Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2023 / Paper 1 / 5 / a

The products lie below the reactants, while the transition state lies above both. On an energy-versus-reaction-coordinate plot, the forward activation energy is the vertical separation $E_{\mathrm{TS}}-E_{\mathrm{R}}$, not the difference between product and reactant energies. For an exothermic elementary path, $\Delta E=E_{\mathrm{P}}-E_{\mathrm{R}}<0$, and the reverse barrier is $E_{a,-}=E_{a,+}-\Delta E>E_{a,+}$.

The Arrhenius equation isThe exponential gives the strongly temperature-dependent energetic factor. For a simple bimolecular collision picture, $A$ collects the encounter frequency per reactant concentration and the fraction of encounters with a suitable orientation and other dynamical requirements. Thus a collision is not automatically a successful reaction even when its energy is sufficient.

$A$ has the same units as $k$, which depend on the rate law; it is not always a frequency in ${\mathrm{s}}^{-1}$. More generally it is an effective prefactor containing statistical and dynamical contributions rather than a literal universal collision count. Treating it as temperature independent is the approximation requested here.