Cold packs used in sports medicine contain ammonium nitrate and water in separate compartments. When the pack is squeezed, the compartments break and the ammonium nitrate dissolves, causing the pack to feel cold. Explain, in terms of energy and bond breaking and bond making, why the pack feels cold, and describe what a reaction profile for this type of reaction would look like compared to one for an exothermic reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cold packs used in sports medicine contain ammonium nitrate and water in separate compartments. When the pack is squeezed, the compartments break and the ammonium nitrate dissolves, causing the pack to feel cold.
Model answer (5 marks)
The dissolution of ammonium nitrate is endothermic – energy is absorbed from the surroundings. The energy required to break the ionic bonds in solid NH4NO3 and the H–O bonds in water exceeds the energy released when new bonds (e.g. NH4+–H2O, NO3––H2O) are formed. Consequently the pack feels cold because heat is taken from the surrounding air and the pack’s temperature falls.
On a reaction profile the reactants (solid NH4NO3 and water) lie at a lower energy level than the products (aqueous NH4+ and NO3–). The curve rises to a transition state and then falls to a higher‑energy product plateau – the opposite of an exothermic profile where the products are at a lower energy level than the reactants.
On a reaction profile the reactants (solid NH4NO3 and water) lie at a lower energy level than the products (aqueous NH4+ and NO3–). The curve rises to a transition state and then falls to a higher‑energy product plateau – the opposite of an exothermic profile where the products are at a lower energy level than the reactants.
Examiner tips
- Use the word ‘endothermic’ and explain heat absorption from surroundings
- Show that bond breaking > bond making energy
- Mention higher energy products on the profile
- Contrast with exothermic profile
Common mistakes
- Saying the reaction is exothermic
- Ignoring the energy difference between bond breaking and making
- Not describing the reaction profile shape
Mark scheme (5 marks)
- The reaction/dissolving process is endothermic
- Energy is taken in from the surroundings, causing the temperature of the surroundings to decrease
- More energy is required to break bonds in the reactants than is released when bonds are formed in the products
- On the reaction profile, the products are at a higher energy level than the reactants
- This is the opposite of an exothermic reaction profile, where the products are at a lower energy level than the reactants
Key terms in this question
bond breaking · bond making · reaction profile
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Physical chemistry questions
- Hydrogen gas can be produced by reacting methane with steam at high temperatures…
- A student dissolves citric acid in water and measures the pH. She then dissolves…
- Baking powder contains sodium hydrogencarbonate. When baking powder is added to …
- Hand warmers contain iron powder. When the pack is opened and exposed to air, th…