Explaining rate factors with collisions
Key idea: Reaction rate depends on successful collisions per second. Higher temperature makes collisions more frequent and gives a greater fraction of particles enough energy to react.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Compare the concentration of two portions of one acid.
- Read the two ways a reaction can speed up.
- Find both early rates, then predict B’s final volume.
- Explain how concentration changes rate without changing the final amount.
- Correct both predictions about doubling the acid volume.
- Decide which reactions speed up at higher pressure.
- Find surface areas before and after cutting.
- Compare the faces before and after cutting.
- Predict which sample starts faster and explain why.
- Read both reasons heating speeds up a reaction.
- Choose the complete explanation and find the missing link.
- Write a complete explanation, then mark your answer.
- Explain whether adding more reacting gas increases the rate.
- Explain why rate rises more than collision frequency.
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ContinueParticles react only when they collide with enough energy. Concentration, pressure, particle size and temperature each change the rate of a reaction by changing how often those successful collisions happen.
Compare volume and concentration
Check your understanding 1 More acid, or more concentrated acid?
A bottle contains 1.0 mol/dm³ hydrochloric acid. You pour 50 cm³ into one beaker and 100 cm³ into another. Which beaker has the higher concentration?
Show answer
Count successful collisions
Reacting particles must collide to react. Not every collision works: a collision leads to reaction only if the particles have enough energy when they meet. The rate depends on the number of successful collisions per unit time.
A change can increase the rate in two ways:
- the reacting particles collide more often;
- a greater fraction of the collisions have enough energy to react.
Concentration, pressure and particle size work through the first. Temperature works through both.
The four factors
- Higher concentration: there are more reactant particles per unit volume, so collisions occur more frequently and rate increases.
- Higher pressure for reacting gases: the gas particles are closer together, so collisions occur more frequently and rate increases.
- Smaller solid particles: the same mass has a larger total surface area, so collisions at the solid surface occur more frequently and rate increases.
- Higher temperature: particles have more kinetic energy and move faster. They collide more frequently and, more importantly, a greater fraction of collisions have enough energy to react. Both effects increase the rate.
Concentration: more particles per unit volume
Concentration is the amount of solute in a fixed volume of solution. A larger volume of the same acid has the same concentration: it contains more acid particles, but not more per unit volume.
Two runs use the same length of magnesium ribbon (0.048 g) with 50 cm³ of dilute hydrochloric acid at 20 °C. The acid is in excess in both. Only the acid concentration changes. Hydrogen volumes are compared at the same temperature and pressure.
| Time / s | 0 | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|---|
| Hydrogen, run A (1.0 mol/dm³ acid) / cm³ | 0 | 20 | 34 | 42 | 46 | 48 | 48 |
| Hydrogen, run B (0.5 mol/dm³ acid) / cm³ | 0 | 10 | 19 | 27 | 34 | 40 | 44 |
Check your understanding 2 Read the data before explaining the difference
Use the table above. Calculate each average rate over 0–10 s in cm³/s. Has run B finished at 60 s? Predict its final volume from the equal magnesium amounts and excess acid.
Show answer
Worked example 1
Compare two concentrations from raw data
Problem
Use both rows of the table. Which run is faster, what does each run make in total, and why does concentration change the rate?
Show solution step by step
Check the comparison is fair
Method
List what changed and what stayed the same.Reason
A difference in rate can be linked to one factor only if everything else was controlled.
Working
Answer
Same magnesium, acid volume and temperature; only the concentration differs.
Compare the same interval
Method
Calculate the average rate for 0–10 s in each run.
Reason
Use the same initial interval to compare the two runs near their starts.
Working
Answer
A: 20 ÷ 10 = 2.0cm³s⁻¹. B: 10 ÷ 10 = 1.0cm³s⁻¹.
Read the final amounts
Method
Look at where each row stops changing.Reason
The final amount depends on how much magnesium there is, not on how fast it reacts.
Working
Answer
A stays at 48 cm³ from 50 s. B is still rising at 60 s; it has the same magnesium, so it will also reach 48 cm³.
Explain with collisions
Working
Answer
Run A is faster. The more concentrated acid has more acid particles per unit volume, so they collide with the magnesium surface more often. There are more successful collisions per unit time, so the rate is higher. Both runs make the same amount of hydrogen.
Quick check
Do not read B’s 44 cm³ at 60 s as “less hydrogen”. B has not finished. Compare rates over the same interval, and final amounts only once both curves have levelled off.
Check your understanding 3 Twice the acid volume
Two equal magnesium ribbons are fully immersed, with the same exposed surface, in 50 cm³ and 100 cm³ portions of 1.0 mol/dm³ acid at the same temperature. Acid is in excess in both runs. A student predicts that the larger portion gives twice the initial rate and twice the final hydrogen amount. Correct both predictions and explain each correction.
Compare your answer against concentration, successful collisions per unit time and the amount of magnesium that reacts.
Show answer
Pressure: crowd reacting gas particles together
Compressing the same amount of reacting gas at constant temperature raises its pressure and puts more reactant particles in each unit volume. This is the gas version of increasing concentration. A liquid hardly compresses under ordinary laboratory pressure changes, so the concentration of dissolved reactants changes very little. Pressure does not appreciably change the rate of a reaction between solutions by this collision-frequency mechanism.
Guided practice 2
Where does pressure matter?
Problem
Look at the state of each reactant
Hints
Hint 1
state symbols
Read the state symbol of every reactant.Hint 2
product
A gas that is made is a product, not a reacting particle.Show solution step by step
Gaseous reactants
Method
Apply the pressure effect to nitrogen monoxide and oxygen.Reason
Doubling the pressure of the same amount of gas halves its volume, so the particles are twice as crowded.Working
Answer
More gas particles per unit volume → more frequent collisions → higher rate.Solutions and solids
Working
Answer
An ordinary increase in pressure does not appreciably change the rate of the other two reactions by this mechanism: their reactants are solutions and a solid. Giving off a gas does not make pressure a factor.
Quick check
Particle size: expose more solid surface
Only particles at the surface of a solid can collide with the other reactant, so exposing more surface makes collisions more frequent. Cutting a solid exposes new faces only if the pieces are separated so the other reactant can reach them.
Check your understanding 4 Predict the exposed area
A solid cube has edge length 2 cm. Cut it into eight separate cubes, each with edge length 1 cm, so acid can reach every face. Use six faces per cube: total area = number of cubes × 6 × edge length². Calculate the area before and after cutting, then compare with the diagram.
Show the steps Optional working
Show the steps Optional working
Show answer
Compare the separated cube surfaces
Try it yourself 3
Explain a surface-area effect
Problem
Build the surface-to-rate chain
Hints
Hint 1
same mass
The amount of marble is controlled; focus on how much solid is exposed.Show solution step by step
Compare exposed marble
Method
Compare the surface the acid can actually reach.Reason
Pressing grains together hides surfaces inside the pellet, even though the grain sizes and total masses match.Working
Answer
A has more initially accessible surface than B.Connect exposure to rate
Method
Increase collisions at the marble surface per unit time.Reason
More exposed sites allow acid particles to meet marble particles more frequently.Working
Answer
A has more successful collisions per unit time and a faster initial reaction. Grains of the same size do not mean the same surface is open to the acid.
Quick check
Temperature: more collisions can succeed
Temperature is the only factor here that works in both ways: particles collide more often, and a greater fraction of collisions have enough energy to react. An answer that gives only the first is incomplete.
Spot the mistake 4
“More collisions” is not the whole answer
Two answers
Two students explain why a reaction is faster at 50 °C than at 20 °C. P: “The particles move faster, so they collide more often. More collisions per second means a faster rate.” Q: “The particles have more kinetic energy on average and move faster, so they collide more often. More importantly, a greater fraction of the collisions have enough energy to react. There are more successful collisions per unit time, so the rate is higher.” Which answer is complete, and what is missing from the other?
Compare the two answers
Show solution step by step
Test answer P
Method
Check what P says about each collision.Reason
P counts collisions but says nothing about whether they have enough energy to react.
Working
Answer
P: faster particles → more collisions. Only part of the account.
Accept answer Q
Working
Answer
Q adds the main effect: a greater fraction of collisions have enough energy, so there are more successful collisions per unit time.
Common mistake
More frequent collisions are real, but they are not the whole reason and not the main one. A temperature answer that stops at “collide more often” is incomplete.
Exam-style question 1
Explain the temperature effect
Examination question
A reaction is faster at 40 °C than at 20 °C. Explain this difference using the G3 Science particle model. [4 marks]
Build a linked particle explanation
Show solution step by step
Change particle motion
1 markMethod
State that reacting particles move faster at the higher temperature.
Reason
Heating increases their motion.Working
Answer
higher temperature → faster-moving particles
Change encounters
1 markMethod
State that particles meet more often.Reason
Faster motion produces more collisions in a given time.
Working
Answer
faster motion → more collisions
Explain successful collisions
1 markMethod
State that a greater fraction of collisions have enough energy to react.
Reason
Heating increases particle kinetic energy; more encounters alone are not the complete account.
Working
Answer
higher kinetic energy → greater fraction of collisions able to react
Reach the rate
1 markMethod
Conclude that reactant is used or product is formed more quickly.
Reason
More successful collisions per unit time increase the measured rate.
Working
Answer
more successful collisions per unit time → higher rate
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Keep the answer at the stated course depth; activation energy is not needed.
Build a clear explanation
A full explanation states the factor that changed, its effect on the reacting particles, and how that gives more successful collisions per unit time. For temperature, give both reasons: the particles collide more often, and a greater fraction of collisions have enough energy to react.
- Saying a larger volume of acid means a higher concentration. Concentration is particles per unit volume.
- Saying pressure affects a reaction between solutions. Pressure is a rate factor here only when reacting particles are gases.
- Saying smaller solid particles have less mass when the experiment uses the same total mass.
- Explaining temperature with “more frequent collisions” alone.
- Crediting one factor when the comparison changed two.
Check your understanding 5 Cut a cube
A 3 cm cube of marble is cut into twenty-seven 1 cm cubes, which are separated so acid can reach every face. Calculate the total surface area before and after, then explain what happens to the rate with the same acid.
Show answer
Before: 6 faces × (3 cm × 3 cm) = 54 cm². After: 27 cubes × 6 faces × (1 cm × 1 cm) = 162 cm². The mass is unchanged but the surface area triples. More marble is exposed, so acid particles collide with it more often, giving more successful collisions per unit time and a higher rate.
Check your understanding 6 Judge a pressure claim
Nitrogen monoxide and oxygen react in a sealed container of fixed volume. More of both gases is pumped in at the same temperature, which raises the pressure. A student says, “The volume has not changed, so the rate cannot change.” Is the student right? Explain.
Compare particles per unit volume, collision frequency and successful collisions per unit time. Compression is not the only way to crowd reacting gases.
Show answer
No. More gas in the same volume means more gas particles per unit volume: a higher concentration of each reactant. The particles collide more often, so there are more successful collisions per unit time and the rate increases. The pressure rises for the same reason.
Check your understanding 7 A small warming, a large effect
A reaction mixture is warmed from 20 °C to 30 °C at constant volume, with the same reactant amounts. Its rate roughly doubles, but the number of collisions per second rises by only a few per cent. A student writes, “Heating must have increased the concentration.” Correct the student, and explain why the rate rises so much more than the collision frequency.
Compare the small change in collision frequency with the larger change in the fraction that can react. State why the concentration explanation is wrong under the given conditions.
Show answer
Heating does not change the concentration: the same particles are in the same volume. The particles move faster, which gives only slightly more collisions per second. Most of the increase comes from the greater fraction of collisions that have enough energy to react, so successful collisions per unit time rise far more than collisions overall.
Common mistakes
- Saying particles collide more, without “per unit time”.
- Explaining a temperature rise only by more frequent collisions, and leaving out the greater fraction with enough energy.
- Saying a larger surface area gives particles more energy. It gives more particles exposed to collisions.
- Saying more concentrated solutions make more product, when the question is about rate.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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