Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Intermolecular Forces Exam Questions
Free Edexcel International A Level Chemistry exam questions for Intermolecular Forces, with a mark scheme for every pack. Covers London forces, permanent dipole-dipole forces, hydrogen bonding, boiling temperature trends, solubility and the anomalous properties of water.
Intermolecular Forces exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Intermolecular Forces
Intermolecular forces in Unit 2 of Edexcel International A Level Chemistry explains the physical properties of simple molecular substances. You learn how London forces, permanent dipole-dipole forces and hydrogen bonds arise, and use them to explain trends in boiling temperatures in alkanes, alcohols and hydrogen halides, as well as solubility in water and non-aqueous solvents. The topic builds on electronegativity and shapes of molecules from Unit 1. Examiners frequently ask you to draw hydrogen bonds with lone pairs and partial charges, compare boiling temperatures of named compounds and explain why ice is less dense than water.
What you need to know
- London forces Instantaneous dipoles arise from uneven electron distribution and induce dipoles in neighbouring molecules. They increase with the number of electrons and with greater surface contact, so straight-chain alkanes boil higher than branched isomers.
- Permanent dipole-dipole forces These act between polar molecules, such as HCl or propanone, in addition to London forces.
- Hydrogen bonding A hydrogen bond forms between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F atom. The three atoms are in a straight line, with a bond angle of about 180 degrees at H.
- Boiling temperature trends Hydrogen fluoride, water and ammonia have anomalously high boiling temperatures compared with the hydrides of the rest of their groups because of hydrogen bonding. HCl to HI boiling temperatures increase because London forces increase.
- Water and ice Ice has an open lattice held by hydrogen bonds, so it is less dense than liquid water. Water also has a relatively high boiling temperature and surface tension.
- Solubility Alcohols dissolve in water because they form hydrogen bonds with water; solubility falls as the hydrocarbon chain gets longer. Non-polar substances dissolve in non-polar solvents because similar London forces form.
Common mistakes in Intermolecular Forces
- Saying covalent bonds break when a simple molecular substance boils. Only intermolecular forces are overcome.
- Drawing hydrogen bonds without the lone pair or partial charges. Show delta positive H, delta negative O, N or F and the lone pair, with a dashed line.
- Claiming hydrogen bonds are the strongest force in every case. Large molecules can have London forces strong enough to outweigh them.
- Explaining London forces only by molecular mass. The key factor is the number of electrons, plus the shape and contact area.
- Thinking any molecule with H has hydrogen bonds. The hydrogen must be bonded directly to N, O or F.
Try three Intermolecular Forces exam-style questions
1. Explain why the boiling temperature of water (100 C) is much higher than that of hydrogen sulfide (-60 C). (3 marks)
Show the mark scheme answer
Water molecules form hydrogen bonds; because O is very electronegative and has lone pairs, and H is bonded to O; H2S has only weaker dipole-dipole and London forces, so less energy is needed to separate its molecules.
2. Explain why pentane has a higher boiling temperature than 2,2-dimethylpropane, although both are C5H12. (2 marks)
Show the mark scheme answer
Pentane is a straight chain with a larger surface contact between molecules; so London forces are stronger and more energy is needed to overcome them.
3. Draw a diagram to show a hydrogen bond between two water molecules. (2 marks)
Show the mark scheme answer
Two water molecules with delta positive H and delta negative O, and a lone pair on O; dashed line from H on one molecule to the lone pair on O of the other, roughly linear O to H to O.
Intermolecular Forces FAQs
What does Edexcel International A Level Chemistry Intermolecular forces cover?
It covers London forces, permanent dipole-dipole forces and hydrogen bonding, and uses them to explain boiling temperatures, solubility and the properties of water and ice.
Which unit is Intermolecular forces examined in?
Intermolecular forces is part of Unit 2, building on electronegativity and molecular shapes from Unit 1.
How do I compare boiling temperatures in an exam?
Identify every type of intermolecular force present in each substance, say which is stronger and why (number of electrons, contact area or hydrogen bonding), then link it to the energy needed to separate molecules.
