Intermolecular Forces

Question 1

When does hydrogen bonding occur?

Answer 1

Between a molecule that contains a hydrogen atom attached to fluorine, oxygen or nitrogen with a lone pair of electrons and a polarised molecule containing hydrogen

Question 2

What is the bond angle in hydrogen bonding?

Answer 2

180°

Question 3

Why does hydrogen bonding occurs?

Answer 3

The lone pair of electrons is attracted to the exposed nucleus of the hydrogen atom

Question 4

What sort of intermolecular force is the strongest?

Answer 4

Strongest
Hydrogen bonding
Dipole dipole interactions
London forces

Can affect physical properties eg boiling temperature

Question 5

What is the trend in boiling temperatures in the alkene? Why?

Answer 5

Increase as chain length increases because London forces increase as the chain then increases so there are more places for the forces to operate
Branching molecules have lower melting temperatures because sidechains interfere with the packing together of the molecules, increasing the distance between the molecules which reduces the intermolecular forces

Question 6

What does volatility mean?

Answer 6

How easily molecules escape from a liquid

A volatile liquid has a lower melting point

Question 7

Do alkanes or alcohols have melting points? Why?

Answer 7

Alcohols

London forces are present in both but alcohols also form hydrogen bonds and dipole dipole interactions

Question 8

Why do hydrogen bonds form in alcohols?

Answer 8

The oxygen atom is negatively polarised so is attracted to the positively polarised hydrogen atom of another molecule

Question 9

What is insolvent cold when it doesn’t dissolve in water?

Answer 9

Non-aqueous solvent

Question 10

What does solubility mean?

Answer 10

The mass of the sun you to that results in 100 g of solvent at a particular temperature

Question 11

What are the general rules of solubility?

Answer 11

Highly polarised solids such as ionic salts dissolve in polar solvents but not in a non-polar solvent
Organic substances dissolved in water that not hexane
Nonpolar substances don’t dissolve in water but do in hexane
Nonpolar liquids are miscible
Polar liquids and water are miscible
A polar liquid and nonpolar liquid are immiscible

Question 12

What happens as an ionic solid dissolves in water?

Answer 12

The polarised water molecules are attracted to ions within the lattice, spreading individual ions throughout the water

Question 13

What is hydration?

Answer 13

The process of water molecules arranging themselves around an ion

Question 14

What is hydration enthalpy?

Answer 14

The energy released when a substance dissolves in water
It will be an hour to balance the energy needed to break the lattice
If there is insufficient energy a substance is insoluble in water

Question 15

Why alcohol soluble in water?

Answer 15

They have polar hydroxide groups that can hydrogen bond to the water molecules

Question 16

How does the solubility of the alcohols change with increasing carbon chain length?

Answer 16

Decreases because the smaller proportion of the molecule is polar

Question 17

Why are nonpolar substances in soluble in water?

Answer 17

The forces between the hexane molecules are much weaker than the hydrogen bonds between the water molecules so they cannot disrupt the water structure

Question 18

Why do organic liquids mix?

Answer 18

Only weak London forces throughout the mixture

Question 19

What does the size of London forces depend on?

Answer 19

The size of the electron clouds
Larger atoms don’t hold onto their outer electron so tightly, allowing them to be deformed more easily so instantaneous dipoles and induced dipoles are more likely