- activated complex
- (also, transition state) unstable combination of reactant species representing the highest energy state of a reaction system
- activation energy (Ea)
- energy necessary in order for a reaction to take place
- Arrhenius equation
- mathematical relationship between the rate constant and the activation energy of a reaction
- average rate
- rate of a chemical reaction computed as the ratio of a measured change in amount or concentration of substance to the time interval over which the change occurred
- bimolecular reaction
- elementary reaction involving the collision and combination of two reactant species
- catalyst
- substance that increases the rate of a reaction without itself being consumed by the reaction
- collision theory
- model that emphasizes the energy and orientation of molecular collisions to explain and predict reaction kinetics
- elementary reaction
- reaction that takes place precisely as depicted in its chemical equation
- frequency factor (A)
- proportionality constant in the Arrhenius equation, related to the relative number of collisions having an orientation capable of leading to product formation
- half-life of a reaction (tl/2)
- time required for half of a given amount of reactant to be consumed
- heterogeneous catalyst
- catalyst present in a different phase from the reactants, furnishing a surface at which a reaction can occur
- homogeneous catalyst
- catalyst present in the same phase as the reactants
- initial rate
- instantaneous rate of a chemical reaction at t = 0 s (immediately after the reaction has begun)
- instantaneous rate
- rate of a chemical reaction at any instant in time, determined by the slope of the line tangential to a graph of concentration as a function of time
- integrated rate law
- equation that relates the concentration of a reactant to elapsed time of reaction
- intermediate
- molecule or ion produced in one step of a reaction mechanism and consumed in another
- method of initial rates
- use of a more explicit algebraic method to determine the orders in a rate law
- molecularity
- number of reactant species (atoms, molecules or ions) involved in an elementary reaction
- overall reaction order
- sum of the reaction orders for each substance represented in the rate law
- rate constant (k)
- proportionality constant in the relationship between reaction rate and concentrations of reactants
- rate expression
- mathematical representation relating reaction rate to changes in amount, concentration, or pressure of reactant or product species per unit time
- rate law
- (also, rate equation) mathematical equation showing the dependence of reaction rate on the rate constant and the concentration of one or more reactants
- rate of reaction
- measure of the speed at which a chemical reaction takes place
- rate-determining step
- (also, rate-limiting step) slowest elementary reaction in a reaction mechanism; determines the rate of the overall reaction
- reaction mechanism
- stepwise sequence of elementary reactions by which a chemical change takes place
- reaction order
- value of an exponent in a rate law, expressed as an ordinal number (for example, zero order for 0, first order for 1, second order for 2, and so on)
- termolecular reaction
- elementary reaction involving the simultaneous collision and combination of three reactant species
- unimolecular reaction
- elementary reaction involving the rearrangement of a single reactant species to produce one or more molecules of product