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Define rate constant. What is pseudo first order reaction? Explain with one example. |
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Answer» Solution :Rate constant : The rate constant of a chemical reaction is defined as the rate of the chemical reaction when the concentration (or active masses) of each reactant has unit value, i.e., 1 mol the case of solution and the pressure is 1 atm in case of gases. Pseudo first ORDER reaction : A reaction which has higher order true rate LAW but experimentally found to behave as first order is called pseudo first order reaction. Explanation : Consider an ACID hydrolysis reaction of an ester like methyl acetate. `CH_(3)COOCH_(3_((AQ)))+H_(2)O_((l)) overset(H_((aq))^(+))to CH_(3)COOH_((aq))+CH_(3)OH_((aq))` Since the reaction involves two substances, ester and water, it is a bimolecular reaction and the true rate law should be, `"Rate"=k'[CH_(3)COOCH_(3)]xx[H_(2)O]` Hence the reaction is expected to have second order kinetics. However experimentally it is found that the reaction follows first order kinetics. This is because solvent water being a large excess, its concentration remains constant. Hence,`[H_(2)O]="constant"=k"` `"Rate"=k'[CH_(3)COOCH_(3)]xx[H_(2)O]` `=k'[CH_(3)COOCH_(3)]xxk''` `=k'xxk''xx[CH_(3)COOCH_(3)]` iF `k'xxk''=k`, then Rate= `k[CH_(3)COOCH_(3)]`. This indicates that Pecond order true rate law is forced into first order rate law. Therefore this bimolecular reaction which APPEARS of second order is called pseudo first order reaction. |
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