Collision Theory & Rates of Reaction
Chemistry: IB Diploma Higher Level, topic 5.1.1
Collision Theory
In order for particles to react, they must collide with sufficient energy and in the correct orientation.
Collision theory
- Collision theory states that for particles to react they must first collide with:
- sufficient energy (greater than the activation energy)
- correct orientation
- The activation energy (Ea) is the minimum energy required for a collision to result in a reaction.
- To speed up the rate of reaction we need to increase the frequency of collisions and/or increase the energy of the colliding particles so that more collisions are successful.
Maxwell-Boltzmann distribution
- To better understand this, we need to consider the Maxwell-Boltzmann distribution.
- This distribution shows that at a given temperature a distribution of kinetic energies for the particles is available and only a percentage of particles will have an energy greater than the Ea.
Factors Affecting Rate of Reaction
The rate of a given chemical reaction is influenced by temperature, particle size, concentration or pressure, and the presence/absence of a catalyst.
Temperature
- Increasing the temperature has two effects on the particles to increase the rate of reaction:
- particles have more kinetic energy at higher temperatures and this causes more frequent collisions
- the increased kinetic energy also means that more collisions will now be successful as more collisions now have an energy greater than the activation energy.
Temperature cont.
- At higher temperatures the curve of the Maxwell-Boltzmann distribution shifts to the right (higher energy).
Particle size
- Decreasing the particle size increases the surface area to volume ratio.
- Increasing the surface area causes more contact between reactants.
- This results in more frequent collisions between reacting particles and a faster rate of reaction.
Concentration or pressure
- Increasing the concentration increases the frequency of collisions and the rate of reaction is increased.
- Increasing the pressure for gases has the same effect as increasing concentrations of solutions.
Catalysts
- Catalysts are substances that increase the rate of reaction without being consumed by the process.
- Catalysts work by offering an alternative reaction pathway for the reaction with a lower activation energy.
- Lowering the activation energy means that more colliding particles now have an energy greater than the activation energy and more collisions become successful.
Catalysts
Test yourself on Collision Theory & Rates of Reaction
Which of the following describes the requirements for a successful collision?
How does a catalyst speed up the rate of reaction?