We perform a quantum study of the dissociative adsorption of H-2 molecules on the Cu(100) surface. By calculating the reaction probability at specific surface sites, we find that, even at low collision energies, reaction is not necessarily dominated by the site with the lowest potential barrier to reaction. By controlling the vibration and collision energy of the molecules one could, in principle, selectively induce reaction at different specific surface sites, probing them individually. We also suggest an experiment that could provide additional evidence for our predictions.