CHEM 1312 Chapter 11 Chemical Kinetics

reaction rate

how quickly concentration changes over time

average rate

Δ[concentration] / Δt over a time interval (slope of secant line)

instantaneous rate

rate at one specific moment (slope of tangent line)

rate (with stoichiometry)

rate law

rate = k[A]^m[B]ⁿ

rate constant (k)

fixed value for a given reaction at a given temperature; doesn’t change with concentration

reaction order

exponent on a reactant’s concentration in the rate law; must be determined exponentially

overall reaction order

sum of all individual orders (exponents, m+n)

method of initial rates

technique using multiple trials (changing one concentration at a time) to determine reaction orders from data

0th order k unit

M/s

1st order unit of k

1/s

2nd order unit of k

1/(M*s)

3rd order unit of k

1/(M²*s)

integrated rate law

equation relating concentration directly to time (rather than rate to concentration)

Zero-order integrated rate law

first order integrated rate law

second order integrated rate law

half life (t_1/2)

time required for concentration to drop to half its value

zero order half life

first order half life

second order half life

activation energy (E_a)

minimum energy molecules need upon collision for a reaction to occur

Arrhenius equation

Linearized Arrhenius equation

two-point arrhenius equation

elementary step

a single molecular event/collision; cannot be broken down further

molecularity

number of molecules colliding in an elementary step (unimolecular, bimolecular, termolecular)

intermediate

species produced in one step and consumed in a later step; does not appear in overall equation or final rate law

Rate-determining step

the slowest elementary step; controls the overall reaction rate

rate law for an elementary step

read directly from the coefficients

fast equilibrium step substitution

method for eliminating an intermediate step from a rate law using an earlier fast, reversible step

catalyst

speeds up a reaction without being consumed; provides a lower E_a pathway

catalyst v. intermediate

catalyst is present at the start, consumed early, regenerated later

intermediate is produced during the reaction, never present at the start

transition state

highest energy point of an individual elementary step (top of a peak)

overall activation energy (multi-step)

energy of the highest peak in the diagram minus energy of the true starting reactants

ΔH (catalyzed vs. uncatalyzed)

unchanged by a catalyst; only the height of the energy barrier changes