6.2.5 Theories of Catalysis How catalysts work

Surface Adsorption Theory Reactants adsorbed onto the surface This increases concentration of the reactants and so speeds up the reaction May also twist the molecules of reactants so that they find it easier to react Many enzymes work in this way Called Lock and Key Theory

Intermediate Compound Theory Formation of ethene [C2H4] from alcohol [C2H5OH] catalyst reduces activation energy C2H5OH = C2H4 + H2O C2H5OH + H2SO4 = C2H5HSO4 + H2O C2H5HSO4 = C2H4 + H2SO4 180oC

CoCl2 used to catalyse the reaction between tartaric acid and hydrogen peroxide the CoCl2 solution went green as it formed the intermediate compound and then returned to pink as the reaction finished. This was the catalyst returning to it original condition

Autocatalysts In some reactions one of the products of the reaction catalyses the reaction - this product is called an autocatalyst. Mn2+ in MnO4- titrations [permanganate or manganate(VII)] First drop slow to lose purple colour because reaction is slow but second and subsequent drops decolourise faster because the Mn2+ catalyses the reaction

Catalytic Poisons Substances that stop catalysts working are called catalytic poisons e.g. Heavy metals such as lead Hence don’t use leaded petrol if you have a catalytic converter

6.2e Homogeneous and Heterogeneous Catalysts

2 SO2(g) + O2(g) = 2 SO3(g) catalyst V2O5(s) If the catalyst and the reactants are in different phases [states] the catalyst is said to be heterogeneous E.g. Fe in the Haber Process for the industrial production of ammonia The iron [solid] and the nitrogen and hydrogen [gases] are in different phases N2(g) + 3 H2(g) = 2 NH3(g) 2 SO2(g) + O2(g) = 2 SO3(g) catalyst V2O5(s) Part of the Contact Process for the industrial production of sulphuric acid Fe(s)

Homogeneous H2O2 = H2O + ½ O2 Potassium iodide solution catalyses the decomposition of hydrogen peroxide solution Both the catalyst and the reactant are in the same phase (liquid / solution) i.e. there is no boundary between the reactants and catalyst One of the products is a gas but products don’t count only reactants and catalyst KI(aq)