Combustion reactions are relatively easy to carry out and study, and their data can be combined to give enthalpies of other types of reactions. as an illustration, calculate the standard enthalpy of hydrogenation of cyclohexane given that the standard enthalpies of combustion of the two compounds are -3752 kj mol-1 (cyclohexene) and -3953 kj mol-1 (cyclohexane).

ΔH = -201 kJ/mol

Explanation:

The Hess' Law say that the enthalpy of one reaction is equal to the sum of all the steps:

C6H12 + 9O2 --> 6CO2 + 6H2O    ΔH = -3752 kJ

C6H10 + 17/2O2 --> 6CO2 + 5H2O ΔH = -3953 kJ

C6H10 + H2O --> C6H12 + 1/2O2 ΔH = -3953 + 3752

ΔH = -201 kJ/mol

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