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.
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.
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.
Edexcel GCSE Chemistry Notes
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