10:Intermolecular Forces

Question 1

Order the intermolecular forces in order of strength

Weakest → Strongest

Answer 1

Induced dipole-dipole forces

Permanent dipole-dipole forces

Hydrogen bonds

Question 2

When do induced dipole-dipole forces occur

Answer 2

When random electron movement in one molecule cause an instantaneous dipole which in turn causes an induced dipole in the other molecule.

Question 3

Why do instantaneous dipoles occur

Answer 3

Random electron movements cause an excess of electron density on one side of the molecule.

Question 4

What affects the strength of induced dipole-dipole forces.

Answer 4

• The number of electrons in the molecule

- The surface area of the molecule

Question 5

What effect does symmetry have on dipoles

Answer 5

If the molecule is highly symmetrical then the individual dipoles may cancel out, so that even though there are polar bonds, the molecule as a whole is not polar.

Question 6

What is a hydrogen bond

Answer 6

The intermolecular attraction between hydrogen covalently bonded to N, O or F on one molecule and the lone pair on N, O or F in another molecule.

Question 7

What can hydrogen bonds be between

Answer 7

Hydrogen and either Fluorine, Oxygen or Nitrogen

Question 8

What anomalous properties does water have (unique properties)

Answer 8

Relatively high melting and boiling points
Water is less dense as ice than liquid state
High surface tension

Question 9

Why is water less dense as a solid than as a liquid

Answer 9

It has an open lattice structure- the hydrogen bonds keep all of the water molecules fixed in position, so on average the spaces between molecules are slightly larger than in water.