Chemical Kinetics

$\text{Rate} = \frac{\Delta[\text{product}]}{\Delta t} = -\frac{\Delta[\text{reactant}]}{\Delta t}$

Measured by: gas volume, mass loss, colour change, conductivity.

For a reaction to occur, particles must:

1. Collide with each other
2. Have energy ≥ activation energy ($E_a$)
3. Have correct orientation

A graph showing the distribution of kinetic energies among particles:

• Most particles have moderate energy
• Few have very low or very high energy
• Area under curve = total number of particles
• Area to the right of $E_a$ = particles that can react

Effect of temperature increase:

• Curve shifts right and flattens
• More particles exceed $E_a$
• Rate increases

Effect of catalyst:

• $E_a$ is lowered
• More particles exceed the new, lower $E_a$
• Distribution curve unchanged

$\text{Rate} = k[A]^m[B]^n$

• $m$, $n$ = orders (determined experimentally)
• $k$ = rate constant
• Overall order = $m + n$

Determining Orders

Compare experiments where one concentration changes:

• [A] doubles, rate doubles → order 1
• [A] doubles, rate ×4 → order 2
• [A] doubles, rate unchanged → order 0

$k = Ae^{-E_a/RT}$

$\ln k = \ln A - \frac{E_a}{RT}$

Plot $\ln k$ vs $1/T$: gradient = $-E_a/R$

The rate-determining step is the slowest step. The rate equation reflects the species in the RDS.

If rate = $k[A][B]$, the RDS involves one molecule of A and one of B.

• Rate depends on collision frequency and energy
• Factors: temperature, concentration, surface area, catalysts
• Maxwell-Boltzmann: temperature shifts curve; catalyst lowers $E_a$
• HL: Rate equations from experimental data; Arrhenius equation links $k$ to $T$
• RDS determines the rate equation