The enthalpy change for the reaction
Cls, graphite) + 1/2O2(g) --> CO(g) cannot be measured directly since some carbon
dioxide is always formed in the reaction.
It can be calculated using Hess's Law and the enthalpy changes of combustion of
graphite and of carbon monoxide.
C(s, graphite) + O2(g) --> CO2 AH=-394 kJmol-1
CO(g) + 1202(8) --> CO2 AH=-283 kJmol-1