Rate Laws and Equilibrium Constants for Elementary Reactions

Molecularity is the number of molecules reacting in a single elementary step. Unimolecular reactions involve one molecule, bimolecular reactions involve two, and termolecular reactions involve three. Higher molecularity reactions are rarer because simultaneous collisions of multiple molecules are statistically unlikely.

The rate constant, denoted by 'k', is a proportionality constant that is concentration-independent but temperature-dependent. It determines how fast a reaction proceeds at a given temperature. As temperature increases, the rate constant increases, causing the overall reaction rate to accelerate.

For reversible elementary reactions, the equilibrium constant Kc equals the ratio of the forward reaction rate constant to the reverse reaction rate constant. When the forward rate constant far exceeds the reverse rate constant, Kc is much greater than one, indicating strong product favorability at equilibrium.

The rate law expresses how the reaction rate depends on reactant concentrations. The reaction rate is proportional to reactant concentrations raised to their stoichiometric coefficients. This mathematical relationship allows prediction of how changes in concentration affect the speed at which reactants convert to products.

Reactions proceed through multi-step mechanisms where each elementary step is a single process. Intermediates are species that appear only between successive elementary steps—they are produced in one step and consumed in a later step. Understanding elementary steps is essential for analyzing reaction mechanisms using the rate limiting step approximation.

Termolecular reactions require three molecules to collide simultaneously, which is statistically improbable. Unimolecular and bimolecular reactions are far more common because they require fewer molecules to interact at once. The rarity of higher molecularity reactions reflects the low probability of multi-molecule collisions occurring together.

At equilibrium, the rates of forward and reverse reactions are equal, meaning reactants convert to products at the same rate as products convert back to reactants. This dynamic balance determines the equilibrium constant Kc, which reflects the ratio of forward to reverse rate constants at that temperature.