of reaction will be very slow but with the increase in time the rate of reaction increases. Auto catalysis is observed in the following reactions. CH COOC H +H O CH COOH+C H OH Acetic acid acts as the autocatalyst 2AsH 2As+3H Arsenic acts as an autocatalyst Negative Catalysis In certain reactions, presence of certain substances, decreases the rate of the reaction. Ethanol is a negative catalyst for the following reaction.
(i) 4CHCl +3O 4COCl +2H O+2Cl Ethanol decreases the rate of the reaction (ii) 2H O 2H O+O In the decomposition of hydrogen peroxide, dilute acid or glycerol acts as a negative catalyst. XII U10-Surface XII U10-Surface - - - - . . Theories of Catalysis For a chemical reaction to occur, the reactants are to be activated to form the activated complex.
The energy required for the reactants to reach the activated complex is called the activation energy. The activation energy can be decreased by increasing the reaction temperature. In the presence of a catalyst, the reactants are activated at reduced temperatures in otherwords, the activation energy is lowered. The catalyst adsorbs the reactants activates them by weakening the bonds and allows them to react to form the products.
As activation energy is lowered in presence of a catalyst, more molecules take part in the reaction and hence the rate of the reaction increases. The action of catalysis in chemical reactions is explained mainly by two important theories. They are (i) the intermediate compound formation theory (ii) the adsorption theory. The intermediate compound formation theory A catalyst acts by providing a new path with low energy of activation.
In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or decompose to give products and the catalyst is regenerated. Consider the reactions: A+B → AB ( ) A+C → AC (intermediate) ( ) C is the catalyst AC+B → AB+C ( ) Activation energies for the reactions ( ) and ( ) are lower compared to that of ( ). Hence the formation and decomposition of the