Intermolecular Forces

Question 1

What are intermolecular forces? (1 mark)

Answer 1

⦾ Forces between molecules (1)

Question 2

What are the 3 types of IMFs? (3 marks)

Answer 2

1. VWFs (induced dipole-dipole) (1)
2. Permanent dipole-dipole (1)
3. Hydrogen (1)

Question 3

What are Van der Waals forces? (1 mark)

Answer 3

⦾ Cause all atoms + molecules to be attracted to each other (1)

Question 4

Explain how Van der Waals forces are created? (8 marks)

Answer 4

⦾ e- in charge clouds move v. fast (1)
⦾ any moment, more likely to be one side (1)
⦾ (creates) temporary dipole (1)
⦾ causes another temporary dipole in opposite direction on neighbouring atom (1)
⦾ 2 dipoles attracted (1)
⦾ Continues (chain reaction) (1)
⦾ Dipoles created + destroyed all the time (1)
⦾ Overall atoms attracted to each other (1)

Question 5

Explain (fully) the forces that hold iodine molecules together? (4 marks)

Answer 5

⦾ Iodine atoms in molecule held in pairs via strong covalent bonds (1)
⦾ molecules held in molecular lattice arrangement (1)
⦾ via weak VWFs (1)
⦾ crystal stucture (1)

Question 6

What are the factors that affect Van der Waals forces strength? How? (4 marks)

Answer 6

⦾ Bigger molecules have bigger electron clouds - stronger VWFs (2)
⦾ Molecule shape - long + straight lie closer than branched - stronger VWFs between (2)

Question 7

What happens to VWFs when you boil a liquid? (2 marks)

Answer 7

⦾ need to overcome IMFs (1)

⦾ so particles escape liquid surface (1)

Question 8

Explain why liquids with stronger VWFs have higher boiling points? (1 mark)

Answer 8

⦾ More energy needed to overcome stronger VWFs (1)

Question 9

What other properties can VWFs affect? (2 marks)

Answer 9

⦾ Melting point (1)

⦾ Viscosity (1)

Question 10

What is the trend in strength of VWFs and boiling points down the noble gases? (3 marks)

Answer 10

⦾ no. of e- increase (1)
⦾ VWFs increase (1)
⦾ bp. increases (1)

Question 11

What is the trend in VWFs and boiling points as alkane chains get longer? (4 marks)

Answer 11

⦾ no. of e- increase (1)
⦾ area over which VWFs can act increases (1)
⦾ therefore stronger (1)
⦾ bp. increases (1)

Question 12

What are permanent dipole-dipole forces? (2 marks)

Answer 12

⦾ weak electrostatic forces of attraction (1)

⦾ between 𝛿+ and 𝛿− charges on neighbouring molecules (1)

Question 13

What would happens when an electrostatically charged rod is placed next to a jet of polar liquid? e.g. water
(1 mark)

Answer 13

⦾ Liquid moves towards rod (1)

Question 14

Explain how the liquid is able to move towards the electrostatically charged rod? (1 mark)

Answer 14

⦾ Polar liquids contain molecules with permanent-dipoles (1)

Question 15

Explain why it doesn’t matter if the rod is +vely or -vely charged as to why the polar liquid is still able to move? (2 marks)

Answer 15

⦾ Polar molecules in liquid turn around (1)

⦾ (so) oppositely charged end attracted to rod (1)

Question 16

What effect does increased polarity of the liquid have on the attraction between the rod and the jet? (2 marks)

Answer 16

⦾ Stronger electrostatic attraction between rod + jet (1)

⦾ greater deflection (1)

Question 17

What is the effect on non-polar liquids when placed near charged rod? (1 mark)

Answer 17

⦾ no effect (1)

Question 18

What is Hydrogen bonding? (1 mark)

Answer 18

⦾ When H covalently bonded to F, N or O (1)

Question 19

How do F, N and O bond with hydrogen? (2 marks)

Answer 19

⦾ V. electronegative (1)

⦾ draw bonding e- away from H atom (1)

Question 20

Explain how Hydrogen bonding works? (3 marks)

Answer 20

⦾ Polarised bond (1)
⦾ H’s v. small so v. high charge density (1)
⦾ forms weak bonds with lone e- pairs on F, N or O (1)

Question 21

Explain how hydrogen bonding works in water?

2 marks

Answer 21

⦾ H bonding between (1)

⦾ 𝛿+H and lone e- pair on 𝛿−O (1)

Question 22

Explain how hydrogen bonding works in ammonia?

2 marks

Answer 22

⦾ H bonding between (1)

⦾ 𝛿+H and lone e- pair on 𝛿−N (1)

Question 23

State and explain the effect of hydrogen bonding on properties of substances? (3 marks)

Answer 23

⦾ Higher mp.s/bp.s (1)
⦾ (because) extra energy needed (1)
⦾ (to) break H bonds (1)

Question 24

What is an unusual property of water? (1 mark)

Answer 24

⦾ Ice less dense than liquid water (1)

Question 25

Explain the effect of hydrogen bonding when liquid water cools to form ice? (4 marks)

Answer 25

⦾ Molecules make more H bonds (1)
⦾ arrange themsleves into regular lattice structure (1)
⦾ increased distance between molecules as ice compared to liquid (due to H bonding) (1)
⦾ (therefore) ice less dense than water (1)

Question 26

What are the 3 physical properties of simple covalent compounds? (3 marks)

Answer 26

1. Electrical conductivity (1)
2. Melting point (1)
3. Solubility (1)

Question 27

Explain the 3 physical properties of simple covalent compounds? (6 marks)

Answer 27

Electrical conductivity:
⦾ don’t conduct electricity (1)
⦾ (because) no free ions or e- to carry charge (1)

Melting point:
⦾ Low mp.s (1)
⦾ (because) weak forces between molecules easily broken (1)

Solubility:
⦾ Some dissolve in water (1)
⦾ (depending) on how polarised molecules are (1)

Question 28

What is the main factor that determines the melting/boiling point of a substance?

Answer 28

⦾ Strength of induced dipole-dipoleforces

Question 29

What are the 2 main factors that affect the overall strength of intermolecular forces:

⦾ Down group 7 hydrides from HCl to HI?

(4 marks)

Answer 29

⦾ Polarity of molecules decreases (1)
⦾ (so) strength of permanent dipole-dipole interactions decrease (1)

⦾ No. of e- increases in molecule (1)
⦾ (so) strength of induced dipole-dipole interactions increases (1)

Question 30

How are the boiling points affected:

⦾ Down group 7 hydrides from HCl to HI?

(3 marks)

Answer 30

⦾ Increases (1)
⦾ Increased strength of induced dipole-dipole interactions (1)
⦾ (has) greater effect on bp. than decreasing strength of permanent dipole-dipole interactions (1)

Question 31

What is the strength of induced dipole-dipole interactions for 2 molecules with similar number of electrons? (1 mark)

Answer 31

⦾ Similar (1)

Question 32

Which group 7 hydride has the highest boiling point?

1 mark

Answer 32

⦾ HF (1)