Aids to Chemistry - Metals and non-metals reveiw
Francine Taylor Campbell, Contributor

Students at Holy Childhood High in St. Andrew prepare to leave for home after a long day at school. - Carlington Wilmot Photo

REVIEW OF POINTS

* "Metals lose their valence electrons to form a 'sea of electrons', which are said to be delocalised. These electrons are mobile and can bind to the cations to form a strong metallic bond.

* "Non-metals ionise by gaining electrons, thus they are called oxidising agents.

* "Non-metals react with metals to form ionic compounds and with other non-metals to form covalent compounds.

* "Metals are mixed to form alloys in order to improve certain physical properties of the metals.

Examine the questions below:

1. Metals, both in the pure state and as alloys, are used in the making of many items.

(a) Give ONE use each for copper and lead other than for making containers. Give the physical property that makes each metal suitable for the use specified.

(b) For one named alloy of aluminium state.

(i) the main constituents of the alloy
(ii) a reason for the use of the alloy instead of the pure metal.

(c) If you were provided with containers made of the metals aluminium, zinc, copper, magnesium and lead, state, giving a reason in each case, which one(s)

(i) would NOT be suitable for use as cooking utensils

(ii) would be suitable to store

* sodium hydroxide solution
* a solution of iron (II) sulphate
* citrus juice

(d) Describe the role of the following metals: iron, magnesium and zinc in living systems.

ANSWERS

1. Copper is used to make electrical cables as it is a good electrical conductor. Lead is used to make protective shield to protect from X-ray and radioactivity, as the radiation cannot penetrate the lead.

(b) Magnalium is an alloy of aluminium consisting of 70 per cent Al and 30 per cent Mg. It is used in the construction of aircraft, as it is just as light as aluminium but stronger and more corrosion resistant.

(c) Magnesium and zinc would not make suitable cooking utensils as they would react with the solutions in the containers and make people sick.

All the metals mentioned can be used to store sodium hydroxide solution. However, only copper and lead containers could be used to store a solution of iron (II) sulphate.

Comment: Based on the reactivity of the metals, none would be more reactive than sodium so that it could displace it from solution. Copper and lead are less reactive than iron and would not be able to displace it from solution.

Copper would be suitable to store citrus juice.

Comment: Citrus juice contains a weak acid and only metals that would not react with it could be used.

(d) Iron is an essential metallic element which forms part of the haemoglobin structure found in red blood cells. In plants it is also necessary for the formation of chlorophyll.

Magnesium is also an important component of the chlorophyll molecule in plants.

Zinc is a trace element needed for certain enzymatic activities.

Question 2

2. Aluminium, copper and iron are the most common metals used in making cooking utensils.

(a) Indicate the methods used for the extraction of aluminium and iron and give a reason, in each case, for the use of that method.

(b) Account for the following:

(i) Certain types of foods cause pitting (eating away of small areas of the metal), especially in aluminium and iron pots.

(ii) In moist air, the surface of copper pots first turns black very quickly and then gradually goes green. Your explanation must also include relevant equations.

(iii) Why is it likely to cost a householder less money for electricity if rain water is habitually used in an electric kettle instead of tap water.

ANSWERS

2. To extract aluminium, the molten ore (bauxite - Al2O3.2H2O) is electrolysed. This method is used as aluminium is a reactive metal.

Reducing agents such as carbon or carbon monoxide are reacted with the ore (haematite ­ Fe2O3) to extract iron. A less powerful method of reduction is used as iron is not very reactive.

(b) (i) The moisture and the weak acids in these foods can react with the aluminium and iron pots. This is usually referred to as corrosion.

(ii) The black colour on the copper is due to the formation of copper oxide.

2 Cu (s) + O2 (g) == 2CuO (s)

The change to a green colour is due to the reaction between the copper oxide and carbon dioxide in the atmosphere. Note the carbon dioxide in the moist atmosphere, could also react with the water vapour to form the weak carbonic acid (H2CO3).

CuO (s) + CO2 (g) == CuCO3 (s)
CuO (s) + H2CO3 (aq) == CuCO3
(s) + H2O (can be lost to the atmosphere)

(iii) Rain water is said to be soft as it does not contain dissolved calcium and magnesium ions which make water hard. Tap water is hard water and when boiled, 'fur' which is CaCO3 and MgCO3 is deposited in the kettle. This can eventually cause blockage in the kettles and requires more energy to bring the water to boil.

* Francine Taylor-Campbell is an independent contributor.