Rates of reaction and collision theory
Some reactions are over in a flash and some take years. You need to measure a rate, read it from a graph, and explain with collision theory why temperature, concentration, surface area and catalysts change it.
Part 1 of 3: Learn it
In short
- Mean rate = quantity of product formed (or reactant used) ÷ time taken.
- Particles must collide with at least the activation energy to react.
- More frequent collisions or more energetic collisions mean a faster reaction.
Where this is in your specification
Spec points: AQA 4.6.1, Edexcel Topic 7, OCR C5.2
| Board | Topic: Rates of reaction and equilibrium |
|---|---|
| AQA 8462 | 4.6 (8462); 5.6 (8464) |
| Edexcel 1CH0 | Topics 4 and 7 |
| OCR J248 | C5 |
Measuring the rate
- Volume of gas given off, collected in a gas syringe or an upturned measuring cylinder (cm³/s).
- Loss of mass as a gas escapes, on a balance (g/s).
- Time for a solution to go cloudy, such as the disappearing cross with sodium thiosulfate and acid.
Reading a rate graph
On a graph of product against time, the steeper the line the faster the reaction. It is steepest at the start, when there are most reactant particles, then levels off when one reactant is used up.
For the rate at one moment (Higher), draw a tangent to the curve at that time and find its gradient.
Collision theory and the four factors
| Change | Effect on rate | Why |
|---|---|---|
| Higher temperature | faster | particles move faster, so they collide more often and a bigger share of collisions carry enough energy to react |
| Higher concentration (or gas pressure) | faster | more particles in the same volume, so more frequent collisions |
| Larger surface area (smaller pieces) | faster | more particles exposed, so more frequent collisions |
| Catalyst | faster | provides a different pathway with a lower activation energy; it is not used up |
A reaction loses 1.2 g of mass in 60 s. What is the mean rate?
Show the answer
1.2 ÷ 60 = 0.02 g/s.
Part 2 of 3: See it worked
Worked examples
Example 1
Marble chips react with acid and give off 60 cm³ of carbon dioxide in the first 30 s. What is the mean rate over that time?
- mean rate = volume ÷ time
- = 60 ÷ 30
- = 2.0 cm³/s
Answer: 2.0 cm³/s.
Example 2
The same mass of marble is used as powder instead of chips. Explain the effect on the rate.
- Powder has a much larger surface area
- More acid particles can collide with the marble at once
- Collisions are more frequent
Answer: The rate increases, because more frequent collisions happen. The total volume of gas made stays the same if the amounts of reactants are the same.
Common mistakes
- Saying particles collide 'harder' at higher concentration. They collide more often.
- Saying a catalyst gives particles more energy. It lowers the activation energy instead.
- Thinking a faster reaction makes more product. It makes the same amount, sooner.
- Forgetting the units of rate, such as cm³/s or g/s.
Why does the rate slow down as the reaction goes on?
Show the answer
There are fewer reactant particles left, so collisions are less frequent.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
A reaction loses 1.2 g of mass in 60 s. What is the mean rate?
1.2 ÷ 60 = 0.02 g/s.
Why does the rate slow down as the reaction goes on?
There are fewer reactant particles left, so collisions are less frequent.
Define activation energy.
The minimum energy colliding particles need to react.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
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