How to explain the effect of concentration on reaction rate

“When concentration increases, the rate of reaction increases.” Every SPM student can recite that sentence, but very few can earn full marks explaining why. In rate of reaction questions, the marks live in the explanation, not the statement, and the explanation is always built from collision theory. This guide gives you the exact chain of reasoning examiners want, plus two worked examples you can adapt to almost any concentration question.

Start with collision theory

Every rate explanation in SPM rests on one idea: a chemical reaction happens only when reactant particles collide, and only some of those collisions actually lead to a reaction. A collision that leads to a reaction is called an effective collision. For a collision to be effective, two conditions must be met:

1. The colliding particles must have energy equal to or greater than the activation energy (the minimum energy needed for a reaction to occur).
2. The particles must collide in the correct orientation.

The rate of reaction depends on the frequency of effective collisions, how many effective collisions happen per second. Anything that increases that frequency increases the rate. Keep that phrase in your head, because it is the hinge of every answer.

What concentration actually changes

Concentration is the number of solute particles dissolved in a fixed volume of solution. When you increase the concentration of a reactant, you pack more reactant particles into the same volume. The particles are now closer together on average.

Because the particles are more crowded, they collide more often, the frequency of collisions goes up. Since a fixed proportion of collisions are effective, a higher total collision frequency means a higher frequency of effective collisions. More effective collisions per second means a faster reaction.

Notice what concentration does not change. It does not give the particles more energy, and it does not lower the activation energy, so the proportion of collisions that are effective stays the same. Concentration works purely by making collisions more frequent, not more energetic. That distinction matters: temperature and catalysts change the proportion of effective collisions, but concentration only changes how often particles meet.

The model answer

Here is the four-line chain that scores full marks. Memorise its shape and swap in the right reactant:

When the concentration of the acid increases, the number of acid particles per unit volume increases. Therefore the frequency of collisions between the reacting particles increases. This increases the frequency of effective collisions. Therefore the rate of reaction increases.

Every line does a job: statement of the change, more particles per unit volume, more frequent collisions, more effective collisions, higher rate. Skip a line and you drop a mark.

Worked example 1: the disappearing cross

The classic demonstration uses sodium thiosulfate and dilute hydrochloric acid, which react to make a fine yellow precipitate of sulfur that slowly clouds the solution:

Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O

You place the flask over a paper cross and time how long the sulfur takes to hide the cross. A more concentrated sodium thiosulfate solution has more thiosulfate particles per unit volume, so collisions with acid particles are more frequent, effective collisions are more frequent, and the sulfur forms faster, the cross disappears in a shorter time. Because rate is inversely related to time, you plot rate as 1/time: a higher concentration gives a shorter time and therefore a higher 1/time value. You can see the full method in our sodium thiosulfate and acid experiment.

Worked example 2: reading the gas graph

Now take marble chips (calcium carbonate) reacting with hydrochloric acid, collecting the carbon dioxide given off:

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

If you plot the volume of gas against time for two acids, one more concentrated than the other, the more concentrated acid gives a steeper gradient at the start, because the initial frequency of effective collisions is higher, so the initial rate is higher. That is the key phrase for graph questions: higher concentration means a steeper initial gradient. What happens to the maximum volume depends on which reactant runs out first; if the same amount of calcium carbonate is used and the acid is in excess in both runs, both curves level off at the same final volume, the more concentrated one simply gets there sooner. The effect of concentration experiment walks through plotting and comparing these curves.

Common mistakes to avoid

• Writing “more particles” without “per unit volume.” Concentration is about crowding in a fixed volume, so the phrase per unit volume is what earns the mark.
• Saying collisions become “harder” or “more energetic.” They do not. Concentration changes only the frequency of collisions.
• Forgetting the word “effective.” More collisions alone is not enough; examiners want “frequency of effective collisions.”
• Confusing concentration with amount. Doubling the volume of the same acid adds more particles overall but keeps the same concentration, so the rate is unchanged.

Bringing it together

Concentration is the friendliest rate factor to explain because the logic is so clean: more particles per unit volume, more frequent collisions, more frequent effective collisions, higher rate. Drill the four-line answer until it is automatic, then practise attaching it to different reactions and to both the “disappearing cross” and “gas volume” graph styles.

If the collision-theory wording keeps slipping, or if you can say the words but freeze when a graph is involved, that is a quick fix with a teacher watching your phrasing live. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like your rate answers polished against real past-paper questions.