698 CHAPTER 16: Chemical Kinetics
Key Terms
Activation energy The kinetic energy that reactant molecules
must have to allow them to reach the transition state so that a
reaction can occur.
Arrhenius equation An equation that relates the specific rate
constant to activation energy and temperature.
Catalyst A substance that increases the rate at which a reaction
occurs. It remains unchanged when the reaction is complete.
Chemical kinetics The study of rates and mechanisms of chem-
ical reactions and of the factors on which they depend.
Collision theory A theory of reaction rates that states that effec-
tive collisions between reactant molecules must take place for
reaction to occur.
Contact catalyst See Heterogeneous catalyst.
Effective collision A collision between molecules that results in
reaction; one in which molecules collide with proper orienta-
tions and with sufficient energy to react.
Elementary step An individual step in the mechanism by which
a reaction occurs. For each elementary step, the reaction orders
domatch the reactant coefficients in that step.
Enzyme A protein that acts as a catalyst in a biological system.
Half-life of a reactant The time required for half of that reac-
tant to be converted into product(s).
Heterogeneous catalyst A catalyst that exists in a different
phase (solid, liquid, or gas) from the reactants; a contact cata-
lyst.
Homogeneous catalyst A catalyst that exists in the same phase
(liquid or gas) as the reactants.
Integrated rate equation An equation that relates the concen-
tration of a reactant remaining to the time elapsed; has different
mathematical forms for different orders of reaction.
Method of initial rates A method of determining the rate-law
expression by carrying out a reaction with different initial con-
centrations and analyzing the resulting changes in initial rates.
Order of a reactant The power to which the reactant’s con-
centration is raised to the rate-law expression.
Order of a reaction The sum of the powers to which all con-
centrations are raised in the rate-law expression; also called
overall order of a reaction.
Rate constant(also called specific rate constant) An experi-
mentally determined proportionality constant that is different
for different reactions and that, for a given reaction, changes
only with temperature or the presence of a catalyst; kin the
rate-law expression, Ratek[A]x[B]y.
Rate-determining step The slowest step in a reaction mecha-
nism; the step that limits the overall rate of reaction.
Rate-law expression(also called rate law) An equation that
relates the rate of a reaction to the concentrations of the reac-
tants and the specific rate constant; ratek[A]x[B]y. The
exponents of reactant concentrations do not necessarilymatch the
coefficients in the overall balanced chemical equation. The rate-
law expression must be determined from experimental data.
Rate of reaction The change in concentration of a reactant or
product per unit time.
Reaction coordinate The progress along the pathway from
reactants to products; sometimes called “progress of reaction”.
Reaction intermediate A species that is produced and then
entirely consumed during a multistep reaction; usually short-
lived.
Reaction mechanism The sequence of steps by which reactants
are converted into products.
Substrate A reactant in an enzyme-catalyzed reaction.
Thermodynamically favorable (spontaneous) reaction A
reaction that occurs with a net release of free energy, G; a reac-
tion for which Gis negative (see Section 15-15).
Transition state A relatively high-energy state in which bonds
in reactant molecules are partially broken and new ones are par-
tially formed.
Transition state theory A theory of reaction rates that states
that reactants pass through high-energy transition states before
forming products.Exercises
General Concepts
*01.Briefly summarize the effects of each of the four factors
that affect rates of reactions.
*02.Describe the basic features of collision theory and transi-
tion state theory.
*03.What is a rate-law expression? Describe how it is deter-
mined for a particular reaction.
*04.Distinguish between reactions that are thermodynamically
favorable and reactions that are kinetically favorable. What
can be said about relationships between the two?
*05.What is meant by the order of a reaction?*06.What, if anything, can be said about the relationship
between the coefficients of the balanced overallequation
for a reaction and the powers to which concentrations are
raised in the rate-law expression? To what are these powers
related?
*07.Express the rate of reaction in terms of the rate of change
of each reactant and product in the following reactions.
(a) H 2 O 2 (aq)2H(aq)2I(aq)88n
I 2 (aq)2H 2 O()
(b) 2NO(g)Br 2 (g)88n2NOBr(g)
(c) CH 3 COOH(aq)OH(aq)88n
CH 3 COO(aq)H 2 O()