Chapter 4: Chemical bonding
Some compounds may have higher boiling points than expected. This can be due to hydrogen bonding. Figure 4.43 shows a graph of the boiling points of the hydrogen halides, HF, HCl, HBr and HI, plotted against the position of the halogen in the Periodic Table.
Boiling point / °C
+50 0 –50
–100 HF
HCl
HBr
HI
Figure 4.43 The boiling points of the hydrogen halides.
The rise in boiling point from HCl to HI is due to the increasing number of electrons in the halogen atoms as we go down the group. This leads to increased van der Waals’ forces as the molecules get bigger. If hydrogen fluoride only had van der Waals’ forces between its molecules, we would expect its boiling point to be about –90 °C. However, the boiling point of hydrogen fluoride is 20 °C, which is much higher. This is because of the stronger intermolecular forces of hydrogen bonding between the HF molecules.
The peculiar properties of water 1 Enthalpy change of vaporisation and boiling point
Water has a much higher enthalpy change of vaporisation and boiling point than expected. This is due to its extensive hydrogen bonding. Figure 4.44 shows the enthalpy changes of vaporisation of water and other Group 16 hydrides. The rise in enthalpy change of vaporisation from H2S to H2Te is due to the increasing number of electrons in the Group 16 atoms as we go down the group. This leads to increased van der Waals’ forces as the molecules get bigger. If water only had van der Waals’ forces between its molecules, we would expect its enthalpy change to be about 17 kJ mol–1. But the enthalpy change of vaporisation of water is much higher. This is because water is extensively hydrogen bonded. The boiling point of water is also much higher than predicted by the trend in boiling points for the other Group 16 hydrides. This also indicates that much more energy is required to break the bonds between water molecules compared with other hydrides of Group 16 elements. 65
50 H2O Enthalpy change of vaporisation / kJ mol–1
How does hydrogen bonding affect boiling point?
40
30 H2Te H2S
20
H2Se
QUESTION 13 The table lists the boiling points of some Group 15 hydrides.
Hydride
Boiling point / °C
ammonia, NH3
–33
phosphine, PH3
–88
arsine, AsH3
–55
stibine, SbH3
–17
a Explain the trend in the boiling points from phosphine to stibine. b Explain why the boiling point of ammonia does not follow this trend.
10
0
20
40
60 80 100 Number of electrons
120
140
Figure 4.44 Enthalpy changes of vaporisation for Group 16 hydrides plotted against number of electrons present.
2 Surface tension and viscosity
Water has a high surface tension and high viscosity. Hydrogen bonding reduces the ability of water molecules to slide over each other, so the viscosity of water is high. The hydrogen bonds in water also exert a significant downward force at the surface of the liquid. This causes the surface tension of water to be higher than for most liquids.