Why do real gas deviate from ideal behaviour ?
Real gas obey ideal law if pressure is low and temperature is high, but Real gas deviate from ideal behaviour if pressure is high and temperature is low because the assumption of ideal behaviour of gas is based on the kinetic molecular theory of gas,
1. A gas consists of large number of minute particles, called molecules. The molecules are so small that their actual volume is a negligible fraction of the total volume ( space ) occupied by the gas.
2. The molecules are in a state of rapid motion in all possible direction, colliding in a random manner with one another and with the walls of the vessel.
3. The molecular collision are perfectly elastic so that there is no loss of energy when gas molecule is collide with each other or against wall of the vessel.
4. There are no attractive force between molecule or between molecule and wall of the vessel in which gas molecule is contained. The molecule move completely independent of one another.
In order to explain deviation from ideal behaviour, It is necessary to modify the two assumption of kinetic theory of gas.
The following two assumption of kinetic theory do not appear to hold good under all conditions :
1. The volume occupied by the molecule themselves is negligibly small as compared to the total volume occupied by gas molecule :-
This postulates can be justified only under ordinary condition of temperature and pressure. It can be shown by calculations that in some of the common gases , the volume occupied by the molecules themselves, under ordinary condition, is only 0.014 per cent of the total volume of the gas. This is negligible fraction indeed. but if the pressure becomes too high the total volume of the gas will decrease appreciably whereas the volume of the molecule will remain constant because the molecule are incompressible. Hence, under condition of high pressure, the volume occupied by gas molecule will no longer be negligible in comparison with the total volume of the gas.
The same thing happens when the temperature is lowered to a large extent. The total volume of the gas decreases considerably, no doubt, but the volume occupied by the molecule themselves remains practically the same. In this case, too, the volume occupied by the molecules will no longer be negligible. Therefore is not valid at high pressure and low temperatures.
2. The force of attraction between gas molecule is negligible :-
This assumption is valid at low pressure or high temperature becauses under these conditions the molecule lie far apart from one another. But at high pressure or low temperatures, the volume is small and molecule lie closer to one another. The intermolecular force of attraction therefore are appreciably and cannot be ignored. Hence this assumption does not hold under condition of high pressure and low temperature.
Assumption of Kinetic molecular theory of gas :
1. A gas consists of large number of minute particles, called molecules. The molecules are so small that their actual volume is a negligible fraction of the total volume ( space ) occupied by the gas.
2. The molecules are in a state of rapid motion in all possible direction, colliding in a random manner with one another and with the walls of the vessel.
3. The molecular collision are perfectly elastic so that there is no loss of energy when gas molecule is collide with each other or against wall of the vessel.
4. There are no attractive force between molecule or between molecule and wall of the vessel in which gas molecule is contained. The molecule move completely independent of one another.
In order to explain deviation from ideal behaviour, It is necessary to modify the two assumption of kinetic theory of gas.
The following two assumption of kinetic theory do not appear to hold good under all conditions :
1. The volume occupied by the molecule themselves is negligibly small as compared to the total volume occupied by gas molecule :-
This postulates can be justified only under ordinary condition of temperature and pressure. It can be shown by calculations that in some of the common gases , the volume occupied by the molecules themselves, under ordinary condition, is only 0.014 per cent of the total volume of the gas. This is negligible fraction indeed. but if the pressure becomes too high the total volume of the gas will decrease appreciably whereas the volume of the molecule will remain constant because the molecule are incompressible. Hence, under condition of high pressure, the volume occupied by gas molecule will no longer be negligible in comparison with the total volume of the gas.
The same thing happens when the temperature is lowered to a large extent. The total volume of the gas decreases considerably, no doubt, but the volume occupied by the molecule themselves remains practically the same. In this case, too, the volume occupied by the molecules will no longer be negligible. Therefore is not valid at high pressure and low temperatures.
2. The force of attraction between gas molecule is negligible :-
This assumption is valid at low pressure or high temperature becauses under these conditions the molecule lie far apart from one another. But at high pressure or low temperatures, the volume is small and molecule lie closer to one another. The intermolecular force of attraction therefore are appreciably and cannot be ignored. Hence this assumption does not hold under condition of high pressure and low temperature.
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