Rate Expressions of Chemical Reactions

The rate of a chemical reaction is a function of the temperature, the composition of the reacting mixture, and, if present, the catalyst. The relationship between the reaction rate and these parameters is commonly called the rate expression or, sometimes, the rate law. Chemical kinetics is the branch of chemistry that deals with reaction mechanisms and provides a theoretical basis for the rate expression. When such information is available, we use it to obtain the rate expression. In many instances, the reaction rate expression is not available and should be determined experimentally. 

For most chemical reactions, the rate expression is a product of two functions, one of temperature, k T), and the second of species concentrations, h C/s)  

The function k(T) is commonly called the reaction rate constant. However, note that the reaction rate depends on the temperature. The term rate constant comes about because k T) is independent of the composition and is constant at isothermal 

For most chemical reactions, k(J) relates to the temperature by the Arrhenius 

The value of the rate constant at a given temperature is readily calculated when both parameters in the Arrhenius equation, and Ea, are known. However, it is convenient to calculate die value of the rate constant at one temperature on the basis of its value at a different temperature, using only the activation energy. To obtain a relationship between the values of the reaction rate constant at two temperatures, Ti and T2, we take the log of Eq. 3.3.2 for each and combine the two equations to obtain 

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Definition of fluid properties and incorporation of constitutive and user-defined functions (UDFs) (e.g., rate expressions of chemical reactions)... 

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