Extraction of metals

1. Native metals refer to those metals that are found uncombined (by themselves), because they are unreactive. These include silver, gold and platinum.

2. Minerals and rocks which contain metal compounds are called ores.

3. Examples of ores include bauxite (Al), haematite (Fe) and zinc blende (Zn).

4. Metals are extracted from their ores by reacting with a reducing agent such as carbon or hydrogen. Metals that are more reactive are extracted by electrolysis.

5. In electrolysis, an electric current is used to split the molten metal ore to produce the pure metal.

6. Iron is extracted from its ore in the blast furnace. Coke (C), limestone and iron oxide are heated in the furnace. Carbon monoxide reduces the iron oxide to form iron, and limestone removes the impurities of silica.

7. Iron produced from the blast furnace contains carbon, which is usually removed by reacting with oxygen. Low- or high-carbon steel can be produced based on the amount of carbon present.

8. Aluminium oxide is molten and electrolysed using carbon electrodes. The molten aluminium is collected. Cryolite is added to the molten ore to lower the reaction temperature.

QUESTION 1

Lead is a grey metal.

(a) An isotope of lead has the mass number 208. Complete the table to show the number of subatomic particles in an atom of this isotope of lead. (b) When lead is heated in oxygen, lead(II) oxide is formed. Write a word equation for this reaction.

(c) Based on the position of lead in the reactivity series, outline a reaction that could be used to obtain lead from lead (II) oxide.

(d) Which substance becomes oxidised during this reaction?

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

ANSWERS

a. From the periodic table, lead has an atomic number of 82. If the mass number of the isotope is 208, then the number of subatomic particles can be determined.

c. Lead is below iron in the reactivity series. Metals lower in the series can be extracted by reacting their metal oxides with a reducing agent such as carbon or hydrogen. Heating lead (II) oxide with carbon results in the formation of lead and carbon monoxide.

PbO (s) + C (s) (r) Pb (s) + CO (g)

COMMENT: Note that the reducing agent can be carbon or carbon monoxide when the carbon is burnt. This example is seen in the blast furnace.

d. From the equation, carbon is the reducing agent which becomes oxidised during the reaction.

e. To extract aluminium, the molten ore (bauxite – Al O .2H 2O) 2 3 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.

QUESTION 2

The following statement was made in an article: `The important metal aluminium is made by the electrolytic reduction of alumina’.

a. Outline the method used for obtaining aluminium from alumina. Include in your account an explanation of the term electrolytic reduction. (5 marks)

b. Limestone is a common constituent of rocks. i. What is the chemical name for limestone? ii. Name the elements present in limestone. iii. Name a substance, other than limestone, which is used in the manufacture of glass.

iv. Name the two elements present in the substance given in (iii).

v. Why is limestone added to a blast furnace for the production of iron? (5 marks)

ANSWERS

2. (a) In the production of aluminium, the bauxite ore

(Al O .2H O) is purified and dissolved in molten cryolite (Na AlF ) 2 3 2 3 6 and heated to a high temperature. The molten solution is electrolysed using graphite (C) electrodes. Al2O3 forms the ions Al3+ and O2-. These ions migrate to the electrodes; Al3+ to the cathode and O2- to the anode.

Al3+ (l) + 3e (r) Al (l)

Since the aluminium ion gained three electrons, it is reduced and the electrons are provided/transported by the electric current. This is called electrolytic reduction. The molten aluminium is collected at the bottom of the electrolytic cell.

2O2- (l) (r) O (g) + 4e.

2

At the anode, oxygen gas is formed, but this generally reacts with the electrodes (C) to form CO , which causes the ‘eating

2 away’ of the electrodes.

C(s) + O (g) (r) CO (g)

2 2

(b) (i) The chemical name for limestone is calcium carbonate (CaCO2).

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

(iii) Silicon dioxide (SiO2) is used in the manufacture of glass. (iv) The elements present in (iii) are silicon and oxygen.

(v) 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 (s) CaO (s) + CO (g); CaO (s)+ SiO (s)

2 2

CaSiO (calcium silicate or slag).

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Francine Taylor-Campbell is an independent contributor. Send feedback to kerry-ann.hepburn@gleanerjm.com.