Describe the effect of concentration, pressure, surface area and temperature on the rate of reaction, and explain these effects using collision theory

What this dot point is asking

SEAB wants you to describe how four factors (concentration, pressure for gases, surface area and temperature) change the rate of a reaction, and to explain each using collision theory. The skill examiners reward is not just stating that a change speeds the reaction up, but explaining it in terms of the frequency and energy of collisions between particles.

The answer

Collision theory

For a reaction to happen, particles must collide with one another, and the collision must be successful: the particles must collide with at least a minimum energy, the activation energy, and in the right orientation. Not every collision leads to a reaction; only the energetic, well-aimed ones do. Anything that increases the frequency of collisions, or the proportion that have enough energy, speeds the reaction up.

Concentration

Increasing the concentration of a solution puts more reactant particles in the same volume. The particles are closer together, so they collide more often. More frequent collisions mean more successful collisions per second, so the rate increases. Diluting the solution has the opposite effect.

Pressure (for gases)

Increasing the pressure of a gas squeezes the same number of particles into a smaller volume, so they are closer together and collide more often. As with concentration, more frequent collisions increase the rate. Pressure has this effect only for reactions involving gases.

Surface area

Breaking a solid into smaller pieces (or using a powder) increases its surface area. More of the solid is exposed to the other reactant, so there are more frequent collisions at the surface, and the rate increases. This is why powdered solids react faster than large lumps.

Temperature

Increasing the temperature gives the particles more kinetic energy, so they move faster. This has two effects:

1. They collide more often (more frequent collisions).
2. More importantly, a greater proportion of collisions have energy above the activation energy, so a larger fraction of collisions are successful.

Because of the second effect, temperature has a particularly strong influence on rate: a modest rise in temperature can roughly double the rate.

Examples in context

Example 1. Flour mills and dust explosions. Fine flour or coal dust has an enormous surface area, so it can react with oxygen so fast that it explodes if ignited. This hazard is the surface-area effect taken to an extreme, and it is why dust is carefully controlled in mills and mines.

Example 2. Storing food in a fridge. Cooling food slows the reactions that spoil it, because at lower temperature the particles move more slowly and fewer collisions have the activation energy. The same collision-theory reasoning that speeds reactions when heated explains why chilling preserves food.

Try this

Q1. State two factors that increase the rate of a reaction. [1 mark]

• Cue. Any two of: higher concentration, higher temperature, larger surface area, higher pressure (for gases).

Q2. Explain, using collision theory, why increasing the concentration of a solution increases the rate. [2 marks]

• Cue. More reactant particles in the same volume means the particles collide more often, so there are more successful collisions per second and the rate increases.

Q3. Explain why a reaction speeds up when the temperature is raised. [3 marks]

• Cue. The particles gain kinetic energy and move faster, so they collide more often and, more importantly, a greater proportion of collisions have energy above the activation energy, so more collisions are successful.