Chemistry Catalysts Essay

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• A catalyst is a substance that changes the speed of a chemical reaction, but is not used up in the reaction. • Catalysts that speed up reactions work by providing an alternative route with a lower activation energy (catalysts do not alter G, H or S for a reaction, only the activation energy). • Catalysts speed the rate at which an equilibrium is reached by speeding up the forward and reverse reaction - they do not alter the position of the equilibrium. • Catalysts that slow down reactions are called inhibitors (or negative catalysts). Heterogeneous catalysis a) What is a heterogeneous catalyst? • A catalyst that is in a different phase to the reactants is a heterogeneous catalyst. • The catalyst is usually a solid and the reaction takes place on the surface. • The bulk of industrial processes involve heterogeneous catalysis. e.g. e.g. e.g. Haber process Contact process making methanol 3 H2 + N2 ¾ 2 NH3 2 SO2 + O2 ¾ 2 SO3 CO + 2 H2 ¾ CH3OH b) How does a heterogeneous catalyst work? Fe catalyst V2O5 catalyst Cr2O3 catalyst • At least one of the reactants is adsorbed onto the surface (i.e. forms bonds to the atoms in the solid surface). • The places on the surface where molecules are adsorbed are called active sites. • In an effective catalyst, the molecules can move about the surface, bonding to different active sites. • The adsorption of reactants onto the surface can result in increased reaction in a number of ways: a) adsorption onto the surface effectively concentrates the reactants, i.e. brings them closer together than in the gas phase, so increasing the likelihood of collision; b) it may weaken some of the bonds in the molecule, making reaction easier; c) it may position the molecule in a favourable orientation for reaction. • For the catalyst to work, molecules must be adsorbed onto the surface. However if: adsorption is too weak: not many

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