6.3 Intermolecular Forces & 6.4 Hydrogen Bonding

Question 1

What are intermolecular forces?

Answer 1

• Weak interactions between the dipoles of different molecules

Question 2

What are the 3 types of intermolecular forces?

Answer 2

• Induced dipole-dipole interactions (London forces)
• Permanent dipole-dipole interactions
• Hydrogen bonding

Question 3

What do intermolecular forces determine?

Answer 3

• Physical properties
• E.g. melting and boiling points

Question 4

What do covalent bonds determine?

Answer 4

• The identity of molecules
• Their chemical reactions

Question 5

List the 4 types of bonds in order of the weakest to the strongest.

Answer 5

• London forces
• Permanent dipole-dipole interactions
• Hydrogen bonding
• Single covalent bonds

Question 6

What are London forces/ induced dipole-dipole interactions? What else can they be called?

Answer 6

• They are weak, attractive forces between induced dipoles in different molecules
• They exist between all molecules, regardless of what other forces are present
• They can also be called dispersion forces

Question 7

How do London forces work?

Answer 7

• The natural fluctuation in electron density in atoms produces a changing dipole in a molecule
• At any moment, an instantaneous dipole exists, but its position is constantly changing
• The instantaneous dipole induces a dipole on a neighbouring molecule
• The induced dipole then induces further dipoles on neighbouring molecules, which then attract one another
• Induced dipoles are only temporary, so a dipole that was there may no longer exist shortly after, but this whole process would just happen all over again

Question 8

What affects the strength of London forces, and why?

Answer 8

• The more electrons there are in each molecule, the stronger the London forces
  This is because:
• The more electrons there are in each molecule, the larger the instantaneous and induced dipoles
• The stronger the induced dipole-dipole interactions
• The stronger the attractive forces

Question 9

How does the strength of London forces impact the melting/ boiling points of substances?

Answer 9

• The stronger the London forces are, the more energy is needed to overcome the intermolecular forces
• This increases the melting/ boiling point

Question 10

What are permanent dipole-dipole interactions?

Answer 10

• Attractive forces between the permanent dipoles in different polar molecules

Question 11

How are polar molecules bonded together within a substance, and how does this affect their melting/ boiling point?

Answer 11

• There are both London forces and permanent dipole-dipole interactions acting between the molecules
• As energy is needed to break the permanent dipole-dipole interactions as well as the London forces, polar molecules have higher melting/ boiling points

Question 12

What do simple molecules form in the solid state? Describe their structure.

Answer 12

• A simple molecular lattice
• Molecules are held together by weak intermolecular forces
• The atoms within each molecule are bonded together strongly by covalent bonds

Question 13

Describe the melting/boiling points of simple molecular lattices, and explain why they are like this.

Answer 13

• They have low melting/ boiling points
• This is because their weak intermolecular forces can be broken even with a small amount of energy

Question 14

Are non-polar simple molecular substances soluble in non-polar solvents, and why?

Answer 14

• They usually are
• This is because intermolecular forces form between the molecules and the solvent
• These interactions weaken the intermolecular bonding in the simple molecular substance until they break, so the compound dissolves

Question 15

Are non-polar simple molecular substances soluble in polar solvents?

Answer 15

• They usually aren’t
• When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the molecules in the solvent
• This is because the bonding within the polar solvent is too strong to be broken

Question 16

Do polar simple molecular substances dissolve in polar solvents?

Answer 16

• They do, similarly to how an ionic lattice dissolves in water
• However, this depends on the strength of the dipole and can be hard to predict

Question 17

What makes certain biological molecules capable of dissolving in both polar and non-polar solvents?

Answer 17

• They have hydrophilic and hydrophobic parts
• The hydrophilic part would be polar (and usually contain oxygen) and would be able to interact with water
• The hydrophobic part would be non-polar (and made of a carbon chain)

Question 18

Can simple molecular substances conduct electricity? Why?

Answer 18

• They have no moving, charged particles
• Nothing can therefore complete an electrical circuit, so they don’t conduct electricity

Question 19

What is a hydrogen bond? Required.

Answer 19

• A special type of permanent dipole-dipole interaction found between molecules containing:
• an electronegative atom (such as F, O and N) with a lone pair of electrons
• a hydrogen atom bonded to an electronegative atom

Question 20

How are hydrogen bonds drawn?

Answer 20

• They are shown with a dashed line
• The line is drawn between the lone pair of electrons on the electronegative atom and the hydrogen atom on the other molecule
• The dipoles are drawn on the bonded atoms
• You then label the hydrogen bond

Question 21

Why does water have anomalous properties?

Answer 21

• It has hydrogen bonds between its molecules

Question 22

What is special about water’s density, and why does it have this property?

Answer 22

• Water in the solid form (ice) is less dense than water in the liquid form
• This is because hydrogen bonds hold water molecules in the solid state far apart in an open lattice structure

Question 23

What is special about the melting and boiling points of water, and why are they notable?

Answer 23

• They are relatively high
• This is because hydrogen bonds are the strongest type of intermolecular force, so a lot more energy is needed to overcome them when compared to London forces, for example

Question 24

What are 2 other special properties of water, and why does it have them?

Answer 24

• High surface tension
• High viscosity
• It has hydrogen bonding

Question 25

Why might some substances with hydrogen bonding have higher melting/boiling points than others?

Answer 25

• The more hydrogen bonds a molecule can form, the higher its melting/ boiling point