2.8 (a) a model of CO2
(b) model of H2O
Figure 2.23 The strong covalent bonds within molecules do not break easily.
Figure 2.24 Forces between molecules have to be overcome for a substance to boil. These are intermolecular forces.
Covalent bonds are about 100 times stronger than weak intermolecular forces. The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points. Substance molecules
CO
CO2
SO2
SO3
Melting point °C
–205
–55.6
–72
17
Boiling point °C
–191.5
–78.5
–10
45
2
Suggest why the boiling point of carbon dioxide, CO2, is higher than the boiling point of carbon monoxide, CO.
3
Propane, C3H8, has a melting point of –188 °C and butane, C 4H10, –140 °C. Suggest why.
4
Nitrogen has a very strong triple covalent bond but a very low boiling point of –195.8 °C. Explain why.
Predicting the properties of small molecules There are weak intermolecular forces between the molecules, which affect the physical properties of small molecules. • As they have weak intermolecular forces between the molecules they are easy to separate so the substances have low melting points. • As there are no charge carriers available (neither free electrons nor free ions) they do not conduct electricity. However, it is difficult to predict trends in boiling points and melting points as there are other factors involved. Boiling point and melting point not only depend on the size of the molecule but on other factors such as its shape. So CH4 (tetrahedral shape, at –164 °C) has a much lower boiling point than CO2 (linear, at –78.5 °C) as the molecules cannot get as near to one another. 5
Explain why pure water does not conduct electricity very well, but sea water does.
6
Pentane is a straight chain molecule. 2,2-dimethylpropane has exactly the same molecular formula but is more like a sphere. Explain which is likely to have the higher boiling point.
7
Fluorine, F2, has a boiling point of –188°C and chlorine, Cl2, a boiling point of –34°C. Suggest why they have different boiling points.
DID YOU KNOW? Predicting boiling points is not as easy as it seems as there are other forces at work too which depend on the shape of the molecule as well.
KEY INFORMATION The atomic mass of C is 12 and of S is 32.
Google search: 'intermolecular forces'
75054_P054_091.indd 71
71
5/24/16 8:51 PM