Ideal Gases and Factors Affecting Them

The ideal gases are the ones that have elastic collisions between their molecules. There exist no intermolecular attractive forces in these ideal gases. Coming to reality, there is no such thing alike ideal gases. Gases simply exhibit the ideal behaviour under specific conditions of pressure and temperature.

Instead, when we speak about the ideal gases, the following assumptions can be taken into the consideration:

• The ideal gases are combined of molecules that are in constant motion, and in random directions.

• The ideal gas molecules behave as rigid spheres.

• The total collisions are elastic.

• The temperature of the gas is always directly proportional to the average kinetic energy of the molecules.

• Pressure takes place because of the collision between the molecules to the container walls.

The Ideal Gas Law

Behaviour of an ideal gas

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These gases obey three laws, which are given below:

• Charles’ law – This law states that, at any constant pressure and the number of moles, the gas volume is directly proportional to its temperature. The derivative can be given as follows:

V∝ T

• Boyle’s law – This law states that, at any constant temperature and the number of moles, the gas volume is inversely proportional to its pressure. The derivative can be given as follows:

V ∝ 1/p 

• Avogadro law – This law states that, at constant pressure and temperature, the gas volume is directly proportional to the number of its moles. The derivative can be given as follows:

V∝ n

Combining all these three laws, we get the result as follows:

V ∝ nT/p

V = R(nT/p)

Here R is given as the proportionality constant. By re-arranging the above equations we get the result as,

p V = n RT

Where R is called the universal gas constant and is the same for all gases. The above equation is referred to as the ideal gas equation. Thus, it can be said that at constant pressure and temperature, n moles of any gas will contain the same volume.

V = (nRT) / p

Here the terms, n, R, T, p, are constants and so, there will be a fixed volume for every gas under these conditions. This derived equation is applicable to any of the gas which approaches the ideal behaviour. And, the ideal gas law is otherwise called the equation of the state. This is because it defines the relationship between the 4 variables and describes the state of a given gas.

If we think about considering a case, where the pressure, volume, and temperature, varies from T1, V1, p1 - T2, V2, p2, then the resultant gas law can be written as follows:

(p1V1)/T1 = nR

In the same way,

(p1V1)/T1 = nR

=> (p1V1)/T1 = (p2V2)/T2

This is completely a useful equation and is also known as the combined gas law. If the 5 values are identified, then the 6th one can be calculated.

Characteristics of Ideal Gas

Let us look at a few of the important characteristics of an ideal gas:

• They are made up of small particles, which are known as molecules and atoms. Also, currently, this condition is true for all the gases.

• The particles themselves contain 0 volume, but the gas as a whole contains the volume. It means the particles present in the gas contains a negligible amount of volume. So, now, all the particles contain some amount of volume. However, the small it can be, and therefore, this condition is not true for all the gases.

• The forces of attraction present between the particles of the gas are considered to be negligible. So, for sure, the forces of attraction are much feeble, but they do exist. Therefore, this condition is not true for all the gases.

• The collisions present between the individual gas molecules are given as perfectly elastic. We also know that no collision is either perfectly inelastic or perfectly elastic. Therefore, this condition is not true for all the gases.

• Always, the gas particles will be in a random motion. This is completely true because the particles of a gas are moving and hence the gas contains properties such as expansion. And, this is true for all the gases.

These are a few of the primary and important characteristics of ideal gases and also the reason they are either true or false in the case of real gases.

Properties of an Ideal Gas

An ideal gas contains a number of properties, where, the real gases often exhibit behaviour very close to that of ideal gases. A few of the properties of an ideal gas can be listed as follows:

• An ideal gas has a large number of identical molecules.

• When compared to the volume occupied by the gas, the volume occupied by the molecules themselves is negligible.

• Also, the molecules obey Newton's laws of motion, and they travel in a random motion.

• Finally, the molecules experience the forces only during collisions, where any collisions are completely elastic and can take a negligible amount of time.