A reaction happens when particles collide with enough energy. Every factor that speeds a reaction up does it by making collisions more frequent, more energetic, or both.
This lesson is part of rates of reaction. It gives the reasons behind the results in comparing factors affecting rate.
What sentence do I write?
Use three parts: the change, what it does to the particles, and the result on the rate. For example: “When the temperature is higher, the particles move faster, so they collide more often and with more energy, and the rate increases.”
Leave out any one part and you usually lose a mark.
| Factor | What changes for the particles | Result |
|---|---|---|
| Higher concentration | More particles in the same volume | More frequent collisions |
| Higher temperature | Faster particles with more energy | More frequent and more energetic collisions |
| Smaller pieces | More surface exposed | More frequent collisions |
| Catalyst | A route needing less energy | More collisions lead to reaction |
Worked example: zinc and acid
Zinc powder reacts faster with dilute acid than a zinc granule of the same mass. Explain why.
- Change: the powder has a much larger surface area than one granule.
- Effect on particles: more zinc atoms are exposed, so more acid particles can hit zinc atoms at the same moment.
- Result: collisions are more frequent, so the reaction rate is higher.
The total volume of hydrogen at the end is the same, because the mass of zinc is the same.
The mistake that costs marks
The common slip is to write “the particles move faster” for temperature and stop there. It describes the particles but not the collisions, so the link to the rate is missing.
| Step | Wrong | Right |
|---|---|---|
| Change | Temperature is higher | Temperature is higher |
| Particles | They move faster | They move faster and have more energy |
| Collisions | (missing) | They collide more often and more energetically |
| Result | Faster reaction | The rate increases |
A second slip is to say particles “get bigger” or “expand” when heated. The explanation needs speed and energy, not size.
What about a catalyst?
A catalyst gives the reaction a route that needs less energy. More of the collisions then have enough energy to react, so the rate rises.
The catalyst is not used up and does not end up in the products. Its mass is the same at the end, and the total amount of product is unchanged.
Check yourself
Explain why a 1.0 mol/dm³ acid reacts faster with marble chips than a 0.5 mol/dm³ acid, with all other conditions the same.
Answer
The 1.0 mol/dm³ acid has more acid particles in each cubic decimetre of solution. More acid particles are therefore close to the marble surface at any moment.
Collisions between acid particles and marble happen more often, so the rate of reaction is higher. The particles have the same energy as before, because the temperature has not changed.
What to study next
Continue with calculating a rate from a supplied table, which puts numbers on the rates you have explained. Then test the whole cluster with the rates of reaction practice set.
If you want a teacher to check your written explanations with you, see online one-to-one Science tuition.