Collision theory, concentration and pressureAQA A-Level Chemistry: Revision notes
Section 1
Collision theory
Collision theory says that a reaction can only occur when reactant particles collide, and the collision must be successful. Particles must collide with energy equal to or greater than the activation energy (Ea).
The activation energy is the minimum energy that colliding particles must have in order to react. It is the energy needed to break the bonds in the reactants so that new bonds can form.
The rate of reaction therefore depends on how many successful collisions happen per second, not just on how many collisions.
Defining activation energy as the energy released, or the energy to break all the bonds. It is the minimum energy needed for a collision to lead to a reaction.
Section 2
Why most collisions do not lead to reaction
In a gas or solution, particles collide billions of times every second, yet reactions are not instantaneous. This is because most colliding particles have less energy than the activation energy. When they collide they simply rebound without reacting.
Only the small fraction of collisions in which the particles have energy equal to or greater than Ea are successful. This is why a fast reaction is one with a high number of successful collisions per second, not one where every collision works.
Link three ideas in an answer: collision frequency, energy equal to or greater than Ea, and the number of successful collisions per second.
Section 3
Effect of concentration
Increasing the concentration of a reactant in solution means there are more particles per unit volume. The particles are closer together, so collisions are more frequent. The proportion of collisions with energy ≥ Ea is unchanged (the temperature is the same), but there are more collisions per second and so more successful collisions per second. The rate of reaction increases.
Example: the reaction of marble chips with 2.0 mol dm⁻³ hydrochloric acid is faster than with 1.0 mol dm⁻³ acid because there are more acid particles per unit volume, so more frequent collisions with the surface of the marble.
In the thiosulfate and acid experiment, halving the concentration of thiosulfate increases the time for the cross to disappear, since the rate is lower.
Saying that increasing concentration gives particles more energy, or lowers the activation energy. Neither changes.
Section 4
Effect of pressure on gases
For gases, increasing the pressure (at constant temperature) squeezes the same number of particles into a smaller volume. There are more gas particles per unit volume, which is equivalent to a higher concentration. Collisions become more frequent, so there are more successful collisions per second and the rate increases.
Doubling the pressure at constant temperature doubles the concentration of each gas.
Pressure has almost no effect on the rate of reactions involving only solids or liquids, as these cannot be compressed. Adding an unreactive gas such as argon at constant volume does not change the concentrations of the reactants, so does not change the rate.
In a gas, higher pressure means higher concentration, so give the same explanation as for concentration.
Must know
- Reactions need collisions with energy ≥ activation energy
- Most collisions are unsuccessful as the particles have energy < Ea and rebound
- Higher concentration or pressure: more particles per unit volume, more frequent collisions, more successful collisions per second
- Concentration and pressure do not change Ea or the proportion with energy ≥ Ea
- Inert gas at constant volume has no effect on rate
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Collision theory, concentration and pressure
- A student investigates the reaction between marble chips (calcium carbonate, in excess) and 50.0 cm³ of hydrochloric acid, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), by measuring the volume of carbon dioxide produced using a gas syringe. She repeats the experiment using 2.0 mol dm⁻³ acid in place of 1.0 mol dm⁻³ acid. All other conditions, including the temperature and the mass and size of the marble chips, are kept the same.Explain why most collisions between particles do not lead to a reaction.2 marks
- Nitrogen monoxide reacts with oxygen in the gas phase at constant temperature: 2NO(g) + O₂(g) → 2NO₂(g). A mixture of the gases is held in a sealed cylinder fitted with a movable piston.Explain, in terms of particles, why halving the volume of the cylinder increases the rate of reaction.2 marks
- A student investigates how concentration affects the rate of the reaction between sodium thiosulfate and hydrochloric acid: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l). In each experiment 5.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid is added to the thiosulfate solution at 20 °C, and the time taken for a cross viewed through the mixture to disappear is measured. Experiment 1 uses 50.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution and takes 36 s. Experiment 2 uses 25.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution mixed with 25.0 cm³ of water, and takes 72 s.Explain, in terms of collisions, why the cross disappears more slowly in experiment 2.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).