Dot/Cross diagrams, Intermolecular Forces

Question 1

What are the 3 main types of intermolecular forces>

Answer 1

• London forces
• Permanent dipole-permanent dipole bonds
• Hydrogen bonding

Question 2

What do London forces do?

Answer 2

They cause all atoms and molecules to be attracted to each other

Question 3

Describe how London forces come about:

Answer 3

• Electrons in charge clouds move quickly, causing the atom to have a temporary dipole
• Dipole can induce another dipole, which they are then attracted to each other
• Dipoles are constantly created and destroyed, but the overall effect is that the atoms are attracted to each other

Question 4

Describe how iodine molecules are held together in a lattice:

Answer 4

• Iodine atoms are held by strong covalent bonds to form I2 molecules
• Molecules are then held together by London forces

Question 5

Describe a simple molecular structure:

Answer 5

When atoms are held together by covalent bonds to form molecules which are held together by London forces

Question 6

Why do larger molecules have stronger London forces?

Answer 6

Larger electron clouds

Question 7

Why do solids with stronger London forces have higher melting and boiling points?

Answer 7

Because melting solids involves overcoming intermolecular forces

Question 8

How are alkane molecules held together?

Answer 8

By London forces

Question 9

Why do longer carbon chains have stronger london forces?

Answer 9

Because there is more surface contact and more electrons interact

Question 10

Why do branched alkanes have lower melting and boiling points?

Answer 10

They can’t pack as closely together and their molecular surface contact is small compared to straight chain alkanes

Question 11

Which molecule have permanent dipole-permanent dipole bonds?

Answer 11

Polar molecules

Question 12

What are permanent-dipole permanent dipole bonds?

Answer 12

Weak electrostatic forces of attraction between molecules

Question 13

Why do molecules with permanent dipole-permanent dipole bonds have higher melting and boiling points than those with just London forces?

Answer 13

Because they have London forces as well as permanent dipole-permanent dipole bonds

Question 14

What is the strongest type of intermolecular force?

Answer 14

Hydrogen bonding

Question 15

When is the only case that hydrogen bonding occurs?

Answer 15

When hydrogen is bonded to fluorine, nitrogen or oxygen

Question 16

Why can hydrogen bonding occur with only fluorine, nitrogen or oxygen?

Answer 16

They are very electronegative so can draw bonding electrons away from the hydrogen atom

Question 17

As hydrogen bonding is the strongest type of intermolecular force, what does this mean for the substances which can form hydrogen bonds?

Answer 17

They have high melting and boiling points

Question 18

Why does hydrogen flouride have the highest melting/boiling point out of the group 7 hydrides?

Answer 18

Can form hydrogen bonds, whereas the other group 7 hydrides cannot form hydrogen bonds

Question 19

After fluorine, why does the melting/boiling points of the group 7 hydrides increase as you go down the group?

Answer 19

More electrons means the strength of the London forces increases, which overides the decreasing strength of the permanent dipole-permanent dipole bonds

Question 20

Why does water have the highest melting/boiling point out of the group 6 hydrides?

Answer 20

Water is the only group 6 hydride which forms hydrogen bonds

Question 21

In terms of hydrogen bonds, explain why ice floats on water:

Answer 21

In ice, the water moleculesare arranged so that there is a maximum number of hdyrogen bonds, which ‘wastes’ lots of space

The effect is that ice is far less dense than water, therefore it floats

Question 22

Why do alcohols have higher melting/boiling points than similar alkanes?

Answer 22

They can form hydrogen bonds, which makes them stronger