Factors Affecting the Extent of Adsorption

Have you ever wondered how gas masks can efficiently evade poisonous gases or how soaps and detergents effectively remove grease stains? The answer lies in the phenomena of surface chemistry that deal with the processes occurring at the interphase between two bulk substances. Adsorption is one such phenomenon that occurs at the surface of materials and has far-reaching implications in our day-to-day life.

What is Adsorption?

The surface of solids tends to attract and retain molecules with which it comes into contact. These molecules remain on the surface and do not penetrate the bulk of the solid. Adsorption is the process by which molecular species accumulate on the surface rather than going deeper into the volume of a solid or liquid. The substance which accumulates or concentrates on the surface is called adsorbate, and the material or medium on which adsorption takes place is termed as adsorbent. Since adsorption is a surface phenomenon. Finely divided substances with a large surface area serve as suitable adsorbents.

How does Adsorption differ from Absorption?

Adsorption can be differentiated from absorption based on:

• Definition: Adsorption is the loose adherence of gases, liquids, or dissolved solids onto the surface of another solid or liquid. In absorption, atoms, molecules, or ions enter the bulk of another solid or liquid material.

• Nature: While adsorption is a surface phenomenon, absorption is a bulk process.

• Reaction rate: The rate of adsorption increases until equilibrium is reached. Absorption occurs at a uniform rate.

• Heat exchange: While adsorption is exothermic, absorption is an endothermic process.

• Temperature: Adsorption is seen at lower temperatures. However, absorption is unaffected by temperature.

• Concentration: Concentration of the adsorbed substance changes within the medium. But the concentration of absorbed substances remains constant throughout the medium.

• Application: Adsorption is used in air conditioners, water purifiers, chillers, etc. Absorption finds application in refrigerants, ice production, cold storage, etc.

What are the Different Types of Adsorption?

Adsorption of gases on solids is mainly of two types:

• Physical adsorption or physisorption: Gas molecules accumulate on the surface of solids via weak van der Waal forces. This process is non-specific and reversible. Physisorption is favoured at low temperatures, but as the temperature rises, chemical adsorption begins. 

• Chemical adsorption or chemisorption: Gas molecules accumulate on the surface of solids via ionic or covalent bonds. Since chemical bonds are involved, the process is highly specific, irreversible in nature, and requires high activation energy.

Factors affecting Adsorption of Gases on Solids

The extent of adsorption of a gas on a solid depends on the following factors:

• Nature of the Adsorbate and Adsorbent

Easily liquefiable gases like carbon dioxide, ammonia, chlorine, etc. have higher adsorption than elemental gases like oxygen, nitrogen, etc. But why are easily liquefiable gases adsorbed more readily? The reason is that easily liquefiable gases have higher intermolecular forces of attraction and are therefore more strongly adsorbed. An adsorbent that is porous and finely-powdered such as charcoal and Fuller's earth adsorb more when compared to hard and non-porous materials.

• The Surface Area of the Adsorbent

A larger surface area of the solid adsorbent allows more adsorption to occur. Also, smaller particle size imparts more surface area.

• Pressure

Adsorption increases with an increase in pressure of the adsorbate gas, and this increase is most significant at low temperatures. The extent of adsorption is directly proportional to small pressure ranges, but it achieves a limiting value at high pressures when all the adsorption sites are saturated.

• Temperature

Since adsorption is an exothermic process, as per Le-Chatelier's principle, a rise in temperature decreases the extent of adsorption. But this is true only for physical adsorption. In the case of chemical adsorption, the requirement of high energy of activation causes the extent of adsorption initially to increase with a rise in temperature but then gradually falls with rising temperatures.

What are Some Examples of Adsorption?

Let us look at some examples that demonstrate adsorption:

• Silica gel helps dry the air because the water molecules present in the air get adsorbed on the surface of silica gel.

• If charcoal is added to a coloured solution of organic dye and shaken well, the solution turns colourless because the dye molecules get adsorbed on the charcoal surface.

• If a gas like chlorine or ammonia is confined in a closed vessel with powdered charcoal, the pressure inside it decreases because the charcoal surface adsorbs the gas molecules.

Key Learnings from the Chapter - 

• Adsorption is the process by which molecules are stuck on the solid surface

• The molecule stuck on the surface is called absorbents 

• The surface which accumulated the molecules is the absorbate

• Two types of adsorption namely, physical and chemical adsorption

• Factors like pressure and temperature can affect the process of adsorption

Watch the video lectures on the topic to get a better understanding of the concepts.