Measurements of the influence of reactant alignment on the rates of chemical reactions provide direct information concerning the atomic motions necessary for chemical transformation. Data presented here show that at low collision energy, the dissociative adsorption of deuterium (D-2) on the (111) surface of copper has a much higher probability for broadside than for end-on collisions. Furthermore, this steric preference is sensitive to the kinetic energy of the incident molecule, almost disappearing as the energy increases to 0.8 electron volt. This study shows that the dynamic conditions of a surface chemical reaction can profoundly influence the associated steric requirements.