The Deviation Of Real Gas From Ideal Gas Behavior - Practice Questions & MCQ

Ideal Gas: 
These gases obey gas laws under all the conditions of temperature and pressure,

• No gas is ideal in reality (hypothetical). 

• No force of attraction is present between molecules in them. 

• Volume of molecules is negligible to the total volume of the gas (container).

Real Gas:
These gases obey gas laws only at high temperature and low pressure. 

• All the gases are real. 

• Here the force of attraction between molecules cannot be neglected at high pressure and low temperature.

• Here volume occupied by gas molecule is not negligible specially at high pressure and low temperature.

Behaviour of Real Gases: Deviation from Ideal Gas Behaviour and Compressibility factor Z



The extent of deviation of a real gas from ideal gas behaviour is expressed in terms of compressibility factor Z. It is an empirical correction for the non-ideal behaviour of real gases which allows the simple form of the combined gas law to be retained It is given as:

When Z = 1 (ideal gas behaviour)

When Z < 1 (negative deviations)

When Z > 1 (positive deviations)

When Z < 1 gas is more compressible 

When Z > 1 gas is less compressible

• For He and H₂ , Z > 1 as PV>RT [as a/V² = 0 ] that is, positive deviations.
• At very Low Pressure: PV RT (as a/V²  and b are neglected) that is, Z 1 so nearly ideal gas behaviour. 
• At Low Pressure: PV<RT that is, Z < 1 so negative deviation 
• At Moderate Pressure : PV = RT i.e, Z = 1 so ideal gas behaviour 
• At High Pressure: PV > RT (as b can not be neglected). that is, Z > I so positive deviation.
• An increase in temperature shows a decrease in deviation from ideal gas behaviour.

Plot of pV vs p for real gas and ideal gas

Plot of pressure vs volume for real gas and ideal gas

Van der Waal’s Equation
Van der waal's equation is a modification of the ideal gas equation that takes into account the non-ideal behaviour of real gases Van der Waal 's equation modified kinetic theory of gases by considering these two points of kinetic theory of gases not to be fully correct or are not followed by real gases.
For an ideal gas the force of attraction between gaseous molecules is negligible and the volume of gaseous molecules is negligible to the total volume of the gas. These two assumptiions are not followed by real gases.

He made following two corrections :

• Volume Correction
  According to him, at high pressure the volume of the gas becomes lower so volume of molecules can not be ignored Hence the actual space available inside the vessel for the movement of gas molecules is not the real volume of the gas, actually it is given as:
  V_{real gas} = V - b
  Here V is the volume of the container while b is the volume occupied by gas molecules and it is called co-volume or excluded volume. 
  The excluded volume for 'n' molecules of a gas = 4nV_m or (4 x 4/3𝜋r³)
  Here V_m = Volume of one molecule (4/3𝜋r³
  Thus, the ideal gas equation can be written as:
  P(V-nb) = nRT

• Pressure Correction
  According to him, the gaseous molecules are closer so attraction forces cannot be ignored hence, pressure of the real gas is given as: 
  Pressure of the Real gas = pressure developed due to collisions (P) + pressure loss due to attraction (p')
  
  Here p' is pressure loss due to force of attraction between molecules or inward pull
  
   (for n moles of gas)
  Here 'a' is Van der Waal's force of attraction constant, d is density and V is volume.
  Hence
  

 (for n moles of gas)

• Now ideal gas equation can be written after correction of pressure and volume for n moles
  

• Units of a and b
  
  The values of 'a' and 'b' are 0.1 to 0.01 and 0.01 to 0.001 respectively.

Variation of compressibility factor for some gases

Explanations for Real Gas Behaviour

• At very low pressure for one mole of a gas, the value of 'a' and 'b' can be ignored so Van der Waal's equation becomes equal to ideal gas.
  PV=RT 
• At low pressure, the value of 'nb' or 'b' can be ignored so Van der Waal's equation becomes
  
  

  Hence PV < RT
  so,
  

• At moderate pressures, neither the value of 'a' nor 'b' can be neglected and we have to consider both the value of 'a' as well as 'b' for the calculation of Z.

• At high pressure, the value of 'a' can be ignored so Van der Waal's equation can be written as