What is the rate law?

• A. A description of the equilibrium constant.
• B. A relation between temperature and activation energy.
• C. An expression relating the rate of a reaction to the concentration of the reactants.
• D. A formula linking stoichiometric coefficients to products.

Answer: (C)

The rate law is an expression that relates how fast a reaction occurs to the concentrations of the reactants (usually with a rate constant and exponents that indicate the reaction orders). It shows how the rate changes as [A], [B], etc. change, not the equilibrium position or the energy barrier.

This concept is distinct from the equilibrium constant, which describes the balance between forward and reverse reactions at equilibrium, and from activation energy or temperature effects, which influence how fast the reaction goes overall but not the form of how rate depends on concentrations. The exponents in the rate law aren’t necessarily the same as the stoichiometric coefficients and are determined experimentally (though for an elementary step they match the stoichiometry). For a reaction with two reactants, the rate law has the form rate = k [A]^m [B]^n, where m and n are the orders with respect to each reactant.