GCSE · Chemistry · AQA · Spec 8462

Collision theory and activation energy

Particles in a reaction mixture are crashing into each other all the time. So why doesn't everything react instantly? Because most of those crashes do absolutely nothing.

Why do most collisions do nothing?

Cool: slow particles

Temperature: 20 °C. State: Cool: slow particles. static

Temperature20 °C

Cool mixture. The particles drift slowly, so they don't meet very often. When they do, most bumps are too gentle: the particles just bounce apart and nothing reacts.

This is a model of a reacting gas mixture, and the temperatures are just for illustration. The dots show how fast the particles are moving. The caption under the picture tells you what that does to their collisions.

Exam line: Higher temperature → particles move faster → they collide more often AND more of the collisions have at least the activation energy → more successful collisions each second → faster rate.
Watch out: Heating does not lower the activation energy. The energy barrier stays exactly where it was. What changes is how many particles now have enough energy to get over it.

Concentration

?

Reason it through

Why does a more concentrated acid react faster, at the same temperature?

Link 1 of 4

First link · your turn

You swap to a more concentrated acid. What's different inside the beaker?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Collision theory · Proportion

Double the particles, double the collisions
050100150200drag to raise the concentration →

Concentration: 1/4. Reactant particles in the same volume 10. Collisions each second (model) 50. Successful collisions each second (model) 5

Reactant particles in the same volume10Collisions each second (model)50Successful collisions each second (model)5

A model beaker at a fixed temperature. Drag the concentration and watch the collisions keep pace. The numbers are made-up model values, chosen to show the pattern.

Exam line: In the simple collision model, the number of collisions each second is proportional to the concentration of a reactant, as long as the temperature and everything else stay the same. Double one, and you double the other.
Watch out: This only works when you change ONE thing. If the temperature changed as well, the share of collisions with enough energy would change too, and the pattern would no longer be a simple doubling.

Collision theory · Surface area

Cut it up and see what gets exposed

A 3 cm cube of a reacting solid has a surface area of 54 cm². You cut it into 1 cm cubes (27 of them). What is the TOTAL surface area of all the small cubes?

Your estimate

150 cm²

0 cm²300 cm²

Exam line: Smaller pieces of a solid have a bigger surface area to volume ratio. More of the solid's particles are exposed, so collisions with the other reactant happen more often and the rate increases.

Watch out: Cutting doesn't change how much solid you have: the volume stays the same. It only changes how much of it is on the outside, where the other reactant can reach it.

Collision theory · Sorting the factors

More often, or harder?

For each change, decide what it does to the collisions between reactant particles. Tick one set, both sets, or neither.

  • A Collisions happen more often
  • B More collisions have at least the activation energy
  1. Use a more concentrated acid
  2. Raise the pressure of a reacting gas
  3. Grind a lump of solid into powder
  4. Heat the reaction mixture
  5. Cool the reaction mixture
  6. Add water to the acid

Exam line: Concentration, gas pressure and surface area make collisions more frequent. Temperature makes them more frequent AND more energetic. That's why temperature needs a two-part explanation.

Collision theory · Your turn

Predict and explain: heating a reaction

A student carries out a reaction at 20 °C and then repeats it at 40 °C. Everything else stays the same. Predict how the rate of reaction changes and explain your prediction using collision theory. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

Exam line: A full temperature answer has two collision ideas, not one: collisions are more frequent, and more of them have at least the activation energy. Then link them to more successful collisions each second.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Collision theory: particles can react only when they collide with each other AND with sufficient energy.
  • Activation energy is the minimum amount of energy that particles must have to react.
  • Increasing concentration (solutions), pressure (gases) or surface area (solids) increases the frequency of collisions, so the rate increases.
  • Increasing temperature increases the frequency of collisions AND makes collisions more energetic, so the rate increases.
  • Cutting a solid into smaller pieces increases its surface area to volume ratio, exposing more particles to collisions.
  • With everything else kept the same, collision frequency is proportional to concentration: double one, double the other.

The big picture

Reacting particles have to collide, and collide with at least the activation energy, the minimum energy they must have to react. Higher concentration, higher gas pressure and a bigger surface area (smaller pieces, so a larger surface area to volume ratio) all make collisions more frequent. Higher temperature makes collisions more frequent and more energetic. Either way, more successful collisions each second means a faster rate of reaction.

Key points

1Only collisions with at least the activation energy lead to a reaction; the rest just bounce apart.
2The rate of reaction depends on the number of successful collisions each second.
3Concentration, gas pressure and surface area change how OFTEN particles collide, not how hard.
4Temperature is the only one of the four factors that changes both how often and how hard particles collide.
5Raising the temperature does not change the activation energy; it gives more particles enough energy to reach it.

Worked example

Problem

Two gases react inside a sealed container. The container is squeezed into a smaller space, so the pressure of the gases goes up. The temperature does not change. Predict and explain the effect on the rate of reaction.

⚠ Watch out

Thinking that every collision makes the particles react. Most collisions don't: only collisions with at least the activation energy succeed. So a faster rate means more SUCCESSFUL collisions each second, not just more collisions.

🧠

Memory hook

Rate = how many hits COUNT each second. Crowd them (concentration, pressure), expose them (surface area) or speed them up (temperature). Only heat makes the hits harder as well as more frequent.

✓

Check yourself

Cover the page. Name the two things particles need in order to react, define activation energy, and say why heating is the only factor that needs a two-part answer.

Flashcards

(12)
According to collision theory, what two things must happen for particles to react?
They must collide with each other, and they must collide with sufficient energy.
What is the activation energy?
The minimum amount of energy that particles must have to react.
Does every collision between reactant particles cause a reaction?
No. Only collisions with at least the activation energy succeed; the rest just bounce apart.
In collision theory, what does the rate of reaction depend on?
The number of successful collisions each second.
Why does increasing the concentration of a solution increase the rate?
There are more reactant particles in the same volume, so collisions are more frequent, giving more successful collisions each second.
Why does increasing the pressure of a reacting gas increase the rate?
More gas particles are squeezed into the same volume, so they collide more frequently.
What happens to the surface area to volume ratio when a solid is cut into smaller pieces?
It increases: the total surface area goes up while the volume stays the same.
Why does a larger surface area to volume ratio increase the rate?
More of the solid's particles are exposed, so collisions with the other reactant happen more often.
What are the TWO effects of raising the temperature on collisions?
Collisions become more frequent, and they become more energetic, so more of them have at least the activation energy.
Which of concentration, pressure, surface area and temperature changes the energy of the collisions?
Only temperature. The other three change only how often the particles collide.
Does raising the temperature change the activation energy?
No. The activation energy stays the same; more particles now have enough energy to reach it.
With everything else the same, the concentration of a reactant is tripled. What happens to the number of collisions each second?
It triples: collision frequency is proportional to concentration in the simple collision model.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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