2.2.2—Collision theory
- Syllabus
- First assessment 2025
- Objective
- 2.2.2
- Level
- HL
Collision Theory
A successful collision needs sufficient kinetic energy to overcome activation energy and a suitable orientation of the reacting particles.
Temperature raises average kinetic energy and changes collision frequency and the fraction of particles with energy at least Ea. Collision frequency alone does not guarantee reaction.
At one temperature, only the fraction of collisions above Ea and with a productive orientation can react. Raising temperature increases that fraction, not the energy of every particle by the same amount. Use collision frequency to explain concentration or pressure effects and the energy distribution to explain the stronger temperature effect.
Explaining Rate with Collision Theory
Assessment in practice
Representative question
Question 1
[Maximum number: 3]
Explain, using collision theory, how an increase in temperature increases the reaction rate.
Any 3 of the following:
temperature increases kinetic energy/speed of molecules more frequent collisions
more molecules have E≥Ea at higher temperature
larger ratio/percentage of collisions are successful
M2 requires time reference for probability, chance, or number of
collisions.
3 max
Rate and Mechanisms Summary
Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.
Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.