Effect of concentration and pressure — AQA A-Level Chemistry
Test yourself on Effect of concentration and pressure with AQA A-Level practice questions.
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Effect of concentration and pressure explained
Concentration counts particles per unit volume.
Read the full explanation
Double the concentration of a reactant in solution and there are twice as many of its particles in every cubic decimetre, so any given particle of the other reactant meets one twice as often. Collisions therefore become more frequent, and since the same fixed proportion of them has energy greater than or equal to Eₐ, successful collisions also become more frequent and the rate rises. Note what does not change: the energy distribution of the particles is set by temperature alone, so the proportion of collisions that succeed is unaltered. This also explains why a reaction is fastest at the start and slows as it proceeds, since the reactants are steadily consumed and their concentrations fall.
The qualitative effect of a change in the pressure of a gas on collision frequency.
For gases, pressure plays the role concentration plays in solution. Squeezing a fixed amount of gas into a smaller volume packs the same number of molecules into less space, so the concentration of every gaseous reactant rises and collisions become more frequent. The result is a greater frequency of successful collisions and a faster rate, with the proportion of successful collisions unchanged because the temperature and therefore the energy distribution are the same. The reasoning depends on the molecules being brought closer together, which is why pumping in an unreactive gas at constant volume does not speed the reaction up: the total pressure rises, but the partial pressures of the reactants, and so their concentrations, are untouched.
Students should be able to: explain how a change in concentration or a change in pressure influences the rate of a reaction.
Gas pressure increases act via increased concentration (particles per unit volume), raising collision frequency. Raising solution concentration or gas pressure brings reacting particles closer together. Collisions happen more often. The proportion of collisions with energy ≥ Eₐ is fixed by temperature. A greater collision frequency scales up successful collisions, increasing the rate. The energy distribution remains untouched, so the Maxwell–Boltzmann curve is unchanged. Example: Compressing a gas mixture halves its volume, doubling the concentration of gaseous reactants, which directly increases the frequency of successful collisions.
Your focus
- Explain an increase in rate on increasing concentration using a chain from particle density to successful collision frequency.
- State what a concentration change does not alter about the distribution of molecular energies.
- Explain why a reaction is fastest at the start and slows as time passes.
Show all 11 objectives
- Distinguish the reasoning used for a concentration change from that used for a temperature change.
- Explain the effect of increasing the pressure of a gaseous reaction mixture on the rate, working from volume to concentration to collision frequency.
- Predict the effect on rate of adding an unreactive gas at constant volume and justify the prediction.
- Identify when a pressure argument is inappropriate because the reactants are not gases.
- Contrast the effect of pressure on the rate of a reaction with its effect on the position of equilibrium.
- Explain how an increase in gas pressure increases concentration and collision frequency.
- Explain the effect of concentration on reaction rate in solutions.
- Justify why the Maxwell-Boltzmann distribution is unchanged by pressure.
Effect of concentration and pressure exam tips
Marking Points
- one mark for stating that a higher concentration means more particles in a given volume
- one mark for concluding that collisions become more frequent
- one mark for referring to more frequent successful collisions, not just more collisions
- one mark for explaining a falling rate during a reaction by the decreasing concentration of reactants
- one mark for stating that increasing the pressure forces the same number of molecules into a smaller volume
- one mark for identifying the resulting increase in concentration of the gaseous reactants
- one mark for concluding that collisions, and therefore successful collisions, become more frequent
- one mark for recognising that adding an inert gas at constant volume leaves the reactant concentrations, and the rate, unchanged
- Increased gas pressure increases concentration (particles per unit volume), leading to higher collision frequency.
- A greater collision frequency increases the frequency of successful collisions.
- The proportion of successful collisions remains unchanged if temperature is constant.
- The rate of reaction increases directly in step with the frequency of successful collisions.
Examiner Tips
- 💡Write the chain in order: more particles per unit volume, more frequent collisions, more frequent successful collisions.
- 💡For a reaction that slows over time, run the same chain in reverse and say fewer particles as reactants are used up.
- 💡Keep energy language out of a concentration answer unless you are contrasting it with a temperature change.
- 💡Convert pressure into concentration in your first sentence; every later mark follows from that step.
- 💡Check whether the reactants are gases before you use a pressure argument at all.
- 💡Watch for the inert gas trap, where the total pressure rises at constant volume.
- 💡Treat pressure and concentration as acting on collision frequency, while temperature acts on the proportion of successful collisions.
- 💡Always use the phrase 'per unit volume' when discussing concentration to secure the mark.
Common Mistakes
- writing that the particles move faster at higher concentration, which confuses concentration with temperature
- saying the proportion of successful collisions increases, when only their frequency does
- using the words chance or probability of collision, which examiners ignore
- omitting the word successful, so the answer scores only the first mark
- assuming rate is always proportional to concentration, before the order has been determined experimentally
- claiming a pressure increase gives the molecules more energy and so a greater proportion of successful collisions
- applying a pressure argument to a reaction between substances in solution
- assuming any rise in total pressure must speed the reaction up, whatever causes it
- writing more collisions without the word frequent or successful
- confusing the effect of pressure on rate with its effect on the position of an equilibrium
- Denying that concentration reasoning (particles per unit volume) applies to gas phase pressure changes; correct by treating pressure as gas concentration.
- Explaining a concentration change using activation energy; correct by referencing collision frequency.
- Stating particles collide harder; correct by stating they collide more often.