The method chosen to extract a metal depends on its position in the reactivity series compared with carbon. (a) Define reduction in terms of oxygen and in t...

This question tests the meaning of reduction and how the reactivity series determines the method of extraction.

(a) Reduction has two linked definitions. In terms of oxygen it is the loss of oxygen [1]. In terms of electrons it is the gain of electrons [1]. A useful memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

(b) Carbon can only remove oxygen from, that is reduce, the oxides of metals that are less reactive than carbon itself [1]. Metals more reactive than carbon hold onto their oxygen too strongly for carbon to displace it, so an electric current (electrolysis) is needed to break the compound down [1]. This is why the reactivity of carbon acts as the dividing line between the two extraction methods.

(c) Carbon reduces copper(II) oxide to copper, forming carbon dioxide:

One mark for formulae, one for balancing [2] (2 Cu, 2 O and 1 C on each side).

(d) Electrolysis is usually more expensive because it needs a large amount of electrical energy, and electricity is costly to supply [1].

(e) Even though sodium is cheap, carbon still cannot extract it because sodium is more reactive than carbon [1]; carbon cannot remove the oxygen from sodium's compound (cannot displace sodium), so electrolysis of the molten compound must be used [1]. Cost of the raw metal is irrelevant to whether carbon reduction is chemically possible; only the reactivity comparison matters.