The Ideal Gas

In the earlier chapter properties of an ideal gas were discussed. It was also mentioned that any gas at very low pressure when its density is very small, may be treated as an ideal gas. It essentially means that the laws of ideal gas may be applied to any gas when much larger volume of space is available to each gas molecule in comparison to its size. It is, however, evident that an ideal gas is only a conceptual gas that does not exist in real terms.

Difference between an Ideal Gas and The Real Gas

Finite molecular size and attraction between molecules

A real gas differs from an ideal gas in two respects: first, the molecules of a real gas are not point particles but have a finite size. This means that the actual volume available to the gas molecules for their motion is restricted by the amount of the volume occupied by the molecules themselves. Secondly, in the case of a real gas the gas molecules attract each other. The force of molecular attraction, called the Van der Waals force, originates from the net electrostatic force of attraction between the electron cloud of one constituent atom and the nucleus of the other atom of the molecule minus the force of repulsion between the nuclei and the electron clouds of the atoms in the molecule. Since electrostatic forces have infinite range, the net electrostatic force of attraction exceeds beyond the molecular dimensions. The Van der Waals force may also be looked as the net resultant of the forces of attraction between the electron cloud of one molecule and the nuclei of the other molecule and the forces of repulsion between the electron clouds and nuclei of the two molecules as shown in Fig. 2.1. This leaked or residual force of attraction (Van der Waals force) is responsible for the molecular attraction in real gases. Since an attractive force may be derived from a negative potential, the potential between two real gas molecules may be represented as shown in Fig. 2.2.