Kinetic Theory

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CBSE Learning Objectives – Key Concepts & Skills You Must Know

Understand the molecular nature of matter and its implications.
Explain the behavior of gases based on kinetic theory.
Apply the law of equipartition of energy to different types of gases.
Calculate specific heat capacities for monatomic, diatomic, and polyatomic gases.
Analyze the mean free path and its significance in gas behavior.
Utilize the ideal gas equation in various scenarios involving pressure, volume, and temperature.

CBSE Revision Notes & Quick Summary for Last-Minute Study

Chapter 12: Kinetic Theory

12.1 Introduction

Kinetic theory explains the behavior of gases based on the idea that gases consist of rapidly moving atoms or molecules.
Developed in the 19th century by Maxwell, Boltzmann, and others.
Provides a molecular interpretation of pressure and temperature, consistent with gas laws.

12.2 Molecular Nature of Matter

Atomic Hypothesis: Matter is made up of atoms, which are in perpetual motion.
Atoms combine to form molecules, with properties depending on the nature and ratio of constituent atoms.

12.3 Behaviour of Gases

Properties of gases are easier to understand due to the large distances between molecules.
Ideal gas behavior is described by the equation:

$PV = nRT$

where:
- P = pressure
- V = volume
- n = number of moles
- R = universal gas constant
- T = absolute temperature

12.4 Kinetic Theory of an Ideal Gas

The kinetic theory relates pressure, volume, and temperature of gases.
The average kinetic energy of gas molecules is given by:

$E = \frac{3}{2} k_B NT$

where:
- k_B = Boltzmann constant
- N = number of molecules

12.5 Law of Equipartition of Energy

States that energy is distributed equally among all degrees of freedom in thermal equilibrium.
Each degree of freedom contributes $\frac{1}{2} k_B T$ to the energy.

12.6 Specific Heat Capacity

The molar specific heats of gases can be determined using the law of equipartition of energy.

12.7 Mean Free Path

The mean free path (l) is the average distance a molecule travels between collisions:

$l = \frac{1}{\sqrt{2} n \sigma}$

where:
- n = number density of molecules
- σ = collision cross-section.

Points to Ponder

Pressure exists everywhere in a fluid, not just on the walls.
The mean free path in a gas is significantly larger than intermolecular distances.
The law of equipartition of energy applies to all degrees of freedom.
Molecules do not settle due to their high speeds and collisions.
The average of a squared quantity is not the square of the average.

Exercises

Estimate the fraction of molecular volume to the actual volume occupied by oxygen gas at STP.
Show that the molar volume of an ideal gas at STP is 22.4 liters.
Analyze the plot of PV/T versus P for oxygen gas at different temperatures.
Estimate the mass of oxygen taken out of a cylinder given initial conditions.
Calculate the volume of an air bubble as it rises from a lake.
Estimate the total number of air molecules in a room.
Estimate the average thermal energy of a helium atom at various temperatures.
Compare the number of molecules in vessels containing different gases at the same temperature and pressure.
Determine the temperature at which the root mean square speed of argon equals that of helium.
Estimate the mean free path and collision frequency of a nitrogen molecule in a cylinder.

CBSE Exam Tips, Important Questions & Common Mistakes to Avoid

Common Mistakes and Exam Tips

Common Pitfalls

Misunderstanding Pressure in Fluids: Students often think pressure is only exerted on the walls of a container. Remember, pressure exists everywhere in a fluid, and equilibrium means pressure is the same on both sides of any layer of gas.
Exaggerated Intermolecular Distances: Many students have an exaggerated idea of the intermolecular distance in gases. At ordinary pressures and temperatures, this distance is only about 10 times the interatomic distance in solids and liquids.
Confusing Average Values: The average of a squared quantity, such as <V²>, is not equal to the square of the average, <v>². Be cautious when interpreting average values in problems.
Law of Equipartition Misapplication: The law states that energy for each degree of freedom in thermal equilibrium is ½ k T. Each vibrational mode contributes two degrees of freedom, which can lead to confusion in calculations.

Exam Tips

Understand the Ideal Gas Law: Familiarize yourself with the ideal gas equation (PV = µRT) and its implications for real gases, especially under varying conditions of pressure and temperature.
Practice Kinetic Theory Concepts: Ensure you can derive and apply the kinetic theory equations, particularly those relating to temperature and average kinetic energy.
Use Units Consistently: When calculating specific heats or other properties, always keep track of your units to avoid errors.
Review Mean Free Path Calculations: Be prepared to calculate the mean free path and understand its dependence on molecular size and density.

CBSE Quiz & Practice Test – MCQs, True/False Questions with Solutions

Multiple Choice Questions

Energy for each degree of freedom in thermal equilibrium is

\frac{1}{2} k_B T

Energy is equally distributed among all molecules regardless of their state.

Energy is only present in kinetic form in gases.

Energy is constant for all temperatures.

Correct Answer: A

Solution:

The law of equipartition of energy states that the energy for each degree of freedom in thermal equilibrium is

\frac{1}{2} k_B T

Gas A

Gas B

Both have the same speed

Cannot be determined

Correct Answer: A

Solution:

The root mean square speed of a gas is given by

v_{rms} = \sqrt{\frac{3k_BT}{m}}

, where

m

is the molar mass. Since gas A has a lower molar mass than gas B, it will have a higher root mean square speed.

Pressure is only exerted on the walls of the container.

Pressure is exerted uniformly throughout the gas, including on any layer within the gas.

Pressure is only exerted at the bottom of the container due to gravity.

Pressure is exerted only when the gas molecules collide with each other.

Correct Answer: B

Solution:

The kinetic theory of gases states that pressure is exerted uniformly throughout the gas, including on any layer within the gas, due to the constant motion and collisions of gas molecules.

E

2E

E/2

3E

Correct Answer: A

Solution:

The average kinetic energy of a gas molecule is given by

\frac{3}{2}k_BT

, which depends only on the temperature. Therefore, at the same temperature, both helium and nitrogen molecules have the same average kinetic energy.

RMS speed is the same for all gases at the same temperature.

RMS speed is higher for gases with higher molar mass.

RMS speed is independent of the temperature of the gas.

RMS speed is higher for gases with lower molar mass.

Correct Answer: D

Solution:

The root mean square speed is given by

v_{rms} = \sqrt{\frac{3k_BT}{m}}

, where

m

is the molar mass. Thus, gases with lower molar mass have higher rms speed at the same temperature.

Atoms are indivisible and eternal particles.

Atoms are little particles that move around in perpetual motion.

Atoms are static and do not interact with each other.

Atoms are large particles that form the visible matter.

Correct Answer: B

Solution:

Richard Feynman described the atomic hypothesis as the idea that all things are made of atoms, which are little particles in perpetual motion.

PV = \frac{1}{3} nV m v^2

PV = \frac{2}{3} nV m v^2

PV = \frac{3}{2} nV m v^2

PV = nV m v^2

Correct Answer: A

Solution:

The kinetic theory of gases gives the relation

PV = \frac{1}{3} nV m v^2

, where

n

is the number of molecules,

m

is the mass of a molecule, and

v^2

is the mean square speed.

Increase in potential energy

Increase in kinetic energy due to increased molecular collisions

Decrease in intermolecular forces

Conservation of volume

Correct Answer: B

Solution:

When a gas is compressed, the molecules collide more frequently, increasing their kinetic energy and thus the temperature of the gas.

22.4 liters

44.8 liters

11.2 liters

33.6 liters

Correct Answer: B

Solution:

According to Avogadro's law, at the same temperature and pressure, the volume of a gas is directly proportional to the number of moles. Therefore, 2 moles of gas would occupy 44.8 liters.

The temperature increases.

The temperature decreases.

The temperature remains constant.

The temperature fluctuates randomly.

Correct Answer: A

Solution:

When a gas is compressed by a piston, its temperature increases due to the increase in kinetic energy of the molecules.

^{235}\text{U}

^{238}\text{U}

Both have the same average speed

Cannot be determined

Correct Answer: A

Solution:

At a fixed temperature, the average kinetic energy is the same for both isotopes. Therefore, the isotope with the smaller mass,

^{235}\text{U}

, will have a larger average speed.

The average distance a molecule travels before colliding with another molecule.

The average distance a molecule travels in one second.

The average distance a molecule travels before it stops moving.

The average distance between the walls of the container.

Correct Answer: A

Solution:

The mean free path is defined as the average distance a molecule travels before colliding with another molecule, which is a key concept in understanding gas behavior.

Air molecules are too light to be affected by gravity

High speeds and incessant collisions keep them suspended

Intermolecular forces are too strong

Air pressure counteracts gravitational force

Correct Answer: B

Solution:

Molecules of air remain suspended due to their high speeds and frequent collisions, which prevent them from settling under gravity.

They do not settle on the ground due to their high speeds and incessant collisions.

They settle on the ground due to gravity.

They remain static and do not move.

They only move when the temperature changes.

Correct Answer: A

Solution:

According to the kinetic theory, molecules of air do not settle on the ground because of their high speeds and incessant collisions, which keep them in motion.

Pressure is directly proportional to volume.

Pressure is inversely proportional to volume.

Pressure is equal to volume.

Pressure is independent of volume.

Correct Answer: B

Solution:

Boyle's Law states that the pressure of a gas is inversely proportional to its volume at constant temperature.

1.5 eV

2.5 eV

3.5 eV

4.5 eV

Correct Answer: B

Solution:

The average thermal energy is given by

\frac{3}{2} k_B T

. Converting to electron volts,

\frac{3}{2} \times 1.38 \times 10^{-23} \times 6000 \text{ J} = 1.242 \times 10^{-19} \text{ J}

. Converting to eV (1 eV =

1.602 \times 10^{-19} \text{ J}

), the energy is

\frac{1.242 \times 10^{-19}}{1.602 \times 10^{-19}} \approx 2.5 \text{ eV}

Atoms are indivisible and cannot be broken down into smaller parts.

Atoms consist of a nucleus and electrons, and the nucleus is made up of protons and neutrons.

Atoms are only theoretical and have not been observed.

Atoms are larger than molecules.

Correct Answer: B

Solution:

Atoms consist of a nucleus made up of protons and neutrons, with electrons orbiting the nucleus. This is a well-established scientific fact.

The temperature of the gas decreases.

The temperature of the gas remains constant.

The temperature of the gas increases.

The internal energy of the gas decreases.

Correct Answer: C

Solution:

In an adiabatic compression, work is done on the gas, which increases its internal energy, leading to an increase in temperature.

It is the average distance a molecule travels before colliding with another molecule.

It is the distance between two molecules at rest.

It is the average distance a molecule travels in a vacuum.

It is the minimum distance required for a molecule to escape the gas.

Correct Answer: A

Solution:

The mean free path is the average distance a molecule travels before colliding with another molecule, which is significant in understanding the behavior of gases.

The pressure doubles.

The pressure is halved.

The pressure remains the same.

The pressure quadruples.

Correct Answer: A

Solution:

According to Boyle's Law,

PV = \text{constant}

. If the volume is halved, the pressure must double to maintain the constant product, assuming temperature remains constant.

6.02 × 10²³ molecules

1.24 × 10²⁵ molecules

3.01 × 10²⁴ molecules

5.00 × 10²⁴ molecules

Correct Answer: B

Solution:

Using the ideal gas law

PV = nRT

, where

n = \frac{PV}{RT}

, we find the number of moles and then multiply by Avogadro's number

N_a = 6.02 × 10^{23}

to get the total number of molecules.

The rms speed of helium is greater than that of argon.

The rms speed of argon is greater than that of helium.

The rms speed of both gases is the same.

The rms speed is independent of the type of gas.

Correct Answer: A

Solution:

The rms speed of gas molecules is given by the formula

v_{rms} = \sqrt{\frac{3k_B T}{m}}

, where

k_B

is the Boltzmann constant,

T

is the temperature, and

m

is the molecular mass. Since helium has a lower molecular mass than argon, its rms speed is greater at the same temperature.

The root mean square speed is directly proportional to the molar mass.

The root mean square speed is inversely proportional to the square root of the molar mass.

The root mean square speed is independent of the molar mass.

The root mean square speed is inversely proportional to the molar mass.

Correct Answer: B

Solution:

The root mean square speed of gas molecules is inversely proportional to the square root of their molar mass.

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

The pressure of a gas is directly proportional to its temperature.

The volume of a gas is inversely proportional to its pressure.

The energy of a gas is proportional to its volume.

Correct Answer: A

Solution:

Avogadro's hypothesis states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

Equal volumes of all gases at equal temperature and pressure have the same number of molecules.

Different gases have different numbers of molecules in the same volume at STP.

Avogadro's hypothesis only applies to solids.

Avogadro's hypothesis is not related to the kinetic theory.

Correct Answer: A

Solution:

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

Gases consist of rapidly moving atoms or molecules.

Gases have strong inter-atomic forces similar to solids.

Gases are made up of stationary particles.

Gases do not follow Avogadro's hypothesis.

Correct Answer: A

Solution:

The kinetic theory explains that gases consist of rapidly moving atoms or molecules, and this is consistent with gas laws and Avogadro's hypothesis.

The rms speed doubles.

The rms speed is halved.

The rms speed remains the same.

The rms speed decreases by a factor of

\sqrt{2}

Correct Answer: D

Solution:

The root mean square speed

v_{rms}

is given by

v_{rms} = \sqrt{\frac{3k_B T}{m}}

, where

m

is the molar mass. If the molar mass is doubled, the rms speed decreases by a factor of

\sqrt{2}

Intermolecular distances in gases are much smaller than in solids and liquids.

Intermolecular distances in gases are about the same as in solids and liquids.

Intermolecular distances in gases are much larger than in solids and liquids.

Intermolecular distances in gases are irrelevant to their behavior.

Correct Answer: C

Solution:

In gases, the intermolecular distances are much larger compared to those in solids and liquids, allowing molecules to move freely.

The speed remains the same.

The speed decreases.

The speed increases.

The speed becomes zero.

Correct Answer: C

Solution:

When a gas molecule rebounds off a moving wall, its speed increases due to the additional velocity imparted by the moving wall.

1.5 cm³

2.0 cm³

2.5 cm³

3.0 cm³

Correct Answer: B

Solution:

Using the combined gas law

\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}

, where

P_1 = 5 \text{ atm}

(pressure at 40 m depth),

V_1 = 1.0 \text{ cm}^3

T_1 = 285 \text{ K}

P_2 = 1 \text{ atm}

, and

T_2 = 308 \text{ K}

. Solving for

V_2

, we get

V_2 = \frac{P_1 V_1 T_2}{P_2 T_1} = \frac{5 \cdot 1.0 \cdot 308}{1 \cdot 285} = 2.0 \text{ cm}^3

The pressure decreases.

The pressure remains constant.

The pressure increases.

The pressure becomes zero.

Correct Answer: C

Solution:

According to the ideal gas law,

PV = nRT

, if the volume

V

decreases while the temperature

T

remains constant, the pressure

P

must increase to maintain the equality.

Pressure is only exerted on the walls of the container.

Pressure is exerted everywhere in the fluid, not just on the walls.

Pressure is only exerted at the bottom of the container.

Pressure is exerted only when the gas is in motion.

Correct Answer: B

Solution:

The kinetic theory states that pressure exists everywhere in a fluid, not just on the walls of the container.

It states that all gases have different numbers of molecules at the same temperature and pressure.

It states that equal volumes of all gases at the same temperature and pressure have the same number of molecules.

It states that gases do not have molecules.

It states that gases have a fixed number of molecules regardless of conditions.

Correct Answer: B

Solution:

Avogadro's hypothesis states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

The average speed decreases.

The average speed remains the same.

The average speed increases.

The average speed becomes zero.

Correct Answer: C

Solution:

According to kinetic theory, the average speed of gas molecules increases with an increase in temperature.

Rate of diffusion is directly proportional to molecular mass.

Rate of diffusion is inversely proportional to molecular mass.

Rate of diffusion is independent of molecular mass.

Rate of diffusion is equal to molecular mass.

Correct Answer: B

Solution:

The rate of diffusion of gases is inversely proportional to the square root of their molecular masses.

Argon molecules have a higher average speed than chlorine molecules.

Chlorine molecules have a higher average speed than argon molecules.

Both gases have the same average speed.

The average speed depends on the volume of the container.

Correct Answer: A

Solution:

At the same temperature, lighter gas molecules (argon in this case) have higher average speeds compared to heavier gas molecules (chlorine) due to their lower molar mass.

It is independent of temperature.

It decreases with increasing temperature.

It is directly proportional to the absolute temperature.

It is inversely proportional to the volume.

Correct Answer: C

Solution:

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

300 K

400 K

500 K

600 K

Correct Answer: D

Solution:

The rms speed

v_{rms}

is given by

v_{rms} = \sqrt{\frac{3k_B T}{m}}

. Setting the rms speeds equal,

\sqrt{\frac{3k_B T_{Ar}}{M_{Ar}}} = \sqrt{\frac{3k_B T_{He}}{M_{He}}}

. Solving for

T_{Ar}

T_{Ar} = T_{He} \times \frac{M_{Ar}}{M_{He}}

. Given

T_{He} = 253 \text{ K}

M_{Ar} = 39.9 \text{ u}

M_{He} = 4.0 \text{ u}

T_{Ar} = 253 \times \frac{39.9}{4.0} = 600 \text{ K}

The internal energy of the gas increases.

The internal energy of the gas decreases.

The internal energy of the gas remains constant.

The internal energy change depends on the type of gas.

Correct Answer: C

Solution:

For an ideal gas, the internal energy depends only on temperature. During an isothermal process, the temperature remains constant, so the internal energy of the gas does not change.

It states that equal volumes of all gases at the same temperature and pressure have the same mass.

It states that equal volumes of all gases at the same temperature and pressure have the same number of molecules.

It states that equal volumes of all gases at the same temperature and pressure have the same energy.

It states that equal volumes of all gases at the same temperature and pressure have the same density.

Correct Answer: B

Solution:

Avogadro's hypothesis states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules, which is fundamental to the molecular theory of gases.

The volume occupied by 1 mole of any gas at STP is 22.4 liters.

The volume occupied by 1 mole of any gas at STP is 10 liters.

The volume occupied by 1 mole of any gas at STP is 1 liter.

The volume occupied by 1 mole of any gas at STP is 100 liters.

Correct Answer: A

Solution:

At standard temperature and pressure, 1 mole of any ideal gas occupies 22.4 liters.

Gases have a fixed shape and volume.

Gases have negligible intermolecular forces.

Gases are composed of large molecules.

Gases have strong intermolecular forces.

Correct Answer: B

Solution:

Gases are easier to understand because their molecules are far apart, and intermolecular forces are negligible.

It states that equal volumes of all gases at the same temperature and pressure have the same number of molecules.

It suggests that gases have different numbers of molecules at the same conditions.

It implies that gases do not have a fixed number of molecules.

It indicates that gases have a variable number of molecules depending on their volume.

Correct Answer: A

Solution:

Avogadro's hypothesis states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules, which is a fundamental concept in understanding gas behaviour.

It is the average distance a molecule travels before colliding with another molecule.

It is the distance between two molecules at rest.

It is the distance a molecule travels in one second.

It is the distance a molecule travels in one minute.

Correct Answer: A

Solution:

The mean free path is the average distance a molecule travels before colliding with another molecule.

The mean free path is directly proportional to the density of the gas.

The mean free path is inversely proportional to the density of the gas.

The mean free path is independent of the density of the gas.

The mean free path is proportional to the square root of the density of the gas.

Correct Answer: B

Solution:

The mean free path of gas molecules is inversely proportional to the density of the gas. As the density increases, the number of molecules per unit volume increases, leading to more frequent collisions and a shorter mean free path.

The pressure doubles.

The pressure remains the same.

The pressure halves.

The pressure quadruples.

Correct Answer: A

Solution:

According to Boyle's Law, for a given mass of gas at constant temperature, the pressure of a gas is inversely proportional to its volume. Therefore, if the volume is halved, the pressure doubles.

It is a universal constant related to the energy per degree of freedom.

It is a measure of the volume of a gas.

It represents the mass of a gas molecule.

It is the force between two gas molecules.

Correct Answer: A

Solution:

The Boltzmann constant

k_B

is a universal constant that relates the energy per degree of freedom to temperature.

10 Å

100 Å

1000 Å

10,000 Å

Correct Answer: C

Solution:

The mean free path is much larger than the interatomic distance, typically around 1000 times the size of the molecule, which is consistent with the given conditions.

The temperature of the gas decreases.

The temperature of the gas increases.

The gas molecules stop moving.

The gas molecules expand.

Correct Answer: B

Solution:

When a gas is compressed by a piston, the molecules collide more frequently, increasing the temperature of the gas.

The mean free path is much shorter than the interatomic distance in solids.

The mean free path is about the same as the interatomic distance in solids.

The mean free path is much longer than the interatomic distance in solids.

The mean free path is irrelevant to the interatomic distance in solids.

Correct Answer: C

Solution:

In gases, the mean free path is much longer than the interatomic distance in solids, as gas molecules are much farther apart.

Gas molecules are in constant random motion and are far apart from each other.

Gas molecules have more mass than those in solids and liquids.

Gas molecules are tightly packed together.

Gas molecules are stationary and do not move.

Correct Answer: A

Solution:

In gases, molecules are in constant random motion and are much farther apart than in solids and liquids, leading to a lower density.

The rate of diffusion is directly proportional to the square root of the molar mass.

The rate of diffusion is inversely proportional to the square root of the molar mass.

The rate of diffusion is independent of the molar mass.

The rate of diffusion is directly proportional to the molar mass.

Correct Answer: B

Solution:

According to kinetic theory, the rate of diffusion of gases is inversely proportional to the square root of their molar masses.

\frac{1}{2} k_B T

k_B T

\frac{3}{2} k_B T

2 k_B T

Correct Answer: A

Solution:

According to the law of equipartition of energy, the energy for each degree of freedom in thermal equilibrium is

\frac{1}{2} k_B T

The mean free path remains unchanged.

The mean free path is halved.

The mean free path is doubled.

The mean free path becomes zero.

Correct Answer: B

Solution:

The mean free path is inversely proportional to the pressure of the gas. Thus, doubling the pressure halves the mean free path.

The average distance a molecule travels without colliding.

The distance between two molecules at rest.

The average speed of molecules in a gas.

The time taken for a molecule to collide with another.

Correct Answer: A

Solution:

The mean free path is the average distance a molecule can travel without colliding with another molecule.

The molecules are too light to be affected by gravity.

The molecules are in constant motion and collide with each other, which prevents them from settling.

The molecules are held up by the pressure of the air above them.

The molecules are attracted to the walls of the room, keeping them suspended.

Correct Answer: B

Solution:

According to kinetic theory, air molecules are in constant motion and collide with each other, which keeps them suspended and prevents them from settling due to gravity.

The speed of the molecules decreases.

The speed of the molecules remains the same.

The speed of the molecules increases.

The speed of the molecules becomes zero.

Correct Answer: C

Solution:

When a gas is compressed by a piston, its temperature rises, leading to an increase in the speed of the gas molecules.

It is the distance a molecule travels before colliding with another molecule.

It is the distance between two molecules at rest.

It is the distance a molecule travels in a vacuum.

It is the distance a molecule travels in a solid.

Correct Answer: A

Solution:

The mean free path is the average distance a molecule travels before colliding with another molecule.

It is the average distance a molecule travels before colliding with another molecule.

It is the average distance between molecules in a gas.

It is the distance a molecule travels in one second.

It is the average distance a molecule travels in a vacuum.

Correct Answer: A

Solution:

The mean free path is defined as the average distance a molecule travels before colliding with another molecule, as described in the kinetic theory of gases.

The gas molecules are very far apart and rarely collide.

The gas molecules are densely packed and frequently collide.

The gas molecules are stationary and do not move.

The gas molecules are only influenced by external forces.

Correct Answer: A

Solution:

A mean free path that is 1000 times the size of the molecule indicates that the molecules are far apart, making collisions infrequent, which is characteristic of gases.

Gas molecules are in constant random motion and collide elastically.

Gas molecules are at rest until they collide with each other.

Gas molecules are only influenced by gravitational forces.

Gas molecules move in a straight line until they reach the container wall, where they stop.

Correct Answer: A

Solution:

The kinetic theory of gases states that gas molecules are in constant random motion and their collisions are elastic, meaning there is no net loss of kinetic energy.

1 atm

2 atm

4 atm

6 atm

Correct Answer: C

Solution:

According to the ideal gas law, for a constant volume, the pressure of a gas is directly proportional to its temperature (in Kelvin). Therefore, if the temperature doubles from 300 K to 600 K, the pressure will also double, resulting in a new pressure of 4 atm.

It states that equal volumes of all gases at the same temperature and pressure contain different numbers of molecules.

It states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

It states that the volume of a gas is directly proportional to the number of molecules, regardless of temperature and pressure.

It states that the volume of a gas is inversely proportional to the number of molecules at constant temperature and pressure.

Correct Answer: B

Solution:

Avogadro's hypothesis states that equal volumes of all gases at the same temperature and pressure contain the same number of molecules.

The molecule speeds up after the collision.

The molecule slows down after the collision.

The molecule's speed remains unchanged.

The molecule stops moving.

Correct Answer: A

Solution:

When a molecule collides with a moving wall, it rebounds with a speed that is increased relative to the wall's motion.

The molecules move faster.

The molecules move slower.

The molecules stop moving.

The molecules remain at the same speed.

Correct Answer: A

Solution:

When a gas is heated, the kinetic energy of its molecules increases, causing them to move faster.

Each degree of freedom contributes an energy of

\frac{1}{2} k_B T

to the total energy.

Each degree of freedom contributes an energy of

k_B T

to the total energy.

Each degree of freedom contributes an energy of

\frac{1}{3} k_B T

to the total energy.

Each degree of freedom contributes an energy of

2 k_B T

to the total energy.

Correct Answer: A

Solution:

The law of equipartition of energy states that each degree of freedom contributes an energy of

\frac{1}{2} k_B T

to the total energy of a system in thermal equilibrium.

The pressure remains constant.

The pressure doubles.

The pressure halves.

The pressure quadruples.

Correct Answer: B

Solution:

According to the ideal gas law,

PV = nRT

, if the volume and amount of gas are constant, the pressure is directly proportional to the temperature. Therefore, doubling the temperature from 300 K to 600 K doubles the pressure.

The mean free path increases.

The mean free path decreases.

The mean free path remains unchanged.

The mean free path becomes zero.

Correct Answer: A

Solution:

When the temperature of a gas is increased while keeping the pressure constant, the average speed of the gas molecules increases. This leads to an increase in the mean free path, as the molecules are moving faster and have more energy to travel longer distances between collisions.

The pressure increases.

The pressure decreases.

The pressure remains the same.

The pressure becomes zero.

Correct Answer: A

Solution:

According to the kinetic theory, increasing the temperature of a gas increases the average kinetic energy of its molecules, which in turn increases the pressure if the volume is kept constant.

Pressure is directly proportional to the average kinetic energy.

Pressure is inversely proportional to the average kinetic energy.

Pressure is independent of the average kinetic energy.

Pressure is equal to the average kinetic energy.

Correct Answer: A

Solution:

According to the kinetic theory, pressure is directly proportional to the average kinetic energy of the gas molecules.

3 atm

4 atm

1.5 atm

2.5 atm

Correct Answer: A

Solution:

For an isochoric process, the pressure and temperature are related by the equation

\frac{P_1}{T_1} = \frac{P_2}{T_2}

. Given

P_1 = 2 \text{ atm}

T_1 = 300 \text{ K}

, and

T_2 = 450 \text{ K}

, we can solve for

P_2

P_2 = \frac{P_1 \cdot T_2}{T_1} = \frac{2 \cdot 450}{300} = 3 \text{ atm}

The speed decreases.

The speed remains the same.

The speed increases.

The speed becomes zero.

Correct Answer: C

Solution:

When a molecule rebounds off a moving wall, its speed increases relative to the wall's speed.

22.4 liters

24.0 liters

20.0 liters

18.0 liters

Correct Answer: A

Solution:

The molar volume of an ideal gas at STP is 22.4 liters.

The pressure decreases.

The pressure increases.

The pressure remains the same.

The pressure becomes zero.

Correct Answer: B

Solution:

According to the kinetic theory, increasing the temperature of a gas increases the kinetic energy of its molecules, leading to an increase in pressure if the volume is constant.

It relates the average kinetic energy of particles in a gas with the temperature of the gas.

It is used to calculate the gravitational force between gas molecules.

It determines the speed of sound in a gas.

It measures the intermolecular forces in a gas.

Correct Answer: A

Solution:

The Boltzmann constant

k_B

relates the average kinetic energy of particles in a gas with the temperature of the gas, as described in the kinetic theory.

It states that all molecules have the same speed.

It provides a way to calculate the average kinetic energy per degree of freedom.

It explains why gases have different colors.

It determines the volume of a gas at STP.

Correct Answer: B

Solution:

The law of equipartition of energy states that each degree of freedom contributes ½ k_B T to the energy, allowing calculation of average kinetic energy per degree of freedom.

Both temperature and pressure decrease.

Temperature decreases, but pressure increases.

Both temperature and pressure increase.

Temperature increases, but pressure decreases.

Correct Answer: C

Solution:

In an adiabatic compression, no heat is exchanged with the surroundings. The work done on the gas increases its internal energy, leading to an increase in both temperature and pressure.

Each degree of freedom contributes

k_B T

to the total energy.

Each degree of freedom contributes

\frac{1}{2} k_B T

to the total energy.

Each degree of freedom contributes

2 k_B T

to the total energy.

Each degree of freedom contributes

3 k_B T

to the total energy.

Correct Answer: B

Solution:

The law of equipartition of energy states that each degree of freedom contributes

\frac{1}{2} k_B T

to the total energy of a system in thermal equilibrium.

v_{238} = v_{235}

v_{238} > v_{235}

v_{238} < v_{235}

Cannot be determined from the given information

Correct Answer: C

Solution:

According to the kinetic theory, the average speed of gas molecules is inversely proportional to the square root of their molar masses. Since

^{238}UF_6

has a greater molar mass than

^{235}UF_6

v_{238} < v_{235}

The molecules are in constant motion and only collide with the container walls.

The molecules are at rest and only move when the temperature increases.

The molecules are in constant motion and collide with each other and the container walls.

The molecules move in a fixed pattern and do not collide with each other.

Correct Answer: C

Solution:

According to the kinetic theory, gas molecules are in constant motion and frequently collide with each other and the walls of the container, which is consistent with the behavior of gases at STP.

The mean free path increases.

The mean free path decreases.

The mean free path remains unchanged.

The mean free path becomes zero.

Correct Answer: B

Solution:

Increasing the pressure reduces the mean free path as molecules are closer together, leading to more frequent collisions.

True or False

Correct Answer: True

Solution:

At absolute zero, the temperature is 0 Kelvin, meaning that the kinetic energy of gas molecules, which depends on temperature, is also zero.

Correct Answer: False

Solution:

Even at the end of the nineteenth century, there were famous scientists who did not believe in the atomic theory.

Correct Answer: True

Solution:

The mean free path in gases is about 100 times the interatomic distance in solids and liquids, as mentioned in the excerpt.

Correct Answer: False

Solution:

The pressure of a fluid exists everywhere within the fluid, not just on the walls of its container. Any layer of gas inside the volume of a container is in equilibrium because the pressure is the same on both sides of the layer.

Correct Answer: False

Solution:

Richard Feynman considered the discovery that 'Matter is made up of atoms' to be very significant.

Correct Answer: True

Solution:

The kinetic theory states that the average kinetic energy of molecules is directly proportional to the absolute temperature, as shown in the equation

E = \frac{3}{2} k_B NT

Correct Answer: True

Solution:

The law of equipartition of energy states that the energy for each degree of freedom in thermal equilibrium is ½ k_B T.

Correct Answer: True

Solution:

Boyle's Law, which describes the inverse relationship between pressure and volume of a gas at constant temperature, was indeed discovered by Robert Boyle in 1661.

Correct Answer: True

Solution:

The kinetic theory explains that molecules of air remain in motion due to their high speeds and constant collisions, preventing them from settling on the ground.

Correct Answer: True

Solution:

Boyle discovered the law named after him in 1661, which explains the behavior of gases.

Correct Answer: True

Solution:

At standard temperature and pressure (STP), the mass of 22.4 liters of any gas is indeed equal to its molecular weight in grams, which corresponds to one mole of the gas.

Correct Answer: False

Solution:

Pressure exists everywhere in a fluid, not just on the walls of its container.

Correct Answer: False

Solution:

In the Vaiseshika school of thought, space (Akasa) was thought to have no atomic structure and was considered continuous and inert.

Correct Answer: True

Solution:

Boyle's law, which describes the inverse relationship between the pressure and volume of a gas at constant temperature, was discovered by Robert Boyle in 1661.

Correct Answer: True

Solution:

The law of equipartition of energy states that each degree of freedom contributes ½ k_B T to the energy in thermal equilibrium, as described in the excerpt.

Correct Answer: True

Solution:

In a gas, the mean free path is indeed about 1000 times the size of the molecule, indicating the average distance a molecule travels between collisions.

Correct Answer: False

Solution:

At a fixed temperature, the average speed is inversely proportional to the square root of the mass. Therefore, the lighter isotope will have a higher average speed.

Correct Answer: True

Solution:

According to the atomic hypothesis, atoms exhibit attractive forces when slightly apart and repulsive forces when pushed too close.

Correct Answer: True

Solution:

The kinetic theory states that gases at low pressures and high temperatures behave approximately as ideal gases, following the equation PV = nRT.

Correct Answer: True

Solution:

The mean free path is the average distance a molecule travels in a gas without colliding with another molecule.

Correct Answer: False

Solution:

The kinetic theory of gases assumes that inter-atomic forces are negligible for gases, which allows the theory to explain the behavior of gases based on the idea of rapidly moving atoms or molecules.

Correct Answer: True

Solution:

The Vaiseshika school of thought, founded by Kanada, considered atoms to be eternal, indivisible, infinitesimal, and the ultimate parts of matter.

Correct Answer: False

Solution:

While John Dalton is credited with the modern atomic theory, the concept of atoms existed in ancient Indian and Greek philosophies long before him.

Correct Answer: True

Solution:

The mean free path in a gas is indeed about 100 times the interatomic distance in solids and liquids, highlighting the relatively large distances between molecules in gases.

Correct Answer: True

Solution:

Richard Feynman, a renowned physicist, emphasized the importance of the discovery that matter is composed of atoms.

Correct Answer: False

Solution:

In dynamic equilibrium, molecules collide and change their speeds during collisions, but the average properties remain constant.

Correct Answer: False

Solution:

The kinetic theory was developed in the nineteenth century by Maxwell, Boltzmann, and others.

Correct Answer: True

Solution:

The kinetic theory was indeed developed in the nineteenth century by scientists like Maxwell and Boltzmann, as stated in the excerpt.

Correct Answer: True

Solution:

The law of equipartition of energy states that the energy for each degree of freedom in thermal equilibrium is

\frac{1}{2} k_B T

. Each quadratic term in the total energy expression of a molecule is counted as a degree of freedom.

Correct Answer: False

Solution:

At ordinary pressures and temperatures, the mean free path in a gas is 1000 times the size of the molecule, not the intermolecular distance.

Correct Answer: True

Solution:

Richard Feynman emphasized the importance of the atomic hypothesis, stating that it is a crucial piece of scientific knowledge.

Correct Answer: True

Solution:

The kinetic theory was developed in the nineteenth century by Maxwell, Boltzmann, and others.

Correct Answer: True

Solution:

The Vaiseshika school, founded by Kanada, developed a detailed atomic picture, proposing that matter is composed of indivisible atoms.

Correct Answer: False

Solution:

The kinetic theory of gases was developed in the nineteenth century by scientists like Maxwell and Boltzmann.

Correct Answer: False

Solution:

Pressure in a fluid exists everywhere, not just on the walls of its container. Any layer of gas inside the volume of a container is in equilibrium because the pressure is the same on both sides of the layer.

Correct Answer: True

Solution:

At STP, 22.4 liters of any gas contains Avogadro's number of molecules, which is

6.02 \times 10^{23}

Correct Answer: True

Solution:

The kinetic theory explains that gases are made up of rapidly moving atoms or molecules, and the inter-atomic forces can be neglected.

Correct Answer: True

Solution:

The intermolecular distance in a gas at ordinary pressures and temperatures is about 10 times the interatomic distance in solids and liquids.

Correct Answer: False

Solution:

At a fixed temperature, the average speed of a gas molecule is inversely proportional to the square root of its mass. Therefore, isotopes with different masses will have different average speeds.

Correct Answer: False

Solution:

The intermolecular distance in a gas is about 10 times the interatomic distance in solids and liquids, while the mean free path is 1000 times the size of a molecule.

Correct Answer: True

Solution:

The Vaiseshika school of thought in ancient India, founded by Kanada, developed an atomic picture in considerable detail, describing atoms as eternal and indivisible.

Correct Answer: True

Solution:

The kinetic theory, which explains the behavior of gases, was indeed developed in the nineteenth century by Maxwell, Boltzmann, and others.

Correct Answer: True

Solution:

Avogadro's hypothesis indeed states that equal volumes of gases at equal temperature and pressure contain the same number of molecules.

Correct Answer: True

Solution:

Boyle's law, discovered in 1661, was an early attempt to explain the behavior of gases by considering them as composed of tiny atomic particles.

Correct Answer: False

Solution:

While ancient theories considered atoms to be indivisible and eternal, modern atomic theory recognizes that atoms are made up of smaller particles like protons, neutrons, and electrons.

Correct Answer: False

Solution:

Molecules of air do not settle on the ground due to their high speeds and incessant collisions, resulting in a slight increase in density at lower heights.

Correct Answer: False

Solution:

The kinetic theory assumes that interatomic forces can be neglected for gases, which is why gases behave differently from solids and liquids.

Correct Answer: True

Solution:

Avogadro's hypothesis states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.

Correct Answer: True

Solution:

The mean free path in a gas is indeed 1000 times the size of the molecule, as stated in the kinetic theory.

Correct Answer: True

Solution:

The average energy per molecule in a gas is indeed given by

\frac{3}{2} k_B T

, where

k_B

is the Boltzmann constant and

T

is the temperature.

Correct Answer: True

Solution:

Kinetic theory explains the behavior of gases based on the idea that the gas consists of rapidly moving atoms or molecules, and the inter-atomic forces, which are short-range forces important for solids and liquids, can be neglected for gases.

Correct Answer: False

Solution:

The law of equipartition of energy states that each vibrational mode contributes two degrees of freedom: one for kinetic energy and one for potential energy.

Correct Answer: False

Solution:

The pressure of a fluid exists everywhere in the fluid, not just on the walls of the container.

Correct Answer: True

Solution:

The law of equipartition of energy states that the energy for each degree of freedom in thermal equilibrium is

\frac{1}{2} k_B T

Correct Answer: True

Solution:

The atomic hypothesis states that all things are made of atoms - little particles that move around in perpetual motion.

Correct Answer: False

Solution:

In the Vaiseshika school of thought, atoms were considered to be indivisible, eternal, and the ultimate parts of matter.

Correct Answer: True

Solution:

The law of equipartition of energy states that each degree of freedom contributes

\frac{1}{2} k_B T

to the energy of a system in thermal equilibrium.

Correct Answer: False

Solution:

The atomic hypothesis was conjectured long before the nineteenth century, with early ideas from ancient India and Greece.

Correct Answer: True

Solution:

In a gas, the mean free path is indeed about 100 times the interatomic distance, which is significantly larger than the size of the molecules themselves.

Correct Answer: True

Solution:

Kinetic theory explains the behavior of gases based on the idea that the gas consists of rapidly moving atoms or molecules.

Correct Answer: True

Solution:

The actual atomic theory got established more than 150 years after Boyle's discovery in 1661.

Correct Answer: False

Solution:

In gases, the mean free path is typically of the order of thousands of angstroms, not a few.

Correct Answer: True

Solution:

Richard Feynman regarded the discovery that 'Matter is made up of atoms' as very significant.

Correct Answer: True

Solution:

Democritus is known for his atomic hypothesis, where the word 'atom' means 'indivisible' in Greek.

Correct Answer: False

Solution:

The atomic hypothesis was suggested long before John Dalton by ancient scholars in India and Greece. Dalton is credited with the modern scientific atomic theory.

Kinetic Theory

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CBSE Learning Objectives – Key Concepts & Skills You Must Know

CBSE Revision Notes & Quick Summary for Last-Minute Study

Chapter 12: Kinetic Theory

12.1 Introduction

12.2 Molecular Nature of Matter

12.3 Behaviour of Gases

12.4 Kinetic Theory of an Ideal Gas

12.5 Law of Equipartition of Energy

12.6 Specific Heat Capacity

12.7 Mean Free Path

Points to Ponder

Exercises

CBSE Exam Tips, Important Questions & Common Mistakes to Avoid

Common Mistakes and Exam Tips

Common Pitfalls

Exam Tips

Multiple Choice Questions

Q1.Which of the following best describes the law of equipartition of energy?

Correct Answer: A

Q2.In a mixture of two gases, A and B, at the same temperature, gas A has a molar mass of 4 g/mol and gas B has a molar mass of 16 g/mol. Which gas will have the higher root mean square speed?

Correct Answer: A

Q3.According to the kinetic theory of gases, which of the following statements is true about the pressure exerted by a gas in a container?

Correct Answer: B

Q4.A helium gas sample is at 300 K. If the average kinetic energy of a helium atom is EEE, what will be the average kinetic energy of a nitrogen molecule at the same temperature?

Correct Answer: A

Q5.In a mixture of gases, which of the following statements about the root mean square (rms) speed of the molecules is true?

Correct Answer: D

Q6.Which of the following best describes the atomic hypothesis as stated by Richard Feynman?

Correct Answer: B

Q7.According to the kinetic theory, what is the relationship between the pressure PPP, volume VVV, and the mean square speed v2v^2v2 of gas molecules?

Correct Answer: A

Q8.A gas in a cylinder is compressed by pushing in a piston, causing its temperature to rise. Which principle of kinetic theory explains this phenomenon?

Correct Answer: B

Q9.If 1 mole of an ideal gas at STP occupies 22.4 liters, how many liters would 2 moles of an ideal gas occupy at the same temperature and pressure?

Correct Answer: B

Q10.According to the kinetic theory, what happens to the temperature of a gas when it is compressed by a piston?

Correct Answer: A

Q11.Consider two isotopes of uranium, 235U^{235}\text{U}235U and 238U^{238}\text{U}238U, both present in uranium hexafluoride gas. If the atomic mass of fluorine is 19 units, which isotope has a larger average speed at a given temperature?

Correct Answer: A

Q12.In the kinetic theory of gases, what does the term 'mean free path' refer to?

Correct Answer: A

Q13.In the context of kinetic theory, why do molecules of air in a room not settle on the ground due to gravity?

Correct Answer: B

Q14.What happens to the molecules of air in a room according to the kinetic theory?

Correct Answer: A

Q15.What is the relationship between pressure and volume in Boyle's Law?

Correct Answer: B

Q16.Estimate the average thermal energy of a helium atom at the temperature on the surface of the Sun (6000 K).

Correct Answer: B

Q17.Which of the following statements is true about the molecular nature of matter?

Correct Answer: B

Q18.A gas is contained in a cylinder with a movable piston. If the gas is compressed adiabatically, which of the following statements is true according to kinetic theory?

Correct Answer: C

Q19.What is the significance of the mean free path in the kinetic theory of gases?

Correct Answer: A

Q20.What is the effect on the pressure of a gas if its volume is halved while keeping the temperature constant, according to Boyle's Law?

Correct Answer: A

Q21.Consider a room at 27°C and 1 atm pressure. If the room has a volume of 25.0 m³, estimate the total number of air molecules present using the ideal gas law.

Correct Answer: B

Q22.Consider two identical containers, one filled with helium gas and the other with argon gas. Both gases are at the same temperature and pressure. Which of the following statements is true about the root mean square (rms) speed of the gas molecules?

Correct Answer: A

Q23.What is the relationship between the root mean square speed of gas molecules and their molar mass?

Correct Answer: B

Q24.What is Avogadro's hypothesis regarding gases?

Correct Answer: A

Q25.Which of the following is a correct statement about Avogadro's hypothesis?

Correct Answer: A

Q26.According to the kinetic theory, which of the following statements is true about the behavior of gases?

Correct Answer: A

Q27.In the context of the kinetic theory, what happens to the root mean square (rms) speed of gas molecules if the molar mass of the gas is doubled while keeping the temperature constant?

Correct Answer: D

Q28.Which of the following statements is true about the intermolecular distance in gases compared to solids and liquids?

Correct Answer: C

Q29.In the context of kinetic theory, what happens to the speed of a gas molecule when it rebounds off a moving wall?

Correct Answer: C

Q30.An air bubble of volume 1.0 cm³ rises from the bottom of a lake 40 m deep at a temperature of 12 °C. To what volume does it grow when it reaches the surface, which is at a temperature of 35 °C, assuming the pressure at the surface is 1 atm and the density of water is 1000 kg/m³?

Correct Answer: B

Q31.If a gas in a closed container is compressed at constant temperature, what happens to its pressure according to the ideal gas law?