Kinetics

Kinetics is the branch of science that studies the rates of chemical processes and the steps by which they occur. It seeks to determine how fast a reaction proceeds and why, often in terms of rate laws that relate the rate to concentrations of reactants. The rate law for a reaction expresses the instantaneous rate as rate = k [A]^m [B]^n … where k is the rate constant and the exponents m, n are the reaction orders. The overall order is the sum.

Factors affecting rates include concentration, temperature, the presence of a catalyst, surface area, pressure for gases, solvent, and light. Temperature rise generally increases rate according to the Arrhenius relation: k = A exp(-Ea/RT). Activation energy Ea is the barrier to reaction. Catalysts provide alternate pathways with lower Ea, increasing rate without changing products. In enzymology, rates depend on substrate concentration, described by Michaelis-Menten: v = Vmax[S]/(Km + [S]), with Km reflecting affinity and Vmax the maximum rate.

Kinetics uses differential equations and their integrated forms. For simple cases: zero-order, [A] decreases at a constant rate; first-order, ln[A] vs t is linear; second-order, 1/[A] vs t is linear. Half-life expressions can depend on initial concentration for some orders. Experimentally, reactions are studied by initial-rate methods and by monitoring concentrations over time through spectroscopy or calorimetry.

Kinetics is distinct from thermodynamics: kinetics concerns speed and pathways, while thermodynamics concerns feasibility and the eventual equilibrium position.

Applications span chemical engineering, environmental science, pharmacology, materials science, and combustion.