because some metals
tend to form a protective layer of an oxide on the surface of metal and
prevent further attack by oxygen. The best example of such metals is
aluminium which, when heated in the air, forms a protective layer of
aluminium (III) oxide (Al2O3) on the surface of a
metal whichprevents further attack by oxygen. Table 9.1 shows the
products formed when certain metals are burned in the air.
Table 9.1: The reaction of metals with oxygen
Metal How it reacts Product
Barium burns with a green flame white solid (barium (II) oxide, BaO)
Calcium burns with a brick-red flame white solid (calcium oxide, CaO)
Sodium burns with a yellow flame white solid (sodium oxide, Na2O)
Potassium burns with a purple flame white solid (potassium oxide, K2O)
Magnesium burns with a white flame white solid (magnesium oxide, MgO)
Iron burns with yellow sparks Blue-black solid (iron (II) oxide, FeO)
Copper doesnot burn, turns black black solid (copper (II) oxide, CuO)
Activity 1
Preparation of oxides by direct combination methods
Aim: to prepare oxides of metals
Materials: magnesium ribbon, aluminium foil, iron filings, gas jar, Bunsen burner and coal tong.
Discussion
  1. What changes did you observe when each of the metals above was heated in a Bunsen flame?
  2. What was the function of oxygen in the reaction?
  3. Write well balanced equations for the reactions that took place.
  4. Write and balance the equations for the reactions when the following metals burn in oxygen:(i)sodium (ii)potassium (iii) iron
Procedure:
  • Lower a piece of burning magnesium ribbon, by means of tongs, into a gas jar of oxygen. Observe and record what happens.
  • Heat the aluminium foil strongly on a Bunsen flame. Observe and record what happens.
  • Perform the same experiment with iron filings. Also, record what happens.
Preparation of metal oxides by indirect methods
This
method involves thermal decomposition of salts. When some salts are
heated, they decompose into oxides and other products as well. If the
anion part of the salt heated contains some oxygen, a portion of this
oxygen may remain bonded to the central metal atom.
However,
this method is limited only to those compounds of metals below sodium
in the electrochemical series. For instance, potassium oxide or sodium
oxide cannot be prepared by action of heat on their carbonates. Thermal
decomposition of some metals is as shown by the following equations:
  • CuCO3(s)→CuO(s)+ O2(g)
  • CaCO3(s)CaO(s)+ CO2(g)
  • ZnCO3(s)→ ZnO(s)+ CO2(g)
Hydroxides
also behave in a similar manner. When hydroxides of metals below sodium
in the electrochemical series are heated, they decompose into
respective oxides, giving off water in the form of steam.
  • Ca(OH)2(s)→ CaO(s)+ H2O(g)
  • Mg(OH)2(s)→ MgO(s)+ H2O(g)
The oxides can also prepared by heating some nitrates and sulphates:
  • 2Pb(NO3)2(s)→ 2PbO(s)+ 4NO2(g)+ O2(g)
  • 2Cu(NO3)2(s)→ 2CuO(s)+ 4NO2(g)+ O2(g)
Nitrates
of silver and mercury are not suitable for preparation of oxides by
thermal decomposition because they decompose to metals directly when
heated. The bond between oxygen and a metal atom is not strong enough to
withstand the thermal energy:
  • 2AgNO3(s)→ 2Ag(s)+ 2NO2(g)+ O2(g)
Activity 2
Preparation of oxides by thermal decomposition of carbonates
Discussion
  1. (a)
    What was the colour of the copper carbonate before heating? (b) What
    was the colour of the residue in the test tube after cooling? What
    substance is this? (c) Which gas was evolved on heating the carbonate?
  2. (a) What happened to lead carbonate whenit was heated? (b) What gas was evolved? (c) What colour was the residue after cooling?
  3. Write well balanced chemical equations for the two experiments performed.
  4. Explain why metal oxides cannot be prepared by thermal decomposition of either potassium or sodium carbonate.
Aim:To prepare metal oxides by thermal decomposition of carbonates
Procedure:
  1. Put a sample of copper carbonate in a test tube.
  2. Place the test tube on a Bunsen flame and heat slowly then strongly.
  3. Observe and record any changes, including testing the gases evolved.
  4. When no further changes take place in the test tube, cool down the contents.
  5. Perform the same experiment with lead carbonate.
The Uses of Metal Oxides
Explain the uses of metal oxides
Metal oxides find a wide range of uses. The following are the uses of the most common oxides:
Uses of calcium oxide (CaO)
  1. Making mortar:Calcium oxide reacts with water to form the hydroxide, Ca(OH)2, known as slaked lime.Mortaris made by mixingslaked lime,sandandwater, and is used in sticking bricks together and in forming smooth surfaces on walls of buildings.
  2. Calcium oxide is used for makingwhitewash,
    which is used in marking sport’s fields, roads and is brushed on walls
    of buildings to give them the white colour. Whitewash is a suspension of
    slaked lime in water.
  3. Cement and concrete:Cementis made by heating togetherlimeorlimestoneandclay. The product is a mixture of calcium silicates and aluminates. Clay is hydrated aluminium silicate. A mixture ofcement,sand,stonesandwatergivesconcrete,
    which on setting becomes extremely hard. It is the materials used for
    making foundations of buildings, pillars, roads, paths, bridges, etc
  4. Soil treatment:In agriculture,quicklime is used to neutralize soil acidity, and it also adds mineral nutrients (Ca2+) to the soil.
  5. Calcium oxide is dissolved in water to make slaked lime, which is used in the softening of water.
  6. Drying agent: Calcium oxide is used for drying ammonia and ethanol.
  7. It is used in the manufacture of bleaching powder, CaOCl2.
  8. Manufacture ofglass:heating a mixture ofsand,sodium carbonateandlimeorlimestonegives glass.
  9. Lining
    of furnaces: It is mixed with magnesium oxide to form the basic lining
    of the furnaces to remove acidic impurities in the form of slag.
  10. it is used in the blast furnace to remove impurities fromiron ore which is removedin the formof slag.
  11. Preparation calcium carbide: Calcium carbide (CaC2) is manufactured in an electric furnace at 2000oC. CaO(s)+ 3C(s)→ CaC2(s)+ CO(g)
Uses of magnesium oxide (MgO)
  1. The oxide is used as a lining material in refractory furnaces, owing its high melting point, which is around 2900oC. It is also used as a refractory agent in the construction of crucibles.
  2. The
    oxide in its solution form (magnesium hydroxide) is commonly used as an
    antacid. This works because magnesium hydroxide is a basic substance,
    which means that, it will neutralize excess acidity and end up
    indigestion, caused by too much hydrochloric acid in the stomach.
  3. It
    is used to manufacture the common chemical reagents in the laboratory
    such a magnesium chloride, magnesium sulphate and magnesium hydroxide.
  4. Magnesium
    oxide is a popular drying agent. In its powder form, it is hydroscopic
    in nature. This makes it suitable for drying different substances.
  5. Insulation: Due to its heat resistance properties, magnesium oxide powder makes an excellent insulator.
  6. Dietary
    supplement: Since it is a good source of magnesium, the oxide is used
    as or in dietary supplements forhumans and animals.
Uses of aluminium (III) oxide (Al2O3)
  1. Aluminium (III) oxide, in the form of bauxite, is used as a source of aluminium.
  2. Owing its rough surface, the oxide is used as an abrasive, i.e. it is used to rub and clean other surfaces.
  3. It is used as an adsorbent in chromatography.
  4. It is used in the lining of furnaces as a refractory material because it has a high melting point (2040oC).
Uses of zinc oxide (ZnO)
  1. Zinc
    oxide is chiefly used in the manufacture of paints and pigments. In
    addition, the oxide is used to manufacture anti-corrosive coatings,
    lubricants, adhesive batteries, fire retardants, plastic, cement, glass
    and ceramics (as a component of glazes).
  2. Manufacture of rubber:
    It is mainly used to activate vulcanization, which aims at improving the
    strength and elasticity of rubber.
  3. Manufacture of cigarette
    filter: As a cigarette filter, zinc oxide helps to remove certain
    harmful compounds from the tobacco smoke, without altering its flavour.
  4. Making concrete: It helps to make the concrete more resistant to water, besides improving the processing time required.
  5. Medical
    uses: Zinc oxide has anti-bacterial properties, for which it is
    extensively used to treat a number of skin conditions. It is topically
    applied to provide relief in skin irritation, diaper rash, minor burns
    and cuts, and for dry and chapped skin. It is added to baby powder,
    anti-dandruff shampoos as well as antiseptic creams and surgical tapes
    due to its medicinal properties. In addition, together with iron oxide,
    it is used to make calamine solution.
  6. Cosmetic uses: the most
    important use of zinc oxide in the cosmetic industry is in the
    preparation of sunscreen lotions and creams. Zinc oxide can absorb
    ultraviolet (UV) radiation of the sun and thereby protect the skin from
    sunburn and other damaging effects of UV radiation.

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