Metals and non-metals
Francine Taylor-Campbell, Contributor

We will examine a few questions this week which focus on the production and uses of metallic and non-metallic compounds.

1. The following statement was written in an article: "The important metal aluminium is made by the electrolytic reduction of alumina". Outline the method used for obtaining aluminium from alumina. Include in your account an explanation of the term electrolytic reduction.

(5 marks)

(a) Limestone is a common constituent of rocks. What is the chemical name for limestone?

(b) Name the elements present in limestone.

(c) Name one substance, other than limestone, which is used in the manufacture of glass.

(d) Name the two elements present in the substance given in (c).

(e) Why is limestone added to a blast furnace for the production of iron?

(5 marks)

ANSWERS

1. In the production of aluminium, the bauxite ore (Al₂O₃.2H₂O) is purified and dissolved in molten cryolite (Na₃AlF₆) and heated to a high temperature. The molten solution is electrolysed using graphite (C) electrodes. Al₂O₃ forms the ions Al³⁺ and O^2-. These ions migrate to the electrodes; Al³⁺ to the cathode and O^2- to the anode.

Al³⁺ (l) + 3e == Al (l) Since the aluminium ion gains three electrons, it is reduced and the electrons are provided or transported by the electric current; this is called electrolytic reduction.

The molten aluminium is collected at the bottom of the electrolytic cell.

2O^2- (l) == O₂ (g) + 4e. At the anode oxygen gas is formed but this generally reacts with the electrodes (C) to form CO₂, which causes the 'eating away' of the electrodes.

C(s) + O₂ (g) == CO₂ (g)

(a) The chemical name for limestone is calcium carbonate (CaCO₃).

(b) The elements present in limestone are carbon, calcium and oxygen.

(c) Silicon dioxide (SiO₂) is used in the manufacture of glass.

(d) The elements present in (c) are silicon and oxygen.

(e) In the blast furnace, limestone is added to remove impurities such as silica (SiO₂). The heat decomposes the limestone to form calcium oxide and carbon dioxide. Calcium oxide then combines with silica to form calcium silicate or slag.

CaCO₃ === CaO + CO₂ then CaO + SiO₂ == CaSiO₃ calcium silicate or slag.

2. (a) The Haber process uses the reversible reaction between nitrogen and hydrogen to make ammonia. The formation of ammonia is exothermic, evolving 46 kilojoules per mole of ammonia formed.

(i)Write an equation which contains all of the information given about the above reaction.

(ii) Explain in terms of collisions between molecules why increasing the pressure increases the rate of this reaction.

(iii) Give two conditions which together with pressure also affect the rate of this reaction.

(iv) Quote the actual conditions used industrially.

(7 marks)

(b) Describe briefly how you would prepare crystals of ammonium sulphate starting with aqueous ammonia.

(3 marks)

ANSWERS

2. (a) (i) N₂ (g) + 3H₂ (g) == 2NH₃ (g) H = —46 kJ mol^-1

(ii) When the pressure is increased in a gaseous reaction, the molecules involved occupy a smaller volume, hence they will collide more. This helps to increase the reaction rate.

(iii) Two other conditions are temperature and a catalyst.

(iv) In the production of ammonia the following conditions are used: 200 atmospheres, 4500C and Fe/Fe₂O₃ catalyst.

(b) To prepare ammonium sulphate, one could react aqueous ammonia with sulphuric acid. The solution can then be heated to remove excess ammonia and water from the ammonium sulphate. The crystals of ammonium sulphate that are formed on cooling can then be filtered, washed to remove impurities and dried between pieces of filter paper.

2NH₃ (aq) + H₂SO₄ (aq) === (NH₄)₂SO₄ (aq)

Try this question for the next lesson.

3. (a) Given samples of the following metals and their aqueous sulphates, describe the experiments by which you could deduce the order of reactivity of these metals:

(zinc, zinc sulphate, copper, copper sulphate, lead and lead sulphate)

(b) State the observations you would look for and how this would lead to the reactivity order.

(c) Give an equation to show the effect of heat on copper carbonate. Suggest why copper carbonate decomposes at a lower temperature than zinc carbonate.

(8 marks)

(d) Name an alloy of copper and zinc. Explain why a bar of the alloy is harder and stronger than a bar of copper.

(2 marks)

Christopher Smith, director of Projects, M&M Jamaica Ltd, hands over bursaries to Tarrant High School students. Tarrant was one of 21 schools from which eight students each received Chance Fund scholarships. - Contributed

Francine Taylor-Campbell is an independent contributor.