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Activation Energy  -  The difference in energy between the reactants and the transition state that is the energy barrier the reactants must overcome to achieve a chemical reaction.
Catalyst  -  A substance that lowers the activation energy for a chemical reaction without being chemically altered by the reaction.
Elementary Step  -  A reaction that represents a single collision or intramolecular step in a reaction mechanism.
Homogeneous Catalyst  -  A catalyst that is in the same phase as the reactants.
Intermediate  -  A species that is both produced and consumed in a chemical reaction. As such, it does not appear in the overall reaction but is proposed to be produced in one elementary step and consumed in another.
Kinetics  -  The study of the rate and mechanism of chemical reactions.
Mechanism  -  The series of elementary steps that combine to produce the path molecules take from reactant(s) to product(s) in a chemical reaction.
Order  -  In the rate law of a reaction, the power to which the concentration of a reagent is raised. Or, the sum of the powers on the concentration terms in the rate law.
Rate  -  The speed of a reaction measured in amount or reagent consumed or product produced per unit time.
Rate Constant  -  The proportionality constant in the rate law expression. This factor is a measure of the intrinsic reactivity of the reaction but is not constant with respect to temperature.
Rate Law  -  An expression of the dependence of the rate of a reaction on the concentrations of reactants.
Rate Limiting Step  -  The slowest elementary step in a mechanism. The rate of the reaction must equal the rate of the slowest step because the reaction can go no faster than its slowest step.
Reaction Coordinate Diagram  -  A plot of free energy versus the reaction coordinate for a reaction that provides a pictorial representation of the lowest energy path from reactants to products.
Steady-State Approximation  -  The assumption that the rate of formation and consumption of a highly reactive intermediate are equal so that the change in intermediate concentration with respect to time is approximated to be zero.
Transition State  -  The species with the highest energy between reactants and products on a reaction coordinate diagram, it is a short-lived species that represents a combination of product-like and reactant-like properties.