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Adsorption is a part of surface chemistry. Surface Chemistry deals with the phenomena occurring on the surface or interfaces. The surface of a solid tends to attract and retain the molecules of the phases with which it comes into contact. The molecules remain only at the surface and not deeper into the bulk. The accumulation of molecular species at the surface and not in bulk is called adsorption. There are two types of Adsorption – Physical Adsorption and Chemical Adsorption. Adsorption occurs in our day-to-day life and on simple surface objects. Adsorption is a surface phenomenon and is affected by factors like nature, surface area, pressure, temperature, and nature of the gas.
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Key Terms: Adsorption, Adsorbate, Adsorbent, Types of Absorption, Factors affecting, Physisorption, Chemisorption, Absorption
Meaning of Adsorption
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The surface of a solid tends to attract and retain the molecules of the phases with which it comes into contact. The molecules remain only at the surface and not deeper into the bulk. The accumulation of molecular species at the surface and not in bulk is called adsorption. Adsorption is a surface phenomenon.
The molecular substance which accumulates at the surface is called adsorbate and the material on the surface where adsorption takes place is called adsorbent.
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| Law of constant proportions | Periodic properties of elements | Law of multiple proportions |
| Atomic mass of elements | Mole concept | Mass percentage |
Types of Adsorption
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Adsorption is mainly divided into two types:
-
Physical Adsorption/Physisorption
Gas molecules accumulate on the surface of a solid via weak van der Waal forces. The process of accumulation is non-specific and reversible. Physical Absorption is favorable at lower temperatures.
-
Chemical Adsorption/Chemisorption
Gas molecules accumulate on the surface of solids via ionic or covalent bonds. The process of accumulation is specific and irreversible and requires high activation energy.
Examples of Adsorption
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Some of the examples of adsorption are:
- The water present in the air gets absorbed on the surface of silica with the help of silica gel.
- When charcoal is added to a colored solution of organic dye and when shaken the solution turns colorless because the molecules of the dye get absorbed on the charcoal surface.
- When gases like chlorine or ammonia are confined in a closed vessel with powdered charcoal, the pressure inside it decreases because the charcoal surface absorbs the gas molecules.
Difference between Adsorption and Absorption
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Adsorption and absorption can happen simultaneously also during the process of sorption but the major difference between adsorption and absorption is:
| Adsorption | Absorption |
|---|---|
| The substance is concentrated on the surface only | The substance is equally divided throughout the surface |
| The concentration increases when on the surface. | The concentration is unaffected by the surface. |
Chemical Factors affecting Adsorption
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The extent of absorption depends on the following factors:
- Nature of the Adsorbate and Adsorbent = The gas of adsorption is related to the nature of the adsorbent. A gas can be adsorbed differently on different surfaces.
- The Surface Area of the Absorbent = The larger the surface area of the solid more is adsorption, the smaller the surface of the area of the solid less is adsorption.
- Pressure = With the increase in the pressure of the adsorbate gas, adsorption also increases. The extent of the absorption is directly proportionate to the pressure.
- Temperature = With the increase in the temperature the extent of adsorption decreases, this is only true for physical adsorption and not for chemical adsorption.
- Nature of gas = If gas is more liquefiable it will be easily absorbed. For example gases like HCI, CI2, and CO2 are more easily adsorbed rather than O2, H2.
Things to Remember
- The extent of adsorption depends on factors like the nature of the adsorbate and adsorbent, the surface area of the adsorbent, pressure, temperature, and nature of the gas.
- The surface of a solid tends to attract and retain the molecules of the phases with which it comes into contact. The accumulation of molecular species at the surface and not in bulk is called adsorption.
- There are two types of adsorption: physical and chemical adsorption.
- Adsorption is a part of surface chemistry. Surface Chemistry deals with the phenomena occurring on the surface or interfaces.
- Adsorption occurs in our day-to-day life like removing coloring matter from solution, controlling humidity, medical industry, and used in making gas masks.
Also Read:
Sample Questions
Ques. Define Adsorption and mention its types. (2 marks)
Ans. The surface of a solid tends to attract and retain the molecules of the phases with which it comes into contact. The molecules remain only at the surface and not deeper into the bulk. The accumulation of molecular species at the surface and not in bulk is called adsorption. Adsorption is a surface phenomenon. The molecular substance which accumulates at the surface is called adsorbate and the material on the surface where adsorption takes place is called adsorbent. There are two types of adsorption: physical and chemical adsorption.
Ques. Mention some examples of adsorption. (2 marks)
Ans. Some of the examples of adsorption are:
- The water present in the air gets absorbed on the surface of silica get with the help of silica gel.
- When charcoal is added to a colored solution of organic dye and when shaken the solution turns colorless because the molecules of the dye get absorbed on the charcoal surface.
- When gases like chlorine or ammonia are confined in a closed vessel with powdered charcoal, the pressure inside it decreases because the charcoal surface absorbs the gas molecules.
Ques. What are the factors which affect adsorption? (1 mark)
Ans. The extent of adsorption depends on factors like the nature of the adsorbate and adsorbent, the surface area of the adsorbent, pressure, temperature, and nature of the gas.
Ques. What are adsorbet and adsorbent? (1 mark)
Ans. The molecular substance which accumulates at the surface is called adsorbate and the material on the surface where adsorption takes place is called adsorbent.
Ques. Difference between Adsorption and Absorption. (3 marks)
Ans. Adsorption and absorption can happen simultaneously also during the process of sorption but the major difference between adsorption and absorption is:
| Adsorption | Absorption |
|---|---|
| The substance is concentrated on the surface only | The substance is equally divided throughout the surface |
| The concentration increases when on the surface. | The concentration is unaffected by the surface. |
Ques. Explain the two types of adsorption. (2 marks)
Ans. Adsorption is mainly divided into two types:
- Physical Adsorption/Physisorption
Gas molecules accumulate on the surface of a solid via weak van der Waal forces. The process of accumulation is non-specific and reversible. Physical Absorption is favorable at lower temperatures.
- Chemical Adsorption/ Chemisorption
Gas molecules accumulate on the surface of solids via ionic or covalent bonds. The process of accumulation is specific and irreversible and requires high activation energy.
Ques. Mention the characteristics of Chemisorption. (2 marks)
Ans. Following are the characteristics of Chemisorption:
- Chemisportion is highly specific and occurs only when there is some chance of chemical bonding between adsorbent and adsorbate.
- Chemisportion is irreversible because it involves a compound foundation.
- Chemisportion increases with the increase in the area of the adsorbent.
- Chemisportion has high enthalpy because it involves compound foundation.
Ques. Mention the characteristics of Physisorption. (4 marks)
Ans. Following are the characteristics of Physisorption:
- Physisorption is not specific because van der Waal’s forces are universal.
- Physisorption is easily reversible since no compound foundation is involved.
- Physisorption increases with the increase in the area of the adsorbent
- Physisorption has low enthalpy because of weak interaction between gas molecules and solid surfaces.
Ques. Mention the differences between Chemisorption and Physisorption. (5 marks)
Ans. The differences between Chemisorption and Physisorption are:
| Physisorption | Chemisorption |
|---|---|
| It arises due to van der Waals forces | It arises because of chemical bond formation. |
| It is not specific. | It is highly specific in nature |
| Physisorption is easily reversible since no compound foundation is involved | Chemisorption is irreversible because it involves compound foundation |
| Physisorption has low enthalpy because of weak interaction between gas molecules and solid surfaces. | Chemisorption has high enthalpy because it involves compound foundation lower |
| Lower temperature is favorable as Physisorption decreases with the increase of temperature. | Higher temperature is favorable as Chemisorption increases with the increase in temperature. |
Ques. What are the applications of adsorption in real life? (4 marks)
Ans. Adsorption occurs in our day-to-day life and on simple surface objects.
- Adsorption helps in the removal of coloring matter from solutions.
- Adsorption helps in controlling humidity.
- Adsorption is also helpful in the medical industry; drugs are used to kill germs by getting adsorbed on them.
- Gas masks are used in coal mines for breathing and to avoid absorbing poisonous gases. Gas masks use the property of adsorption.
Ques. Explain the mechanism of adsorption. (3 marks)
Ans. Adsorption occurs because the surface particles of the adsorbent are in a different environment than the particles inside the bulk. The surface of a solid tends to attract and retain the molecules of the phases with which it comes into contact. The molecules remain only at the surface and not deeper into the bulk. The accumulation of molecular species at the surface and not in bulk is called adsorption.
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