Presentation prepared by: Adnan Akbar Subject:- Chemistry Teacher :- Sir Noor Ali

Kinetic molecular theory The experimental observations about the behavior of gases discussed so far can be explained with a simple theoretical model known as the kinetic molecular theory. This theory is based on the following postulates, or assumptions. Every gas consist of a large number of very small particles called molecules. Gases like He, Ne, Ar have monoatomic molecules. The molecules of a gas move haphazardly, colliding among themselves and with the walls of the container and change their directions. The pressure exerted by a gas is due to the collisions of its molecules with the walls of a container. The collisions among the molecules are perfectly elastic.

The molecules of a gas are widely separated from one another and there are sufficient empty spaces among them. The molecules of a gas have no forces of attraction for each other. The actual volume of molecules of a gas is negligible as compared to the volume of the gas. The motion imparted to the molecules by gravity is negligible as compared to the effect of the continued collisions between them The average kinetic energy of the gas molecules varies directly as the absolute temperature of the gas.

Slo 5.1.1 describe the following diffusion, compression, expansion, motion of molecules, intermolecular forces and kinetic energy in liquids using kinetic molecular theory: Diffusion in liquids: -the process by which molecules spread from areas of high concentration, to areas of low concentration. Diffusion occurs in liquids but more slowly than in gases. The particles are not as free to move about.

Compression in liquids: Liquids cannot be compressed because of the nature of molecules making up Liquids. Unlike, solids, the molecules making up liquids are not close enough to be compressed.

Expansion in liquids: Liquids do not have a definite shape. They take the shape of the container. Thus, we can specify a liquid by its volume. Hence, we can speak of volume expansion only for liquids. Expansion of liquids is much greater than that of solids.

Motion of molecules: In a liquid the force of attraction between the particles is weaker than it is in the solid. It is still strong enough that the particles are held close to each other but they are now free to move. If the liquid is heated, the particles move faster and faster until they overcome the force of attraction between them.

Inter molecular forces The forces holding molecules together are called intermolecular forces. The inter molecular forces bring the molecules close together and give particular physical properties to the substances in gaseous, liquid and solid states. There are four types of these forces. 1- Dipole-dipole forces 2- ion-dipole forces 3-Dipole-induced dipole forces 4- London forces 5- Hydrogen bonding

Slo 5.2.1: explain applications of dipole- forces, hydrogen bonding and London forces Dipole-dipole forces: The positive end of one molecule attracts the negative end of the other molecule and these electrostatic forces of attraction are called dipole-dipole force. The strength of these forces depends upon the electro negativity difference between the bonded atoms and the distance between the molecules. The distances between molecules in the gaseous phase are greater so these forces are very weak in this phase.

In liquids these forces are reasonably strong In liquids these forces are reasonably strong. For example in case of HCl: Greater the strength of these dipole-dipole forces, greater are the values of thermodynamic parameters like melting points, boiling points, heats of vaporization etc.

Instantaneous dipole-induced dipole forces or London forces. The momentary force of attraction created between instantaneous dipole and the induced dipole is called dipole-induced dipole interaction or London forces. Due to London forces boiling points of elements increases down the group.

Hydrogen bonding Hydrogen bonding is the electrostatic force of attraction between a highly electronegative atom and partial positively charged hydrogen atom

Applications of hydrogen bonding: Cleansing action of soaps and detergents: soaps and detergents perform the cleansing action because the polar part of their molecules are water soluble due to hydrogen bonding and the non-polar parts remain outside water , because they are alkyl or benzyl portions and are insoluble in water.

slo 6. 1. 1 describe simple properties of solids e. g slo 6.1.1 describe simple properties of solids e.g. diffusion, compression, expansion, motion of molecules, intermolecular forces and kinetic energy with reference to kinetic molecular theory: As we should remember from the kinetic molecular theory, the molecules in solids are not moving in the same manner as those in liquids or gases. Solid molecules simply vibrate and rotate in place rather than move about. Solids are generally held together by ionic or strong covalent bonding, and the attractive forces between the atoms, ions, or molecules in solids are very strong. In fact, these forces are so strong that particles in a solid are held in fixed positions and have very little freedom of movement. Solids have definite shapes and definite volumes and are not compressible to any extent.

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