Collision theory — AQA A-Level Chemistry
Test yourself on Collision theory with AQA A-Level practice questions.
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Collision theory explained
Particles in a gas or solution are in constant random motion and collide constantly, yet only a tiny minority of those collisions produce a reaction.
Read the full explanation
For a collision to work, the particles must arrive with at least a minimum combined kinetic energy, enough to start breaking the existing bonds and reach the transition state. They must also be correctly oriented, so that the right atoms meet. Most collisions fail on energy: at room temperature only a small fraction of particles are moving fast enough. That is why rate depends on the frequency of successful collisions rather than the total collision frequency, and why examiners want the word successful, or the phrase collisions with energy greater than or equal to the activation energy, rather than simply more collisions.
This energy is called the activation energy.
The activation energy, Eₐ, is the minimum energy that colliding particles must have in order to react. Physically it is the cost of reaching the transition state, the arrangement partway through the reaction where old bonds are stretched and new ones are only partly formed and the potential energy is at a maximum. On an enthalpy profile it is the height from the reactants up to that peak, measured from the reactant level, not from the products and not from the baseline. Every reaction has one, exothermic reactions included, which is why a mixture of petrol and air is stable until a spark supplies the first particles with enough energy. A large Eₐ means only a small fraction of collisions qualify, so the reaction is slow.
Students should be able to: define the term activation energy explain why most collisions do not lead to a reaction.
Two related things are being asked for. First the definition: activation energy is the minimum energy colliding particles must possess for a reaction to occur. Second the explanation of failure, which is a statement about a distribution. At any temperature the particles have a wide spread of kinetic energies, and only the small area under the tail of that distribution beyond Eₐ represents particles energetic enough to react. Most collisions therefore involve particles below the barrier, so the particles simply bounce apart unchanged. Orientation adds a second filter: even an energetic collision can fail if the reacting groups are pointing the wrong way. The proportion of collisions that succeed, not the sheer number of collisions, controls the rate.
Your focus
- State the two conditions a collision must satisfy before it can lead to reaction.
- Explain why a reaction mixture in which billions of collisions occur each second may still react slowly.
- Distinguish between collision frequency and the proportion of successful collisions when accounting for a rate change.
Show all 12 objectives
- Apply collision theory to explain why a reaction is fastest at the start and slows as reactants are used up.
- Define activation energy using the word minimum and referring to colliding particles.
- Label Eₐ and ∆H correctly on an enthalpy profile for an exothermic and an endothermic reaction.
- Explain why a mixture that is thermodynamically unstable can persist unchanged until it is ignited.
- Predict which of two reactions with quoted activation energies is faster at the same temperature, and justify the answer.
- Define activation energy and use the definition to explain why most collisions in a reacting mixture are unsuccessful.
- Explain, in terms of an energy distribution, why only a small fraction of particles can react at a given temperature.
- Give two distinct reasons a collision between reactant particles may fail to produce products.
- Contrast the effect on rate of changing the collision frequency with that of changing the proportion of successful collisions.
Collision theory exam tips
Marking Points
- one mark for stating that particles must collide for a reaction to occur
- one mark for adding that the collision must involve energy greater than or equal to the activation energy
- one mark for referring to the frequency of successful collisions rather than the total number of collisions
- one mark for mentioning correct orientation of the colliding particles as a second requirement
- one mark for defining activation energy as the minimum energy needed for a reaction to occur
- one mark for adding that it is the minimum energy the colliding particles must have
- one mark for marking Eₐ on a profile from the reactant level up to the peak
- one mark for linking a larger activation energy to a smaller proportion of successful collisions and a slower rate
- one mark for the definition, the minimum energy required for colliding particles to react
- one mark for stating that most particles have energy below the activation energy
- one mark for concluding that those collisions leave the particles unchanged, so no reaction occurs
- one mark for citing incorrect orientation as an additional reason why an energetic collision may fail
Examiner Tips
- 💡Use the phrase more frequent successful collisions; both words carry marks in rate explanations.
- 💡Reserve the word collision for particles and the word energy for the barrier, and connect them in one sentence.
- 💡If a question gives a concentration or pressure change, mention collision frequency; if it gives a temperature or catalyst change, mention the proportion of collisions that succeed.
- 💡Include the word minimum; a definition without it does not score.
- 💡On any profile diagram, use two separate arrows: one from reactants to peak for Eₐ and one from reactants to products for ∆H.
- 💡Link the size of Eₐ to the fraction of particles beyond it whenever a question asks about rate.
- 💡Structure a two mark answer as definition then consequence: the minimum energy, and most particles being below it.
- 💡Refer to the proportion of successful collisions rather than the number of collisions when the temperature is unchanged.
- 💡Avoid the words chance and probability; the mark scheme ignores them.
Common Mistakes
- writing that more collisions occur without saying they are successful collisions, which is not credited
- using the words chance or probability of collision, which examiners ignore
- claiming every collision leads to reaction provided the particles touch
- confusing the energy of a collision with the temperature of a single particle
- saying particles need activation energy to collide, rather than to react once they have collided
- measuring Eₐ from the products or from the zero line rather than from the reactants
- defining it as the energy given out by the reaction, confusing it with ∆H
- saying exothermic reactions have no activation energy
- describing Eₐ as the average energy of the particles rather than the minimum needed
- drawing the peak below the reactant level on a profile diagram
- explaining failure by saying the particles do not collide, rather than that they collide without enough energy
- saying the particles do not have enough activation energy, treating Eₐ as something a particle carries
- answering in terms of chance or luck rather than the energy distribution
- assuming all particles at a given temperature have the same energy
- leaving out orientation when the question asks for two reasons