1.5.4 Effects of Forces Between Molecules (Physical Chemistry)

Question 1

Hydrogen bonding in water, causes it to have

Answer 1

anomalous properties

Question 2

Hydrogen bonding in water, causes it to have anomalous properties such as

Answer 2

• high melting and boiling points
• high surface tension
• higher density in the liquid than the solid

Question 3

Water has high melting and boiling points due to

Answer 3

• the strong intermolecular forces of hydrogen bonding between the molecules in both ice (solid H2O) and water (liquid H2O)
• a lot of energy is therefore required to separate the water molecules and melt or boil them

Question 4

Hydrogen bonds are strong intermolecular forces which are harder to break causing water to have a higher melting and boiling point than would be expected for a molecule of such a small size diagram

Answer 4

Question 5

The graph below compares the

Answer 5

enthalpy of vaporisation of different hydrides

Question 6

What is enthalpy of vaporisation

Answer 6

energy required to boil a substance

Question 7

The enthalpy changes increase going from H2S to H2Te due to

Answer 7

the increased number of electrons in the Group 16 elements

Question 8

The enthalpy changes increase going from H2S to H2Te due to the increased number of electrons in the Group 16 elements, this causes an increase in

Answer 8

the instantaneous dipole - induced dipole forces (dispersion forces) as the molecules become larger

Question 9

Based on this, H2O should have a much lower enthalpy change (around 17 kJ mol-1), however, the enthalpy change of vaporisation is almost 3 times larger which is caused by

Answer 9

the hydrogen bonds present in water but not in the other hydrides

Question 10

The high enthalpy change of evaporation of water suggests that instantaneous dipole-induced dipole forces are not the only forces present in the molecule
– there are also strong hydrogen bonds, which cause the high boiling point graph

Answer 10

Question 11

Water has a high

Answer 11

surface tension

Question 12

Surface tension is

Answer 12

the ability of a liquid surface to resist any external forces (i.e. to stay unaffected by forces acting on the surface)

Question 13

The water molecules at the surface of liquid are

Answer 13

bonded to other water molecules through hydrogen bonds

Question 14

These molecules pull downwards

Answer 14

the surface molecules

Question 15

These molecules pull downwards the surface molecules causing

Answer 15

the surface of them to become compressed and more tightly together at the surface, this increases water’s surface tension

Question 16

The surface molecules are pulled downwards due to the hydrogen bonds with other molecules, whereas the inner water molecules are pulled in all directions diagram

Answer 16

Question 17

Solids are denser than their liquids as

Answer 17

the particles in solids are more closely packed together than in their liquid state

Question 18

The water molecules are packed into an

Answer 18

open lattice

Question 19

This way of packing the molecules and the relatively long bond lengths of the hydrogen bonds means

Answer 19

that the water molecules are slightly further apart than in the liquid form

Question 20

Therefore, ice has a lower density than

Answer 20

liquid water by about 9%

Question 21

The ‘more open’ structure of molecules in ice causes it to have a lower density than liquid water diagram

Answer 21