Topic 7
Intermolecular Forces
Intermolecular forces decide whether a substance is a gas, a liquid or a solid, why ice floats, why alcohols boil far above alkanes of the same size and what will dissolve in what. This topic covers London forces, permanent dipole-dipole interactions and hydrogen bonding, the anomalous properties of water, boiling temperature trends and the choice of solvents, all examined in Unit 2.
Revision Notes
6 revision notes, in the order the specification teaches them.
London Forces
How moving electrons create instantaneous and induced dipoles, and why electrons and shape decide their strength.
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Permanent Dipole-Dipole Forces
Which molecules have a permanent dipole, how polar molecules attract, and a routine for unfamiliar molecules.
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Hydrogen Bonding
The requirement, the four-feature exam diagram, water, ammonia and hydrogen fluoride, and predicting other cases.
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Anomalous Properties of Water
Why water boils at 100 °C, why ice floats and why water has a high surface tension.
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Boiling Temperature Trends
Alkane chain length and branching, alcohols against alkanes, and the hydrogen halides explained by forces.
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Choosing Solvents
Hydration of ions, alcohols in water, why halogenoalkanes do not dissolve, and non-aqueous solvents.
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Intermolecular Forces Map
Edexcel International A Level Chemistry · Unit 2 · Topic 7A Intermolecular Forces
The whole section on one A4 page: bonds or forces, London forces, permanent dipoles, hydrogen bonds, H₂O, NH₃ and HF, and why water is odd.
Boiling Temperatures and Solvents Map
Edexcel International A Level Chemistry · Unit 2 · Topic 7B Physical Properties
The whole section on one A4 page: chain length, branching, alcohols against alkanes, the hydrogen halides, water as a solvent and other solvents.
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Specification
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7.1
7.2
7.3
- i
understand the following anomalous properties of water resulting from hydrogen bonding: its high melting and boiling temperature when compared with similar molecules
Revise - ii
understand the following anomalous properties of water resulting from hydrogen bonding: the density of ice compared to that of water
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7.4
- 7.4
be able to predict the presence of hydrogen bonding in molecules analogous to those mentioned in 7.2
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7.5
- i
understand, in terms of intermolecular forces, physical properties shown by substances, including: the trends in boiling temperatures of alkanes with increasing chain length
Revise - ii
understand, in terms of intermolecular forces, physical properties shown by substances, including: the effect of branching in the carbon chain on the boiling temperatures of alkanes
Revise - iii
understand, in terms of intermolecular forces, physical properties shown by substances, including: the relatively low volatility (higher boiling temperatures) of alcohols compared to alkanes with a similar number of electrons
Revise - iv
understand, in terms of intermolecular forces, physical properties shown by substances, including: the trends in boiling temperatures of the hydrogen halides HF to HI
Revise
7.6
- i
understand factors that influence the choice of solvents, including: water, to dissolve some ionic compounds, in terms of the hydration of the ions
Revise - ii
understand factors that influence the choice of solvents, including: water, to dissolve simple alcohols, in terms of hydrogen bonding
Revise - iii
understand factors that influence the choice of solvents, including: water, as a poor solvent for compounds (to include polar molecules such as halogenoalkane), in terms of inability to form hydrogen bonds
Revise - iv
understand factors that influence the choice of solvents, including: non-aqueous solvents, for compounds that have similar intermolecular forces to those in the solvent
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