Power Law Models and Elementary Rate Laws

The dependence of the reaction rate. on the concentrations of the species present. fn(C ). is almost without exception determined by experimental observation. Although the functional dependence on concentration may be postulated from theory, experiments are necessary to confinn the proposed form. One of the most common general forms of this dependence is the power law model. Here the rate law is the product of concentrations of the individual reacting species, each of which is raised to a power, for example. 

The exponents of the concentrations in Equation (3-3) lead to the concept of reaction order. The order of a reaction refers to the powers to which the concentrations are raised in the kinetic rate law, In Equation (3-3), the reaction is a order wirfi respect to reactant A, and p order with respect to reactant B. The overall order of the reaction. , is 

The units of are always in terms of concentration per unit time while the units of the specific reaction rate. kJ, will vary with the order of the reaction, Consider a reaction involving only one reactant, such as 

Consequently, the rate laws corresponding to a zero-, first-, second-, and third-order reaction, together with typical units for the corresponding rate constants. are  

However, for many reacting systems, the activity coefficients, y, do not change appreciably during the course of the reaction, and they arc adsorbed into the specific reaction rate  

The exponents of the concentrations in Equation (3-3) lead to the concept of reaction order. The order of a reaction refers to the powers to which the con- 

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