Reaction Profiles (AQA GCSE Chemistry)

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Reaction Profiles

Activation energy

  • For atoms or particles to react with each other in a chemical system they must first of all come into contact with each other in a collision
  • A number of factors come into play when analysing collisions such as energy, orientation, and number of collisions per second
  • In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful, that is for the particles to react together
  • This minimum amount of energy is called the activation energy
  • Different reactions have different activation energies, depending on the chemical identities involved
  • Reactions which have higher activation energies require more energy to start than those with lower activation energies

Exam Tip

All reactions have an activation energy as the chemical bonds in the reactant molecules have to be broken first

Reaction profiles

  • Reaction profiles (sometimes called energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
  • The energy of the reactants and products are displayed on the y-axis and the reaction pathway is shown on the x-axis
  • Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
  • The difference in height between the energy of reactants and products represents the overall energy change of a reaction
  • The initial increase in energy, from the reactants to the peak of the curve, represents the activation energy required to start the reaction
  • The greater the initial rise then the more energy that is required to get the reaction going e.g. more heat needed

2-graphs-1

Reaction profile of an exothermic reaction and an endothermic reaction

  • Energy is given out in exothermic reactions
  • The energy of the products will be lower than the energy of the reactants, so the change in energy is negative
  • This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
  • Energy is taken in endothermic reactions
  • The energy of the products will be higher than the energy of the reactants, so the change in energy is positive
  • This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants

Exam Tip

You should be able to draw clear and fully labelled reaction profile diagrams for both types of reactions and you should be able to identify a reaction from its reaction profile.

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Stewart

Author: Stewart

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.