Describe how concentration, temperature, surface area and a catalyst change the speed of a reaction, and explain these using the idea of colliding particles

What this dot point is asking

The syllabus wants you to describe how concentration, temperature, surface area and a catalyst change the speed (rate) of a reaction, and to explain each one using the collision idea. The big picture is that a reaction only happens when particles collide with enough energy, so anything that makes collisions more frequent or more energetic speeds the reaction up.

The answer

What "rate" means and how to measure it

The rate of a reaction is how fast reactants turn into products. A fast reaction finishes in a short time; a slow one takes longer. You can measure rate by timing how quickly a product appears, for example by collecting a gas and measuring its volume each minute, or by timing how long a mixture takes to turn cloudy.

The collision idea

Particles must collide, and collide with enough energy, for a reaction to happen. A collision with enough energy is a successful collision. The more successful collisions there are each second, the faster the reaction. Every factor below works by changing how often, or how energetically, particles collide.

Concentration

A higher concentration means the particles are more crowded together. They collide more often, so there are more successful collisions each second and the reaction is faster. A more dilute solution reacts more slowly.

Temperature

A higher temperature gives the particles more energy, so they move faster. They collide more often and, more importantly, more of the collisions have enough energy to react. So the reaction is faster. A lower temperature slows it down.

Surface area

Breaking a solid into smaller pieces (or a powder) gives it a larger surface area. More particles are exposed, so there are more places for collisions to happen. The reaction is faster. A single large lump has a small surface area and reacts slowly.

A catalyst

A catalyst speeds up a reaction without being used up. It works by giving the particles an easier path that needs less energy, so more collisions are successful. Because it is not used up, a small amount can be used again and again.

Examples in context

Example 1. Keeping food in the fridge. A fridge slows the reactions that make food go off by lowering the temperature, so particles collide less often and with less energy. This is collision theory used every day to make food last longer.

Example 2. Catalytic converters in cars. A car's catalytic converter uses a catalyst to speed up reactions that turn harmful exhaust gases into safer ones. Because the catalyst is not used up, it keeps working for the life of the car, showing why catalysts are so valuable in industry.

Try this

Q1. State two factors that would speed up the reaction between zinc and dilute acid. [2 marks]

• Cue. Any two of: higher acid concentration, higher temperature, larger surface area (smaller pieces of zinc), or adding a catalyst.

Q2. Explain, using particles, why a reaction is faster at a higher temperature. [2 marks]

• Cue. The particles have more energy and move faster, so they collide more often and more of the collisions have enough energy to react.

Q3. State what a catalyst does to a reaction and one reason it is useful. [2 marks]

• Cue. A catalyst speeds up the reaction without being used up, so a small amount can be reused many times.