The factors that affect the rate of reaction (concentration, temperature, surface area and catalysts), how rate is measured, collision theory and activation energy, and how each factor changes the frequency or energy of collisions.

What this dot point is asking

CCEA Double Award wants the four factors that change reaction rate, how rate is measured, and collision theory with activation energy to explain each factor. The explanation questions are worth the most, so learn how each factor changes the frequency or energy of collisions.

The factors that affect rate

Measuring rate

Rate is how fast reactants turn into products. You can measure it by following something that changes:

• The volume of gas produced (using a gas syringe).
• The loss in mass as a gas escapes (on a balance).
• The time for a precipitate to hide a cross (turbidity).

On a graph of product against time, the steeper the gradient, the faster the rate. The line levels off when the reaction finishes.

Collision theory and activation energy

The more frequent the successful collisions, the faster the reaction. Anything that increases either the frequency of collisions or the proportion that have the activation energy will speed up the reaction.

How each factor works

• Concentration (or pressure): more particles in the same volume, so they collide more often - more frequent collisions.
• Surface area: breaking a solid into smaller pieces exposes more particles, so there are more frequent collisions at the surface.
• Temperature: particles move faster, so they collide more often AND with more energy, so more collisions exceed the activation energy.
• Catalyst: provides an alternative pathway with a lower activation energy, so a greater proportion of collisions succeed; the catalyst is not used up.

Examples in context

Example 1. Storing food in a fridge. Cooling food slows the reactions that make it go off, because the particles collide less often and with less energy. This is the temperature effect on rate used to keep food fresh.

Example 2. Catalytic converters in cars. A catalyst in the exhaust speeds up the reactions that turn harmful gases into less harmful ones, without being used up. This shows a catalyst lowering the activation energy in a real application.

Try this

Q1. Name two factors that increase the rate of a reaction. [2 marks]

• Cue. Any two of: higher concentration, higher temperature, larger surface area, a catalyst.

Q2. What is the activation energy? [1 mark]

• Cue. The minimum energy needed for a collision to result in a reaction.