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In the last lesson you learned that the rate of a reaction depends on how often particles collide and on how many of those collisions are successful. Now we look in detail at the four everyday factors a chemist can change to control a rate: temperature, concentration, pressure and surface area. (The fifth factor, a catalyst, gets a lesson of its own.) The key skill is not just knowing that each factor speeds a reaction up, but being able to explain why using collision theory — because "explain" questions on rate are among the most common in the whole of your OCR Gateway Combined Science course.
By the end of this lesson you should be able to state how temperature, concentration, pressure and surface area each affect the rate of a reaction, and explain each effect in terms of the frequency and energy of collisions.
This lesson is mostly AO2: you apply collision theory to a given situation to explain why changing each factor changes the rate, building on the AO1 recall from the previous lesson.
The reason this lesson matters so much is that examiners rarely ask you simply to name a factor. They give you a situation — a reaction that speeds up when something is changed — and ask you to explain the change through collisions. Get the reasoning right for each factor and you will handle almost any rate question.
Each factor increases the rate, but they do not all work in the same way. Two mechanisms are at work across the four factors: making collisions more frequent, and making collisions more energetic. This table sorts them out; the rest of the lesson takes each one in turn.
| Factor (increased) | How it changes collisions | Which mechanism? |
|---|---|---|
| Temperature | Particles move faster: collisions are more frequent and more reach the activation energy | Frequency and energy |
| Concentration | More particles in the same volume: collisions are more frequent | Frequency only |
| Pressure (gases) | Particles squeezed closer: collisions are more frequent | Frequency only |
| Surface area | More particles exposed: collisions are more frequent | Frequency only |
Exam Tip: Only temperature changes the energy of the collisions. Concentration, pressure and surface area change only the frequency. A frequent misconception is to write that a higher concentration "gives the particles more energy" — it does not; it only makes them collide more often.
Raising the temperature speeds a reaction up in two separate ways, and this is the factor most often asked about, so it is worth learning carefully.
First, giving the particles more heat energy makes them move faster. Faster-moving particles cover more ground in a given time, so they meet and collide more frequently. On its own, this would make the reaction somewhat faster.
Second — and this is the more important effect — because the particles are moving faster, a greater proportion of their collisions have at least the activation energy. In other words, more of the collisions are successful. It is this energy effect that makes temperature so powerful: even a fairly small rise in temperature can cause a large increase in rate, because it lifts many more collisions over the activation-energy barrier.
So a full answer about temperature has two halves: collisions become more frequent (particles move faster) and more collisions are successful (more reach the activation energy). If you can add that the energy effect is the larger of the two, you have the top-band answer.
Exam Tip: For temperature, always give both effects: more frequent collisions and more energetic collisions (more reaching the activation energy). Writing only "the particles move faster and collide more" misses the more important half of the answer.
Increasing the concentration of a solution means packing more reactant particles into the same volume. With more particles in a given space, any one particle meets another sooner, so the particles collide more frequently. More frequent collisions means a faster rate.
Notice what concentration does not do: it does not change how fast the particles move or how much energy they have, so it does not change the proportion of collisions that are successful. It changes only the frequency. This is why a dilute acid reacts more slowly than a concentrated one with the same metal — there are simply fewer acid particles in reach of the metal at any moment.
Exam Tip: Explain concentration in one clean sentence: a higher concentration means more particles in the same volume, so collisions are more frequent and the rate increases. Do not mention energy or the activation energy for concentration — that is the wrong mechanism.
Pressure matters only for reactions between gases. Increasing the pressure squeezes the same number of gas particles into a smaller volume. The particles are then closer together, so — exactly as with a more concentrated solution — they collide more frequently, and the rate increases.
In fact, increasing the pressure of a gas is really the gaseous version of increasing the concentration of a solution: both crowd the particles into a smaller space so that collisions happen more often. The mechanism is the same (frequency of collisions), and again the energy of the collisions is unchanged.
Exam Tip: Treat pressure of a gas and concentration of a solution as the same idea explained the same way: particles closer together → more frequent collisions. Pressure only applies to gases — do not use it to explain a reaction between solutions or solids.
The surface area of a solid affects how fast it reacts. When a solid reacts with a liquid or a gas, the reaction can only happen on the surface of the solid, because only the particles at the surface can be reached and hit by the other reactant. The particles locked away inside the lump cannot react until the ones around them have been used up.
Breaking a solid into smaller pieces, or grinding it into a powder, increases its total surface area. This exposes more particles to the other reactant, so collisions become more frequent and the rate increases. A powder therefore reacts faster than a few large lumps of the same substance and the same total mass.
This is why a chemist grinds a solid to a powder to make it react quickly. It is also why fine dusts can be dangerous: a fine powder of flour, custard powder or coal has such an enormous surface area that it can burn extremely fast — fast enough to cause an explosion in a mill or store.
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