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Intermolecular Forces Chemistry Tutorial

Key Concepts

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Types of Intermolecular Forces

Three types of force can operate between covalent molecules:

  1. Dispersion Forces: also known as
    ⚛ London Forces (named after Fritz London who first described these forces theoretically 1930)
    ⚛ Weak Intermolecular Forces
    ⚛ van der Waal's Forces2 (namd after the person who contributed to our understanding of non-ideal gas behaviour).
  2. Dipole-dipole interactions
  3. Hydrogen bonds

1. Dispersion Forces (London Forces, Weak Intermolecular Forces, van der Waal's Forces):

2. Dipole-dipole Interactions

3. Hydrogen Bonds

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Effect of Intermolecular forces on Melting Points and Boiling Points of Molecular Covalent Substances

Since melting or boiling result from a progressive weakening of the attractive forces between the covalent molecules, the stronger the intermolecular force is, the more energy is required to melt the solid or boil the liquid.

If only dispersion forces are present, then the more electrons the molecule has (and consequently the more mass it has) the stronger the dispersion forces will be, so the higher the melting and boiling points will be.

Consider the hydrides of Group 14 elements, all of which are non-polar molecules, so only dispersion forces act between the molecules.

The Group 14 hydrides are:
    CH4 (molecular mass ≈ 16)
    SiH4 (molecular mass ≈ 32)
    GeH4 (molecular mass ≈ 77)
    SnH4 (molecular mass ≈ 123)
    All of these molecules can all be considered non-polar covalent molecules.

As the mass of the molecules increases, so does the strength of the dispersion force acting between the molecules.

As the strength of the dispersion forces acting between the molecules increases, more energy is required to weaken the attraction between the molecules resulting in higher boiling points.

So, as the mass of the Group 14 hydride molecules increase, the boiling point of the substance increases as shown by the graph below:

If a covalent molecule has a permanent net dipole then the force of attraction between these molecules will be stronger than if only dispersion forces were present between the molecules.
As a consequence, this substance will have a higher melting point or boiling point than similar molecules that are non-polar in nature.

Consider the boiling points of the hydrides of Group 17 (halogen) elements.

The hydrides of Group 17 (halogen) elements are:
    HF (molecular mass ≈ 20)
    HCl (molecular mass ≈ 37)
    HBr (molecular mass ≈ 81)
    HI (molecular mass ≈ 128)

All of these molecules are polar, the hydrogen atom having a partial positive charge (Hδ+) and the halogen atom having a partial negative charge (Fδ-, Clδ-, Brδ-, Iδ-).

As a consequence, the stronger dipole-interactions acting between the hydride molecules of Group 17 elements results in higher boiling points than for the hydrides of Group 14 elements as seen above.

The boiling point of the hydrides of Group 17 elements is shown in the graph below:

With the exception of HF, as the molecular mass increases, the boiling point of the hydrides increase.
HF is an exception because of the stronger force of attraction between HF molecules resulting from hydrogen bonds acting bewteen the HF molecules.
Weaker dipole-dipole interactions act between the molecules of HCl, HBr and HI.
So HF has a higher boiling point than the other molecules in this series.

There is a full discussion of the effect of intermolecular forces on the group 16 hydrides in the Trends in Group 16 Hydrides tutorial.

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Effect of Intermolecular Forces on Solubility

In general like dissolves like:

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1. Do not confuse the terms "intermolecular" forces and "intramolecular" forces.
Intermolecular forces act between discrete molecules.
Intramolecular forces act between the atoms making up the molecule (known as covalent bonds)

2. Some Chemists refer to all intermolecular forces as Van der Waal's forces, others use the term Van der Waal's forces synonymously with London forces or dispersion forces.
It is probably best to avoid using the term Van der Waal's forces at all and use one of the other, unambiguous, terms instead.

3. A discussion of how polar covalent molecules and ionic compounds dissolve in water can be found in the tutorial Water as a Solvent.