Representing Energy Changes in Reactions

Specification Point 7.16:
  • Draw and label reaction profiles for endothermic and exothermic reactions, identifying activation energy.
  • Reaction profiles (sometimes called energy level diagrams) are graphical representations of the heat changes in chemical reactions.
  • The enthalpy of the reactants and products are displayed on the y-axis.
  • 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 activation energy in a reaction profile diagram is the energy difference between the reactants and the highest point on the curve.

Exothermic Reaction

Reaction Profile Exo, Edexcel GCSE Chemistry

Reaction profile of an exothermic 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 enthalpy (ΔH) is negative.
  • This is represented on the reaction profile with a downwards arrow as the energy of the products is lower than the reactants.

Endothermic Reaction

Reaction profile of an endothermic reaction

  • Energy is taken in endothermic reactions.
  • The energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive.
  • This is represented on the reaction profile with an upwards arrow as the energy of the products is higher than the reactants.

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