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activated complex
(also, transition state) unstable combination of reactant species formed during a chemical reaction
activation energy (Ea)
minimum energy necessary in order for a reaction to take place
Arrhenius equation
mathematical relationship between a reaction’s rate constant, activation energy, and temperature
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 two reactant entities
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 in a single step, 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
entities produced in one step of a reaction mechanism and consumed in a subsequent step
method of initial rates
common experimental approach to determining rate laws that involves measuring reaction rates at varying initial reactant concentrations
number of reactant entities 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 a rate law
rate expression
mathematical representation defining reaction rate as change in amount, concentration, or pressure of reactant or product species per unit time
rate law
(also, rate equation) (also, differential rate laws) 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 diagram
used in chemical kinetics to illustrate various properties of a 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 (for example, zero order for 0, first order for 1, second order for 2, and so on)
termolecular reaction
elementary reaction involving three reactant entities
unimolecular reaction
elementary reaction involving a single reactant entities
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