Real Gas

What is a Real Gas?

The term ‘real gas’ usually refers to a gas that does not behave like an ideal gas. Their behaviour can be explained by the interactions between the gaseous molecules. These intermolecular interactions between the gas particles is the reason why real gases do not adhere to the ideal gas law.

Therefore,

Real gases can be defined as non-ideal gases whose molecules occupy a given amount of space and have the ability to interact with each other.

Table of Contents

• Recommended Videos
• Compressibility Factor for a Real Gas
• Examples of Real Gases
• Frequently Asked Questions – FAQs

It is important to note that in most cases, the behaviour of a real gas is almost the same as that of an ideal gas. Therefore, in many applications, a detailed analysis of the deviation of real gases from ideal behaviour is unnecessary. Reasonably accurate calculations can be conducted by applying the ideal gas equation to these real gases. However, it is important to note that a gas must be considered as a real gas when it is approaching its condensation point.

Furthermore, almost all gases must be considered as real gases as they approach their critical points. Other situations in which gases can be considered as real gases include situations in which the pressure applied to the gas is very high, and in order to explain the Joule-Thomson effect. It is also important to note that the deviation of a real gas from the behaviour of an ideal gas can be expressed in terms of the compressibility factor (also known as the gas deviation factor or the compression factor; usually denoted by the symbol ‘Z’).

Recommended Videos

What are the different factors that must be considered while dealing with Real Gases?

In order to understand how real gases behave, several factors must be considered. The different factors that must be considered while dealing with real gases are listed below.

• Compressibility effects on the real gas.
• The varying specific heat capacities of different real gases.
• The effects of Van der Waals forces on the interactions between the molecules of the real gas.
• The non-equilibrium thermodynamic effects that may arise in the system.
• The variable composition of the gas and the changes in the composition of the gas as a result of the molecular dissociation along with the elementary reactions that may take place.

The Compressibility factor for a Real Gas at High Pressure is

The compressibility factor of natural gas (which corrects for the ratio of actual volume to ideal volume) is roughly 0.5% correction in volume per 100 psi of pressure for an orifice meter under normal pressure and temperature conditions.

(P+a/V2)(V−b)=RT

at high-pressure a/V2 can be neglected

PV−Pb = RT

PV = RT + Pb

PV/RT = 1+Pb/RT

Z = 1+Pb/RT

Z>1 at high pressure

Examples of Real Gases

It is important to understand that almost all gases can behave as real gases when they are placed in appropriate conditions. For example, under standard conditions of temperature and pressure (usually abbreviated as STP), the behaviour of air can be estimated with the help of the ideal gas law. This is because air behaves like an ideal gas under standard conditions for temperature and pressure. However, when the pressure applied to the air is increased to a very high magnitude, the same air starts to exhibit notable deviations from the ideal gas law and begins to exhibit the behaviour similar to that of a real gas.

Furthermore, any increase in the absolute temperature of the gas can also have a similar effect. This is because an increase in the absolute temperature of the gas results in an increase in the average kinetic energy of the molecules of the gas. This, in turn, results in an increase in the number of interactions between the molecules of the gas. Therefore, an increase in the absolute temperature of the air can also trigger notable deviations from ideal behaviour and make it a real gas.

Thus, almost all gas can behave like ideal gases as well as real gases. When the conditions are relatively ambient, most gases are known to exhibit almost ideal behaviour. However, under relatively extreme conditions in which either the temperature of the gas is raised to a very high value, the pressure on the gas is raised to a very high value, or both the temperature and the pressure associated with the gas is raised to extremely high values, most gases will deviate from ideal behaviour and become real gases.

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