The sorption of hydrogen and deuterium by copper and palladium I-The behaviour of copper and copper oxides
The investigation of homogeneous reactions of deuterium and of deuterides has shown that the heavy isotopic molecules react in general less readily than the light, owing to the greater energy of activation required in the deuterium reactions. This result is qualitatively in accordance with theoretical expectations, the reason for the difference being mainly due to the larger zero point energy of the hydrogen compounds. Quantitative analysis of reactions whose mechanism is well established has, however, shown that the ratio of the velocities is often considerably less than that expected if all the zero point energy is contributed to the energy pool in the activating collision. The smaIler ratio of velocity is due to the fact that the zero point energy of the activated state cannot be neglected, the difference for H and for D reaction often being considerable. For example, in the reaction Br + H 2 → HBr [DBr] + H [D] the difference in the energy of activation is 1·50 kg cal, and therefore the difference in zero point energies in the activated state is 0·3 kg cal; similarly for Cl + H 2 (D 2 ) → HCI (DCI) + H (D) the respective figures are 1·23 and 0·55 kg cal, while for the reaction H + H 2 (para) = H 2 (ortho) + H and D + D 2 (ortho) = D 2 (para) + D, the difference in the activated state is no less than 1·20 kg cal. The position with regard to heterogeneous reactions of hydrogen is much less satisfactory and therefore any further means of obtaining information in tin branch of kinetics is of value. Deuterium at first sight promised to be a useful tool in helping to discriminate between various hypotheses. Unfortunately, it is difficult to predict theoretically exactly what will happen in any well-defined instance even with a complete experimental knowledge of the mechanism of the reaction. The state of affairs is analogous to that obtaining with homogeneous reactions, it being necessary to determine experimentally the behaviour of the two isotopes in a variety of heterogeneous processes.