By Hess Law,
For finding the standard enthalpy of formation of FeO(s)
Fe(s)+CO(g)→C(graphite)+FeO(s) −155
(graphite)+O2(g)→CO2(g) −393
CO2(g)→CO(g)+12O2(g) 282
∴△Hr (For the formation of FeO)(S)
i.e. for Fe(S)+12O2(g)→FeO(S)
=−155−393+282=−266
By Hess law, For finding the standard enthalpy of formation of Fe2O3(S)
2Fe(S)+3CO(g),→3c(graphite)+Fe2O3(S) −492
3c(graphite)+3O2(g)→3CO2(g) −3×393
3CO2(g)→32O2(g)+3CO(g) 3×282
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯2Fe(S)+32O2(g)→2CO(g)+Fe2O3(S)
△Hr=−492+(−3×393)+(3×282)
=−825