Additional Science-Chemistry Chapter 4

GCSE AQA Additional Science-Chemistry Chapter 4

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  • Created by: Mel
  • Created on: 24-05-12 10:22

Rates of Reaction

The rate of reaction measures the speed of a reaction or how fast it is and can be found by measuring how much of a reactant is used up or how much of a product is formed in a certain time.

There are different ways we can make these kinds of measurements:

  • Measuring the mass of a reaction mixture
  • Measuring the volume of gas given off
  • Measuring the light transmitted through a solution
  • Measuring the time for a certain amount of reactant to be used or product to be formed

Temperature, concentration, pressure of reacting gases, surface area of reacting solids, and the use of catalysts, are all factors which affect the rate of a reaction.

The rate of reaction is equal to the amount of reactant used divided by the time taken. Or it can expressed as the amount of product formed divided by the time taken (

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Collision Theory

There are five main factors which affect the rate of chemical reactions: temperature, concentration, pressure, surface area, presence of a catalyst.

The collision theory states that particles must collide with enough energy to change into new substances. The minimum energy they need is called the 'activation energy'. If we increase the chance of individual particles reacting, we will also increase the rate of reaction.

Breaking large pieces of a solid into smaller pieces exposes new surfaces which means there are more collisions in the same time and so a powder reacts faster than large lumps of a substance.

Catalysts increase the rate of reaction without being used up. They do this by lowering the activation energy needed. Different reactions need different catalysts. Catalysts are important in industry because they reduce costs.

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The Effect of Temperature

Increasing the temperature increases the speed of the particles in a reaction mixture. This means they collide more often and with more energy, which increases the rate of reaction.

When we heat up a substance, energy is transferred to the particles that make up the substance which means that they move faster. And when particles move faster they have more collisions and more energy. This speeds up the reactions in two ways - the particles will collide more often and they have more energy when they do collide.

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The Effect of Concentration

Increasing the pressure of a reaction involving gases has the same effect. It squashes the gas particles more closely together. This increases the chance that they will collide and react and so speeds up the rate of reaction.

If the concentration of a solution is increased there are more particles dissolved in the same volume. This means the dissolved particles are closer together and so they collide more often. Increasing the concentration of a reactant therefore increases the rate of a reaction.

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Concentration is measured in moles per cubic decimetre (mol/dm3)

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The Effect of Catalysts

Catalysts change the rates of chemical reactions. Most catalysts are used to speed up reactions. The catalyst is not used up so it can be used over and over again. They work by lowering the activation energy of a reaction so that more collisions result in a reaction. Different catalysts are used with different reactions, e.g. iron is used in the Haber process.

Although some catalysts are expensive, they can be economical because they do not need replacing very often. They also reduce the energy costs and time needed for a reaction.

Some catalysts work by providing a surface for the reacting particles to come together.

Catalysts are normally used in the form of powders, pellets so that there is more surface area.

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