A student dissolves ammonium nitrate in water. The temperature of the solution decreases. Explain why the temperature decreases, in terms of energy changes in bond breaking and bond forming.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When ammonium nitrate dissolves in water, the solution becomes noticeably colder.
Model answer (5 marks)
Energy is absorbed to break the ionic bonds in the ammonium nitrate lattice and to separate the ions.
When the ions are hydrated, new bonds are formed between the ions and water molecules.
However, the energy required to break the lattice bonds is greater than the energy released by hydration.
Thus the overall process is endothermic.
Heat energy is taken from the surroundings, so the temperature of the solution falls.
When the ions are hydrated, new bonds are formed between the ions and water molecules.
However, the energy required to break the lattice bonds is greater than the energy released by hydration.
Thus the overall process is endothermic.
Heat energy is taken from the surroundings, so the temperature of the solution falls.
Examiner tips
- Use the term ‘endothermic’ to show understanding of the overall energy change.
- Show the sequence: bond breaking → energy absorbed, bond forming → energy released, then compare magnitudes.
- Mention that heat is drawn from the surroundings, causing the temperature drop.
Common mistakes
- Failing to state that the lattice bonds are broken and that hydration bonds are formed.
- Confusing the direction of heat flow – writing that heat is released instead of absorbed.
- Using vague phrases like ‘energy changes’ without specifying bond breaking and forming.
Mark scheme (5 marks)
- Energy is needed / absorbed to break bonds (between ions in the lattice / between ammonium nitrate particles)
- Energy is released when new bonds form (between ions and water molecules / during hydration)
- More energy is absorbed (breaking bonds) than is released (forming bonds)
- The overall process is endothermic
- The solution / surroundings loses energy / heat energy is transferred from the surroundings to the reaction, so temperature of the solution decreases
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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