Cambridge International AS Level Chemistry
van der Waals’ forces
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increasing number of electrons (and protons) in the molecule increasing the number of contact points between the molecules – contact points are places where the molecules come close together.
Differences in the size of the van der Waals’ forces can be used to explain the trend in the enthalpy change of vaporisation and boiling points of the noble gases. Figure 4.35 shows how these vary with the number of electrons present. (The enthalpy change of vaporisation is the energy required to convert a mole of liquid into a mole of gas.) b
20
Kr Ar
0
Ne He 0
20 40 Number of electrons
electron movements result in temporary dipole
atom without dipole
δ+ δ – repulsion
δ– δ– δ+
δ– δ+
attraction
δ+ as atoms approach each other, the dipole on one atom induces a dipole on another
Figure 4.34 How van der Waals’ forces arise.
You can see that both the enthalpy change of vaporisation and the boiling points of the noble gases increase as the number of electrons increases. This is because the van der Waals’ forces between the atoms are increased with an increasing number of electrons. So, more energy is needed to change the liquid into vapour and the boiling point is higher. The effect of increasing the number of contact points can be seen by comparing the boiling points of pentane (boiling point 36 °C) and 2,2-dimethylpropane (boiling point 10 °C) (Figure 4.36). These compounds have equal numbers of electrons in their molecules. The molecules in pentane can line up beside each other so there are a large number of contact points. The van der Waals’ forces are higher, so the boiling point is higher. The molecules of 2,2-dimethylpropane are more compact. The surface area available for coming into contact with neighbouring molecules is smaller. The van der Waals’ forces are relatively lower, so the boiling point is lower. The van der Waals’ forces between individual atoms are very small. However, the total van der Waals’ forces between very long non-polar molecules such as poly(ethene) molecules (see page 211) can be much larger. That is why poly(ethene) is a solid at room temperature. Xe Kr
Xe 10
δ – δ+
–100
Boiling point / °C
a
Enthalpy of vaporisation / kJ mol–1
62
Noble gases such as neon and argon exist as isolated atoms. Noble gases can be liquefied, but at very low temperatures, so there must be very weak forces of attraction between their atoms. These weak forces keep the atoms together in the liquid state. Bromine is a non-polar molecule that is liquid at room temperature. The weak forces of attraction are keeping the bromine molecules together at room temperature. These very weak forces of attraction are called van der Waals’ forces. Van der Waals’ forces exist between all atoms or molecules. So, how do van der Waals’ forces arise? The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short moment, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ+ end of the dipole in one molecule and the δ– end of the dipole in a neighbouring molecule (Figure 4.34). These dipoles are always temporary because the electrons clouds are always moving. Van der Waals’ forces are sometimes called temporary dipole–induced dipole forces. Van der Waals’ forces increase with
60
Ar –200 Ne –270
He 0
20 40 Number of electrons
60
Figure 4.35 a Enthalpy changes of vaporisation and b boiling points of the noble gases plotted against the number of electrons present.