Friday, November 13, 2009

EFFECT OF TEMPERATURE,PRESSURE

EFFECT OF TEMPERATURE

Physical adsorpotion
It occurs rapidly at lower temperature and decreases with increase in temperature.

Chemical adsorption
It increases with increase in temperature and then decreases with the increase of temperature, physical adsorption changes to chemical adsorption.
For ex. Nitrogen is physically adsorbed on iron at 190°c. But when the temperature is raised to 500°c, it is chemisorbed to from a nitride.

EFFECT OF PRESSURE
According to Le-chatlier’s principlel adsorption increases with the increase of pressure. Decreases of pressure causes desorption.

Reversible nature
Physical adsorption
It is a reversible process. The gas adsorbed on to a solid can be removed (desorbed) under reverse conditions of temperature and pressure.

Chemical adsorption
It is an irreversible process because a strong chemical bond is formed between adsorbent and adsorbate.

Heat of adsorption
It is defined as the energy liberated when 1g mole of gas is adsorbed on the solid surface.
In physical adsorption, gas molecules concentrate on the solid surface. Thus, it is similar to the condensation of a gas to liquid. So, adsorption is an exothermic process. The attraction between the gas molecules and solid surface are due to weak vander waal’s forces. Therefore, the heat of adsorption is very small (about 5kcal / mol) for physical adsorption.
In chemical adsorption, the attractive forces are due to the formation for true chemical bonds. Therefore, the heat of adsorption is large (20 to 100 kcal/mol)

Thickness of adsorbed layer of gas
Langmuir showed that at low pressure, the physically adsorbed gas forms only one thick molecular layer. However, above a certain pressure, multi-molecular thick layer is formed. In Chemistry ion, the adsorbed gas is one-molecule thick since chemical combination takes place with the adsorbent surface directly.

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