Quick answer: An exothermic reaction releases energy to the surroundings and its temperature rises; an endothermic reaction absorbs energy and its temperature falls. This Chemistry 9-10 lab on energy changes gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Some salts make the water hot when they dissolve and others make it cold, which is why cold packs and hand warmers can be built from them. You measure the temperature change for three salts, classify each dissolving process and turn the thermometer readings into an energy change per mole.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-4
Aim
To find which of three salts dissolves exothermically and which endothermically, and to calculate the energy change per mole of salt dissolved.
Equipment
- Polystyrene cup with lid and a beaker to support it
- Thermometer (−10 to 110 °C) and a stirrer
- Measuring cylinder (25 cm³), balance reading to 0.1 g, stopwatch
- Ammonium nitrate (5.0 g), anhydrous calcium chloride (2.0 g), sodium chloride (5.0 g)
- Distilled water, spatula, paper towel
Safety
Wear safety goggles. Calcium chloride is an irritant and gets hot, so add it slowly. Ammonium nitrate must be kept away from fuels and heat. Wash hands afterwards and do not eat or drink in the lab.
Method
- Place the cup in the beaker and add 25 cm³ of water (25 g). Record the temperature after one minute of stirring.
- Weigh the first salt and add it all at once. Replace the lid immediately.
- Stir gently and record the lowest or highest temperature reached.
- Empty and rinse the cup, then repeat with each of the other salts using fresh water at room temperature.
Results
Sample results with 25 g of water each time:
| Salt | Mass / g | Start / °C | End / °C |
|---|---|---|---|
| Ammonium nitrate | 5.0 | 21.0 | 9.0 |
| Calcium chloride | 2.0 | 21.0 | 34.5 |
| Sodium chloride | 5.0 | 21.0 | 19.5 |
Analysis questions
Question 1 Temperature changes
Calculate the temperature change for each salt and classify each dissolving process as exothermic or endothermic.Question 2 Energy transferred
Use q = m × c × ΔT with m = 25 g of water and c = 4.2 J/g/°C. Calculate q in kJ for each salt, ignoring the sign.Question 3 Moles of salt
Calculate the moles of each salt used (Ar: N = 14, H = 1, O = 16, Ca = 40, Cl = 35.5, Na = 23).Question 4 Energy change per mole
Calculate ΔH in kJ/mol for each salt, including the correct sign.Question 5 Comparing with accepted values
The accepted value for ammonium nitrate is about +26 kJ/mol.1. Calculate the percentage difference between your value and the accepted one.
2. Suggest why the experimental value is smaller in size.
Question 6 Sources of error
Explain why a lid and a polystyrene cup are used and give one further source of error.Question 7 Conclusion and use
State which salt is best for a cold pack and which for a hand warmer, and suggest one improvement to the method.
