Just something I've always wondered about...
transition okorder /... for ex-- X (one reactant) + catalyst(transition element) ------X.catalyst(intermediate unstable compound) X.catalyst + Y (other reactant) --------XY(product) + catalyst how the change in oxidation state of transition elements helps the reacton through the formation of intermediates may be seen from reaction in between SO2 and O2 to form SO3 in presence of V2O5 ... V2O5 + SO2 ------V2O4 + SO3 2V2O4 + O2 ------2V2O5 in the above reaction vanadium changes its oxidation state from +5 to +4 and again to +5.. another example is reaction in between iodide and persulphate ions in presence of Fe(III) as catalyst... 2I(-) + S2O8(2-) ---------I2 + 2SO4(2-) (Fe(III) is present as catalyst) the reaction is believed to take place as follows: 2Fe(3+) + 2I(-) ------2Fe(2+) + I2 2Fe(2+) + S2O8(2-) ------2Fe(3+) + 2SO4(2-) (3)in number of cases transition elements provide a suitable large surface area with free valencies on which reactants are absorbed ...as a result concentration of reactants on surface of catalysts increases..hence rate of reaction increases...this is known as adsorption theory.... according to adsorption theory : there are free valencies on surface of solid transition metals because of the incomplete d-subshelll.. so the mechanism of catalysis involve followin five steps: (1) diffusion of reactant molecules towards surface of catalyst... (2) adsorption of reactant molecules on surface of catalyst by forming loose bonds with catalyst due to free valencies... (3)occurence of chemical reactions between reactant and catalyst forming an intermediate.. (4)desorption of product molecules from surface due to its lack of affinity for the catalyst surface thereby making the surface free for fresh adsorption of reactant molecules... (5)diffusion of product molecules away from surface of catalyst...
there are so much catalysts made up of transition metals. because one of the characteristics of transition metals is can be made to catalyst. one of the catalyst that is mostly used is iron fillings which is used as catalyst to make ammonia from nitrogen gas and hydrogen gas.
activation energy has got nothing to do with this. the main reason that the transition metal oxides are catalysts is that they have vacant d orbitals. the reagents are adsorbed on the metal surface and the d orbitals serve as a sort of clamp between the substrates and reagents. for example if you consider the hydrogenation of ethylene in palladium catalyst here the d orbitals lead to syn addition of hydrogen by clamping the pi electron cloud . this is a sort of chemisorption.
