CCEA A Level Chemistry
CCEA A Level Chemistry 1.4 Intermolecular Forces Exam Questions
Free CCEA A Level Chemistry revision for 1.4 Intermolecular Forces: what you need to know, common mistakes and exam-style questions with answers. Covers van der Waals forces, permanent dipole-dipole attractions, hydrogen bonding and their effect on melting point, boiling point and solubility.
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About CCEA A Level Chemistry 1.4 Intermolecular Forces
Section 1.4 of Unit AS 1 explains the attractions between molecules. You need van der Waals forces (induced dipole attractions), permanent dipole-dipole attractions and hydrogen bonding, which occurs when hydrogen is bonded to nitrogen, oxygen or fluorine. CCEA expects you to explain trends in melting point, boiling point and solubility of covalent molecular substances using these forces, for example the boiling points of the hydrogen halides, the alkanes and water. This section is closely linked to structure (1.5) and shapes (1.6), since molecular shape decides whether a molecule is polar, and it is used again for alkanes, alcohols, aldehydes and amines.
What you need to know
- Van der Waals forces Temporary dipoles caused by uneven electron distribution induce dipoles in neighbouring molecules. They act between all molecules and increase with the number of electrons, so boiling points of the halogens and alkanes rise with Mr.
- Permanent dipole-dipole Polar molecules such as HCl attract each other because the delta+ end of one is attracted to the delta- end of another. These act in addition to van der Waals forces.
- Hydrogen bonding Forms between a hydrogen atom bonded to N, O or F and a lone pair on an N, O or F atom of another molecule. It is the strongest intermolecular force and explains the high boiling points of H2O, NH3 and HF.
- Water Each water molecule can form two hydrogen bonds, giving a high boiling point, and ice has an open lattice so is less dense than liquid water.
- Solubility Substances that can hydrogen bond with water (for example short chain alcohols) dissolve; non-polar molecules do not, because they cannot replace the hydrogen bonds between water molecules.
Common mistakes in 1.4 Intermolecular Forces
- Saying covalent bonds break when a simple molecular substance boils. Only intermolecular forces are overcome.
- Drawing a hydrogen bond between two hydrogen atoms. It is between delta+ H and a lone pair on N, O or F, shown with a dashed line.
- Saying HCl has a higher boiling point than HI because of dipoles. HI is higher because its larger number of electrons gives stronger van der Waals forces.
- Saying hydrogen bonds form with any hydrogen in a molecule. The H must be directly bonded to N, O or F.
- Forgetting to mention lone pairs in hydrogen bonding diagrams. Show the lone pair and the bond angle near 180 degrees.
Try three 1.4 Intermolecular Forces exam-style questions
1. Explain why the boiling point of water is much higher than that of hydrogen sulfide. (3 marks)
Show the mark scheme answer
Water forms hydrogen bonds between molecules; H2S has only van der Waals and dipole-dipole forces as S is not electronegative enough; more energy needed to overcome hydrogen bonds.
2. Draw a diagram to show a hydrogen bond between two ammonia molecules. (2 marks)
Show the mark scheme answer
N-H of one molecule with H labelled delta+ and N delta-; dashed line from H to lone pair on N of second molecule.
3. Explain the trend in boiling points from fluorine to iodine. (2 marks)
Show the mark scheme answer
Boiling point increases; more electrons so stronger van der Waals forces need more energy to overcome.
1.4 Intermolecular Forces FAQs
What does CCEA A Level Chemistry 1.4 Intermolecular Forces cover?
It covers van der Waals forces, permanent dipole-dipole attractions and hydrogen bonding, and how they affect melting point, boiling point and solubility.
Which unit is 1.4 Intermolecular Forces examined in?
It is examined in Unit AS 1: Basic Concepts in Physical and Inorganic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
How do I answer boiling point comparison questions?
Name the intermolecular forces in each substance, say which are stronger and why, and finish by linking stronger forces to more energy being needed to separate the molecules.
