Introduction to ideal and real gases

Unit-5

GAS MIXTURES AND THERMODYNAMIC RELATIONS

Properties of Ideal gas, real gas - comparison. Equations of state for ideal and real gases. vander Waal's relation - Reduced properties – Compressibility factor Principle of Corresponding states - Generalized Compressibility Chart. Maxwell relations - TdS Equations - heat capacities relations - Energy equation, Joule-Thomson experiment - Clausius - Clapeyron equation

INTRODUCTION TO IDEAL AND REAL GASES

1. Ideal gas:

An ideal gas is an imaginary substance that obeys the relation pV = mRT or pv = RT. So, the equation of state is also known as ideal-gas equation. At low pressure and high temperature, the density of a gas decreases. At that time, the gas behaves as an ideal gas.

2. Real gas:

At high pressure, the gases start to deviate from ideal-gas behaviour. So, the deviation should be accounted. A factor is introduced to account the deviation of ideal-gas behavior, known as compressibility factor. Therefore, compressibility factor is a measure of deviation of the ideal-gas behaviour. So, the state equation for real gases is given by

pv = ZRT

where

Z = Compressibility factor.

The compressibility factor can also be expressed as

where

vactual = Specific volume of real gases

videal = Specific volume of ideal gases.

For ideal gases, Z = 1. But, for real gases, Z is greater than or less than unity.

The graph is plotted between pressure, temperature and compressibility factor to account the deviation of ideal gas behaviour as shown in Figure 5.4.