The collision-induced reaction of gas-phase atomic hydrogen with chlorine atoms chemisorbed on a:;silicon (001)-(2 x 1) surface is studied by use of the classical trajectory approach. The model is based on reaction zone atoms interacting with a finite number of primary system silicon atoms, which are coupled to the heat bath. The potential energy;of the H ... Cl interaction is the primary driver of the reaction, and in all reactive collisions, there is an efficient flow of energy from this interaction to the Cl-Si bond. All reactive events occur in a single impact collision on a subpicosecond scale, following the Eley-Rideal mechanism, These events occur in a localized region around the adatom site on the surface. The reaction probability is dependent upon the gas temperature and largest near 1000 K, but it is essentially independent of the surface temperature. Over the surface temperature range of 0-700 K and gas temperature range of 300 to 2500 K, the reaction probability lies below 0.1. The reaction energy available for the product state is small, and most of this energy is carried away by the desorbing HCl in its translational and vibrational motions. The Langevin equation is used to consider energy exchange between the reaction zone and the surface.