Real gas Introduction – Deviation,  Attraction force, molecules irregularities

Real gas Introduction – Deviation,  Attraction force, molecules irregularities

Introduction to Real Gases

A real gas is a gas that does not obey the ideal gas law perfectly under all conditions. Unlike ideal gases, real gases have intermolecular forces, and their molecules occupy a finite volume, leading to deviations from ideal behavior.

At low pressure and high temperature, most gases behave like ideal gases, but at high pressure and low temperature, real gases show significant deviations due to molecular interactions and volume effects.

Deviation of Real Gases from Ideal Behavior

Ideal gases follow the ideal gas equation:

$$PV=nRT$$

However, real gases deviate due to the following reasons:

1. Intermolecular Forces – Real gas molecules experience attractive and repulsive forces, which affect pressure and volume.
2. Finite Molecular Volume – Gas molecules are not point masses; they occupy space, affecting compression.
3. High Pressure Effects – At high pressures, the volume of molecules becomes significant, reducing free space for movement.
4. Low Temperature Effects – At low temperatures, attractive forces become dominant, pulling molecules closer and reducing pressure.

This deviation is represented by the compressibility factor (Z):

Z=PVnRTZ = \frac{PV}{nRT}Z=nRTPV​

• If Z = 1, the gas behaves ideally.
• If Z < 1, the gas is more compressible due to attractive forces.
• If Z > 1, the gas is less compressible due to repulsive forces.

Attraction Forces in Real Gases

Real gases experience intermolecular forces, which influence their behavior:

1. Van der Waals Forces – Weak forces that cause gases to deviate from ideal behavior.
2. Dipole-Dipole Interactions – Occur in polar gases like HCl, SO₂.
3. London Dispersion Forces – Present in non-polar gases like O₂, N₂, He, and Ar.
4. Hydrogen Bonding – Strong forces in gases like NH₃ and H₂O vapor.

These forces reduce gas pressure as molecules attract each other, deviating from the ideal gas law.

Molecular Irregularities in Real Gases

Real gases exhibit the following molecular irregularities:

1. Finite Size of Molecules – Unlike ideal gases, real molecules occupy space, affecting their compressibility.
2. Non-Elastic Collisions – Molecules experience energy loss during collisions due to intermolecular attractions.
3. Variable Speed and Movement – Some molecules move faster or slower due to interaction forces.
4. Liquefaction – At low temperatures, strong intermolecular forces cause gases to turn into liquids (e.g., CO₂ at high pressure forms dry ice).

Van der Waals Equation (Correction for Real Gases)

To account for deviation in real gases, Van der Waals modified the ideal gas equation:

(P+aV2)(V−b)=RT\left( P + \frac{a}{V^2} \right) (V – b) = RT(P+V2a​)(V−b)=RT

Where:

• a = Correction for intermolecular attraction (higher for stronger forces).
• b = Correction for finite volume of molecules (larger for bigger molecules).

This equation improves accuracy in predicting gas behavior under high pressure and low temperature.

Conclusion

Real gases do not follow the ideal gas law under all conditions due to attractive forces, molecular volume, and irregularities. These factors lead to deviations that can be corrected using the Van der Waals equation. Understanding real gas behavior is essential in applications like liquefied gas storage, refrigeration, and chemical engineering.

Real gas Introduction – Deviation,  Attraction force, molecules irregularities

The gas laws pV = nRT

IDEAL AND REAL GASES

Lecture 4 – Real Gases