Elementary Rate Laws and the Principle of Mass Action

Elementary Rate Laws and the Principle of Mass Action 

We use the notation A + B = C to describe the stoichiometry of a reaction. The notation A + B — C is used to mean a specific molecular encounter. 

Because the rates of change of all the components are simply related at all times in single-step reactions, the rate of reaction may be defined as 

At equilibrium R is zero, which we interpret to mean that the rate of reaction and its inverse precisely balance. To determine the rate of the forward process in a single-step reaction note that a reaction occurs in one step only if all reactants are close together at the same time. Stated differently the reacting molecules must collide essentially simultaneously. Since collision probability is proportional to the product of the number density of the molecules which participate in a kinetic event, the equilibrium forward and reverse reaction rates are 

For a discussion of progress variables and chemical equilibria see, e.g., F. T. Wall, Chemical Thermodynamics, 3rd ed. (San Francisco Freeman, 1974), Chapter 10. The pressure dependence of k in single-step reactions is usually small this problem is treated by C. A. Eckart, Ann. Rev. Phys. Chem. 23,239 (1972). Gas-phase unimolecular reaction, for which A is a sensitive function of p, is not a single-step process (Section 5.4). 

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