The ΔH∘f for CO2(g),CO(g) and H2O(g) are −393.5,−110.5 and −241.8kJ mole−1 respectively. The standard enthalpy change in kJ for the reaction CO2(g) + H2(g) → CO(g) + H2O(g) is
+41.2
(i) C(s) + O2(g) → CO2(g);ΔH = −393.5kJmol−1
(ii) C(s) + 12O2(g) → CO(g);ΔH = −110.5kJmol−1
(iii) H2(s) + 12O2(g) → H2O(g);ΔH = −241.8kJmol−1
For getting CO2(g) + H2(g) → CO(g) + H2O(g), add
(ii) and (iii) and subtract (i).
Thus −110.5 − 241.8 + 393.5 = 395.3
= 41.2kJmol−1