AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Forces between molecules: permanent dipole–dipole forces induced…
Full specification wording
Forces between molecules: Forces between molecules: permanent dipole–dipole forces induced dipole–dipole (van der Waals, dispersion, London) forces hydrogen bonding.
Three forces act between molecules.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Three forces act between molecules. Induced dipole-dipole forces, also called van der Waals, dispersion or London forces, act between every molecule: the constantly moving electrons create an instantaneous dipole which induces a matching dipole in a neighbour, and the strength grows with the number of electrons and with the surface area in contact. Permanent dipole-dipole forces act in addition wherever the molecule has a permanent dipole, with δ⁺ of one molecule attracted to δ⁻ of the next. Hydrogen bonding is the strongest and needs two things: a hydrogen atom bonded directly to nitrogen, oxygen or fluorine, and a lone pair on a nitrogen, oxygen or fluorine atom of another molecule.
What earns the marks
- Identifying induced dipole-dipole forces as present between all molecules.
- Adding permanent dipole-dipole forces only where the molecule has a permanent dipole.
- Hydrogen bonding identified only where hydrogen is bonded directly to nitrogen, oxygen or fluorine.
- A diagram showing the lone pair and the hydrogen bond drawn to it.
- The order of strength, hydrogen bonding strongest and induced dipole forces weakest for small molecules.
Common mistakes and how to put them right
Claiming that molecules which hydrogen bond have no induced dipole-dipole forces.
Assuming any molecule containing hydrogen can hydrogen bond, so including methane or hydrogen chloride.
Drawing the hydrogen bond to the wrong atom or omitting the lone pair it must reach.
Describing intermolecular forces as bonds within the molecule being broken.
Exam tips
- List every force present, not just the strongest; induced dipole forces are always there.
- In a hydrogen bond diagram show the lone pair, the partial charges and a roughly linear arrangement.
- Check the hydrogen is bonded directly to nitrogen, oxygen or fluorine before claiming a hydrogen bond.
Try the lesson
The start of the Forces between molecules: Forces between molecules: permanent dipole–dipole forces induced dipole–dipole (van der Waals, dispersion, London) forces hydrogen bonding. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of intermolecular force acts between all molecules, whether polar or non-polar?
Listen
Three forces act between molecules. Induced dipole-dipole forces, also called van der Waals, dispersion or London forces, act between every molecule: the constantly moving electrons create an instantaneous dipole which induces a matching dipole in a neighbour, and the strength grows with the number of electrons and with the surface area in contact. Permanent dipole-dipole forces act in addition wherever the molecule has a permanent dipole, with δ⁺ of one molecule attracted to δ⁻ of the next. Hydrogen bonding is the strongest and needs two things: a hydrogen atom bonded directly to nitrogen, oxygen or fluorine, and a lone pair on a nitrogen, oxygen or fluorine atom of another molecule. A strong answer earns marks for these points. identifying induced dipole-dipole forces as present between all molecules. adding permanent dipole-dipole forces only where the molecule has a permanent dipole. hydrogen bonding identified only where hydrogen is bonded directly to nitrogen, oxygen or fluorine. a diagram showing the lone pair and the hydrogen bond drawn to it. Exam tip: List every force present, not just the strongest; induced dipole forces are always there.
Key words
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induced dipole-dipole forces
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permanent dipole-dipole forces
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hydrogen bonding
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order of strength
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See every definition
- induced dipole-dipole forces
- Act between every molecule, as constantly moving electrons create an instantaneous dipole which induces a matching dipole in a neighbour.
- permanent dipole-dipole forces
- Act in addition wherever the molecule has a permanent dipole, with the δ⁺ of one molecule attracted to the δ⁻ of the next.
- hydrogen bonding
- The strongest force, needing a hydrogen atom bonded directly to nitrogen, oxygen or fluorine and a lone pair on a nitrogen, oxygen or fluorine atom of another molecule.
- order of strength
- Hydrogen bonding is strongest and induced dipole forces weakest for small molecules.
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