Rate of Reaction » Factors Affecting the Rate

Speeding up and slowing down

Some reactions are over in a flash, like a firework exploding. Others take years, like iron slowly rusting. Chemists often want to control how fast a reaction goes. A factory wants to make a product quickly. A food company wants its products to react slowly so they stay fresh.

The speed of a reaction is its rate. In the last lesson you learned how to measure it. In this lesson you will learn what changes it. There are five factors to know:

• the concentration of reactants in solution
• the pressure of reacting gases
• the surface area of solid reactants
• the temperature
• the presence of a catalyst

You need to be able to recall how changing each one affects the rate. The reasons why are taught in the lesson on Collision Theory and Activation Energy, so here we focus on what happens.

Key terms:

• Concentration: how much of a substance is dissolved in a given volume of solution. A concentrated solution has lots dissolved. A dilute solution has only a little.
• Surface area: the total area of the outside of a solid that is open to the other reactant.
• Catalyst: a substance that speeds up a reaction and is not used up.

The five factors

Everyday examples

You already see these factors every day.

• Surface area: a sugar lump takes longer to dissolve in tea than the same mass of caster sugar. Flour dust in the air can burn extremely fast, yet a bag of flour will not catch fire easily.
• Temperature: food goes off more slowly in a fridge than on a warm kitchen worktop. Cooking is quicker at a higher oven temperature.

How to investigate a factor

To find out how one factor affects the rate, you run the same reaction several times and change only that factor. You must also measure the rate each time. Remember from the last lesson that you can do this by measuring how fast a product forms or how fast a reactant is used up.

Two common methods are:

1. Measuring the volume of gas produced. Use a gas syringe, or collect the gas over water in an upside-down measuring cylinder. Record the volume every 10 seconds. A faster reaction gives gas faster.
2. Timing a change you can see. Some reactions make the mixture go cloudy. This cloudiness is called turbidity. You can time how long it takes for a cross under the flask to disappear. Other reactions change colour, so you time how long the colour takes to change. A shorter time means a faster rate.

Both methods are used in Required practical 5, which is covered in its own lesson, Required Practical: Rates of Reaction. That investigation should involve developing a hypothesis, for example "if the concentration goes up, the rate will go up".

Making it a fair test

If you change more than one thing at a time, you cannot tell which change caused the result. So in a fair test, use the three kinds of variable you met earlier in the course:

• change only the independent variable (for example, the concentration of the acid)
• measure the dependent variable (for example, the volume of gas after 30 seconds)
• keep the control variables the same (for example, temperature, mass of solid, volume of acid, size of the pieces)

Repeating the experiment and finding a mean makes your results more reliable.

Exam-style question

A student reacts zinc with dilute sulfuric acid and measures the volume of hydrogen made. Zinc powder gives a faster reaction than zinc granules of the same mass. (a) Give the reason for the difference. (b) Give two other ways to increase the rate of this reaction. (c) Name one variable the student must keep the same.