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

What this dot point is asking

SEAB wants you to explain how concentration, temperature, surface area and catalysts change the rate of a reaction, and to give every explanation in terms of collision theory. You should also be able to read a rate from a graph of product volume or mass against time. The collision-theory explanations are the core marks, so each factor needs a clear cause-and-effect sentence.

The answer

What rate of reaction means

The rate of reaction is how fast reactants are used up or products are formed. It can be measured by following the volume of gas given off, the loss in mass, or the time for a fixed change (such as a cross disappearing through cloudy solution).

Collision theory

For a reaction to happen, particles must collide, and they must collide with enough energy. The minimum energy needed for a successful collision is the activation energy. Anything that increases either the frequency of collisions or the fraction of collisions that succeed will increase the rate.

Concentration

Increasing the concentration of a dissolved reactant puts more particles in the same volume. The particles collide more frequently, so there are more successful collisions per second and the rate increases. (For gases, increasing the pressure does the same thing.)

Temperature

Increasing the temperature has two effects. The particles move faster, so they collide more often. More importantly, a greater fraction of particles have energy above the activation energy, so a much greater proportion of collisions are successful. The second effect dominates, which is why a small rise in temperature gives a large rise in rate.

Surface area

Breaking a solid into smaller pieces, or grinding it to a powder, increases its surface area. More of the solid's particles are exposed for the other reactant to collide with, so the frequency of successful collisions increases and the rate goes up.

Catalysts

A catalyst speeds up a reaction without being used up. It works by providing an alternative pathway with a lower activation energy, so a greater fraction of collisions have enough energy to react. The catalyst is chemically unchanged at the end and can be reused.

Reading rate from a graph

On a graph of product (volume or mass change) against time, the steeper the curve, the faster the rate. The curve is steepest at the start (most reactant present) and flattens as reactants run out; it becomes horizontal when the reaction has finished.

Examples in context

Example 1. Storing food in a fridge. Lowering the temperature slows the chemical reactions that spoil food, because the particles collide less often and fewer collisions exceed the activation energy. This is collision theory applied to everyday preservation, the reverse of speeding a reaction up.

Example 2. Catalytic converters in cars. A catalytic converter uses platinum and rhodium catalysts to speed up the reactions that turn harmful exhaust gases into less harmful ones, working at the temperatures inside an exhaust. The catalyst lowers the activation energy and is not consumed, so it lasts the life of the car, a key industrial use of catalysts.

Try this

Q1. State three ways to increase the rate of a reaction between a solid and a solution. [3 marks]

• Cue. Increase the concentration of the solution, increase the temperature, and increase the surface area of the solid (e.g. use a powder); a catalyst also works.

Q2. Explain, using collision theory, why a powdered solid reacts faster than a single lump of the same mass. [2 marks]

• Cue. The powder has a larger surface area, so more particles are exposed and collisions with the other reactant are more frequent, giving more successful collisions per second.

Q3. State what a catalyst does to the activation energy and whether it is used up. [2 marks]

• Cue. A catalyst lowers the activation energy (provides an alternative pathway) and is not used up.