Kinetic Theory of Gases
1. Gaseous particles are in motion. Particles are made
up of molecules and as such molecules are in motion.
The moving molecule theory of gases is known as
the kinetic theory of gases.
2. Actual volume of a gas is negligible as compared to
the empty space between molecules.
3. Molecules experience attractive forces and therefore
these are in rapid, random (Zig-Zag) motion.
4. Molecules collide each other but there is no loss of
kinetic energy.
Boyle’s Law
Volume of a fixed mass of a gas at constant
temperature is inversely proportional to the pressure of
the gas.
Charles’ Law
The volume of a given mass of gas at constant
pressure varies directly with absolute temperature.
Mathematically it can be seen as:
V a T at constant P and n
Avogadro’s Hypothesis
At constant temperature and pressure, volume of a
gas varies directly with no. of moles of the gas.
Mathematically it can be seen as :
V a n at constant T and P.
Equation of State or Ideal Gas Equation
It is a combined gas law and is established with the help
of Boyle’s law, Charle’s law and Avogadro’s hypothesis. i.e.
PV = nRT where R is gas constant and is equal to
8.314 JK-1 mol-1
Real and Ideal Gas
Gases which obey Boyle’s law, Charle’s law,
Avogadro’s law and equation of state are called ideal,
or perfect gases and those which show deviations from
these laws are called non-ideal or real gases. Almost all
gases show deviation from these laws and hence almost
all gases are non-ideal, or real gases.
Vander Waal Equation
Since none of the gases is known to behave according
to the ideal gas equation, these gases are called as real
gases.
P (V-nb) = nRT (for n mole of the gas.)
It is the modification of the ideal gas equation PV = nRT
where pressure correction ‘a’ as well as volume correction
‘b’ have been applied; a and b are Vander Waal’s constants
whose values depend on the nature of the gas.
Liquefaction of Gases
When gases are compressed and cooled (i.e.
subjected to high pressure and low temperature), they
are converted to liquid. The process of gas ® liquid is
known as liquifaction and the temperature at which a
gas is converted to liquid is known as the liquifaction
temperature. At high pressure and low temperature,
attractive forces between gaseous molecules is high and
molecules are drawn together to form a liquid. The
liquifaction of gases is indicative of the attractive forces
between the gaseous molecules.
Critical Temperature
The temperature at which a gas can be liquified is
GASEOUS BEHAVIOUR CHRONICLE
called liquifaction temperature and this liquifaction
temperature is also called critical temperature because
above this temperature, the gas cannot be liquified no
matter how high the pressure is. Thus above the critical
temperature gaseous state exists, at the critical
temperature liquifaction occurs and below critical
temperature liquid state exists. Critical temperature
depends on the attractive forces present in the gaseous
molecule. Thus attractive forces in HCl (H+d – Cl–d) is
greater than that in CO2 (O–d ¬C+d+d ®O–d ) and so
critical, temperature of HCl is higher than that of CO2
i.e., HCl is liquified at a higher temperature than CO2.
Deviation from Ideal Gas Behaviour
Gas behaves like ideal gas at high temperature and
low pressure. If gases behave like ideal gas through
obeying PV=nRT gas equation, it means that at high
temperature – high pressure, PV = nRT is valid but at
(i) low temperature – high pressure and
(ii) low temperature – low pressure, there is deviation.
Deviation from ideal gas behaviour is less at high
temperature and greater at low temperature.
Inversion Temperature
It is the temperature at which the gas neither shows
heating effect nor shows cooling effect on expansion.
Diffusion
Inter mixing of gases irrespective of the force of
gravity is known as diffusion. It refers to the flow of
molecules from a region of high concentration to a region
of low concentration.
Effusion
The passage of gases through a small aperture under
pressure is known as effusion. Graham’s law of diffusion
is also applicable to effusion.
Graham’s Law of Diffusion
Under similar conditions of temperature and
pressure, the rates of diffusion of gases are inversely
proportional to the square roots of their molecular masses
or their densities or directly proportional, to their
pressures.
