Reaction Rate Equations The Mass Action Law

Although a proper definition of the rate of reaction is necessary, we cannot do much with it until we find how the rate depends on the variables of the system such as temperature, total pressure, and composition. In general terms, we must set the rate definition equal to a mathematical expression that correlates properly the effects of such variables. That is, 

Again let us work through the example of a specific (but hypothetical) system, this time the irreversible reaction 

In many cases we shall see that the individual orders p, q, and r have numerical values that are the same as the stoichiometric coefficients of the reaction, a, b, and c. In this event it is said that order corresponds to stoichiometry, there is in general, however, no restriction on the numerical values of p, q, and r determined on the basis of experimental observation, including negative values. 

If the reaction is reversible, A, B, C. can be formed from the products L, M. and a corresponding form of the law of mass action applies. For this case equation (1-18) becomes 

Here u and w are the orders of the reverse reaction with respect to L and M, kf the forward rate constant, and k the reverse rate constant. The same comments as given earlier pertain to the relationship between the apparent orders and the stoichiometric coefficients I and m. 

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