The standard enthalpy of formation of ethane ( C ₂ H ₆ , g) is -92 kJ mol ⁻¹ . The enthalpy of sublimation of carbon (graphite) is 718 kJ mol ⁻¹ , and the bond dissociation enthalpy of H ₂( g ) is 436 kJ mol ⁻¹ . If the bond enthalpy of the C - C bond is 346 kJ mol ⁻¹ , the average C - H bond enthalpy in ethane is ________ kJ mol ⁻¹ . (Nearest integer)

Step-by-step solution

The standard enthalpy of formation of ethane corresponds to the reaction: 2 C ( s, graphite ) + 3 H ₂( g ) C ₂ H ₆( g ) Using Hess's Law, the enthalpy of formation can be related to atomisation energies and bond enthalpies: H ^ _ f = Energy required to break reactant bonds (atomisation) - Energy released upon forming product bonds Energy required for reactants: For 2 C ( s ) , atomisation energy = 2 H _ sub ( C ) = 2 718 = 1436 kJ For 3 H ₂( g ) , bond breaking energy = 3 E ( H - H ) = 3 436 = 1308 kJ Total energy