Kinetic Molecular Theory

Determine the
chemical properties of
matter.

Affect the
physical
properties of
matter.

Based on physical appearance you can distinguish between solids and liquids

However, substances that are gases at room
temperature usually display similar physical
properties despite their different composition

• The kinetic theory of gases originated

in the ancient idea that matter consists
of tiny invisible atoms in rapid motion.
In the 17th century, this idea was
revived and used to explain, among
other phenomena, the properties of
gases.

Kinetic Molecular Theory

• The Kinetic Molecular Theory

describes the behavior of matter in terms of particles in motion.

•The model makes several

assumptions about the size, motion,
and energy of gas particles.

Particle Size

Gases consist of small
particles that are
separated from one
another by empty space.

Particle Motion

Gas particles are in
constant, random
motion.

Collisions between the gas
particles are elastic.

Particle Energy

Temperature is a
measure of the average
kinetic energy of the
particles in a sample of
matter.

As the temperature increases, the velocity increases, and the kinetic energy increases.

There are 2 factors that affect the kinetic energy of the particle; mass and velocity.

Kinetic Molecular Theory

The large amount of space between the gas
particles allow gas to be squeezed into smaller
volume.

Kinetic Molecular Theory

Compression

Expansion

An increase in temperature results
in an increase in the kinetic energy
of the gas, resulting in expansion of
the gas.

The movement of gases or any other
material from an area of high
concentration to an area of low
concentration.

Kinetic Molecular Theory

Diffusion

Effusion

The movement of gases through a tiny
opening.

•Gases consist of large numbers of tiny particles that are far apart
relative to their size.

•Collisions between gas particles and between particles of the
container walls are elastic collisions.

•Gas particles are in continuous, rapid, random motion. They therefore possess kinetic energy, which is energy of motion.

•There are no forces of attraction between gas particles.

•The temperature of a gas depends on the average kinetic energy of the gas.

Kinetic Molecular Theory

Graham’s Law

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

Rate of effusion ᾳ

𝟏

𝐌𝐨𝐥𝐚𝐫 𝐌𝐚𝐬𝐬

• Graham’s law also applies to diffusion.
• The rate of diffusion depends mainly on the mass of the particles.
• Lighter particles diffuse more quickly than heavier particles

𝐑𝐚𝐭𝐞 𝐀
𝐑𝐚𝐭𝐞 𝐁

𝐌𝐨𝐥𝐚𝐫 𝐌𝐚𝐬𝐬 𝐁
𝐌𝐨𝐥𝐚𝐫 𝐌𝐚𝐬𝐬 𝐀

=

Problem

Ammonia has a molar mass of 17.0 g/mol;
hydrogen chloride has a molar mass of 36.5
g/mol. What is the ratio of their diffusion
rates?

Solution

KNOWN

UNKNOWN

molar massHCl = 36.5
g/mol

ratio of diffusion rates
=?

molar massNH3 = 17.0
g/mol

•State the ratio derived from
Graham’s law.

RateNH3
RateHCl

=
molarmassHCl
molarmassNH3

•Substitute molar massHCl = 36.5
g/mol and molar massNH3 = 17.0
g/mol.

36.5g/mol
17.0g/mol = 1.47

The ratio of diffusion rates is 1.47.

Graham’s Law

Pressure

•Pressure is defined as force per unit area.

•Gas particles exert pressure when they collide with the walls of their container.

Pressure

= 𝐅𝐨𝐫𝐜𝐞
𝐀𝐫𝐞𝐚

Air Pressure

•Near the Earth’s surface, the particles of the air are close to one another, and the air is dense, exerting more pressure due to the
gravity. However, as we move away from
the Earth’s surface, the gravity decreases,
the air becomes less dense, and the pressure
decreases.

Air Pressure

•Earth is surrounded by an atmosphere that extends into space for hundreds of kilometers.

•Because the particles in air move in every direction,
they exert pressure in all directions.

•Air pressure varies at different points on Earth.

Measuring Air Pressure

•Italian physicist Evangelista Torricelli was
the first to demonstrate that air exerted
pressure.

•Torricelli invented a device named
barometer to measure atmospheric
pressure.

Atmospheric

pressure

Pressure exerted by
mercury column

vacuum

760mm

Measuring Air Pressure

The barometer is made of an inverted (upside-
down) glass tube standing in a bath of mercury.
Air pressure pushes down on the surface of the
mercury, making some rise up the tube. The greater
the air pressure, the higher the mercury rises.

GasPressure

Manometers measure gas pressure in a
closed container.
A manometer works much like a
barometer, but instead of measuring
atmospheric pressure, it measures the
pressure difference between the inside and outside of a container.

Units of Pressure

The SI unit of pressure is pascal (Pa).

One pascal (Pa) is equal to a force of one Newton per square meter or N/m2.

One atmosphere is equal to 760 mm Hg (millimeter mercury)
or 101.3 kilopascals.

Dalton’s Law of Partial Pressure

When Dalton studied the properties of gases, he found that each gas in a mixture exerts pressure independently of the other gases pressure.

Dalton’s law of partial pressures states that the total pressure of a
mixture of gases is equal to the sum of the pressures of all the gases of the mixture.
The partial pressure of a gas depends on the number of moles, size of the container, and temperature and is independent of the type of gas.
At a given temperature and pressure, the partial pressure of 1mol of
any gas is the same.
Partial pressure can be used to calculate the amount of gas produced in a chemical reaction.

Pressure

(A)

Pressure

(B)

𝐏 𝐓𝐨𝐭𝐚𝐥 =

𝐏𝐀+ 𝐏𝐁

Dalton’s Law of Partial Pressure

Dalton’s Law of Partial Pressure

A container holds a mixture of two different gases. The oxygen in a
container exerts 80 mmHg of pressure on the inside of the container.
The total pressure inside the container is 120 mmHg. What is the
pressure of the other gas in the container?

Answer:
According to Dalton’s law of Partial Pressure
P total = P1 + P2
120 = 80 + Pressure of unknown gas
Pressure of unknown gas = 120 – 80 = 40 mmHg.

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