What are intermolecular forces?
Forces between molecules are of electromagnetic origin.All molecules contain electric charges in motion.These molecules are electrically neutral in the sense that the negative charge of the electron is equal and opposite charge to the positive charge of the nuclei.This does no mean,however that the molecules do not interact electrically.For example when two molecules approach each other the charges are distributed from their usual position in such a way that the average distance between opposite charges in the two molecules is a little smaller than that between like charges.Hence an attractive intermolecular forces results.These forces appear only when molecules are fairly close to each other.Thus these forces are short range forces.The molecules repel each other because there is no way for a molecule to rearrange itself internally to prevent repulsion of the adjacent external electrons.
Graphical description of intermolecular forces:
We can describe intermolecular forces graphically by considering the molecules spherically symmetrical.Figure shows how potential energy of two molecules and the force between them changes with their separation.Here we can imagine one molecule to be fix at O.The force at any point is found from F =-dU/dr ,where U is the potential energy.Two forces act between the molecules:
- The repulsive force which predominates at short distances
- The attractive force which predominates at long distances
We can see from the graph that when the molecules are close to each other the repulsive force predominates,while at greater distances the attractive force is larger.The resultant force is:
- Repulsive from O to M
- Attractive from M to B but increasing with distance
- Attractive from B to infinity but decreasing with distance.
There is a position where the two forces balance,shown by M on the graph.This is the equilibrium position for molecules in the solid.
The potential energy is a minimum at this point.The separation distance between the two molecules at which the mutual potential energy is zero is called distance of closest approach.Any disturbance from this position would produce a force tending the return the molecule to M.The force of attraction between the molecules increases as the molecules are separated from M to B.The breaking point is at B,since beyond this point the force of attraction decreases with increasing separation.
For a molecule to be completely separated from its neighbour it must gain an amount of energy F,represented by CM on the diagram.The latent heat of vaporization for the two molecules is CM when there is no residual attractive force.This length also represents the latent heat of vaporization for the whole material.In a solid the distance OM is some 2-3 ×10-10m and you can see that around this point the force between the molecules varies approximately linearly with distance.