Collision Theory
Molecular basis of reaction rates
Rate Expression
Rate = Z × f × p
- Z = collision frequency (collisions per second)
- f = fraction of collisions with E ≥ Ea
- p = steric factor (proper orientation)
Three Requirements for Reaction
1. Molecules must collide
Higher concentration or temperature → more collisions → faster rate
2. Sufficient energy (E ≥ Ea)
f = e^(-Ea/RT) from Boltzmann distribution
3. Proper orientation
Reactive sites must align; p often < 1 (especially for large molecules)
Temperature Effect
Increasing T:
- Increases Z (molecules move faster, collide more often)
- Increases f (larger fraction with E ≥ Ea)
- Overall: rate typically doubles for every 10°C rise near room temperature
Connection to Arrhenius Equation
Collision theory provides molecular interpretation of k = A e^(-Ea/RT):
- A (pre-exponential factor) ∝ Z × p
- e^(-Ea/RT) = f (energy fraction)