The reaction for the hydrogenation of cyclohexene is: $\text{C}_6\text{H}_{10} + \text{H}_2 \rightarrow \text{C}_6\text{H}_{12}$ According to Hess's Law, the enthalpy of reaction can be calculated from the enthalpies of combustion of reactants and products: $\Delta H_{\text{hydrogenation}} = \Sigma \Delta H_c (\text{reactants}) - \Sigma \Delta H_c (\text{products})$ $\Delta H_{\text{hydrogenation}} = [\Delta H_c(\text{C}_6\text{H}_{10}) + \Delta H_c(\text{H}_2)] - [\Delta H_c(\text{C}_6\text{H}_{12})]$ Given: $\Delta H_c(\text{C}_6\text{H}_{10}) = -3800 \text{ kJ mol}^{-1}$ $\Delta H_c(\text{H}_2) = -241 \text{ kJ mol}^{-1}$ $\Delta H_c(\text{C}_6\text{H}_{12}) = -3920 \text{ kJ mol}^{-1}$ Substituting the values: $\Delta H_{\text{hydrogenation}} = [-3800 + (-241)] - [-3920]$ $\Delta H_{\text{hydrogenation}} = -4041 + 3920 = -121 \text{ kJ mol}^{-1}$