Catalysis is the increase in the rate of a chemical reaction by the addition of a reagent — the catalyst — that is not itself consumed. The catalyst works by opening up a route between starting material and product with a lower activation barrier than the uncatalyzed process. Catalyzed reactions have lower activation energy (rate-limiting free energy of activation) than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced naturally, such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.