Intermolecular forces

Question 1

What are London/van der Waals forces? How does strength vary with electrons and branching?

Answer 1

London forces act as an induced dipole between molecules

How strength varies:
* Higher number of electrons = stronger London forces, because dipoles can be larger
* Straighter chain molecules = stronger london forces, because there is more surface area contact, and can pack closer together

Question 2

How do London forces form between molecules?

Answer 2

1. There is an uneven distribution of electrons
2. resulting in an instantaneous dipole being set up
3. this induces a dipole on the other molecule
4. the molecules are attracted to each other because of the difference in charge

Question 3

What are permenant dipole forces?

Answer 3

between polar molecules, where different atoms of that molecule have different delta charges

the delta positive and delta negative regions of neighbouring polar molecules attract each other and hold the molecules together in a lattice-like structure

Question 4

What are hydrogen bonds?

Answer 4

A hydrogen bond is a particularly strong permenant dipole-permenant dipole attraction between the lone pair of electrons on a very electronegative atom (N, O or F) and a hydrogen atom directly covalently bonded to another very electronegative N, O or F atom

Question 5

Explain hydrogen bonding in H2O, NH3, and HF

Answer 5

Hydrogen bonds only act between hydrogen and the most electronegative atoms:
* Nitrogen
* Oxygen
* Fluorine

The lone pair of these atoms form a bond with a delta positive hydrogen from another molecule

Question 6

Water’s anomalous properties: why high melting and boiling temperature?

Answer 6

Despite water having weakish London forces between molecules due to low electron number,

Water has hydrogen bonds between molecules, which are stronger than both london and permenant dipole forces, requiring more energy to overcome

Question 7

Water’s anomalous properties: density of ice compared to water?

Answer 7

• Hydrogen bonds between water molecules in ice hold it in a lattice structure, with more space between molecules
• H-bonds are also longer than covalent bonds so ice is less dense than water

Question 8

Predicting the presence of hydrogen bonding in molecules

Answer 8

Criteria for hydrogen bonding:
* H and either O, N or F
* O, N or F have lone pairs
* the molecules involved are polar

Question 9

Trends in the boiling temperatures of alkanes with increasing chain length

Answer 9

• Alkanes with increased chain length have more electrons per molecule
• Meaning the London forces between these molecules are stronger
• Requiring more energy to overcome

Question 10

What is the effect of branching in the carbon chain on the boiling points of alkanes?

Answer 10

• Branching of alkane chains weakens the London forces between molecules because they are less able to pack tightly together
• The distance over which the London forces act is increased, weakening them

Question 11

Why do alcohols have higher boiling temperatures compared to alkanes with similar numbers of electrons?

Answer 11

Alcohols have hydrogen bonds between molecules, alkanes do not

hydrogen bonds are much stronger and require more energy to overcome

Question 12

Trends in boiling temperatures of hydrogen halides (HF –> HI)

Answer 12

• HF has the highest b.pt because it has hydrogen bonds between its molecules
• HCl –> HI, b.pt increases because the number of electrons in each molecule increase and so do the London forces

Question 13

What is hydration?

Answer 13

Hydration is the process of adding water molecules to an ion to form a hydrated ion

Question 14

Why is water chosen to dissolve ionic compounds? (In terms of hydration)

Answer 14

• Because of its hydrogen bonding capabilities, it can dissolve some ionic compounds by solvating the individual ions

Question 15

Why is water chosen to dissolve simple alcohols? (In terms of hydrogen bonding)

Answer 15

water can dissolve some alcohols by forming hydrogen bonds with their hydroxyl group

Question 16

Why is water a bad solvent for compounds (including polar molecules such as halogenoalkanes) (In terms of inability to form hydrogen bonds)

Answer 16

• both water and alcohols are poor solvents for the dissolving of some polar molecules such as halogenoalkanes that cannot form hydrogen bonds
• non-aqueous solvents are used for compounds with the same type of intermolecular force

Question 17

Why can solvents dissolve a solute with the same type of intermolecular force between their molecules?

Answer 17

The energy released when new London forces are formed would be greater than the energy required to seperate solvent molecules’ and solute particles’ London forces

Question 18

Why are polar solutes less soluble in non-polar solvents and vice versa?

Answer 18

Energy released when new London forces are formed would be less than the energy required to seperate hydrogen bonds/permenant dipole forces between molecules

Question 19

For what kind of intermolecular-force-compounds are non aqueous sovents used?

Answer 19

Non aqueous solvents are used for compounds with the same type of intermolecular force