rateconstante

Rate constant, denoted k, is the proportionality factor in the rate law that links the speed of a chemical reaction to the concentrations of the reactants. For a reaction aA + bB → products, the instantaneous rate can be written as rate = k [A]^m [B]^n, where m and n are the reaction orders with respect to A and B. The overall order is m+n, and the rate constant k depends on the mechanism and conditions; it is not a universal constant. In elementary reactions, m and n may equal the stoichiometric coefficients, but in complex mechanisms they are determined experimentally.

The units of k depend on the overall order: for a first-order process, k has units of s^-1; for a second-order process, k has units of M^-1 s^-1; higher orders have corresponding units.

Temperature has a strong influence on k. The Arrhenius equation k = A exp(-Ea/RT) describes this dependence, where Ea is the activation energy and A is the pre-exponential factor. Changes in temperature, solvent, pressure, or catalysts can change k by altering Ea and/or A. Catalysts provide alternative pathways with lower activation energy, increasing k at a given temperature.

k is determined experimentally by fitting rate data to the rate law; it can also be estimated from theoretical models such as transition state theory for elementary steps. Understanding k for a given reaction allows prediction of its rate under specified conditions and is essential in kinetic modeling and reactor design.