Rate of Reaction: The Four Factors, and Four Experiments to Prove Them
The rate of a reaction is how fast reactants are used up or products are formed, and four things change it: surface area, concentration, temperature and a catalyst. All four work the same way underneath. Particles have to collide before they can react, and the collision only counts if they hit hard enough and the right way round. Anything that makes collisions more frequent, or makes more of them effective, speeds the reaction up. This page covers the theory and four experiments you can run to prove it, two of which need nothing but vinegar and baking soda.
Quick answers
| Question | Answer |
|---|---|
| Does increasing surface area increase reaction rate? | Yes. Breaking a solid into smaller pieces exposes more of it, so more particles are available to be hit and collisions happen more often |
| Does a smaller surface area react faster? | No, slower. A single folded piece of magnesium takes far longer to dissolve than the same mass cut up small |
| Does concentration affect the rate? | Yes. More particles in the same volume means more collisions per second |
| Does it change how much product forms? | No. Rate and yield are different things. This is the distinction most learners get wrong |
| Does volume affect the rate? | Not on its own. Adding water to a fixed amount of acid lowers the concentration, and that is what slows it down |
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 12 |
| Term | 2 |
| Topic | Chemical change, rate and extent of reaction |
| Status | Not a prescribed formal assessment, but examinable |
| Time needed | Two periods for all four experiments |
What reaction rate means
Some reactions take years. Rusting is one. Others are over before you can blink, like an explosion. Reaction rate is how we compare them.
Reaction rate is the increase in the concentration of a product, or the decrease in the concentration of a reactant, per unit time.
The study of reaction rates is called chemical kinetics.
Collision theory
You can measure a rate without knowing why it is what it is. To explain it you have to think about what the particles are doing, and that is what collision theory is for.
Collision theory says that reacting particles must collide with each other before a reaction can happen. But not every collision produces a reaction. For a collision to be effective, two conditions have to be met at once:
- The particles must collide with enough kinetic energy
- They must collide with the correct orientation relative to one another
Particles that bump into each other gently, or at the wrong angle, simply bounce apart unchanged.
Activation energy
Activation energy is the minimum energy that colliding particles must have for a reaction to occur.
The word minimum is the one that carries the marks. A collision below that threshold achieves nothing no matter how well aimed it is.
This is also how a catalyst works, and it is worth being precise about it. A catalyst lowers the activation energy. It does not give the particles more energy. It lowers the bar rather than raising the jumpers, which means a larger fraction of the collisions already happening become effective.
A catalyst takes part in the reaction but is regenerated by the end, so it is not used up and can be recovered afterwards.
The factors that affect reaction rate
Surface area
Only the surface of a solid can be reached by the other reactant. Break the solid up and you expose more of it.
That is why wood shavings burn faster than a log, and why a dust explosion can happen in a coal store or a feed mill when a solid lump of the same material would simply sit there.
Concentration
A higher concentration means more reactant particles in the same volume, so collisions happen more often and the rate goes up.
Pressure
For reactions between gases, raising the pressure squeezes more molecules into the same volume. The effect is the same as increasing concentration, for the same reason.
Temperature
Raising the temperature does two things at once, and a full-mark answer names both.
Particles gain kinetic energy, so they move faster and collide more often. And a larger fraction of those collisions carry more energy than the activation energy, so more of them are effective.
Most learners give the first reason and stop. The second is the one that matters.
Catalyst
A positive catalyst increases the rate by lowering the activation energy, so a greater fraction of collisions become effective. See above.
Four experiments
Two of these four need nothing from a laboratory supplier. The concentration and temperature experiments run on white spirit vinegar and bicarbonate of soda from any supermarket. If your budget is thin, start there.
1. Surface area: magnesium and hydrochloric acid
Three learners, three beakers each holding about 100 ml of dilute hydrochloric acid, and three 2 cm pieces of magnesium ribbon.
- The first learner folds their piece as small as possible.
- The second cuts theirs into three or four pieces and folds each.
- The third cuts theirs into as many tiny pieces as they can.
- At a signal, all three drop their magnesium in at the same moment.
- Watch which one disappears first.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
What you should see: the finely cut magnesium finishes first and the folded piece last. Same mass of metal, same acid, same temperature. The only difference is how much of the metal the acid can reach.
If the magnesium barely reacts, the ribbon is tarnished. Rub it bright with fine sandpaper before the lesson.
2. Concentration: vinegar and baking soda
Three test tubes:
- 15 ml vinegar, undiluted
- 15 ml vinegar plus 10 ml water
- 15 ml vinegar plus 25 ml water
All three learners add half a teaspoon of baking soda at the same moment, then time how long each takes to stop bubbling.
CH3COOH(aq) + NaHCO3(s) → CH3COONa(aq) + H2O(ℓ) + CO2(g)
What you should see: the undiluted vinegar fizzes hardest and finishes first.
Then point out what did not change. All three produce the same total volume of gas, because the baking soda was the same in all three. Concentration changed the rate, not the amount. That single observation answers more exam questions than anything else on this page.
3. Temperature: the same two ingredients, three temperatures
Each learner measures exactly 25 ml of vinegar into a beaker. One stays at room temperature. One is warmed in hot water to about 35 °C, and must not go above 40 °C. One is warmed in just-boiled water to at least 65 °C. Record all three temperatures, then add half a teaspoon of baking soda to each at the same moment.
What you should see: the hottest one erupts. The difference between 20 °C and 65 °C is dramatic, and this is the one a class remembers.
Use a 250 ml beaker for the hot one and stand back. Bicarbonate into hot vinegar foams a long way up.
4. The iodine clock, which gives you numbers
The fizzing experiments show you a difference. This one measures it. The solution stays colourless and then turns blue-black all at once, so there is no argument about when to stop the watch.
It runs on three solutions: A acidified sodium thiosulphate, B potassium iodate, and C starch.
For concentration: three conical flasks with 50 ml of Solution A diluted with 20 ml, 50 ml and 100 ml of water. Add 10 ml of Solution C to each. Pour in 50 ml of Solution B mixed with 50 ml of water, all three at the same moment, and time the colour change.
For temperature: the same setup, but with the water at room temperature, about 60 °C and near boiling.
The most concentrated goes blue first. The hottest goes blue first.
Aim for a colour change between 10 and 40 seconds. Test it once before the lesson. Faster and learners cannot time it, slower and the lesson drags. If it is too fast, dilute Solution B.
What you need
Chemicals
- Magnesium ribbon
- Dilute hydrochloric acid
- Sodium thiosulphate, potassium iodate and soluble starch, for the iodine clock
You supply
White spirit vinegar and bicarbonate of soda from a supermarket, a heat source, and distilled water.
Safety precautions
- Goggles on for all four experiments, including the vinegar ones. They fizz over
- Dilute hydrochloric acid is corrosive. Rinse any splash under a running tap
- Potassium iodate is an oxidising agent. Keep it away from the starch and anything organic while it is dry
- The near-boiling water is the real hazard in this topic. Use the beaker, not your hands
- Magnesium and acid give off hydrogen. Fine in an open beaker, but no flames nearby
If it does not work
| What you see | What caused it |
|---|---|
| Magnesium barely reacts | The ribbon is tarnished, or the acid is too dilute. Rub the ribbon bright |
| All three magnesium pieces finish together | They were not cut differently enough. The third one needs lots of very small pieces |
| The vinegar experiments show no clear difference | The baking soda was not added simultaneously. That is the whole comparison |
| The blue colour never appears | Solution B too dilute, or the starch was mixed into cold water and never dissolved |
| The blue appears instantly | Solution B too concentrated. Dilute it and test again |
| Iodine clock times are inconsistent | The solutions were not poured at the same moment, or the flasks were not swirled |
| Hot vinegar foams over the bench | Beaker too small. Use the 250 ml |
The mistake worth teaching to
Learners confuse rate with amount. Diluting the vinegar makes the reaction slower, but the same mass of baking soda still produces the same total carbon dioxide. Heating it makes it faster, not bigger.
Almost every exam question on this topic is built on that distinction. A class that has watched three tubes fizz at different speeds and finish in the same place is far better placed to answer them than a class that has only read about it.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, covering all four experiments with the variables, results tables, equations and collision theory questions set out
- Marking memorandum, with the mark allocation, model answers and a note on the six places learners most often drop marks
Buy this experiment
We are putting together a complete Rate of Reaction Kit for Grade 12, with the glassware, thermometers, stopwatch, magnesium ribbon and the three iodine clock chemicals in one box, plus an annual refill of the consumables. Both coming shortly.
In the meantime every item in the apparatus table above is in stock and sold separately. And remember that two of the four experiments need nothing from us at all.