Estimate Δr H for C₂H₄(g) + H₂(g) → C₂H₆(g) from the mean bond enthalpies: C=C 611, H-H 435, C-C…

Hess's law: the enthalpy change of a reaction is the sum of the enthalpy changes of any set of steps that lead from the same start to the same end. Equations add like algebra: reverse one and its $\Delta H$ changes sign, multiply one and its $\Delta H$ is multiplied. For gases, $\Delta_r H \approx \sum(\text{bonds broken}) - \sum(\text{bonds formed})$, using mean bond enthalpies.