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class 11 chemistry ncert chapter 5
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UNIT 5 STATES OF MATTERTHE GASEOUS STATE
PHYSICAL PROPERTIES OF GASES
Gases have neither definite shape nor definite volume.
The particles are far apart from one another.
The intermolecular forces of attraction are very weak.
Gases are highly compressible.
Gases exert pressure equally in all directions.
Gases have much lower density than solids and liquids.
Gases mix evenly and completely in all proportions without any mechanical aid.
INTERMOLECULAR FORCES
The forces of attraction and repulsion between interacting particles like atoms and
molecules are called intermolecular forces.
Intermolecular Forces are of the following types.
VANDER WAALS FORCES
Vander Waals Forces are weak, short range electrostatic attractive intermolecular
forces between uncharged molecules.
DISPERSION FORCES OR LONDON FORCES
London Force is the weakest intermolecular force.
London Force is a temporary attractive force that results when the electrons in two
adjacent atoms occupy positions that make the atoms form temporary dipoles.
Atoms and non-polar molecules are electrically symmetrical and have no dipole
moment.
But in an atom, at a particular moment, the nucleus is shifted towards one side and
the electrons, to the other side.
So a temporary dipole is created.
This results in the development of instantaneous dipole on the adjacent atom for a
very short time.
These temporary dipoles of atoms attract each other.
This force of attraction between temporary dipoles is termed as London forces or
dispersion forces.
These forces are important only at short distances.
DIPOLE-DIPOLE FORCES
Dipole-Dipole Forces are attractive forces between the positive end of one polar
molecule and the negative end of another polar molecule.
These forces act between the molecules possessing permanent dipole.
These molecules interact with the neighbouring molecules.
This interaction is stronger than the London forces.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY ERNAKULAM Page | 1
, But it is weaker than ion-ion interaction because only partial charges are involved.
The attractive force decreases with the increase of distance between the dipoles. E.g.
HCl
DIPOLE INDUCED DIPOLE FORCES
Dipole-induced dipole forces are weak attractive forces.
These forces operate between the polar molecules and the non-polar molecules.
Permanent dipole of the polar molecule induces dipole on the electrically neutral
molecule by deforming its electronic cloud.
Thus an induced dipole is developed in the other molecule.
The attraction between these molecules is termed as dipole–induced dipole force.
THERMAL ENERGY
Thermal Energy is the energy of a body arising from motion of its atoms or molecules.
It is directly proportional to the temperature of the substance.
It is the measure of the average kinetic energy of the particles of the matter and is
thus responsible for movement of particles.
This movement of particles is called thermal motion.
THE GAS LAWS
1. BOYLE’S LAW
The law states that at constant temperature, the pressure of a fixed amount of gas is
inversely proportional to its volume.
1
Mathematically it can be written as P
V
1
P = a constant x
V
PV = a constant
Consider a fixed mass of gas at a constant temperature.
Let its pressure be P1 when its volume is V1
Let the pressure be P2 when its volume is V2.
Then according to Boyle’s law, P1V1 = P2V2
GRAPHICAL REPRESENTATION OF BOYLE’S LAW
A graph is plotted between PV against P at a given temperature.
The graph shows that PV remains constant.
The graph obtained by plotting volume against pressure at a given temperature is
called an Isotherm.
PREPARED BY HAIZEL G. ROY, HSST (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY ERNAKULAM Page | 2
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