The Law of Mass Action and Reaction Order

The rate, or speed or velocity, at which a reaction proceeds is a measure of how quickly reactants (S) are consumed or products (P) are formed  

Evaluation of Enzyme Inhibitors in Drug Discovery, by Robert A. Copeland ISBN 0-471-68696-4 Copyright 2005 by John Wiley Sons, Inc. 

The reaction rate is directly proportional to the product of reactant concentrations raised to the power of their respective stoichiometric coefficients. 

For the simple reaction we have looked at so far, we are dealing with one reactant, S, and we require only one mole of S to produce one mole of product P. Hence the law of mass action dictates that the reaction rate will be directly proportional to [S]1. Such a reaction is referred to as a first-order reaction because there is only one reactant concentration term in the rate equation (Equation A1.2). 

Now let us consider some other types of reactions that might be encountered in a biological system. Consider, for example, the formation of a dimer, by the combination of two monomer molecules  

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