. Catalysis In Berzelius identified certain substances loosen the bond in the reacting molecules and increased the rate of the reaction. But he also found these substances didn’t undergo any change chemically. In order to indicate the property, he gave them the name catalyst.
(In greek, kata-wholly, lein-to loosen). XII U10-Surface XII U10-Surface - - - - Later it was identified that there were many substances which retarded the speed of a reaction. Hence a catalyst is defined as a substance which alters the rate of chemical reaction without itself undergoing chemical change. The phenomenon which involves the action of a catalyst is called catalysis.
Positive and negative catalysis: In positive catalysis, the rate of a reaction is increased by the presence of catalyst but in negative catalysis, the rate of reaction is decreased by the presence of a catalyst. The two main types of catalysis (i) Homogeneous catalysis and (ii) Heterogeneous catalysis Homogeneous catalysis In a homogeneous catalysed reaction, the reactants, products and catalyst are present in the same phase. Illustration ( ): 2SO +O +[NO] 2SO +[NO] (g) (g) (g) (g) (g) ® In this reaction the catalyst NO, reactants, SO and O , and product, SO are present in the gaseous form. Illustration ( ): In the decomposition of acetaldehyde by I catalyst, the reactants and products are all present in the vapour phase.
CH CHO +[I ] CH +CO +[I ] ( ) ( ) ( ) ( ) ( ) g g g g g Let us consider some examples in which the reactants, products and catalyst are present in aqueous solution. ( ) Hydrolysis of cane sugar with a mineral acid as catalyst Sucrose Glucose Fructose (l) (l) (l) (l) C H O +H O H2SO C H O + C H O (l) ( ) Ester hydrolysis with acid or alkali as catalyst Ethyl acetat a e cetic acid ethyl alcohol CH COOC H + H O H2SO4 CH COOH +C H OH (l) (l) (l) (l)