Which bonds are broken when a simple molecular liquid boils?

None. Only the intermolecular forces are overcome.

What is another name for London forces?

Instantaneous dipole–induced dipole forces.

How does a London force arise?

An instantaneous dipole in one particle induces a dipole in a neighbour, and the opposite partial charges attract.

Which substances have London forces as their only intermolecular force?

Non-polar substances such as noble gases, alkanes and halogens (Br₂, I₂).

How do London forces change with the number of electrons?

They get stronger as the number of electrons (size of molecule) increases.

When is a molecule polar?

When it has polar bonds whose dipoles do not cancel.

Why is CCl₄ non-polar although C–Cl bonds are polar?

Its tetrahedral shape is symmetrical, so the bond dipoles cancel.

Which forces act between polar molecules such as ICl?

Permanent dipole–permanent dipole interactions as well as London forces.

What two features are needed for a hydrogen bond?

A δ+ hydrogen bonded to N, O or F, and a lone pair on N, O or F in another molecule.

How many hydrogen bonds can one water molecule form?

Up to four: two through its δ+ hydrogens and two through its lone pairs on oxygen.

Can CH₃OCH₃ molecules hydrogen-bond with each other? Why?

No. Oxygen has lone pairs but no hydrogen is bonded to oxygen.

Why can HCl not form hydrogen bonds with itself?

Chlorine is not one of N, O or F, so it is not electronegative and small enough.

Name three molecules that form hydrogen bonds with themselves.

Water, ammonia and hydrogen fluoride (also ethanol, hydrazine).

Types of intermolecular forceEdexcel International A Level Chemistry: Flashcards

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What are intermolecular forces?

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What are intermolecular forces?: Attractions between neighbouring molecules or atoms, much weaker than covalent bonds.
Which bonds are broken when a simple molecular liquid boils?: None. Only the intermolecular forces are overcome.
What is another name for London forces?: Instantaneous dipole–induced dipole forces.
How does a London force arise?: An instantaneous dipole in one particle induces a dipole in a neighbour, and the opposite partial charges attract.
Which substances have London forces as their only intermolecular force?: Non-polar substances such as noble gases, alkanes and halogens (Br₂, I₂).
How do London forces change with the number of electrons?: They get stronger as the number of electrons (size of molecule) increases.
When is a molecule polar?: When it has polar bonds whose dipoles do not cancel.
Why is CCl₄ non-polar although C–Cl bonds are polar?: Its tetrahedral shape is symmetrical, so the bond dipoles cancel.
Which forces act between polar molecules such as ICl?: Permanent dipole–permanent dipole interactions as well as London forces.
What two features are needed for a hydrogen bond?: A δ+ hydrogen bonded to N, O or F, and a lone pair on N, O or F in another molecule.
How many hydrogen bonds can one water molecule form?: Up to four: two through its δ+ hydrogens and two through its lone pairs on oxygen.
Can CH₃OCH₃ molecules hydrogen-bond with each other? Why?: No. Oxygen has lone pairs but no hydrogen is bonded to oxygen.
Why can HCl not form hydrogen bonds with itself?: Chlorine is not one of N, O or F, so it is not electronegative and small enough.
Name three molecules that form hydrogen bonds with themselves.: Water, ammonia and hydrogen fluoride (also ethanol, hydrazine).

Exam questions on Types of intermolecular force

Argon is a monatomic gas that can be liquefied at 87 K. Liquid argon is used as a coolant in some laboratory experiments.
Explain how an instantaneous dipole on one argon atom leads to an attraction between two argon atoms.2 marks
Chloromethane, CH₃Cl, and ammonia, NH₃, are both polar molecules. Chloromethane contains a polar C–Cl bond. Ammonia contains polar N–H bonds and a lone pair of electrons on the nitrogen atom.
Explain why ammonia molecules can form hydrogen bonds with each other but chloromethane molecules cannot.2 marks
Hydrogen fluoride, water and ammonia are all liquids or easily liquefied gases whose molecules attract each other by hydrogen bonds. A student is learning to recognise which other molecules can behave in the same way.
Describe how a hydrogen bond forms between two molecules of water.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).