# 22.4: Reduction of Metals

The ease with which a metal may be obtained from its ore varies considerably from one metal to another. Since the majority of ores are oxides or can be converted to oxides by roasting, the free-energy change accompanying the decomposition of the oxide forms a convenient measure of how readily a metal may be obtained from its ore. Values of the free energy change per mol O2 produced are given in the table for a representative sample of metals at 298 and 2000 K. A high positive value of ΔGm° in this table indicates a very stable oxide from which it is difficult to remove the oxygen and obtain the metal, while a negative value of ΔGm° indicates an oxide which will spontaneously decompose into its elements. Note how the value of ΔGm° decreases with temperature in each case. This is because a gas (oxygen) is produced by the decomposition, and ΔS is accordingly positive.

Table $$\PageIndex{1}$$: Free-Energy Changes for the Decomposition of Various Oxides at 298 and 2000 K.
Reaction ΔGm° (298 K)/kJ mol–1 ΔGm° (2000 K)/kJ mol–1
$$\frac{2}{3} \ce{Al2O3 \rightarrow \frac{4}{3} AlO_2}$$
+1054
+691
$$\ce{2MgO -> 2Mg + O2}$$
+1138
+643
$$\tfrac{2}{3}\ce{Fe2O3} \rightarrow \tfrac{4}{3}\ce{Fe + O2}$$
+744
+314
$$\text{SnO}_2 \rightarrow \text{Sn} + \text{O}_2$$
+520
+42
$$\ce{2HgO -> 2Hg + O2}$$
+118
–381
$$\ce{2Ag2O -> 4Ag + O2}$$
+22
–331
Formation of CO2
$$\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$$
–394
–396

The two metals in the table which are easiest to obtain from their oxide ores are Hg and Ag. Since the ΔGm° value for the decomposition of these oxides becomes negative when the temperature is raised, simple heating will cause them to break up into O2 and the metal. The next easiest metals to obtain are Sn and Fe. These can be reduced by coke, an impure form of C obtained by heating coal. Coke is the cheapest readily obtainable reducing agent which can be used in metallurgy. When C is oxidized to CO2, the free-energy change is close to – 395 kJ mol–1 over a wide range of temperatures. This fall in free energy is not quite enough to offset the free-energy rise when Fe2O3 and SnO2 are decomposed at 298 K, but is more than enough if the temperature is 2000 K. Thus, for example, if Fe2O3 is reduced by C at 2000 K, we have, from Hess’ law,

$${}_{\text{3}}^{\text{2}}\text{Fe}_{\text{2}}\text{O}_{\text{3}}\text{(}s\text{) }\to \text{ }{}_{\text{3}}^{\text{4}}\text{Fe(}l\text{) + O}_{\text{2}}\text{(}g\text{)}$$ ΔGm° = +314 kJ mol–1

$$\text{C(}s\text{) + O}_{\text{2}}\text{(}g\text{)}\to \text{ CO}_{\text{2}}\text{(}g\text{)}$$ ΔGm° = –394 kJ mol–1

$${}_{\text{3}}^{\text{2}}\text{Fe}_{\text{2}}\text{O}_{\text{3}}\text{(}s\text{) + C(}s\text{) }\to \text{ }{}_{\text{3}}^{\text{4}}\text{Fe(}l\text{) + CO}_{\text{2}}$$ ΔGm° = –82 kJ mol–1

Thus ΔGm° for the reduction is negative, and the reaction is spontaneous.

The two metals in the table which are most difficult to obtain from their ores are Mg and Al. Since they cannot be reduced by C or any other readily available cheap reducing agent, they must be reduced electrolytically. The electrolytic reduction of bauxite to yield Al (the Hall process) is used to produce aluminum.