An atom exchange reaction A + BC = AB + C is exoergic by 115 kcal mol-1 in the gas phase but is found to involve an activation energy barrier of 10 kcal mol-1. Furthermore, calculations of the relevant potential energy surfaces for the reaction show that it is "entrance channel" controlled. With the aid of a labelled diagram, explain what is meant by "entrance channel" controlled reaction and Predict the relative usefulness of external and internal energies of the reactants in promoting this reaction. Explain your reasoning.