Physics

Gas – Properties|Behaviour

Gases consist of loosely bound gas molecules. Each gas molecule may contain one or more atoms bound together. Gases can be classified into monoatomic, diatomic, or triatomic based on the number of atoms in a molecule. In this article, we discuss the chemical and physical properties and the behavior of gas molecules using the kinetic theory of gases.

Physical and chemical properties of gas molecules

Property	Physical absorption	Chemical absorption
Absorption	By weak forces like van der Waal attraction etc	Chemical bond
Selectivity	No	Yes
Absorption heat	Approximate heat of liquefaction (0-20kJ/mol)	Approximate heat of reaction (80-400kJ/mol)
Absorption layer	Monomolecular layer or poly molecular layer	Mono molecular layer
Reversibility	Reversible	Irreversible

Some basic properties of gas molecules

Property	Symbol	Common units
Density	d	g.1 ^-1
Temperature	T	K
Pressure	P	mmHg
Volume	V	cm
Amount of gas	n	mol

The behavior of gas molecules-The kinetic theory of gases

kinetic theory of gases-behavior of gas molecules — Behavior of gas molecules – Kinetic theory of gases

The kinetic molecular theory of gases describes gas molecules as tiny particles which are in constant motion with a lot of empty space between them.

The constant motion of the particles indicates that gases tend to expand and occupy the volume of the container. Hence, they will have lower density and also the tendency to mix with other gases easily.

According to Kinetic Molecular Theory, all gaseous particles are in constant random motion at a temperature above absolute zero. The trajectories of the particles are considered to be in straight lines unless interrupted by a collision with another particle. The collision between particles can cause the transfer of kinetic energy and also the change in direction.

The five postulates of the kinetic theory of gases

The gases are composed of a large number of molecules that are constantly in random movement.
The volume of the gas molecules is negligible compared to the volume occupied by the gas.
The intermolecular interactions are also negligible.
The collision of the gas molecules with each other or with the container wall is usually elastic.
The average kinetic energy of all the molecules is temperature dependant.

Compressibility

Compressibility is the ability of a matter to decrease in volume when under pressure. If we put pressure on a solid or a liquid, then the volume will be changed. The atoms and molecules start to pack-up closer together under pressure and hence the volume decrease.

The kinetic-molecular theory explains that gases are more compressible than either liquid or solids. It is because most of the volume of gas particles is composed of a large number of empty spaces between the gas molecules.

Are compressed gases useful?

•Compressed gases are used in many situations.

•In the hospital, oxygen is often used for patients who have damaged lungs to help them breathe better.

•The Oxygen pump is used in hospitals where it helps to provide medication frequently. This machine uses compressed gases for the mechanism.

•Welding works require very hot flames produced by compresses acetylene and oxygen mixtures.

• Barbeque grills are fuelled by compressed propane.

Factors affecting gas pressure

The gas particles move randomly hence can undergo collision with other particles in the container. Because of these collisions, we are able to describe the pressure of the gas.

The four variables to describe the condition of the gas are :

•Pressure (P).

•Volume (V).

•Temperature (T).

•Amount of the gas (n)y

Pressure and volume

Pressure is affected by the volume of the container. If the volume is decreased, then the gas molecules have less space in which to move around. The frequency of the particles striking the walls of the container will increase. Thus, there will be an increase in pressure. A decrease in gas volume produced increases the gas pressure.

Temperature

The kinetic energy causes the air molecules to move faster and they will impact the container walls more frequently and with more force. And eventually, the pressure of the overall container will increase.

Amount of gas

The amount of gas represent by the symbol ‘n’ is the moles of gas present. We can obtain the n value by dividing the mass of the gas by the molar mass of the gas. The higher the value of n, the higher is the number of gas molecules present.