Rate Laws and Equilibrium Constants for Elementary Reactions

Reactions proceed through multi-step mechanisms, where each elementary step is a single process and intermediates appear only between successive steps.

Elementary reactions are categorized by molecularity which is the number of molecules reacting in one step.

For example, unimolecular reactions involve one molecule, bimolecular reactions involve two, and termolecular reactions involve three; higher molecularity reactions are rarer because simultaneous multi-molecule collisions are unlikely.

The reaction rate is the speed at which reactants convert to products. It is proportional to reactant concentrations raised to their stoichiometric coefficients, as defined by the rate law.

The rate constant, denoted by ‘k’, is a proportionality constant that is concentration-independent but temperature-dependent.

For reversible elementary reactions, the rates of forward and reverse reactions are equal at equilibrium. Here, the equilibrium constant K_c equals the ratio of the rate constants for the forward and reverse reactions.

If the forward reaction rate constant far exceeds the reverse reaction rate constant, then K_c is much greater than one, indicating strong product favorability at equilibrium.