When ammonium chloride dissolves in water, the temperature of the water decreases. Explain what this temperature change tells us about the type of energy change occurring, and describe what happens to the energy of the surroundings during this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student dissolves ammonium chloride crystals in water and measures the temperature of the solution every 30 seconds. The temperature drops by 8 °C over the course of the experiment.
Model answer (4 marks)
The dissolution is an endothermic process – heat is absorbed.
Because the water cools, energy is taken from the surroundings (the water) into the reaction.
Consequently the surroundings lose energy and their temperature falls.
The enthalpy change of the reaction is positive; the products (dissolved ions) have more energy than the reactants (solid NH4Cl).
Because the water cools, energy is taken from the surroundings (the water) into the reaction.
Consequently the surroundings lose energy and their temperature falls.
The enthalpy change of the reaction is positive; the products (dissolved ions) have more energy than the reactants (solid NH4Cl).
Examiner tips
- Use the word "endothermic" to show the type of energy change.
- Explain that heat flows from surroundings to the reaction.
- State that the surroundings lose energy and cool down.
- Mention the positive enthalpy change to link to the energy of products.
Common mistakes
- Saying the reaction is exothermic or that heat is released.
- Confusing the direction of heat flow – claiming energy goes from the reaction to the surroundings.
- Omitting the positive enthalpy change or the fact that the products have higher energy.
Mark scheme (4 marks)
- The reaction is endothermic
- Energy is taken in from the surroundings (by the reaction / by the ammonium chloride)
- The temperature of the surroundings decreases / the surroundings lose energy
- The enthalpy change / heat energy change of the reaction is positive (products have more energy than reactants)
Key terms in this question
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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