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Monday, 28 September 2015

FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 4



 FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 5


Boiling Points:

The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure is known as boiling point of that liquid.
Explanation:
With increase of temperature the K.E of the molecules of a liquid increase due to which the intermolecular attractive forces decrease and hence the vapour pressure of the liquid increases. So we can say that as the temperature of a liquid increases, its vapour pressure also increase and finally a temperature will reach at which the vapour pressure of the liquid rises so much that it becomes equal to the atmospheric pressure. At this condition the temperature of the liquid becomes constant and undergoes no more increase and hence the vapour pressure also undergoes no more increase. This temp is known as boiling point of that liquid. At boiling point the temperature of a liquid does not increase, although heat is continuously provided. It is because of the fact that at becomes equal to the atmospheric pressure. Now highest K.E molecules in the vapour phase enter into the atmosphere and thus some highest K.E molecules go from liquid into vapour phase and at the same time the K.E of the liquid due to heating. Thus the average K.E of the liquid molecules neither increases nor decreases and hence it temperature remains constant. Thus at boiling point the liquid converts into vapour if constantly heated while its temp remains consists. It is important to note that the boiling point of a liquid strictly upon the atmospheric pressure. Thus we can say that the boiling point of a liquid varies with atmospheric pressure. Higher the atmospheric pressure higher will be the of a liquid and vice versa.

Applications:

Some important applications of boiling point are as follow.

(1) Pressure Cooker:
In the pressure cooker the air present above the surface of water is trapped and it has no contact with the outer air. When heat is provided the temp of the liquid increase and hence its Vapour pressure also increases. At 100 degree Celsius the vapor pressure of water becomes equal to the pressure of air in the cooker. None of the water vapours enter into the air but as the air cannot go out. Therefore the vapours of water become a part of the air and thus the pressure of air inside the cooker is increased. Thus with increase of atmospheric pressure (pressure of air inside the cooker) the boiling point of water also increases and thus boiling point of water rises above 100 degree Celsius. Therefore food stuff is cooked earlier in pressure cooker.

(2) Vacuum Distillation:
The process by which the components of a mixture are separated on the basis of difference in their boiling points through evaporation and condensation at reduced pressure is known as vacuum distillation.
In this process a distillation apparatus is used. It consists of a distillation flask containing the mixture whose components are to be separated. A condenser is attached to the distillation flask which helps in the condensation of the evaporated liquid. The other end of condenser is attached with another flask known as “receiving flask”, which receivers’ the separated liquid component of mixture. A vacuum pump is attached to the lower and of condenser which is used for the getting the air out of the distillation apparatus to reduce the pressure of air inside the apparatus. The distillation apparatus is make air tight i.e.: no air can get in or out of it. Vacuum distillation is used for the separation of those liquids from their mixture whose molecules decompose by heating till their boiling point is reached. Thus by reducing the pressure of air inside the dist apparatus the boiling point of the liquid is decreased and can thus by separate without decomposition. For example if we are going to separate glycerin (CH2-CH-CH2) from its mixture then we have to heat the mixture to 290 degree Celsius because (at 1 atmospheric pressure ) the boiling point of glycerin is above 250 degree Celsius, then its molecules decompose. Thus we use vacuum dist. The pressure of air inside the apparatus is decreased from 760mm Hg to 50mm of Hg. With decrease of atmospheric pressure the boiling point of a liquid decreases. Thus at 50 mm of Hg, glycerin boils at 210 degree Celsius instead of 290 degree Celsius. Thus glycerin can now be easily separated from its mixture without decomposition of its molecules.

(3) Food stuff is cooked earlier at plain areas as compared to altitude areas why?
It is because of the fact that with altitude the atmospheric pressure decreases because of O2 decrease. Thus at plain areas the boiling point of water is higher (i.e. 100 degree Celsius) because the atmospheric pressure is lower. That is why the food stuff is cooked earlier at sea level (plain areas) than at altitudes areas .

(4)Steam produces more severe burn than boiling water why?
It is because of the fact that the vapours in the steam are those molecules of water which have highest Kinetic Energy on the other hand all the molecules of boiling are not highest Kinetic Energy molecules some of them have lower Kinetic Energy. As a result the average Kinetic Energy of the vapours in steam is higher than the average Kinetic Energy of molecules of boiling water. But average Kinetic Energy of the molecules of a substance is known as temperature of that substance. Thus we say that steam is hotter than boiling water. That is why steam produces more sever burn than boiling water.

Energies of Phase Changes:

Whenever a matter undergoes some physical or chemical change its energy also changes. The change in energy of a mutter at a constant pressure is known as “enthalpy change”. The different types of enthalpy changes at one atmospheric pressure are as follow.

(1) Heat of Fusion:

The amount of heat absorbed during the conversation of one mole of a substance from solid to liquid state at its meting point is known as heat of fusion that substance

(2) Heat of vaporization:

The amount of heat absorbed during the conversion of one of a substance from liquid to vapour form at its boiling point. Is known as heat of vapour or molar heat of vaporization of that substance.

 (3) Heat of sublimation:

The conversion of a substance from solid into gaseous state without going into liquid state is known as sublimation. “The amount of heat absorbed during the subliming of one mole of a solid to one mole of vapours at a constant temperature and 1atm pressure is known as heat of sublimation

Written by: Asad Hussain

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