Quick answer: Entropy measures how many ways particles and energy can be arranged; it rises from solid to liquid to gas. This Chemistry 11-12 lab on entropy Gibbs energy 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.
How can a solid that takes in heat from its surroundings still dissolve by itself? You dissolve ammonium nitrate in a simple calorimeter, measure the temperature fall, calculate the enthalpy change per mole and then use entropy to explain why the process is spontaneous.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.4
Aim
To measure the enthalpy change on dissolving ammonium nitrate, then use it with a given entropy change to decide why the process is spontaneous even though it is endothermic.
Equipment
- Polystyrene cup with lid, thermometer (0.1 °C divisions) or temperature probe, stirrer
- 100.0 cm³ measuring cylinder or pipette, balance (0.01 g)
- Solid ammonium nitrate: three samples of about 4 g, 8 g and 12 g, each weighed exactly
Safety
Wear eye protection. Ammonium nitrate is an oxidising solid: keep it away from fuels and do not heat it. Wash hands after use and wipe up spills with plenty of water.
Method
- Measure 100.0 cm³ of water into the cup, stir and record the temperature every 30 s for 2 minutes.
- At the 2 minute mark add the weighed ammonium nitrate and replace the lid straight away.
- Stir gently and record the temperature every 30 s until it has been rising for 2 minutes.
- Record the lowest temperature reached.
- Rinse the cup and repeat with the other two masses.
Results
Sample results (Mr of NH₄NO₃ = 80.0; specific heat capacity of the solution taken as 4.18 J g⁻¹ K⁻¹; mass of solution taken as 100.0 g):
| Trial | Mass of NH₄NO₃ (g) | Initial temperature (°C) | Lowest temperature (°C) | Temperature fall (°C) |
|---|---|---|---|---|
| 1 | 4.00 | 21.0 | 18.2 | 2.8 |
| 2 | 8.00 | 21.0 | 15.5 | 5.5 |
| 3 | 12.00 | 21.2 | 12.8 | 8.4 |
Analysis questions
Question 1 Moles dissolved
Calculate the moles of ammonium nitrate dissolved in each trial.Question 2 Heat absorbed
Calculate the heat energy absorbed from the water in each trial using q = mcΔT.Question 3 Enthalpy of solution
Calculate ΔH in kJ/mol for each trial and give the mean. Give the sign.Question 4 Comparing with the data
The data-book value is +25.7 kJ/mol. Calculate the percentage difference of your mean.Question 5 Entropy and spontaneity
Using ΔS = +108.7 J K⁻¹ mol⁻¹ and T = 294 K, calculate ΔG for dissolving and explain why it dissolves although it is endothermic.Question 6 Minimum entropy change
Calculate the minimum value of ΔS that would make dissolving spontaneous at 294 K with your mean ΔH.Question 7 Improving the lab
Suggest one source of error that makes the measured ΔH too small and one improvement.
