What is Activation energy?

Example 8.19 (A rule of thumb)

For $E_a = 53\,\mathrm{kJ}/\mathrm{mol}$, going from 25 to $35\,{}^{\circ}\mathrm{C}$ multiplies $k$ by $\exp\!\big(\frac{53000}{8.314}(\frac{1}{298.15} - \frac{1}{308.15})\big) = 2.0$. The familiar rule “ten degrees more, twice as fast” holds near room temperature for reactions of activation energy around $50\,\mathrm{kJ}/\mathrm{mol}$; for larger $E_a$ the factor is larger.