What is the difference between real and ideal gasses?

A gas is a type of physical state that matter can exist in. When the particles or the molecules of a compound are free to move anywhere inside a container, this compound is called a gas. The gaseous state is different from other two physical states (solid and liquid state) according to the way particles or molecules are packed. A real gas is a gaseous compound that really exists. An ideal gas is a gaseous compound that does not exist in reality but is a hypothetical gas. However, some gaseous compounds show approximately similar behavior to that of ideal gases at a specific temperature and pressure conditions. Therefore, we can apply gas laws for that kind of real gases by assuming that they are ideal gases. Even though the proper conditions are provided, a real gas cannot become100% close to the behavior of an ideal gas due to the differences between real and ideal gas. The main difference between real and an ideal gas is that real gas molecules have intermolecular forces whereas an ideal gas has no intermolecular forces.

Key Areas Covered

1. What is a Real Gas
     – Definition, Specific Properties
2. What is an Ideal Gas
     – Definition, Specific Properties
3. What is the Difference Between Real and Ideal Gas
     – Comparison of Key Differences

Key Terms: Gas, Ideal Gas, Gas Laws, Intermolecular Forces, Real Gas

What is a Real Gas

A real gas is a gaseous compound that really exists in the environment. These real gases are composed of different atoms or molecules that are called particles. These gas particles are in constant motion. A gas particle has a definite volume and mass. Therefore, a gas has a definite volume and a mass. The volume of a gas is considered as the volume of the container in which the gas is kept in.

Some real gases are composed of atoms. For example, Helium gas is composed of Helium atoms. But other gases are composed of molecules. For example, Nitrogen gas is composed of N2 molecules. Therefore, these gases have a mass and a volume.

Furthermore, real gas molecules have intermolecular attractions between them. These attraction forces are called Van Der Waal interactions. These attraction forces are weak. Collisions between real gas molecules are non-elastic. This means when two real gas particles colloid with each other, a change in the energy of the particle and a change in the direction of its movement can be observed.

However, some real gases may behave as ideal gases under low pressure and high temperature conditions. At high temperatures, the kinetic energy of gas molecules is increased. Therefore the motion of gas molecules speed up. This results in less or no intermolecular interactions between real gas molecules.

Therefore, at low pressure and high temperature conditions, we can apply gas laws for real gases. For example, at low pressure and high temperature;

PV / nRT  ≈  1

Where  P is the pressure of the gas,

               V is the volume of the gas,

               n is the number of moles of gas,

               R is the ideal gas constant and

               T is the temperature of the system.

This value is called the compressibility factor. It is a value that is used as a correction factor for the deviation of a property of a real gas from an ideal gas. But for real gases PV ≠ nRT.

Figure 1: Compressibility factor for different gases with respect to that of an ideal gas

Although the value of PV /nRT is not exactly equal to 1, it is an approximately equal value at low pressure and high temperature conditions.

An ideal gas is a hypothetical gas that does not really exist in the environment. The concept of ideal gas was introduced since the behavior of real gases are complicated and different from each other, and the behavior of a real gas can be described with respect to the properties of an ideal gas.

Ideal gases are gaseous compounds that are composed of very tiny molecules that have a negligible volume and a mass. As we already know, all real gases are composed of atoms or molecules that have a definite volume and a mass. The collisions between ideal gas molecules are elastic. This means, there are no changes in the kinetic energy or the direction of the movement of the gas particle.

There are no attraction forces between ideal gas particles. Therefore, particles move here and there freely. However, ideal gases may become real gases at high pressures and low temperatures since the gas particles come close to each other with a reduced kinetic energy that will result in the formation of intermolecular forces.

Figure 2: The behavior of Ideal gas with respect to the He gas and CO2 gas

An ideal gas obeys all the gas laws without any assumptions. The value for PV /nRT for an ideal gas equals 1. Therefore the value for PV is equal to the value for nRT. If this value (compressibility factor) is equal to 1 for a particular gas, then it is an ideal gas.  

Definition

Real Gas: A real gas is a gaseous compound that really exists in the environment.

Ideal Gas: An ideal gas is a hypothetical gas that does not really exist in the environment.

Intermolecular Attractions

Real Gas: There are intermolecular attraction forces between real gas particles.

Ideal Gas: There are no intermolecular attraction forces between ideal gas particles.

Gas Particle

Real Gas: The particles in a real gas have a definite volume and a mass.

Ideal Gas: The particles in an ideal gas do not have a definite volume and a mass.

Collisions

Real Gas: Collisions between real gas molecules are non-elastic.

Ideal Gas: Collisions between ideal gas molecules are elastic.

Kinetic Energy

Real Gas: The kinetic energy of real gas particles is changed with collisions.

Ideal Gas: The kinetic energy of ideal gas particles is constant.

Change in the State

Real Gas: A real gas may behave as an ideal gas at low pressure and high temperature conditions.

Ideal Gas: An ideal gas may behave like a real gas at high pressure and low temperature conditions.

Conclusion

Real gases are gaseous compounds that really exist in the environment. But ideal gases are hypothetical gases that do not really exist. These ideal gases can be used to understand the behavior of real gases. When applying a gas law for a real gas, we can assume that real gasses behave as ideal gasses at low pressure and high temperature conditions. But the accurate way is to use correction factors for the calculations rather than assuming. The correction factors are obtained by determining the difference between real and ideal gas.

References:

1. “Real Gases.” Chemistry LibreTexts, Libretexts, 1 Feb. 2016, Available here. Accessed 6 Sept. 2017.
2. “Compressibility factor.” Wikipedia, Wikimedia Foundation, 11 Aug. 2017, Available here. Accessed 6 Sept. 2017.
3. “Ideal gas.” Wikipedia, Wikimedia Foundation, 30 Aug. 2017, Available here. Accessed 6 Sept. 2017.

Image Courtesy:

1. “Factor Z vs” By Antoni Salvà – Own work (CC BY-SA 4.0) via Commons Wikimedia

IDEAL GAS vs REAL GAS

The states of matter are liquid, solid, and gas which can be recognized through their key characteristics. Solids have strong composition of molecular attraction giving them definite shape and mass, liquids take the form of their container since the molecules are moving that corresponds to one another, and gases are diffused on air since the molecules are moving freely. The characteristics of gases are very distinct. There are gases that are strong enough to react with other matter, there are even with very strong odour, and some can be dissolved in water. Here we will be able to note some differences between ideal gas and real gas. The behaviour of real gases is very much complex while the behaviour of ideal gases is much simpler. The behaviour of real gas can be more tangible by understanding fully the behaviour ideal gas.

This ideal gas can be considered as a “point mass”. It simply means that the particle is extremely small where its mass is almost zero. Ideal gas particle, therefore, does not have volume while a real gas particle does have real volume since real gases are made up of molecules or atoms that typically take up some space even though they are extremely small. In ideal gas, the collision or impact between the particles are said to be elastic. In other words, there is neither attractive nor repulsive energy included throughout the collision of particles. Since there is lack of inter-particle energy the kinetic forces will remain unchanged in gas molecules. In contrast, collisions of particles in real gases are said to be non-elastic. Real gases are made up of particles or molecules that may attract one another very strongly with the expenditure of repulsive energy or attractive force, just like water vapor, ammonia, sulfur dioxide, and etc.

The pressure is much greater in ideal gas as compared to the pressure of a real gas since the particles do not have the attractive forces that enable the molecules to hold back when they will collide at an impact. Hence, particles collide with less energy. Differences that are distinct between ideal gases and real gases may be regarded most clearly when the pressure will be high, these gas molecules are large, the temperature is low, and when the gas molecules excerpt strong attractive forces.

PV=nRT is the equation of ideal gas. This equation is important in its ability to connect together all the fundamental properties of gases. T stands for Temperature and should always be measured in Kelvin. “n” stands for the number of moles. V is the volume which is usually measured in liters. P stands for pressure wherein it is usually measured in atmospheres (atm), but can also be measured in pascals. R is considered ideal gas constant which never changes. On the other hand, since all real gases can be converted to liquids, Dutch physicist Johannes van der Waals came up with an modified version of the ideal gas equation (PV = nRT):

(P + a/V2) (V – b) = nRT. The value of “a” is constant as well as “b”, and therefore should be experimentally determined for each gas.

SUMMARY:

1.Ideal gas has no definite volume while real gas has definite volume.

2.Ideal gas has no mass whereas real gas has mass.

3.Collision of ideal gas particles is elastic while non-elastic for real gas.

4.No energy involved during collision of particles in ideal gas. Collision of particles in real gas has attracting energy.

5.Pressure is high in ideal gas compared to real gas.

6.Ideal gas follows the equation PV=nRT. Real gas follows the equation (P + a/V2) (V – b) = nRT.

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