Combustion of fuels is an exothermic process. Explain what the term exothermic means and describe how the energy profile diagram for an exothermic reaction differs from that of an endothermic reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When methane burns in oxygen, energy is released to the surroundings. When citric acid reacts with sodium hydrogencarbonate, the surroundings become colder.
Model answer (5 marks)
Exothermic means that energy is released from the system to the surroundings.
In an exothermic reaction the temperature of the surroundings rises because heat is transferred outwards.
On an energy‑profile diagram the products lie below the reactants – the vertical drop represents the energy released.
For an endothermic reaction the products are higher in energy than the reactants, so the diagram shows a rise.
Thus ΔH is negative for exothermic reactions and positive for endothermic reactions; the energy difference between reactants and products is larger on the right side for endothermic than for exothermic reactions.
In an exothermic reaction the temperature of the surroundings rises because heat is transferred outwards.
On an energy‑profile diagram the products lie below the reactants – the vertical drop represents the energy released.
For an endothermic reaction the products are higher in energy than the reactants, so the diagram shows a rise.
Thus ΔH is negative for exothermic reactions and positive for endothermic reactions; the energy difference between reactants and products is larger on the right side for endothermic than for exothermic reactions.
Examiner tips
- Use the word ‘released’ to show exothermic; mention heat transfer to surroundings.
- State the temperature change of the surroundings. Show the energy diagram with products lower for exothermic, higher for endothermic. Remember ΔH sign and relative energy differences.
Common mistakes
- Saying ‘energy is absorbed’ instead of ‘released’. Forgetting to mention the temperature rise. Drawing the energy diagram with products higher for an exothermic reaction.
Mark scheme (5 marks)
- In an exothermic reaction, energy is transferred TO the surroundings
- The temperature of the surroundings increases (in an exothermic reaction)
- On the energy profile diagram for an exothermic reaction, the products are at a lower energy level than the reactants
- For an endothermic reaction, the products are at a higher energy level than the reactants (on the energy profile diagram)
- The overall energy change (ΔH) is negative for exothermic and positive for endothermic / the energy difference between reactants and products is greater on the right for endothermic compared to exothermic
Key terms in this question
exothermic · endothermic · energy profile diagram
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
More Energetics questions
- A student investigates the energy change when zinc powder is added to copper sul…
- A student mixes sodium hydroxide solution with hydrochloric acid in a polystyren…
- Explain why breaking bonds requires energy and forming bonds releases energy, an…
- Hydrogen peroxide solution decomposes slowly at room temperature into water and …