A higher concentration means more particles of the reactant in each unit of volume, so they collide more frequently and the rate rises. Examiners reward a full chain of reasoning, not just the word “faster”.
This lesson sits in rates of reaction and follows naturally from reading a rate from a graph.
What does concentration actually change?
Concentration is the amount of solute in a fixed volume, for example mol/dm³. Doubling it puts twice as many reactant particles into the same space.
The particles are not moving faster and they do not have more energy. They simply bump into each other more often. For a reaction to occur, particles must collide with enough energy, so more collisions each second means more successful collisions each second.
How do you write the explanation?
Build it in four links and keep every link:
- The concentration is higher, so there are more particles per unit volume.
- The particles are closer together, so they collide more frequently.
- Each particle has the same energy as before, so the same fraction of collisions are successful. More collisions therefore means more successful collisions per second.
- The rate of reaction increases.
The same reasoning applies to a gas at higher pressure, where the same number of gas particles are squeezed into a smaller volume.
Worked example
Invented data. A student dilutes 25.0 cm³ of acid of concentration 1.00 mol/dm³ by adding water to make 50.0 cm³. Then two reactions are timed to collect the same small volume of gas.
| Acid | Concentration (mol/dm³) | Time to collect 30 cm³ (s) |
|---|---|---|
| Original | 1.00 | 40 |
| Diluted | ? | 80 |
Step 1, find the new concentration. Moles of acid = 1.00 × 0.0250 = 0.0250 mol. The volume is now 50.0 cm³ = 0.0500 dm³.
Step 2, divide. Concentration = 0.0250 ÷ 0.0500 = 0.500 mol/dm³.
Step 3, compare. Rate can be compared as 1 ÷ time. Original: 1 ÷ 40 = 0.025 per second. Diluted: 1 ÷ 80 = 0.0125 per second.
Step 4, explain. The diluted acid has half the concentration, so there are fewer particles per unit volume. Collisions are less frequent, fewer successful collisions occur each second and the rate is lower. In this invented set the rate halved, but that is what these numbers show and not a rule.
What mistake should you watch for?
Mistaken answer: “Higher concentration means the particles have more energy, so more collide.”
The energy claim is wrong. Concentration changes how often particles meet, not how much energy each has. The second mistake to avoid is a sentence that says only “more collisions”, because it never says that the collisions are more frequent or that more of them are successful.
The correction: “There are more particles per unit volume, so collisions are more frequent and more successful collisions happen per second.”
Check yourself
1. 10.0 cm³ of 2.00 mol/dm³ solution is diluted with water to 40.0 cm³. What is the new concentration?
Show answer
Moles = 2.00 × 0.0100 = 0.0200 mol. New volume = 0.0400 dm³. Concentration = 0.0200 ÷ 0.0400 = 0.500 mol/dm³.
2. Explain in two sentences why a reaction with 0.50 mol/dm³ acid is slower than with 1.00 mol/dm³ acid, everything else being the same.
Show answer
The 0.50 mol/dm³ acid has fewer particles per unit volume, so collisions are less frequent. Fewer successful collisions occur each second, so the rate is lower.
3. True or false: doubling the concentration gives particles twice the energy.
Show answer
False. Concentration changes how often particles collide. Their energy depends on temperature.
Where does this lead next?
Try the same four-link chain on solids in powder and lumps without changing the amount. The equation balance reasoning trainer and mole and equation-ratio tutor help you check the amounts before you explain the rate.
If your explanation has the right ideas in the wrong order, a teacher can rehearse the wording with you in online one-to-one Chemistry tuition.