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Question

From the following data at 25C, Calculate the standard enthalpy of formation of FeO(s) and of Fe2O3(s).
ReactionΔrH(KJ/mole)
(1)Fe2O3(s)+3C(graphite)2Fe(s)+CO(g)492
(2)FeO(s)+C(graphite)Fe(s)+CO(g)155
(3)C(graphite)+O2(g)CO2(g)-393
(4)CO(g)+1/2O2(g)CO2(g)-282

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Solution

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)
=155393+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

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