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Heterogeneous and homogeneous catalysis.

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Heterogeneous and homogeneous catalysis A catalyst is something added to a reaction that increases its rate, but does not itself change in concentration. However, it is not true to say a catalyst remains unchanged. Catalysts work by providing a different reaction pathway for the reaction. A reactant will combine weakly with the catalyst to form an activated complex. This activated complex will undergo further reaction to form the products, releasing the catalyst for reuse. The rate of reaction increases because the catalysed reaction pathway has a lower activation energy than that of the uncatalysed reaction. There are two forms of catalysis, homogeneous and heterogeneous. ...read more.


It is now a legal requirement for the exhaust of all new cars sold in the UK to be fitted with a catalytic converter. In the first step in catalytic converters reactant molecules (carbon monoxide and nitrogen monoxide) are absorbed onto the catalyst surface. In this process reactant molecules stick to the catalyst by forming weak chemical bonds. This chemisorption weakens the bonds within the reactant molecules. The absorbed reactant molecules are now much more susceptible to reaction. They are still able to move over the catalyst surface and the products are formed when they collide. In addition, the absorption process effectively concentrates the reactants molecules on the catalyst surface, increasing the frequency of collisions. ...read more.


In this gas phase reaction, chlorine free radicals catalyse the decomposition of ozone into oxygen. When CFC's, reach the stratosphere, ultraviolet light breaks the carbon-chlorine bonds, generating chlorine free radicals. The reaction is: 2O g 3O g The two steps in the mechanism involving the chlorine free radicals are: * Cl� + O ClO� + O * ClO� + O Cl� + O The chlorine free radicals regenerated in the second step are available for further reaction with the ozone molecules. The reaction rate is fast and a few chlorine free molecules rapidly destroy many ozone molecules. The oxygen free radicals are formed continuously in the stratosphere. Ultraviolet light produces oxygen free radicals from oxygen molecules or ozone molecules. ...read more.

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