Elementary Reaction and Equilibrium

If the rate-limiting step in an irreversible second-order reaction to produce P from reactants A and B is the collision of single molecules of A and B, then the reaction rate should be proportional to the concentrations (C) of A and B that is, (kmolm 3) and Cg (kmolm ). The rate of reaction can be given as 

This type of reaction for which the rate equation can be written according to the stoichiometry is called an elementary reaction. Rate equations for such cases can easily be derived. Many reactions, however, are non-elementary, and consist of a series of elementary reactions. In such cases, we must assume all possible combinations of elementary reactions in order to determine one mechanism that is consistent with the experimental kinetic data. Usually, we can measure only the concentrations ofthe initial reactants and final products, since measurements of the concentrations of intermediate reactions in series are difficult. Thus, rate equations can be derived under assumptions that rates of change in the concentrations of those intermediates are approximately zero (steady-state approximation). An example of such treatment applied to an enzymatic reaction is shown in Section 3.2.2. 

The rates of the forward and reverse reactions should balance at the reaction equilibrium. Hence 

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At equilibrium, the rates of the forward and reverse reactions are equal. Because the rates are a function of rate constants and concentrations, we can suspect that there is a relation between rate constants for elementary reactions and equilibrium constants for the overall reaction. 

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