Quick answer: Intermolecular forces are the attractions between separate particles of a substance. This AP Chemistry worksheet on intermolecular forces gives 20 problems from easy to hard: name the main force, which can hydrogen bond? and noble gas boiling points. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These problems train you to name intermolecular forces, rank substances by boiling point and vapour pressure, link forces to properties of liquids and solids, and use the vapour pressure equation. They follow AP Chemistry unit 3.
Work from recall to multi-step reasoning. In the last five problems you must write short arguments in the style of a free-response answer, so always name the force and compare strengths.
Level: AP Chemistry · Curriculum: AP Chem Unit 3
Problem 1 Name the main force ★★★
Name the strongest intermolecular force present in each pure substance.1. Argon, Ar
2. Hydrogen chloride, HCl
3. Ammonia, NH₃
4. Methane, CH₄
Problem 2 Which can hydrogen bond? ★★★
Aarav in Pune lists five molecules: CH₃OH, CH₃OCH₃, HF, CH₃F and CH₃NH₂.1. Identify which molecules can form hydrogen bonds with molecules of the same kind.
2. State the rule you used.
Problem 3 Noble gas boiling points ★★★
Boiling points of four noble gases are 4 K, 27 K, 87 K and 120 K. They are He, Ne, Ar and Kr in some order.1. Match each gas to its boiling point.
2. Name the force involved and explain the trend.
Problem 4 Celsius and kelvin ★★★
Convert, using T(K) = θ(°C) + 273.15.1. Water boils at 100 °C.
2. Nitrogen boils at 77 K.
3. Ethanol boils at 78 °C.
Problem 5 Reading a vapour pressure curve ★★★
1. Estimate the vapour pressure at 60 °C.
2. Estimate the normal boiling point.
3. Estimate the temperature at which the vapour pressure is 50 kPa.
Problem 6 Why ice floats ★★★
Wanjiru in Nairobi drops ice into a glass of water.1. Explain, using hydrogen bonds, why ice is less dense than liquid water.
2. State what happens to the hydrogen bonds when ice melts.
Problem 7 Viscosity of two liquids ★★★
Hexane, C₆H₁₄, flows like water, but ethane-1,2-diol, HOCH₂CH₂OH, is thick and syrupy at the same temperature.1. Name the strongest force in each liquid.
2. Explain the difference in viscosity.
Problem 8 Pentane and neopentane ★★★
Pentane and neopentane, C(CH₃)₄, both have formula C₅H₁₂ and mass 72.2 g/mol, but they boil at 36 °C and 9.5 °C.1. Name the force that explains both boiling points.
2. Explain why neopentane boils lower.
Problem 9 Capillary action ★★★
Water climbs up a thin glass tube and its surface curves upward at the edges. Mercury in the same tube forms a surface that curves downward and sits below the outside level.1. Define cohesive and adhesive forces.
2. Explain the difference between water and mercury.
Problem 10 Classify the solids ★★★
Four white solids were tested in a school lab.| Solid | Melting point | Conducts as solid | Conducts as liquid or in water | Solubility in water |
|---|---|---|---|---|
| W | 801 °C | no | yes | soluble |
| X | 1710 °C | no | no | insoluble |
| Y | 1085 °C | yes | yes | insoluble |
| Z | 95 °C | no | no | insoluble |
2. Give one piece of evidence for each.
Problem 11 Rank vapour pressures ★★★
Rank propanone (CH₃COCH₃), ethanol, water and ethane-1,2-diol from highest to lowest vapour pressure at 25 °C, and justify the order.Problem 12 Energy to vaporise ethanol ★★★
Chiamaka in Lagos evaporates 25.0 g of ethanol, C₂H₅OH (46.07 g/mol), with ΔHvap = 38.6 kJ/mol.1. Calculate the amount of ethanol in mol.
2. Calculate the energy absorbed.
Problem 13 Sweat and cooling ★★★
During a match, Kofi in Accra loses 150 g of sweat by evaporation. Treat it as water with ΔHvap = 40.7 kJ/mol (18.02 g/mol). His body mass is 60.0 kg and its specific heat capacity is 3.5 J/(g·°C).1. Calculate the energy removed.
2. Calculate the temperature drop if this energy were taken from his body with no heat gained.
Problem 14 Spot the error ★★★
A student writes: “When water boils, the O–H bonds break because boiling needs a lot of energy.”1. Identify the error.
2. Rewrite the sentence correctly.
Problem 15 Salt in water and in hexane ★★★
Bilal in Lahore stirs table salt into water and into hexane. It dissolves in water only.1. Name the force between Na⁺ and water molecules.
2. Explain why NaCl does not dissolve in hexane.
Problem 16 Boiling in Kathmandu ★★★
In Kathmandu the air pressure is about 70.0 kPa. Use ΔHvap = 40.7 kJ/mol for water and R = 8.314 J/(mol·K).1. Explain why water boils at a lower temperature there.
2. Calculate the boiling temperature in °C.
Problem 17 Estimate ΔH of vaporisation ★★★
Ethanol has a vapour pressure of 8.0 kPa at 25 °C (298.15 K) and 101.3 kPa at 78.4 °C (351.55 K).1. Calculate ΔHvap.
2. Compare with the data-book value, 38.6 kJ/mol, and suggest a reason for the difference.
Problem 18 Two isomers, two boiling points ★★★
1. Explain, with reference to the structures, why propan-1-ol boils 90 °C higher than methoxyethane.
2. Predict which has the higher vapour pressure at 20 °C.
Problem 19 Hydride trends ★★★
1. Describe the trend for group 14.
2. Identify the anomalies and explain them.
3. Explain why group 14 does not show an anomaly.
Problem 20 Design a comparison of IMFs ★★★
Mei in Singapore wants to compare the strength of intermolecular forces in water, ethanol and propanone by measuring how much each liquid cools a thermometer bulb as it evaporates.1. Describe the procedure and name two variables to control.
2. Predict the order of temperature drops with a reason.
3. State one safety precaution.
