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 separate particles of a substance. These answers go with the AP Chemistry lab on intermolecular forces: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers use the sample data above. Your own readings will differ, but the order of the liquids and the reasoning should match. Always quote numerical answers with units and a sensible number of significant figures.
Water and ethanol: hydrogen bonding; propanone: dipole-dipole. Weaker forces mean more molecules escape per second, so evaporation is faster and the cooling is quicker and larger. The weaker the forces, the larger the cooling; water, with the most hydrogen bonds per molecule, cools least.
Question 4 Why water cools least
Cooling depends on energy removed per second = rate of evaporation × ΔHvap. Water evaporates much more slowly because its vapour pressure at 22 °C (about 2.6 kPa) is far below that of propanone (about 25 kPa), so the rate dominates over the larger ΔHvap.
Question 5 Rate of cooling
Rate = (22.0 − 19.4) / 15 = 2.6 / 15 = 0.17 °C/s.
Question 6 Control of variables
Same volume (1.0 mL) so the same amount evaporates; same air flow so evaporation is not helped more in one run; same starting temperature so the vapour pressure curve starts at the same point. Without control the comparison is not valid.
Question 7 Uncertainty
ΔT is a difference of two readings, so the uncertainties add: ±0.2 °C. Percentage = 0.2 / 3.2 × 100 = 6 percent; for propanone it is 0.2/13.8 = 1.4 percent.
Question 8 Improvement
Use a temperature probe with data logging every second, so the true minimum is captured and the rate of cooling can be plotted. This removes timing error from 15 s readings.
