Quick answer: Intermolecular forces are the attractions between molecules: London forces act between all particles and grow with the number of electrons, permanent dipole–dipole forces act between polar molecules, and hydrogen bonds act when H bonded to N, O or F meets a lone pair. This Chemistry 11-12 lab on intermolecular forces gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Why does ethanol feel colder than butanol on your skin? In this lab you measure the cooling caused by four evaporating liquids and use the data to compare the strength of their intermolecular forces.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Aim
To compare how quickly four liquids evaporate by measuring the temperature fall on a thermometer, and to link the result to intermolecular forces.
Equipment
- Thermometer or temperature probe (to 0.5 °C)
- Four small beakers containing 10 cm³ each of propanone, ethanol, propan-1-ol and butan-1-ol
- Filter paper strips and elastic bands
- Stopclock
- Safety goggles
Safety
All four liquids are flammable and their vapours are harmful: no flames, work in a well-ventilated room, keep lids on the beakers when not in use and wear goggles. Dispose of the wet paper as directed by your teacher.
Method
- Wrap a strip of filter paper round the bulb of the thermometer and secure it with an elastic band.
- Record the room temperature with the dry thermometer.
- Dip the covered bulb into the first liquid for 30 seconds.
- Take it out, hold it in still air and read the temperature every 15 seconds for 3 minutes. Note the lowest temperature.
- Use a fresh paper strip and repeat for each liquid.
Results
Wanjiru in Nairobi recorded these results (thermometer reading ±0.5 °C). Boiling points are from a data book.
| Liquid | Boiling point / °C | Starting temperature / °C | Lowest temperature / °C |
|---|---|---|---|
| Propanone | 56 | 21.0 | 5.0 |
| Ethanol | 78 | 21.0 | 10.5 |
| Propan-1-ol | 97 | 21.0 | 16.0 |
| Butan-1-ol | 117 | 21.0 | 18.5 |
Analysis questions
Question 1 Temperature falls
Calculate the fall in temperature for each liquid.Question 2 Pattern in the data
Describe the relationship between boiling point and temperature fall.Question 3 Why evaporation cools
Explain why the thermometer cools when a liquid evaporates from the paper.Question 4 Propan-1-ol and ethanol
Both are alcohols that form hydrogen bonds. Explain why propan-1-ol cools the thermometer less than ethanol.Question 5 Propanone
Propanone cannot form hydrogen bonds yet shows the biggest fall. Explain.Question 6 Uncertainty
The thermometer reading has an uncertainty of ±0.5 °C. Calculate the uncertainty in a temperature fall and the percentage uncertainty for butan-1-ol.Question 7 Fair test
State three variables to control.Question 8 Improvement
Suggest one way to make the results more reliable.
