The pairwise energy model (PEM) assumes that the cross section for the reaction cross section for the reaction A+BC${leftrightarrow}$B+C, where B and C are isotopes of hydrogen, depends on only the pairwise relative energy Es between A and B. Until now, the PEM has been used to interpret theoretically the isotope effect for the reactions such as $O(^3P)+HD,;Ar^++(H_2,;D_2,and;HD)$. In this paper we carry out extensive quasiclassical trajectory calculations for the three possible reactions $Cl+H_2$ and HD and show that the PEM works very well at high energy. In particular we are able to accurately predict the intramolecular isotope effect at high energy for the reaction of Cl+HD using only the cross section data for $Cl+H_2$. To understand that the PEM works so well at high energy, the internal energy distributions for the products are examined. The distributions for three reactions are different at a fixed relative collision energy E but are approximately same at a fixed pairwise energy Es. This suggests that the PEM works very well at high energy. We believe the conclusions reached here will apply to other A+BC systems.