Quick answer: Entropy S measures the dispersal of energy and matter, and the Gibbs free energy change ΔG = ΔH − TΔS predicts spontaneity: a reaction is spontaneous when ΔG is negative. This AP Chemistry lab on Gibbs free 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.
Why does a cold pack get cold, and yet dissolve without being pushed? You measure the temperature fall when ammonium nitrate dissolves, calculate ΔH, and then use the idea of spontaneity to say what the entropy change must be.
Level: AP Chemistry · Curriculum: AP Chem Unit 9
Aim
To measure the enthalpy change of dissolving ammonium nitrate and use it to find the least entropy increase that makes the process spontaneous.
Equipment
- Ammonium nitrate, 8.00 g
- Distilled water, 100.0 cm³ (100.0 g)
- Polystyrene cup with lid, and a beaker to support it
- Thermometer reading to 0.1 °C
- Balance (0.01 g), stopwatch, stirring rod
Safety
Wear eye protection. Ammonium nitrate is an oxidiser, so keep it away from fuels and heat and use only the amount given. Wash hands after the lab and rinse the solution down the sink with plenty of water.
Method
- Measure 100.0 cm³ of water into the cup and put in the thermometer.
- Record the temperature every 30 s for 90 s.
- At 90 s add the weighed ammonium nitrate, put on the lid and stir gently.
- Record the temperature every 30 s for 3 more minutes.
- Record the lowest temperature reached.
Results
Sample measurements:
| time (s) | temperature (°C) |
|---|---|
| 0 | 21.9 |
| 30 | 21.9 |
| 60 | 21.9 |
| 90 | mixed at 90 s |
| 120 | 17.3 |
| 150 | 16.2 |
| 180 | 16.3 |
| 210 | 16.5 |
| 240 | 16.8 |
Use c = 4.18 J/(g·K) and a total mass of 108.0 g. M(NH₄NO₃) = 80.04 g/mol.
Analysis questions
Question 1 Temperature change
State the temperature change using the lowest temperature reached.Question 2 Heat absorbed
Calculate the heat absorbed from the water, q = mcΔT.Question 3 Enthalpy change per mole
Calculate the amount of NH₄NO₃ and hence ΔH in kJ/mol.Question 4 Why spontaneous
The solid dissolves spontaneously although ΔH is positive. Deduce the sign of ΔS and explain.Question 5 Minimum entropy change
Using ΔH = +25.7 kJ/mol, calculate the least ΔS (in J/(mol·K)) for dissolving to be spontaneous at 295 K.Question 6 Heat gain error
Explain how heat gained from the room affects the measured ΔT and ΔH, and suggest one improvement.Question 7 Larger sample
Predict the temperature fall if 16.00 g of ammonium nitrate were added to the same 100.0 g of water.
