Adsorption

Etymology:

The word adsorption is often confused with absorption. Apparently there’s just a “d” in place of “b” in absorption, but there’s much more in difference.

Definition:

In absorption, a substance enters into another substance and gets distributed uniformly throughout the 2^nd substance

For example when you dip a biscuit in the cup of tea, the biscuit absorbs the tea, i.e. the tea gets distributed uniformly in that particular part of the biscuit which was dipped.

Similar thing happen when you soak a sponge in the tub full of water. The water enters into the sponge from all directions and gets distributed in it uniformly. So we say that the sponge has absorbed the water

Hence absorption phenomenon happens when a substance enters into another substance from all directions and gets distributed in it uniformly

This is not the case with adsorption

Because in adsorption phenomenon, one substance doesn’t get distributed uniformly in another substance. Instead it accumulates only on the surface of 2^nd substance

Simplest example of adsorption phenomenon is, writing on the board with a chalk. The chalk material doesn’t get deep in all layers of the board. Rather it just stays on the surface of the board where we have written.

Thus,

Adsorption is a phenomenon in which one substance accumulates only on the surface of another substance in higher concentration

Examples: Silica Gel, Chalk on the board

The substance which gets adsorbed into another substance is called adsorbate, while the substance which adsorbs the first substance is called adsorbent.

In the example above, the chalk is adsorbate while the board is adsorbent

Normally the adsorbent is in bulk quantity

It is also worth noting that the terms adsorbate and adsorbent are analogous to solute and solvent in case of a true solution

Let’s distinguish adsorption from absorption in more details

Absorption	Adsorption
It is a bulk phenomenon	It is only a surface phenomenon
Distribution of absorbate (solute) is uniform throughout the bulk	Non-uniform distribution of adsorbate (higher on the surface than in the bulk)
It is neither exothermic nor endothermic	It is an exothermic phenomenon
It happens at a uniform rate, from beginning to the end	The rate of adsorption is high in the beginning but it gradually falls down Reason: More surface area is available in the beginning but it declines with time as the surface is pre-occupied with adsorbate particles
It is independent of surface area (because of being a bulk phenomenon)	It is highly dependent on the surface area (because it is a surface phenomenon)

Now that we have learnt the difference between absorption and adsorption, let’s discuss the types of adsorption

Types of adsorption

Based upon the concentration of adsorbate on the surface, we can achieve adsorption in two ways

1) Positive adsorption

2) Negative adsorption

Positive adsorption

If the concentration of adsorbate is more on the surface as compared to its concentration in bulk phase, such an adsorption is called positive adsorption

Examples: Shaking the concentrated solution of KCl with blood charcoal gives us positive adsorption

Negative adsorption

If the concentration of adsorbate is less on surface as compared to its concentration in bulk phase, such an adsorption is called negative adsorption

Examples: When a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption

What is the reason for adsorption?

Consider a molecule present in bulk of the liquid which is surrounded by many other molecules

For this type of molecule we have,

• Balanced forces of attraction
• The molecule is stable
• The molecule is less reactive

Reason: Since the molecule is stable, there is less tendency to make bonds with new molecules, less energy and hence less reactivity

Now consider another molecule near the surface of the liquid. This molecule is surrounded from three sides only, not from all sides.

For this type of molecule we have,

• Unbalanced forces of attraction
• The molecule is unstable
• It is more reactive

Reason: The molecule is unstable hence there’s more tendency towards making new bonds for gaining stability, this corresponds to more energy and hence higher reactivity

In order to gain stability and to balance the unbalanced forces of attraction, the surface molecules will adsorb any other molecule coming into their contact at the surface. And hence adsorption takes place.

The reactivity of the surface molecules drop significantly after this process and this is followed by evolution of the heat, which is known as heat of adsorption.

The amount of heat evolved when one mole of an adsorbate (gas or liquid) is adsorbed on the surface of an adsorbent is called heat of adsorption.

Because heat is evolved in the process of adsorption, it is an exothermic process

After the adsorption phenomenon, the unbalanced and unstable surface molecule with higher energy converts to a balanced and stable molecule with lesser energy.

Types of adsorption (continue)

Adsorption can also be classified based on the strength of intermolecular forces present between the molecules of adsorbate and adsorbent. This way, adsorption is classified into:

1) Physisorption

2) Chemisorption

In Physisorption, the adsorbate and adsorbent molecules at the surface are held by weak Vander Waals Forces. While in chemisorption, these molecules at the surface are held by strong covalent bonds.

This phenomenon was studied by an American chemist Irvin Langmuir

Let’s differentiate Physisorption from chemisorption

Physisorption	Chemisorption
The adsorbate particles are attached to the surface simply by Vander Waals Forces	The particles of adsorbate are held to the surface by chemical bonds, forming surface compounds
It is relatively weak	It is relatively strong
The heat of adsorption is low ( 20KJ mol-1 – 40KJ mol-1 )	The heat of adsorption is high ( 40KJ mol-1 – 200KJ mol-1 )
It is reversible	It is irreversible
It involves the formation of multiple layers of adsorbed particles	It involves the formation of monolayer of adsorbed particles
It is not specific in nature i.e. may form in all conditions	It is specific in nature i.e. occurs only when there is a tendency to form a chemical bond between adsorbate and adsorbent
It occurs appreciably at low temperature. And extent of adsorption decreases as the temperature of the surfaces is raised	It occurs at all temperatures. But the extent of adsorption increases with rise in temperature of the surface

Example of Physisorption: adsorption of gases on charcoal (masks used by minors)

Example of Chemisorption: hydrogenation of a compound in the presence of nickel

Factors affecting adsorption:

1) Nature of adsorbent

As adsorption is a surface phenomenon it depends on the surface of adsorbent. Higher the surface area higher the rate of adsorption and vice versa. Following trends are observed while dealing with nature of the adsorbent

i. Finely Divided Substance : A substance in powdered form provides larger surface area than what it does in solid form. Hence finely divided substances are good adsorbents

ii. Rough Surfaces: Rough surfaces provide more surface area than smooth surfaces. That’s why adsorption in powerful in rough surfaces

iii. Activated Charcoal: Charcoal that has been activated (by heating or treating) is more powerful adsorbent than non-activated charcoal

2) Nature of adsorbate

As far as adsorbate is concerned, easily liquefiable gases are adsorbed to greater extent.

Gas	SO2	NH3	CO2	CH4	CO	N2	H2
Critical Temperature (K)	430	406	304	190	134	126	33
Amount of adsorption (in ml)	380	180	48	16.2	9.3	8.0	4.5

 

 

 

It refers to the gases whose molecules are held by strong inter-molecular forces and have higher tendency to become a liquid

The ease of liquefaction can also be linked to the critical temperature, which is the minimum temperature required for liquefaction of the gas. Above this temperature, the gas cannot be liquefied. The table below shows how critical temperature is linked to adsorption of a gas

From above table we can conclude that,

“Higher the critical temperature of the gas, more easily it is liquefied, hence adsorbed to the greater extent”

And as the critical temperature decreases, ease of liquefaction decreases and so does the adsorption.

3) Temperature

We know that adsorption is an exothermic phenomenon. This means it accelerates by evolution of the heat (i.e. throwing the heat outside). This means the phenomenon favors decrease in heat (temperature) of the system.

Also because adsorption is a surface phenomenon, heat of the system means heat at the surface of the system.

Thus,

“Adsorption phenomenon (rate of adsorption) increases with decrease in temperature of the surface. Similarly increase in temperature of the surface generally halts the adsorption phenomenon”

4) Pressure

Keeping the temperature of the surface constant, increase in pressure at the surface increases adsorption phenomenon up to a certain value of pressure. Any increase in pressure after that particular value will not increase the rate of adsorption, rather it becomes constant