U5.6 · Reaction Energy ProfilePractice · 反应能量图

The activation energy is the gap from the reactants up to the transition state (the peak): 110 − 30 = 80 kJ/mol. Don’t measure from zero, and don’t subtract the products (that gives the reverse activation energy, 60 kJ/mol).

ΔH = energy of products − energy of reactants = 50 − 30 = +20 kJ/mol. The products lie above the reactants, so the reaction is endothermic. ΔH has nothing to do with the activation energy.

Sketch it: the reactants are 65 below the transition state, and the products are 25 above the reactants, so the products are only 65 − 25 = 40 kJ/mol below the peak. For an endothermic reaction, the reverse activation energy is the smaller one.

Exothermic: products below reactants (A and C). Fastest: the smallest activation energy, the height from reactants to the peak. C has the lowest peak, so it is the exothermic and fastest one. B also has a small barrier but its products are higher (endothermic), and A and D have tall barriers.

The higher the energy requirement (the larger Ea), the rarer the qualifying collisions, so heating multiplies them by a bigger factor and k is more sensitive to temperature. From the Arrhenius equation, a 3-fold rise in k over 10 °C corresponds to $E_a$ ≈ 85 kJ/mol and a 2-fold rise to about 53 kJ/mol.