Real and Ideal Gas | Deviation of Real Gas from Ideal Behaviour | Chemistry Grade XI

Real and Ideal Gas

Real and Ideal Gas | Deviation of Real Gas from Ideal Behaviour | Chemistry Grade XI

Real and Ideal Gas | Deviation of Real Gas from Ideal Behaviour

DIFFERENCES BETWEEN REAL GAS AND IDEAL GAS

A gas which obeys the ideal gas equation, PV = nRT under all conditions of temperature and pressure is called an ideal gas. There is no gas which obeys the ideal gas equation under all conditions of temperature and pressure. The gases are found to obey the gas laws if the pressure is low or the temperature is high. Such gases are known as real gas. Some other differences between real and ideal gas are shown in tabular form below;

Real Gas

Ideal Gas

The gas which doesn’t follow ideal gas equation(i.e. PV=nRT) in all conditions of temperature and pressure is called real gas.

The gas which follows ideal gas equation(i.e. PV=nRT) in all conditions of temperature and pressure is called ideal gas.

Real gas exists in nature.

Ideal gas doesn’t exist in nature.

The volume of gas molecules is not negligible as compared to that of total volume of gas.

The volume of gas molecules is negligible as compared to the total volume of gas.

The force of attraction between gases molecules is not negligible.

The force of attraction between gases molecules is negligible.

Real gas behaves as ideal gas at high temperature and low pressure.

Ideal gas behaves as real gas at high pressure and low temperature.

 
DEVIATION OF REAL GAS FROM IDEAL BEHAVIOUR
Real gas behaves as ideal gas at high temperature and low pressure. But it deviates from ideal behavior at high pressure and low temperature. Since the molecules of real gas attracts each other, the gas equation for real gas is:

PV = znRT    Where ‘z’ is correction factor known as compressibility factor.

z = PV/nRT

For ideal gas, z=1 and for real gas z=1

  1. At low temperature and low pressure, ‘z’ is less than one that means real gas is more compressible than ideal gas.
  2. At high temperature and high pressure, ’z’ is more than one that means real gas is less compressible than ideal gas.
  3. At high temperature and low pressure, ‘z’ is equal to 1 i.e. Real gas behaves as ideal gas.

The extent of deviation of real gas from ideal behavior depends on temperature and pressure. The effect of pressure on the deviation of real gas from ideal behavior can be understood by plotting graph i.e. PV against P at room temperature( constant temperature) by taking different gases like H2, N2, He, etc

Real gas behaves ideally as pressure decreases 
Real gas behaves ideally as pressure decreasesSimilarly,the effect of temperature on the deviation of real gas from ideal behavior can be understood by plotting PV against P in graph at different temperature by taking nitrogen gas. Real gas behaves ideally as temperature increases.
 
Reasons for deviation of Real gas from ideal behavior
This is because of following fault in assumptions of kinetic theory of gas;
 
1. The volume of gas molecules is negligibly small as compared to the total volume of gas.
This assumption is valid at high temperature and low pressure. At high temperature and low pressure, the gases molecules possess high kinetic energy and expand in large space. So, volume of a molecule is negligible as compared to the total volume. But at high pressure and low temperature, the molecules of gas comes sufficiently closer to each other, and the volume of gas is small. In such condition, volume of gas molecules is not negligible.
 
2. Force of attraction between gaseous molecules is negligible.
This assumption is only valid at high temperature and low pressure. At high temperature and low pressure, gas molecules possess high kinetic energy which overcomes the force of attraction between molecules. But at high pressure and low temperature, the volume of gas becomes small and gas molecules come sufficiently closer to each other. So they attract to each other.
 
Vander Waal’s Equation (Equation of state for real gas)
J.D Vander Waal modified the ideal gas equation which is known as Vander Waal's equation or equation of state for real gas which is applicable for real gas. Vander Waal's equation is the equation which generalizes ideal gas equation on the basis of possible reasons that real gas does not act ideally. By correcting the volume of molecule and force of attraction between molecules of an ideal gas.
 
The ideal gas equation is:
PV = nRT
 
In fact, real gas molecules attract each other. Therefore the volume of real gas is more compressible than that of ideal gas. Account the volume of molecule of real gas, Vander Waal replaced the volume of an ideal gas from the ideal gas equation with ( V - nb) where,
      ‘V’ is molar volume and
      ‘b’ is volume occupied by 1 mole of molecule.

Similarly, a term ‘an2/v2’ is added to observe pressure which provides molecular attraction where, ‘a’ is Vander Waal's constant (Pressure correction) whose value depends on the nature of gas.

Vander Waal’s equation for ‘n’ mole of real gas is:
 (P + an2 / V2) (v – nb) = nRT
 
For 1 mole ,
 (P + a / V2) (v – b) = nRT 

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