Chemistry 112 Chapter 12

Kinetics

Study of reaction rates

activated complex

(also, transition state) unstable combination of reactant species formed during a chemical reaction

activation energy (E_a)

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

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 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 (t_l/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

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

molecularity

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