AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Reaction profiles: use reaction profiles to identify reactions as exothermic or endothermic
To classify a reaction from its profile, compare the energy of the reactants with the energy of the products.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To classify a reaction from its profile, compare the energy of the reactants with the energy of the products. If the product level is lower than the reactant level, energy has been transferred to the surroundings, so the reaction is exothermic and the overall energy change is negative. If the product level is higher, energy has been taken in from the surroundings, so the reaction is endothermic and the overall energy change is positive. The activation energy is the rise from reactants to the peak and does not by itself decide the classification.
For example: Combustion profiles show products below reactants, so combustion is exothermic.
What earns the marks
- Compare the energy level of the reactants with the energy level of the products on the profile.
- State that a lower product energy level means energy is released to the surroundings, so the reaction is exothermic.
- State that a higher product energy level means energy is absorbed from the surroundings, so the reaction is endothermic.
- Link the direction of the overall energy change to the sign of ΔH: negative for exothermic and positive for endothermic.
- Recognise that activation energy is the energy needed to reach the peak and is separate from the overall energy change.
Common mistakes and how to put them right
Judging the reaction type from the height of the activation energy peak.
Put it right by comparing reactant and product energy levels instead.
Assuming a large activation energy means the reaction is endothermic.
Put it right by explaining that activation energy affects rate, not the overall energy classification.
Ignoring the sign of the overall energy change.
Put it right by stating that exothermic reactions have a negative overall energy change and endothermic reactions have a positive one.
Exam tips
- Quote the reactant and product energy levels or describe their relative heights before naming the reaction type.
- Use the phrase energy transferred to the surroundings for exothermic and energy taken in from the surroundings for endothermic.
- If a profile is unlabelled, trace the curve from left to right and note whether the finishing level is below or above the starting level.
Try the lesson
The start of the Reaction profiles: use reaction profiles to identify reactions as exothermic or endothermic lesson, made from checked teaching notes for this specification point.
Have a go first
On a reaction profile, the energy of the products is lower than the energy of the reactants. What type of reaction is shown?
Listen
To classify a reaction from its profile, compare the energy of the reactants with the energy of the products. If the product level is lower than the reactant level, energy has been transferred to the surroundings, so the reaction is exothermic and the overall energy change is negative. If the product level is higher, energy has been taken in from the surroundings, so the reaction is endothermic and the overall energy change is positive. The activation energy is the rise from reactants to the peak and does not by itself decide the classification. For example, combustion profiles show products below reactants, so combustion is exothermic. A strong answer earns marks for these points. Compare the energy level of the reactants with the energy level of the products on the profile. State that a lower product energy level means energy is released to the surroundings, so the reaction is exothermic. State that a higher product energy level means energy is absorbed from the surroundings, so the reaction is endothermic. Link the direction of the overall energy change to the sign of ΔH: negative for exothermic and positive for endothermic. A common slip: Judging the reaction type from the height of the activation energy peak. Instead, comparing reactant and product energy levels instead. Exam tip: Quote the reactant and product energy levels or describe their relative heights before naming the reaction type.
Key words
Say what each word means before you turn the card.
Exothermic
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Endothermic
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Activation energy
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Overall energy change
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See every definition
- Exothermic
- A reaction in which the product level is lower than the reactant level, so energy has been transferred to the surroundings and the overall energy change is negative.
- Endothermic
- A reaction in which the product level is higher than the reactant level, so energy has been taken in from the surroundings and the overall energy change is positive.
- Activation energy
- The rise from reactants to the peak, which does not by itself decide the classification.
- Overall energy change
- The difference between the energy of the reactants and the energy of the products, negative for exothermic and positive for endothermic.
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