Chemical Kinetics
Chemical kinetics is the study of reaction rates, the factors affecting them, and reaction mechanisms. Thermodynamics tells us whether a reaction is feasible; kinetics tells us how fast it occurs.
Rate of a reaction
The rate is the change in concentration of a reactant or product per unit time.- Average rate = -delta[R]/delta(t) = +delta[P]/delta(t), measured over a time interval.
- Instantaneous rate = -d[R]/dt = +d[P]/dt, measured as delta(t) approaches zero (slope of the tangent on a concentration vs time plot).
- For aA + bB gives cC + dD, the rate term for each species is divided by its stoichiometric coefficient.
- Units: mol L-1 s-1 (or atm s-1 for gases expressed as partial pressure).
Factors affecting rate
Concentration of reactants (pressure for gases), temperature, and catalyst.Rate law, order and molecularity
- Rate law: Rate = k[A]^x[B]^y, determined experimentally - it cannot be predicted from the balanced equation. k is the rate constant.
- Order = x + y, the sum of the powers of concentration terms; can be 0, 1, 2, 3 or a fraction.
- Molecularity: number of species colliding simultaneously in an elementary reaction (1, 2 or 3 only); not defined for complex reactions.
Integrated rate equations
- Zero order: [R] = -kt + [R]0; k = ([R]0 - [R])/t; t(1/2) = [R]0/2k.
- First order: ln[R] = -kt + ln[R]0; k = (2.303/t) log([R]0/[R]); t(1/2) = 0.693/k (independent of [R]0).
- Pseudo first order: higher order reactions that behave as first order because one reactant (e.g. water) is in large excess (hydrolysis of ethyl acetate, inversion of cane sugar).