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Surface ChemistryAdsorption

Adsorption is the process of accumulation of molecules at the surface rather than in the bulk of asolid or liquid which results into the higher concentration of molecules at the surface.

The substance which is absorbed at the surface is called adsorbate and the substance on which it is adsorbed iscalled absorbent. By applying heat or by reducing pressure, the reverse process, i.e. removal of the adsorbedsubstance from the surface occurs, called desorption. The adsorption of gases on the surface of metals is calledocclusion.

Generally, we confuse between the two terms adsorption and absorption, so following are the point differencesbetween the two terms:

Adsorption Absorption

1. It is a surface phenomenon in which

substance is con-centrated only at thesurface of adsorbent.

1. It is a bulk phenomenon where the

substance is uniformly distributed through-out the bulk of the solid.

2. Rate of adsorption is initially high and

then decreases till the equilibrium is

attained.

2. Rate of absorption is uniform throughout

the process.

Most common example which explains the distinction between the adsorption and absorption process is the

interaction of water vapour with silica gel and anhydrous as illustrated in the following figure:

It is found that solids, in finelly divided state act as good absorbents because of large surface area. Some examples ofgood absorbents are charcoal, alumina gel, silica gel, finely divided metals, clay, colloids, etc.

(i) Mechanism of Adsorption


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From the above figure, it is clear that inside the adsorbent or bulk, all the forces acting between the particles aremutually balanced. But on the surface the particles experience only the residual attractive forces by the atoms ormolecules from one side i.e., from bulk side. These forces of adsorbent are responsible for attracting the adsorbateparticles on its surface.

During adsorption the residual forces on the surface decreases. As a result surface energy decreases which appears as

heat. Thus adsorption is an exothermic process (i.e., is always negative). When a gas is adsorbed, its entropy

decreases due to restriction of freedom of movement of its molecules (i.e., is also negative).

As per the thermodynamic requirement, at constant pressure and temperature, must be

negative, so the value of must have sufficiently high negative for a process to be spontaneous. As the adsorption

proceeds, becomes less and less negative and ultimately becomes equal to , so that becomes zeroand the equilibrium state is obtained.

(ii) Types of Adsorption

On the basis of the nature of bond between the adsorbate and adsorbent, adsorption may be of two types physicaladsorption or physisorption and chemical adsorption or chemisorption.

Following table describes the comparison between the two types of adsorption.

Physisorption Chemisorption

1. It involves weak van der Waals forces between

the adsorbate and adsorbent.

1. If involves the chemical bond

(covalent or ionic) between adsorbate

and the adsorbent.

2. In this case, enthalpy of adsorption is low 2. In this case, enthalpy of adsorption

is high

3. No appreciable activation energy is required. 3. High activation energy is required,


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thus also called activated adsorption.

4. Not specific in nature as the van der Waals

forces are same for all types of adsorbate.

4. Highly specific in nature because it

occurs only if there is possibility of

chemical bond between the

adsorbate and adsorbent.

5. Reversible in nature, i.e.,

5. Irreversible in nature.

6. It depends on the physical nature of gas, i.e.,

the gases which are more liquefiable can be

easily adsorbed.

6. It depends on the chemical nature

of gas, i.e., gases which can react with

the adsorbent show chemisorptions.

7. It results into multi-molecular layers onadsorbent surface under high pressure.

7. It results into unimolecular layer onadsorbent.

(iii) Effect of Pressure and Temperature on Adsorption

In both types of adsorption, i.e., physisorption and chemisorption, when the pressure is increased, more of gas isadsorbed because the volume of the gas decreases (Le-chateliers principle).

As the adsorption is an exothermic process, it readily occurs at low temperature and decreases with the rise intemperature according to Le-chateliers principle. But the chemisorption process is very slow at low temperaturebecause of high activation energy, therefore it increases with the rise in temperature like most chemical changes.

It is observed that physisorption of a gas adsorbed at low temperature may change into chemisorption at high

temperature.

(iv) Adsorption Isotherm

A curve which shows the expression of the variation in the amount of gas (x) adsorbed by a definite mass of adsorbent(m) with the pressure (p) at constant temperature, is known as adsorption isotherm.

Freundlich Adsorption Isotherm

Given by Freundlich in 1909, according to which:

Where, k, p and n are constants which depend on the nature of adsorbent and the gas at a particular temperature.

Taking log of both sides,


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Following curve is the representation of Freundlich adsorption isotherm which indicates that at a fixed pressure,there is a decrease in physical adsorption with increase is temperature.

Fig. Plot of against p

(Adsorption isotherm)


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Fig. Plot of against log p(Freundlich isotherm)

The equation corresponds to straight line equation So, theFreundlich isotherm is valid only when a straight line is obtained in the above plot otherwise not valid.

Practically the value of ranges between 0 to 1 (Probable range 0.1 to 0.5)

Freundlich isotherm is not applicable at high pressure because it doesnt explain why the experimental isothermsalways seem to approach saturation at high pressure.

(v) Adsorption of Solution by a Solid :

Solids can also adsorb solutes from solution like they adsorb gases. For example, the litmus solution becomes

colourless when shaken with charcoal. In the presence of magneson reagent, the precipitate of acquires

blue colour which is due to adsorption of magneson.

Freundlichs equation describes the behaviour of adsorption from solution. In this case, instead of pressure,concentration of solution (C) is taken into account. The equation is:

C = equilibrium concentration i.e., when adsorption is complete.

When a plot of against log C is drawn, a straight line is obtained that proves the validity of Freundilichisotherm.

(vi) Some Applications of Adsorption :


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(a) Silica and aluminium gels are used as adsorbents to remove the moisture and control humidity.

(b) Gas mask which consists of activated charcoals or mixture of adsorbents, is used for breathing in coal mines toadsorb poisonous gases.

(c) Adsorbent removes the colouring matter from solutions. For example, animal charcoal removes colours fromsolutions by adsorbing coloured impurities.

(d) Adsorption helps in creating high vacuum. Charcoal removes the traces of air by its adsorption from a vesselevacuated by a vacuum pump to give a very high vacuum.

(e) A number of drugs are used to kill the germs by getting adsorbed on them.

(f) Chromatography has a number of applications in analytical and industrial fields. All chromatographic processesare based on the phenomenon of adsorption.

(g) A mixture of noble gases can be separated by adsorption of gases on coconut charcoal at different temperaturesdue to the difference in degree of adsorption of gases by charcoal.

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