1. Kinetic theory

Introduction

As all molecules of a gas are in a state of rapid and continuous motion, various properties of a gas like pressure, temperature, energy are explained and the kinetic theory was developed by Scottish physicist James Maxwell and Austrian Physicist Ludwig Boltzmann.

Kinetic theory of gases is based on the molecular picture of matter. It correlates the macroscopic properties (like pressure and temperature) of gases to microscopic properties like speed, kinetic energy of gas molecules.

Kinetic theory explains the behaviour of gases based on the idea that the gas consists of rapidly moving atoms or molecules. This is possible as the interatomic forces, which are short range forces that are important for solids and liquids, can be neglected for gases.

Dalton’s Atomic Theory

• Atomic hypothesis was given by many scientists. According to which everything in this universe is made up of atoms.
• Atoms are little particles that move around in a perpetual order attracting each other when they are little distance apart.
• But if they are forced very close to each other then they rebel.

For example: - Consider a block of gold. It consists of molecules that are constantly moving.

• Dalton’s atomic theory is also referred to as the molecular theory of matter. This theory proves that matter is made up of molecules which in turn are made up of atoms.

According to Gay Lussac’s law - when gases combine chemically to yield another gas, their volumes are in ratios of small integers.

Avogadro’s law states that the equal volumes of all gases at equal temperature and pressure have the same number of molecules.

Conclusion: - All these laws proved the molecular nature of gases.

Dalton’s molecular theory forms the basis of Kinetic theory.

Behaviour of Gases

Gases at low pressures and high temperatures much above that at which they liquefy (or solidify) approximately satisfy a relation between their pressure, temperature and volume:

P V= K T . This is the universal relation which is satisfied by all gases. Where P, V, T are pressure, volume and temperature resp. and K is the constant for a given volume of gas. It varies with the volume of gas. K=NK_B

Where N=number of molecules and k_B = Boltzmann Constant and its value never change.

From above two equations we have

P V=NK_BT⇒  PV/NT=K_B Which is the same for all gases.

Consider there are 2 gases :- (P1, V,1, T1) and (P2, V2, T2) where P, V and T are pressure, volume and temperature respectively.

This is Avogadro’s hypothesis, that the number of molecules per unit volume is same for all gases at a fixed temperature and pressure

Conclusion: - This relation is satisfied by all gases at low pressure and high temperature.

According to Avogadro’s hypothesis, the number of molecules per unit volume is the same for all gases at a fixed P and T.

Avogadro number is denoted by NA. Where A denotes Avogadro number.

NA= 6.022 ×10²³. It is a universal value.

Experimentally it has been found that the mass of 22.4 litres of any gas is equal to molecular weight in grams at standard temperature and pressure.

Perfect Gas Equation

Perfect gas equation is given by PV=μRT,

Where P, V are pressure, volume, T =absolute temperature, μ = number of moles and R =universal gas constant.

R= kBNA where kB = Boltzmann constant and NA = Avogadro’s number

This equation tells about the behaviour of gas in a particular situation. If a gas satisfies this equation then the gas is known as Perfect gas or an ideal gas.

Ideal gas: A gas that satisfies the perfect gas equation exactly at all pressures and temperatures.  Ideal gas is a theoretical concept.

• No real gas is truly ideal. A gas which is ideal is known as real gas.
• Real gases approach the ideal gas behaviour for low pressures and high temperatures

Dalton’s Law of partial pressures

Dalton’s law of partial pressure states that the total pressure of a mixture of ideal gases is the sum of partial pressures.

Consider if there are several ideal gases mixed together in a vessel, then the total pressure of that vessel is equal to the sum of partial pressure.

Partial pressure is the pressure exerted by a particular gas if only that gas is present in the vessel.

For example: - Consider if in a vessel there is a mixture of 3 gases, A,B and C.So the partial pressure of A is equal to pressure exerted only by A and considering B and C are not present.

Similarly partial pressure of B is equal to the pressure exerted only by B and considering A and C are not there.  And Similarly for C.

According to Dalton’s law the total pressure of mixture is sum of partial pressure of A, partial pressure of B and partial pressure of C

Therefore P= P1+P2+---total pressure due to the mixture of gases is equal to the sum of the partial pressure of the gas.

Kinetic Theory of an Ideal Gas

Basis and assumptions of Kinetic Theory: -

1. Molecules of gas are in incessant random motion, colliding against one another and with the walls of the container.

2. All collisions are elastic. And total Kinetic energy and momentum are conserved.  In case of an elastic collision total Kinetic energy and momentum before collision is equal to the total Kinetic energy and momentum after collision.
3. The density and the distribution of the molecules is uniform throughout the gas.
4. Between two collisions a molecule moves in a straight path with uniform velocity. But when they come closer they experience the intermolecular forces and as a result their velocities change.
5. There are no intermolecular forces between the molecules of gas except during collisions.
6. There will be no force ,between the molecules. As a result molecules are moving freely as per Newton's first law of motion.
7. At ordinary temperature and pressure the molecular size is very small as compared to intermolecular distance between them.

In the above pictures we can see that molecules moving randomly first and then molecules colliding with each other and change their direct.

The Pressure of an Ideal Gas Based on Kinetic Theory

• Consider a container in the shape of a cube that is filled with an ideal gas. Only one molecule will be considered; the molecule collides with the container's walls and bounces back.
• Let the molecule's velocity while moving be (v_x, v_y, v_z).
• The velocity of the molecule as it bounces back will be (-v_x, v_y, v_z).
• The change in momentum = P_f – P_i where P_f = final momentum and P_i = initial momentum)
•   P_f-P_i= =mv_x-mv_x= -2 mv_x 
• The wall receives this change in momentum as a result of the contact.
• One molecule's momentum delivered to the wall in a collision= 2mv_x

However, because there are so many molecules, we must calculate the overall momentum transferred to the wall by them all.

To figure out how many molecules hit the wall, do the following:

• The area of the wall will be ‘A’. Therefore, in time Δt within a distance of  AvxΔt  all the molecules can hit the wall.
• If n be the number of molecules per unit volume and on average, half of the molecules will hit the wall and half of them will move away from the wall. Therefore,  will hit the wall.

• The total momentum will be:.
• The force exerted on the wall is equal to the rate of change of momentum which will be equal to 
• The Pressure on the wall is equal to  This is true for molecules having velocity v_x

Note:

• The velocity of all the molecules in the gas will not be the same. The velocities of each will be different.
• As a result, the following equation is valid for pressure due to a group of molecules moving at v_x in the x-direction, where n is the number density of that group of molecules.

As a result, the total pressure owing to all such groups may be calculated by adding the contributions due to each molecule. 

• Because the gas is isotropic, the molecules travel at random, meaning that their velocity can be in any direction.
• 
• Therefore, the pressure is equal to   where v2 is the average square speed.

Kinetic Interpretation of Temperature

A molecule's average kinetic energy is proportional to the absolute temperature of the gas. It is unaffected by the ideal gas's pressure, volume, or nature.

For the equation:  , by multiplying both sides by V we will get

   

Also  nV= N ( total number of molecules)

After simplifying the above equation 

Here

‘N’ is the number of molecules in a sample.

EN=12mv2=32kBT ......equation (3)

Therefore, the above equation depicts the average kinetic energy.

So, kinetic energy is directly proportional to the temperature. So, temperature can be identified as a molecular quantity.

Kinetic Theory: Consistent With Ideal Gas Equation and Gas Laws

1. It is consistent with the ideal gas equation:

For the kinetic gas equation: 

For an ideal gas, its internal energy is directly proportional to the temperature. This depicts that internal energy of an ideal gas is only dependent on its temperature, not on pressure or volume.

2. When Kinetic theory is consistent with Dalton’s Law of partial pressure:

The equation for Kinetic theory  

If the mixture of gases is present in the vessel then 

The average Kinetic energy of the molecules of different gases at equilibrium will be equal   

Then the total pressure P is given by

So we have Total pressure P of mixture of gases as equal to the sum of partial pressure of individual gasesP= P₁ + P₂+ P₃............ This is known as Dalton’s law of partial pressure.