Chem 1150 - Chemical Kinetics Lecture Notes

Chemical Kinetics

Study of how fast a reaction goes and the factors that control that rate

Reaction Rate

How fast products appear or how fast reactants disappear (always a positive quantity)

Units in M/s (molarity per second) (mol/L * s)

General Reaction Rate

aA + bB → dD + eE

rate = -(1/a)(ΔA/Δt) = -(1/b)(ΔB/Δt) = (1/d)(ΔD/Δt) = (1/e)(ΔE/Δt)

Instantaneous Rate

Rate that exists at a specific time t (slope of the tangent to the curve at time t)

Initial Rate

The instantaneous rate at t = 0

Average Rate

Average reaction rate that exists over a time period

Rate Law

A + B → C + D

Rate ∝ [A]^x[B]^y

Rate = k[A]^x[B]^y

k = proportionality constant or rate constant

*Note only applies if constant temperature

Units for k

Zeroth Order (Rate = k) → M/s or M s^-1

First Order (Rate = k[A]) → 1/s or s^-1

Second Order (Rate = k[A][B]) → 1/(M * s) or M^-1 s^-1

Third Order (Rate = k[A][B]²) → 1(M² * s) or M^-2 s^-1

Integrated Rate Law (First Order Reactions)

ln[A]_t = -kt + ln[A]₀

Tells us how reactant concentration changes with time.

Conforms reaction equation to a straight line (allows rate constant to be obtained from slope, m = -k)

*only for first order reactions

Half-Life (First Order Reactions)

t_1/2 = 0.693/k

Length of time required for the concentration of a reactant to decrease to half of its initial value

(1/2)^{num of half-lifes} * [A]₀ = [A]_t