IB Chemistry: Intermolecular Forces

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4.3 Intermolecular Forces

4.3.1 Describe the types of intermolecular forces (attractions between molecules that have temporary dipoles ,permanent dipoles or hydrogen bonding) and explain how they arise from the structural features of molecules

One concept you should familiarize yourself with is that the definition of intermolecular forces is different to that of intramolecular forces. Intermolecular forces means the force that holds the bonds between the molecules, whilst intramolecular is the force that holds the bonds within the molecules. Confuse these two concepts, you’re going to lose a lot of marks.

Van Der Waal’s Forces

Theoretically, the electrons in chlorine’s covalent bond are shared equally. However, this is often not the case – electrons density may sometimes tend towards one side of the covalent bond more than the other.  This creates a temporary dipole.

This dipole is only temporary, so the intermolecular forces are weak. Consequently, the melting and boiling points are low – this is because the weak bonds between the molecules can be easily broken.

However, a trend is that as molecular mass increases in structures with Van Der Waal’s Forces, the boiling points and melting points increases. This is because the larger the molecular mass, the greater the interaction between the electrons, and hence the greater interaction between the forces.

An example would be the alkanes.

Alkane Molecular Mass Boiling point / °C
CH4 16 -164
C2H6 30 -89
C3H8 44 -42
C4H10 58 -0.5

As we can see, the trend is very noticeable – as molecular mass increases, so does boiling substance of the alkane.

Dipole-Dipole attraction

This type of bonding is caused by a permanent separation of charge due to differences in electronegativities- as a result, one atom will be slightly positive, the other slightly negative – a good example would be HCl.

Dipole-dipole-interaction-in-HCl-2D.png

As we can see, the Hydrogen is partially positively charged, the chlorine partially negatively charged. We call this a permanent dipole. Opposite charges attract, so the opposite charges on neigbouring molecules, as seen above, will attract each other, creating a much stronger force compared to Van Der Waal’s Forces called a dipole-dipole attraction.  

Hydrogen-Bonding

This is the strongest type of dipole-dipole bonding.

Hydrogen bonding is not really a bond – it’s more like a very strong force of attraction. Hydrogen is first covalently bonded to some very polar, very electronegative element such as oxygen, nitrogen or fluorine. Because of the difference in electronegativities (Hydrogen is not very electronegative), a pair of electrons is pulled out of hydrogen. However, hydrogen only has one pair of electrons, so now with no electrons shielding the nucleus and its relatively small size, it exerts a very strong attraction force on lone pair of electrons of the electronegative element (either oxygen, nitrogen or fluorine). This is hydrogen bonding.

The diagram below illustrates hydrogen bonding in water.

Hydrogen-bonding-in-water-2D.png
So, what’s happening above is that we have two hydrogen molecules floating around. As we can see with the oxygen in the left molecule and the hydrogen in the right molecule, the difference in electronegativities cause them to be strongly attracted. This is hydrogen bonding.

4.3.2 Describe and explain how intermolecular forces affect the boiling points of substances.

If we plot a graph below, with

 

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