Kinetic Molecular
Theory of Gases
Chemistry MYP
Ms. Khan
Kinetic Molecular Theory
▪ kinetic: of, relating to, or resulting from motion
▪ KMT is based on the idea that particles of matter are
always in motion
– theories exist for solids, liquids, and gases
▪ ideal gas: an imaginary gas that perfectly fits all the
assumptions of the kinetic molecular theory of gases
▪ real gas: a gas that does not fully behave according to KMT
– gases are closest to ideal behavior at low pressures and
high temperatures
Kinetic Molecular Theory of Gases
1) gases consist of large numbers of tiny particles that are far
apart relative to their size
2) collisions between gas particles and between particles and
container walls are elastic collisions
 elastic collisions: no net loss of kinetic energy
3) gas particles are in continuous, rapid, random motion –
they possess kinetic energy
 kinetic energy: energy of motion
Kinetic Molecular Theory of Gases
4) there are no forces of attraction or repulsion between gas
particles
 no intermolecular forces (London dispersion, H-bonding,
dipole-dipole)
 immediately bounce apart after colliding
5) the average kinetic energy of gas particles depends on the
temperature of the gas
𝐾𝐸 =
1
𝑚𝑣 2
2
where m = mass, v = velocity
Nature of Gases
Nature of Gases
▪ expansion
– no definite shape, no definite volume
– take shape of container, completely fill container
▪ fluidity
– ability to flow
– particles glide easily past one another
▪ low density
– substance in gaseous state has density about 1/1000 of
the same substance in liquid or solid state
Nature of Gases
▪ compressibility
– particles can be pressed or squeezed together
– reduces sample in size or volume
▪ diffusion
– spontaneous mixing of the particles of two substances
caused by their random motion
Nature of Gases
▪ effusion
– process by which gas particles pass through a tiny opening
Consider the following… (to turn in)
For each characteristic of a gas’s nature, explain how it is
supported by the assumption(s) of the kinetic molecular theory.
expansion – assumption #___
explanation:
fluidity – assumption #___
explanation:
etc.
Graham’s Law of Effusion
Graham’s Law of Effusion
▪ gases at the same temperature have the same kinetic energy
𝐾𝐸𝐴 = 𝐾𝐸𝐵 ∴
1
𝑚𝐴 𝑣𝐴2
2
𝑀𝑀𝐴 𝑣𝐴2 = 𝑀𝑀𝐵 𝑣𝐵2
2
𝑣𝐴
2
𝑣𝐵
=
𝑀𝑀𝐵
𝑀𝑀𝐴
𝒓𝒂𝒕𝒆 𝑨
=
𝒓𝒂𝒕𝒆 𝑩
=
1
𝑚𝐵 𝑣𝐵2
2
where MM = molar mass
∴
𝑣𝐴
𝑣𝐵
=
𝑀𝑀𝐵
𝑀𝑀𝐴
𝒎𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 𝑩
𝒎𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 𝑨
Graham’s Law of Effusion
▪ the rates of effusion of gases at the same temperature and
pressure are inversely proportional to the square roots of
their molar masses
▪ Which gas effuses faster, helium or ethylene oxide, C2H4O?
LITTLE GUYS GO FASTER!
▪ An unknown gas effuses 0.411 times as fast as neon. What is
the molar mass of the unknown gas?
Graham’s Law of Effusion
▪ If a molecule of neon gas travels at an average of 400. m/s
at a given temperature, estimate the average speed of a
molecule of butane gas, C4H10, at the same temperature.
▪ Compare the rate of effusion of carbon dioxide with that of
hydrogen chloride at the same temperature and pressure.