(U1) Intermolecular Forces

Question 1

What does intermolecular mean?

Answer 1

Between neighbouring molecules

Question 2

What are Van Der Waals forces? (3)

Answer 2

• attraction between instantaneous
• and induced dipoles
• on neighbouring molecules

Question 3

What is the effect of molecule size on Van Der Waals?

What does this cause?

Answer 3

• Increased size of forces
• strength increases —> m.p. And b.p. Increase

Question 4

What is permanent dipole-dipole attraction? (4)

Answer 4

• attraction between the positive end (δ+)
• of the permanent dipole on a molecule
• with the negative end (δ-)
• of the permanent dipole of a neighbouring molecule

Question 5

What is a hydrogen bond? (4)

Answer 5

• the attraction between a lone pair of electrons
• of a very electronegative atom on a molecule
• and a hydrogen on a neighbouring molecule
• in which the hydrogen is covalently bonded to a very electronegative atom

Question 6

Name 3 very electronegative atoms

Answer 6

Nitrogen, Oxygen and Fluorine

Question 7

Between:

• hydrogen bonds,
• Van Der Waals forces and
• dipole-dipole forces

Which is strongest?

Answer 7

Hydrogen bonds

Question 8

Why do Van Der Waals forces occur in all molecules?

Answer 8

All molecules have electrons

Question 9

When is a molecule said to have a dipole?

Answer 9

• When it has a δ- and a δ+ end
• due to one atom being more electronegative than the other

Question 10

1. When a molecule has a difference in negativity but doesn’t contain N, O or F:
2. When N, O or F are present

What types of intermolecular force are present?

Answer 10

• Van der Waals
• Permanent dipole-dipole attraction

2.

• Van der Waals
• hydrogen bonds

Question 11

What type of molecules are present in:

1. Hydrogen bonds
2. Permanent dipole-dipoles
3. Van der Waals forces (induced dipole-dipoles)

Answer 11

1. Molecules containing hydrogen and a very electronegative atom i.e. N, O or F
2. Polar molecules
3. All molecules regardless of charge or electronegativity

Question 12

How are hydrogen bonds and permanent dipole-dipole attractions different? (3)

Answer 12

• Hydrogen bonds require a H atom and an N, O or F atom
• therefore the difference in electronegativity is greater in a hydrogen bond
• thus hydrogen bonds are stronger and require more energy to be broken

Question 13

What is the trend in the boiling point of hydrides in groups 5, 6 and 7? (6)

Answer 13

• as you descend the group, the boiling points increase
• due to the molecules having more electrons
• therefore the size of Van der Waals between molecules increases

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• however the first value in each group is an exception as there are also hydrogen bonds present between molecules
• hydrogen bonds require more energy than Van der Waals to overcome
• therefore the presence of both mean the boiling point is highest at the top of the groups