NEET Notes: Gaseous State

NEET Notes: Gaseous State

Introduction to the Gaseous State

The gaseous state is one of the fundamental states of matter, characterized by high compressibility, low density, and indefinite shape and volume. Unlike solids and liquids, gases expand to fill their container and exert pressure in all directions. The study of gases is essential in understanding various physical chemistry concepts, including gas laws, kinetic molecular theory, and real gas behavior, which are important for NEET aspirants.

Characteristics of Gases

• Gases have no fixed shape or volume and expand to fill the entire container.
• They are highly compressible compared to solids and liquids.
• The density of gases is much lower than that of solids and liquids.
• Gas molecules are in constant random motion and move freely in all directions.
• Gases exert pressure on the walls of their container due to molecular collisions.

Gas Laws

Boyle’s Law (Pressure-Volume Relationship)

Statement: At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.

Mathematical Expression:

P ∝ 1/V or PV = constant

Graphical Representation: A hyperbolic curve when plotting P vs. V.

Charles’ Law (Temperature-Volume Relationship)

Statement: At constant pressure, the volume of a given mass of gas is directly proportional to its absolute temperature (Kelvin).

Mathematical Expression:

V ∝ T or V/T = constant

Graphical Representation: A straight-line graph when plotting V vs. T.

Gay-Lussac’s Law (Temperature-Pressure Relationship)

Statement: At constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature.

Mathematical Expression:

P ∝ T or P/T = constant

Graphical Representation: A straight-line graph when plotting P vs. T.

Avogadro’s Law (Volume-Mole Relationship)

Statement: Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

Mathematical Expression:

V ∝ n or V/n = constant

Molar Volume of a Gas: At STP (Standard Temperature and Pressure), 1 mole of any gas occupies 22.4 L.

Ideal Gas Equation

Combining Boyle’s, Charles’, and Avogadro’s laws gives the ideal gas equation:

PV = nRT

Where R is the universal gas constant with different values depending on units.

Applications: Used to calculate gas properties under different conditions.

Kinetic Molecular Theory of Gases

This theory explains the behavior of gases at the molecular level. The main postulates are:

• Gas molecules are in continuous random motion.
• The volume occupied by gas molecules is negligible compared to the total volume of the gas.
• There are no intermolecular forces between gas molecules.
• Collisions between gas molecules are perfectly elastic (no loss of energy).
• The average kinetic energy of gas molecules is directly proportional to the absolute temperature.

Expression for Kinetic Energy

The kinetic energy of a gas molecule is given by:

KE ∝ T

This implies that as temperature increases, molecular motion increases.

Deviation from Ideal Gas Behavior

Real Gases vs. Ideal Gases

Ideal gases follow the ideal gas equation PV = nRT under all conditions.

Real gases deviate from ideal behavior due to intermolecular forces and molecular volume.

Causes of Deviation

• Intermolecular Forces: Real gas molecules experience attractions and repulsions, affecting pressure and volume.
• Finite Molecular Volume: Gas molecules occupy space, making the actual volume greater than predicted by the ideal gas equation.

Van der Waals Equation (for Real Gases)

To correct the ideal gas equation for real gases, Van der Waals introduced:

(P + a/V²) (V - b) = RT

Where:

• a accounts for intermolecular attractions.
• b accounts for finite molecular volume.

Liquefaction of Gases

Critical Temperature and Critical Pressure

• Critical Temperature (Tₐ): The temperature above which a gas cannot be liquefied by pressure alone.
• Critical Pressure (Pₐ): The minimum pressure required to liquefy a gas at its critical temperature.

Methods of Liquefaction

• Linde’s Method: Based on Joule-Thomson effect, where gas expansion cools the gas, leading to liquefaction.
• Claude’s Method: Uses adiabatic expansion to cool and liquefy gases.

Diffusion and Effusion of Gases

Graham’s Law of Diffusion

Statement: The rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

Mathematical Expression:

(r₁/r₂) = √(M₂/M₁)

Application: Used to separate isotopes and in gas identification.

Effusion and Permeability

• Effusion: The passage of gas molecules through a tiny hole without collisions.
• Permeability: The ability of gases to pass through solid materials.

Applications of Gaseous State in Daily Life

• Respiration: Oxygen is inhaled and diffuses into the bloodstream.
• Compressed Gas Cylinders: Used in medical oxygen supply and industrial applications.
• Aerosol Sprays: Use pressurized gases for dispersing liquids.
• Hot Air Balloons: Heated gases expand, making the balloon rise.
• Gas Laws in Meteorology: Predict weather patterns based on atmospheric pressure changes.

Conclusion

The gaseous state and its laws provide fundamental insights into the behavior of gases under different conditions. Understanding gas properties, the kinetic theory, and deviations from ideal behavior is essential for solving problems in physical chemistry, making this chapter a critical part of NEET preparation.