 Sabatier's principle suggests that a plot of rate constant versus hydrogen-absorption energy should form a volcano shape, but this is not observed when oxide-covered metals are excluded. The reaction rate is determined by multiple factors, including intermediate states, and the prediction that highly exothermic hydrogen-absorption decreases the reaction rate does not hold true for all metals, such as nickel, due to its compact orbitals and low hydrogen overlap. This article was authored by Paul Aquino, Fernanda Juarez, Elizabeth Santos, and others.