Collision Theory and Reaction Rates
Collision theory is the fundamental model used to explain how chemical reactions occur and why factors like temperature and concentration alter the reaction speed. It states that for a reaction to happen, reactant particles must collide with enough energy and in the correct physical orientation.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- For a chemical reaction to occur, reactant particles must physically collide with each other.
- Not all collisions lead to a reaction. A collision is only 'successful' if two conditions are met.
- Condition 1 (Energy): The colliding particles must have a combined kinetic energy equal to or greater than the activation energy (Ea).
- Condition 2 (Orientation): The particles must collide in the correct geometric alignment for bonds to break and new bonds to form.
- The rate of a reaction is directly proportional to the frequency of successful collisions per unit time.
- Increasing temperature, concentration, or pressure (for gases) increases the frequency of successful collisions, thus increasing the reaction rate.
Definitions
- Collision Theory
- A model explaining that reaction rates depend on the frequency of collisions between reactant particles that meet the minimum energy and correct orientation requirements.
- Activation Energy (Ea)
- The minimum amount of kinetic energy required for a collision between reactant particles to result in a chemical reaction.
- Successful Collision
- A collision that leads to the formation of products because the particles met both the minimum energy (≥ Ea) and correct orientation requirements.
Worked example
Increasing the concentration of a reactant in a solution increases the rate of reaction. Using collision theory, explain why.
- 1
State the effect of increasing concentration on the particles:
It means there are more reactant particles per unit volume.
- 2
Relate this to the frequency of all collisions:
With more particles in the same space, the particles are closer together and will collide more often.
The total collision frequency increases.
- 3
Connect this to successful collisions:
As the total number of collisions per unit time increases, the number of collisions that meet the energy and orientation requirements (successful collisions) will also increase proportionally.
- 4
Conclude by linking to the reaction rate:
Since the rate of reaction depends on the frequency of successful collisions, the rate increases.
Answer: Increasing the concentration means more reactant particles are present in the same volume. This leads to a higher frequency of total collisions. Consequently, the frequency of successful collisions (those with sufficient energy and correct orientation) also increases, which in turn increases the rate of reaction.
Common mistakes
- ×Forgetting one of the two conditions for a successful collision. A complete answer must mention both sufficient energy (≥ Ea) AND the correct orientation.
- ×Confusing total collision frequency with successful collision frequency. The reaction rate depends specifically on the frequency of successful collisions.
- ×When explaining the effect of temperature, attributing the rate increase only to more frequent collisions. The primary factor is the significantly larger proportion of particles possessing energy greater than the activation energy.
No-calculator tips
- ✓Think in terms of proportions and frequencies, not absolute numbers. The questions will be qualitative, asking for explanations of 'why' or 'what happens when'.
- ✓For temperature effects, remember the relationship is exponential, not linear. A small temperature rise causes a large increase in rate because the number of particles with energy > Ea increases dramatically.
- ✓Visualise the factors: higher concentration = a more crowded room (more bumping); higher temperature = a room where everyone is running (more frequent and more energetic bumping).