Bond enthalpies of carbon to carbon bonds are 348 kJ mol⁻¹ for C-C, 614 kJ mol⁻¹ for C=C and 839 kJ…

Bond enthalpies of carbon to carbon bonds are $348\ \text{kJ mol}^{-1}$ for $\text{C-C}$, $614\ \text{kJ mol}^{-1}$ for $\text{C=C}$ and $839\ \text{kJ mol}^{-1}$ for $\text{C}{\equiv}\text{C}$. Estimate the energy of the first and of the second $\pi$ bond. Why is it easier to add a molecule across a double bond than to break a single bond?