ENERGETICS 67
SPONTANEOUS REACTIONSWe have seen above that for a reaction to "go to completion' AG
must be negative. Enthalpies of reaction often amount to several
hundred kJ mol""l but values of entropy changes are rarely greater
than a few hundred and often very much smaller when no gas is
absorbed (or evolved). Hence at room temperature the term TAS^
can often be disregarded and the sign of AH^ determines the sign
of AG^. However, when AH is small less than approximately
40 kJ mol"^1 , then TAS is important and can result in a negative
value for AG even when AH is positive—i.e. for an endothermic
reaction. In the endothermic dissolution of ammonium nitrate in
water, quoted in the introduction on p. 62, it is the entropy contri
bution which produces the spontaneous reaction since the TAS^ is
greater than AH^ and produces a negative value for AG^. Also in
the introduction the mixing of two gases was mentioned. In this
case the enthalpy of 'reaction' is very small but clearly disorder is
increased by the mixing of the two gases. Thus AS is positive and the
terms — TAS and AG are negative.THE EXTRACTION OF METALS
From the above discussion, we might expect that endothermic
reactions for which the enthalpy change is large cannot take place.
However, a further consideration of the equationclearly indicates that an increase in temperature could result
in a negative value of the free energy, but only if the entropy change
for the reaction is positive ; if the entropy change is negative then
there is no possibility of the reaction occurring. (Note that AH
varies only slightly with temperature.)
Most metals react exothermically with oxygen to form an oxide.
Figure 3.4 shows how the value of AG for this process varies with
temperature for a number of metals (and for carbon), and it can be
seen that in all cases AG becomes less negative as the temperature
is increased. However, the decomposition of these metal oxides into
the metal and oxygen is an endothermic process, and Figure 3.4
shows that this process does not become even energetically feasible
for the majority of metals until very high temperatures are reached.
Let us now consider the reduction of a metal oxide by carbon
which is itself oxidised to carbon monoxide. The reaction will
become energetically feasible when the free energy change for the
combined process is negative (see also Figure 3.3). Free energies.