Modern inorganic chemistry

ENERGETICS 69

like enthalpies, are additive, and the minimum temperature for

energetic feasibility can readily be found.

As an example, consider the reduction of zinc oxide to zinc by the

reaction :

ZnO + C -* Zn + CO

Reference to Figure 3.4 shows that the reduction is not feasible at

800 K, but is feasible at 1300 K. However, we must remember that

energetic feasibility does not necessarily mean a reaction will^4 go' ;

kinetic stability must also be considered. Several metals are indeed

extracted by reduction with carbon, but in some cases the reduction

is brought about by carbon monoxide formed when air, or air-

oxygen mixtures, are blown into the furnace. Carbon monoxide is

the most effective reducing agent below about 980 K, and carbon is

most effective above this temperature.

Since AG^ -

and AG^= -RTlnK,

1

0g-=-

Hence an alternative to Figure 3.4 is to plot Iog 10 K against 1/T

{Figure 3.5); the slope of each line is equal to — A//-e/'2.303jR. A

discontinuity in the line for a given metal-metal oxide system

corresponds to a change in phase (solid, liquid, gas) of the metal or

its oxide (usually the metal). The change in slope is related to the

enthalpy change involved in the change. Thus for magnesium-

magnesium oxide,

2Mg(l) + O 2 (g) -» 2MgO(s) : A/f f = - 1220 kJ mol ~ ]

2Mg(g) + O 2 (g) -> 2MgO(s) : AHf = - 1280 kJ moP [

and hence

2Mg(l) -> 2Mg(g) : A/T^9 - - 260 kJ mol " l

which is twice the enthalpy of vaporisation of one mole of mag-
nesium.
When studying the AG^ — T diagrams we saw that the extrac-
tion of a metal from its compound by a reducing agent becomes
energetically feasible when the free energy change for the combined
process is negative (see also Figure 3.3).