Behaviour of Real Gases

Introduction

The word ‘real gas’ refers to a gas that does not function as an ideal gas. To understand what is ideal gas & the behaviour of gaseous molecules, the interaction between them explains it. The intermolecular behavior of gas particles explains why the ideal gas does not behave like a real gas. Therefore, real gases are characterized as non-ideal gas in which molecules occupy space and interact with one another. The  Dutch physicist Johannes van der Waals made alterations in the ideal gas law to explain the behaviour of real gases by including the effects of the intermolecular forces and molecular size. The Real gas law equation,

=(P+an2/V2) (V-nb)=nRT

n2/V2  – the concentration of gas.

P – pressure

R – a universal gas constant and T is the temperature 

What is Ideal gas and Real Gas Equation?

An ideal gas is defined as gases that obey all the laws of the gas at all temperature & pressure conditions. An ideal gas has the velocity as well as the mass that has no volume. The volume taken up by the gas is small and compared to the total volume. The triple point does not exist and does not condense too. 

The equation of ideal gas laws of the hypothetical ideal gas is called the general gas equation. For many conditions, the behavior of several gases is reasonably approximated but it has many limitations. Émile Clapeyron was first described in 1834 as the variation of the empirical law of Boyle, the law of Avogadro, the law of Charles, and the law of Gay-Lussac. In the equation form, 

pV=nRT

Why do gases deviate from the ideal behaviour?

The term ‘real gas’ does not behave like an ideal gas and its behavior can be explained by the behavior of gaseous molecules. Therefore, real gases are defined as non-ideal gases and occupy a given amount of space. In most cases, the behavior of real gas is almost the same as the ideal gas. Therefore, a detailed analysis is required for the deviation from real gas to ideal gas behavior. The reasonable calculations are conducted by applying the ideal gas equation to the real gases. While it is crucial to note that the gas should be considered as a real gas when it is deviating to the condensation point. 

On the other hand, almost all the gases are considered as the real gases as they deviate to their critical points. Other instances in which gases are considered as real gases include the instances in which high pressure is applied to the gas and to explain the Joule-Thomson effect. It is also important to note that the real gases deviate from ideal gases can be explained in terms of the compressibility factor ( also known as the compression factor denoted as the ‘Z’).

But, the ideal gas rate is RT/P. Therefore, the compressibility factor can be defined as the ratio of real gas volume to ideal gas volume, i.e,

Compressibility factor Z=  Vreal=Videal. 

At every low pressure and high temperature, all the gases act as the ideal gases. So when the pressure is decreased, the real gas show ideal behavior at the low pressure when the value of the z tends to unite.

Depending on the temperature and pressure, it is to be remembered that the value of Z can be less than or greater than unity. The compressibility factor shows the value of z corresponding to the pressure. 

What are the Factors that are Considered While Dealing with the behavior of Real Gases?

To understand the behavior of Real Gases, several factors must be considered while dealing with real gases. 

• The effects of Compressibility on real gas.
• The varying heat capacities of the real gas.
• The effects of Van der Waals forces between the interaction of the molecules of real gas.
• The effects of non-equilibrium thermodynamics can arise in the system.
• The changes in the composition of the gas because of the molecular dissociation

(P+aV2)(V−b)=RT

at high pressure, aV2 can be neglected

PV−Pb = RT

PV = RT + Pb

PVRT = 1+PbRT

Z = 1+PbRT

Z>1 at high pressure

Examples of behaviour of real gases:

To understand what ideal gases are, we have to understand first that all gases behave as the real gases when they are situated in the appropriate conditions. For instance, under the standard conditions for pressure and temperature, the behavior of the air can be estimated with the help of the ideal gas law. Because the air behaves like the ideal gases under the standard conditions for pressure and temperature. While, when the pressure is applied on the air, it is increased to a very high magnitude, the same air exhibits deviations from the ideal gas law and starts to show the behavior the same as the real gas. Afterward, any surge in the absolute temperature of the gas can also show similar effects. Because the surge in the absolute temperature of the gas can increase the average kinetic energy of the gas. As a result, the increase in the number of iterations of the molecules of gas. Therefore, the surge in the absolute temperature of the air can trigger deviations from ideal behavior and make it a real gas.

Conclusion

Almost all gases behave like ideal gases as well as real gases. Most of the real gas shows ideal behavior at relatively ambient conditions. While the temperature of the gas increases to a high value under relatively extreme conditions. The pressure of the gas also rises to the high temperature and both the pressure and temperature are increased to extremely high values and gases will deviate from ideal to real gases.