Catalysis

In 1836, the Swedish chemist Berzelius noticed that certain substances, when present during a chemical reaction, loosened the bonds within the reacting molecules and thereby increased the rate of the reaction, while the substances themselves came out of the reaction chemically unchanged. To describe this property, he coined the term catalyst, from the Greek 'kata' (wholly) and 'lein' (to loosen). It was later recognised that some substances have the OPPOSITE effect — retarding, rather than speeding up, a reaction's rate — so the modern definition covers both directions: a catalyst is a substance that ALTERS the rate of a chemical reaction without itself undergoing any permanent chemical change, and the overall phenomenon involving the action of a catalyst is called catalysis.

Positive and negative catalysis. In positive catalysis, the presence of the catalyst INCREASES the rate of reaction; in negative catalysis, the presence of the catalyst instead DECREASES the rate. Both are genuine forms of catalysis, since in each case the substance alters the rate without being consumed.

Catalysed reactions fall into two broad families depending on whether the catalyst shares a phase with the reactants: homogeneous catalysis and heterogeneous catalysis.

Homogeneous catalysis. Here the reactants, products and catalyst are all present together in the SAME phase. In the oxidation of $\text{SO}_2$ to $\text{SO}_3$ using nitric oxide as catalyst, $\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} + [\text{NO}]\text{(g)} \rightarrow \text{SO}_3\text{(g)} + [\text{NO}]\text{(g)}$, catalyst, reactants and product are all gases. In the decomposition of acetaldehyde catalysed by $\text{I}_2$, $\text{CH}_3\text{CHO(g)} + [\text{I}_2]\text{(g)} \rightarrow \text{CH}_4\text{(g)} + \text{CO(g)} + [\text{I}_2]\text{(g)}$, everything is again in the vapour phase. Reactions with all species in aqueous solution are equally homogeneous: the acid-catalysed hydrolysis of cane sugar, $\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_6\text{H}_{12}\text{O}_6\text{(glucose)} + \text{C}_6\text{H}_{12}\text{O}_6\text{(fructose)}$, and the acid-catalysed hydrolysis of ethyl acetate to ethyl alcohol and acetic acid, $\text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH}$, are both homogeneous, with the mineral acid catalyst dissolved right in the aqueous mixture. …