Grade 12 → Surface chemistry ↓
Adsorption and its types (Physicosorption and Chemisorption)
In the fascinating world of surface chemistry, the concept of adsorption plays a key role in understanding how particles interact with surfaces. Adsorption is the process by which atoms, ions, or molecules from a gas, liquid, or dissolved solid stick to a surface. This process is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. Adsorption is a surface phenomenon, meaning that it occurs only at the surface level of substances.
What is adsorption?
Adsorption can be defined as the adhesion of atoms, ions, biomolecules or molecules of a gas, liquid or dissolved solid to a surface. This process forms a film of the adsorbent on the surface of the adsorbent. The adsorbent is the substance that is deposited on the surface of the adsorbent, while the adsorbent is the substance to which the particles stick.
Visual example:
This illustration shows gas molecules (the adsorbent) sticking to the surface of a solid (the adsorbent).
The adsorption process can be affected by several factors, such as the nature of the adsorbent and adsorbed substance, temperature, pressure, and surface area. Generally, a larger surface area facilitates greater adsorption because more particles can stick to the surface.
Types of absorption
Adsorption can be classified into two types depending on the nature of the forces involved: physisorption and chemisorption. Each type exhibits different characteristics and is driven by different forces.
Physical adsorption
Physisorption is driven by weak van der Waals forces. These forces are relatively weak compared to chemical bonds, so physisorption is usually reversible. Here, the adsorbent is bound to the adsorbent surface through weak interactions, and there is no significant change in the electronic structure of the adsorbent molecules.
Some of the salient features of physisorption are as follows:
- Weak Force: The forces involved are weak van der Waals forces.
- Reversibility: Since the forces are weak, the process is usually reversible.
- Low activation energy: The energy required to initiate physical absorption is usually low.
- Multi-layer adsorption: It can form multiple layers of adsorbate molecules.
- Temperature effect: Generally, physisorption decreases with increase in temperature because higher kinetic energy opposes the adsorption process.
Example: The adsorption of nitrogen gas (N 2) on the surface of activated charcoal is an example of physical absorption. Nitrogen molecules stick to the surface due to weak van der Waals forces.
Chemical absorption (chemical absorption)
Chemisorption involves the formation of strong chemical bonds between the adsorbent and the surface of the adsorbent. This type of adsorption results in significant changes in the electronic structure of the adsorbent, making it more stable.
Some important features of chemisorption are as follows:
- Strong forces: Chemical bonds are strong, such as covalent or ionic bonds.
- Irreversible: Because of the strong bonds, chemisorption is usually irreversible.
- High activation energy: This process requires higher activation energy than physisorption.
- Monolayer adsorption: It usually involves a monolayer of adsorbate molecules.
- Temperature effect: Chemical absorption generally increases with increasing temperature because this provides energy to overcome activation barriers.
Example: The adsorption of hydrogen gas (H 2) on the surface of a metal such as nickel involves the formation of a chemical bond between the hydrogen molecules and the metal, which is an example of chemisorption.
Comparison of physisorption and chemisorption
Both physisorption and chemisorption play important roles in various chemical processes. The differences between these two types of adsorption are significant:
| Aspect | Physical absorption | Chemical absorption |
|---|---|---|
| Force of attraction | Weak van der Waals force | Strong chemical bonds |
| Reversibility | Generally reversible | Usually irreversible |
| Activation energy | Less | High |
| Temperature dependence | Decreases as the temperature increases | Increases as the temperature rises |
| Layer creation | Multiple layers possible | Usually a monolayer |
Applications of absorption
Adsorption has numerous applications in industry and technology. Here are some notable examples:
- Gas mask: Activated charcoal absorbs toxic gases, allowing clean air to be inhaled.
- Water purification: Activated carbon filters use adsorption to trap impurities in water.
- Industrial catalysis: Metal surfaces adsorb reactant molecules, increasing reaction efficiency.
- Chromatography: A technique for separating components based on adsorption in a mixture.
- Medicinal uses: Adsorption is used in drug delivery systems and detoxification.
Factors affecting absorption
Several factors can affect the extent of absorption:
- Nature of adsorbate and adsorbent: Molecular structure, polarity and chemical nature affect adsorption.
- Surface area of the adsorbent: Larger surface area provides more space for adsorption.
- Temperature: As temperature increases, physical adsorption generally decreases, while chemisorption may increase.
- Pressure of the gas adsorbed: Higher pressure generally results in greater adsorption, up to saturation.
The study and application of adsorption is an integral part of various scientific and technological advancements. Understanding the intricate details of physisorption and chemisorption is the key to optimizing processes in catalysis, separation technologies, and environmental science.
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