States Of Matter

CHAPTER 5
STATES OF MATTER
INTERMOLECULAR FORCES
Intermolecular forces are the forces of attraction and repulsion between interacting particles (atoms and molecules).
Attractive intermolecular forces are known as van der Waals forces. van der Waals forces include dispersion forces or London forces, dipole-dipole forces, and dipole-induced dipole forces. A particularly strong type of dipole-dipole interaction is hydrogen bonding.

Dispersion Forces or London Forces
Observed between non-polar molecules.
Atoms and nonpolar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed. But, at a particular instant of time, a dipole may develop momentarily. Therefore, electronic charge distribution in one of the atoms, becomes unsymmetrical. This instantaneous dipole distorts the electron density of the other atom which is close to it and as a consequence a dipole.
Eg:- Between He atoms.
Dipole–Induced Dipole Forces
It operates between a polar molecules and a non-polar molecule.
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.
Eg:- Between HCl and H2
Dipole - Dipole Forces
It acts between polar molecules.
The attractiveforce decreases with the increase of distance between the dipoles.
Eg:- Between HCl molecules.

Hydrogen bond
Hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electronegative atom (F, O or N) of another molecule.

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.

THE GAS LAWS
Boyle’s Law
At constant temperature, the pressure of a fixed amount of gas is inversely proportional to its volume.
P1V1 = P2V2 Each curve corresponds to a different constant temperature and is known as an isotherm.

Charles’ Law
It states that at constant pressure, the volume of a fixed mass of a gas is directly proportional to its absolute temperature.

=

Each line of the volume vs temperature graph is called isobar.

The lowest hypothetical or imaginary temperature at which gases are supposed to occupy zero volume is called Absolute zero.

Gay Lussac’s Law
It states that at constant volume, pressure of a fixed amount of a gas is directly proportional to temperature. = Each line of the Pressure vs temperature graph is called isochore. Avogadro Law
It states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules. = A constant
IDEAL GAS EQUATION pV = nRT, R is called gas constant.
R = 8.314 Pa m3 K–1 mol–1
= 8.314 × 10–2 bar L K–1 mol–1
= 8.314 J K–1 mol–1
Combined gas law
P1V1 = P2V2 T1 T2 Dalton’s Law of Partial Pressures
It states that the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases.
PTotal = P1+P2+P3+......(at constant T, V)
Pressure exerted by saturated water vapour is called aqueous tension.
PDry gas = PTotal – Aqueous tension.
Density and Molar Mass = n = m/M = = where d is the density.
Partial pressure in terms of mole fraction
Suppose at the temperature T, three gases, enclosed in the volume V, exert partial pressure P1, P2 and P3 respectively. then,
P1 = P2 = P3 = where n1 n2 and n3 are number of moles of these gases.
PTotal = P1 + P2 + P3 = + = x1

Thus, P1 = x1 PTotal

KINETIC MOLECULAR THEORY OF GASES

The actual volume of the molecules is negligible in comparison to the empty space between them.
There is no force of attraction between theparticles of a gas at ordinary temperature and pressure.
Particles of a gas are always in constant and random motion.
Particles of a gas move in all possible directions in straight lines. Pressure is exerted by the gas as a result of collision of the particles with the walls of the container.
Collisions of gas molecules are perfectly elastic. Total energy of molecules before and after the collision remains same.
At any particular time, different particles in the gas have different speeds and hence different kinetic energies.
Average kinetic energy of the gas molecules is directly proportional to the absolute temperature.

BEHAVIOUR OF REAL GASES
At low pressure and high temperature gases show ideal behavior.

Compressibility factor, Z =

For ideal gas Z = 1 at all temperatures and pressures because pV = n RT.

REAL GAS EQUATION
PIdeal = PReal +
VIdeal = VReal – nb
Hence, ( P + ) (V – nb) = nRT, where a and b are van der Waals constants.
This equation is known as van der Waals equation.
CRITICAL CONSTANTS
CRITICAL TEMPERATURE:
It is the highest temperature at which a gas can be liquefied by applying pressure.
30.98 °C is the critical temperature (TC) of carbon dioxide.

CRITICAL PRESSURE:
It is the pressure of 1 mol of a gas at critical temperature.
73 atm is the critical pressure of CO2
CRITICAL VOLUME:
It is the volume of 1 mol of a gas at critical temperature.
LIQUID STATE
Pressure exerted by an individual gas on the walls of the container is known as partial vapour pressure.
The temperature at which vapour pressure of a liquid is equal to the external pressure is called boiling temperature.
At 1 atm pressure boiling temperature is called normal boiling point.
Normal boiling point of water is 1000C.
If pressure is 1 bar then the boiling point is called standard boiling point.
Standard boiling point of water is 99.60C.

Surface Tension
Surface tension is defined as the force acting per unit length perpendicular to the line drawn on the surface of liquid.

It is denoted by Greek letter γ (Gamma).

It has dimensions of kg s–2 and in SI unit it is expressed as N m–1.

A molecule in the bulk of liquid experiences equal intermolecular forces from all sides. The molecule therefore does not experience any net force. But for the molecule on the surface of liquid, net attractive force is towards the interior of the liquid.

Liquids tend to minimize their surface area. The molecules on the surface experience a net downward force and have more energy than the molecules in the bulk. Therefore, liquids tend to have minimum number of molecules at their surface.

Hence, small drops of mercury form spherical bead instead of spreading on the surface, Particles of soil at the bottom of river remain separated but they stick together when taken out, Liquid rises (or falls) in a thin capillary.

Viscosity

Viscosity is a measure of resistance to flow which arises due to the internal friction between layers of fluid as they slip past one another while liquid flows.
When a liquid flows over a fixed surface, the layer of molecules in the immediate contact of surface is stationary.
The velocity of upper layers increases as the distance of layers from the fixed layer increases. This type of flow in which there is a regular gradation of velocity in passing from one layer to the next is called laminar flow.

‘ η ’ is proportionality constant and is called coefficient of viscosity.
SI unit of viscosity coefficient is 1 newton second per square metre (N s m–2)
Greater the viscosity, more slowly the liquid flows.
Hydrogen bonding and van der Waals forces are strong enough to cause high viscosity.
Windowpanes of old buildings become thicker at the bottom than at the top. This is because glass is an extremely viscous liquid. It is so viscous that many of its properties resemble solids. It flows down very slowly.