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GCSE Chemistry Revision

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GCSE Chemistry revision

Factors which affect the rates of chemical reactions

Rate of reaction

AQA 4.6.1.2 Foundation & Higher
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Key knowledge

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What is Collision Theory?

  • Collision theory states that particles must collide with each other with sufficient energy in order for a chemical reaction to occur.
  • The minimum energy required for a successful collision is called the activation energy.
  • If colliding particles do not reach the activation energy, they simply bounce apart and no reaction takes place.

What Determines the Rate of Reaction?

  • The rate of reaction depends on two key factors: the energy of the particles and the frequency of collisions between them.
  • The more energy particles have, the more likely they are to exceed the activation energy during a collision.
  • The more frequently particles collide, the greater the number of successful collisions overall, increasing the rate of reaction.

Effect of Temperature on Rate of Reaction

  • Increasing the temperature gives particles more energy, causing them to move faster.
  • Faster-moving particles collide more frequently and with greater energy, making it more likely that collisions will exceed the activation energy.
  • Overall, a higher temperature leads to a greater number of successful collisions and therefore a faster rate of reaction.

Effect of Concentration and Pressure on Rate of Reaction

  • Concentration (for solutions) and pressure (for gases) both refer to the number of particles per unit volume.
  • Increasing either concentration or pressure means there are more particles in the same volume, which increases the frequency of collisions.
  • A higher frequency of collisions leads to more successful collisions and a faster rate of reaction.

Effect of Surface Area on Rate of Reaction

  • Increasing the surface area of a solid reactant increases the area over which collisions with other reactants can take place.
  • For example, powdered magnesium has a much higher surface area to volume ratio than a solid block of the same mass.
  • A greater surface area leads to more frequent collisions between reactants, increasing the rate of reaction.

What is a Catalyst?

  • A catalyst is a substance that speeds up a chemical reaction without being used up in the process.
  • Because catalysts are neither reactants nor products, they are not included in the reaction equation.
  • Catalysts work by providing an alternative reaction pathway that has a lower activation energy.

Reaction Profiles and Activation Energy

  • A reaction profile is a diagram that shows how the energy of chemicals changes throughout a reaction.
  • The activation energy is represented on a reaction profile as the difference between the energy level of the reactants and the peak of the curve.
  • When a catalyst is present, the reaction profile shows a lower peak, indicating a reduced activation energy and a higher proportion of successful collisions.

Examples of Catalysts

  • Transition metals such as cobalt and nickel are common examples of catalysts used in chemical reactions.
  • Enzymes are biological catalysts produced by living organisms and are studied in biology as well as chemistry.
  • Different catalysts are suited to different reactions, making the term 'catalyst' a broad category of substances.