The method and the results are on the intermolecular forces lab. Do the analysis yourself first, then check the answers below.
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. These answers go with the Chemistry 11-12 lab on intermolecular forces: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The model answers link each temperature fall to the forces between molecules. Mention hydrogen bonds, dipole–dipole forces and London forces by name, and comment on the uncertainty of the thermometer.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Propanone: 21.0 − 5.0 = 16.0 °C. Ethanol: 21.0 − 10.5 = 10.5 °C. Propan-1-ol: 21.0 − 16.0 = 5.0 °C. Butan-1-ol: 21.0 − 18.5 = 2.5 °C.
Question 2 Pattern in the data
The higher the boiling point, the smaller the temperature fall. Liquids with weaker intermolecular forces evaporate faster and cool the bulb more.
Question 3 Why evaporation cools
The molecules with the highest kinetic energy escape from the surface, because they can overcome the intermolecular forces. The mean kinetic energy of the remaining liquid falls, so its temperature falls, and heat flows from the thermometer into the liquid. The thermometer cools.
Question 4 Propan-1-ol and ethanol
Both form hydrogen bonds. Propan-1-ol has more electrons (32 against 26), so its London forces are stronger, it evaporates more slowly, and the fall is smaller (5.0 against 10.5 °C).
Question 5 Propanone
Propanone has only dipole–dipole and London forces, which are weaker than hydrogen bonds. Its molecules escape easily, so it evaporates quickly and cools the thermometer most: 16.0 °C.
Question 6 Uncertainty
Fall = start − lowest, so the uncertainties add: 0.5 + 0.5 = ±1.0 °C. For butan-1-ol: 1.0 ÷ 2.5 × 100 = 40 %.
Question 7 Fair test
Same volume of liquid, same size of paper, same dipping time, and the same airflow and room temperature.
Question 8 Improvement
Use a temperature probe with a data logger and repeat each liquid three times, then take the mean, so that the smallest fall (butan-1-ol) is measured with a lower percentage uncertainty.
