When magnesium ribbon is added to dilute hydrochloric acid, the temperature of the solution rises. Explain what this observation tells us about the type of energy change occurring, and describe how the enthalpy change for this reaction would be represented on a reaction profile.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium reacts with dilute hydrochloric acid according to the equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Model answer (4 marks)
The observation that the solution temperature rises shows that the reaction releases energy to the surroundings, i.e. it is exothermic.
On a reaction profile the reactants are drawn at a higher energy level than the products. The vertical drop from reactants to products represents the enthalpy change, which is negative for this reaction.
Thus the enthalpy change ΔH is negative, indicating that the products are at a lower energy level than the reactants.
On a reaction profile the reactants are drawn at a higher energy level than the products. The vertical drop from reactants to products represents the enthalpy change, which is negative for this reaction.
Thus the enthalpy change ΔH is negative, indicating that the products are at a lower energy level than the reactants.
Examiner tips
- Use the word ‘exothermic’ to show understanding of energy transfer.
- Show the reaction profile with a downward arrow from reactants to products.
- State that ΔH is negative and explain it represents a drop in energy.
- Link the temperature rise to energy being released to the surroundings.
Common mistakes
- Saying the reaction is endothermic or ignoring the temperature rise.
- Not showing the reaction profile or misplacing the energy levels.
- Using ‘positive ΔH’ instead of ‘negative ΔH’.
Mark scheme (4 marks)
- The reaction is exothermic
- Energy is transferred to the surroundings, causing the temperature to increase
- On the reaction profile, the products are at a lower energy level than the reactants
- The overall enthalpy change (ΔH) is negative / the difference in energy between reactants and products represents a negative enthalpy change
Key terms in this question
enthalpy change · reaction profile
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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