AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The qualitative effect of changes in concentration on collision frequency.
Full specification wording
Effect of concentration and pressure: The qualitative effect of changes in concentration on collision frequency.
Concentration counts particles per unit volume.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Concentration counts particles per unit volume. 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.
What earns the marks
- Stating that a higher concentration means more particles in a given volume.
- Concluding that collisions become more frequent.
- Referring to more frequent successful collisions, not just more collisions.
- Explaining a falling rate during a reaction by the decreasing concentration of reactants.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Effect of concentration and pressure: The qualitative effect of changes in concentration on collision frequency. lesson, made from checked teaching notes for this specification point.
Have a go first
A solution of hydrochloric acid is made more concentrated by dissolving more hydrogen chloride gas in the same volume of water. What happens to the frequency of collisions between the acid particles?
Listen
Concentration counts particles per unit volume. 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. A strong answer earns marks for these points. stating that a higher concentration means more particles in a given volume. concluding that collisions become more frequent. referring to more frequent successful collisions, not just more collisions. explaining a falling rate during a reaction by the decreasing concentration of reactants. Exam tip: Write the chain in order: more particles per unit volume, more frequent collisions, more frequent successful collisions.
Key words
Say what each word means before you turn the card.
particles per unit volume
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collision frequency
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successful collisions
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proportion unchanged
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See every definition
- particles per unit volume
- Concentration counts particles per unit volume, so doubling the concentration of a reactant in solution gives twice as many of its particles in every cubic decimetre.
- collision frequency
- Any given particle of the other reactant meets one twice as often when concentration is doubled, so collisions become more frequent.
- successful collisions
- The same fixed proportion of collisions has energy greater than or equal to Eₐ, so these become more frequent and the rate rises.
- proportion unchanged
- The energy distribution of the particles is set by temperature alone, so the proportion of collisions that succeed is unaltered.
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