Intermolecular Forces

Question 1

Intermolecular forces…

Answer 1

break/weaken when substances change phase

Question 2

Strength and nature of intermolecular force determines…

Answer 2

physical properties such as melting and boiling points

Question 3

The stronger the intermolecular force

Answer 3

The more energy is required to overcome them and thus higher boiling point

Question 4

Intermolecular Forces are a type of

Answer 4

Van der Waal’s force

Question 5

Types of Intermolecular forces

Answer 5

• Dipole-dipole
• Induced Dipole (London forces)
• Hydrogen bonding

Question 6

Strength of intermolecular forces

Answer 6

London forces < Dipole-Dipole forces (intermediate strength) < Hydrogen Bonding

Question 7

Induced Dipole Force

Answer 7

• Very weak temporary forces
• Temporary dipole can induce (create) dipole in neighbouring molecules and interaction between opposing poles occur
• Strong enough to allow liquification of He₂, N₂ , H₂ at low temperature

Question 8

Induced Dipole Forces occur when

Answer 8

A non-polar molecule forms a temporary dipole (by chance) and induces a temporary dipole in adjacent molecule (temporary attract)

Question 9

Solids with London Forces:

Answer 9

• Carbon Dioxide (CO₂)
• Iodine (I₂)

Question 10

Factor affecting strength of London Forces

Answer 10

Larger atom/molecules: more electrons, easier to form temporary dipole (larger in size). Stronger force

Question 11

Hydrogen Bonds

Answer 11

• Special form of dipole-dipole forces, much stronger
• Strongest type of intermolecular force

Question 12

Hydrogen Bonding occurs

Answer 12

Between molecules where hydrogen is bonded to F, N or O (all of which are small and highly electronegative)

Question 13

Conditions for hydrogen bonding

Answer 13

• Molecule must contain highly electronegative atom (N, O, F) linked to hydrogen atom
• Size of electronegative atom should be small (the smaller, the greater electrostatic attraction)
• Highly electronegative atom must have at least one lone pair

Question 14

What constitutes a hydrogen bond:

Answer 14

Negative end of one molecule with highly electronegative atom attracts the positive end of adjacent atom (strong polarity)

Question 15

Water molecules

Answer 15

• 4 Hydrogen bonds
• Each lone pair makes hydrogen bond

Question 16

Ammonia molecule

Answer 16

• 2 Hydrogen bonds
• Only one lone pair
• In group of ammonia molecules there aren’t enough lone pairs to to go around to satisfy all the hydrogen
• On average each ammonia molecule forms one hydrogen bond using it’s lone pair and one involving one of its δ+ hydrogens (other H’s wasted)

Question 17

Hydrogen fluoride molecule

Answer 17

• 2 Hydrogen bonds
• Shortage of hydrogens
• On average, each molecule can only form one hydrogen bond using its δ+ hydrogen and one involving one of its lone pairs (other lone pairs wasted)

Question 18

Explain why the boiling point of ammonia is so much higher than phosphine

Answer 18

• Ammonia has hydrogen bonds between its molecules
• Phosphine has dipole-dipole attraction between its molecules
• Hydrogen bonds are stronger than dipole-dipole attraction
• More energy is needed to overcome the stronger hydrogen bonds and therefor ammonia has a higher boiling point than phosphine

Question 19

Explain why the boiling point of (F₂) is lower than (Br₂)

Answer 19

• Both F₂ and Br₂ have weak London forces
• F₂ is a smaller atom with less electrons
• F₂ forms temporary dipole less readily and thus has weaker London forces than Br₂
• Less energy is needed to break the water intermolecular force between F₂ atoms and thus F₂ has a lower boiling point than Br₂

Question 20

Explain why the boiling point of water is higher than ammonia

Answer 20

• Both water and ammonia have hydrogen bonds between their molecules
• Water can form 4 hydrogen bonds while ammonia can only form 2 hydrogen bonds
• Therefore water has stronger intermolecular force
• More energy is needed to overcome the stronger intermolecular force in water. Therefore water has a higher boiling point than ammonia

Question 21

Properties of Water

Answer 21

• When water freezes, each water molecule makes 4 H bonds with adjacent water molecules (resulting rigid, open lattice structure)
• Open lattice structure accounts for ice being less dense than water
• Water molecules slightly closer together (liquid phase)
• Strength of H bond accounts for unusually high BP (allows it to be liquid)