What Is Adsorption? Key Concepts and Physics Applications Explained
All solid substances have the capabilities to attract to their surfaces molecules of solutions or gases with which they are contacted is called adsorption. Here in this article, we will understand the adsorption meaning, types of adsorption like chemical adsorption, physical adsorption, isothermal adsorption. Also, we will discuss different adsorbate and adsorbent. So, let’s discuss adsorbate and adsorbent before any discussion. Well, the solids that are used to adsorb dissolved substances or gases are known as adsorbents and the molecules that are adsorbed by solids are collectively known as adsorbate. Let’s take an example of charcoal. Charcoal is an adsorbent as it removes impurities and poisons from air or stream in gas masks.
Define Adsorption
Adsorption is the process of collecting molecules by the internal surface or external surface of solids or by liquid surface. Adsorption refers to the collecting of molecules by the external surface or internal surface (walls of capillaries or crevices) of solids or by the surface of liquids. You may have confusion with the word adsorption and absorption. Absorption is the process of penetrating the substance of blocks of amorphous liquids or solids, or into the actual interior crystals. Sometimes a solid takes up the liquid or gas without specifying a particular process of absorption or absorption, such process is called sorption.
Difference Between Adsorption and Absorption
Most people have confusion between these two terms as they sound similar but both the processes are different. Adsorption is the surface phenomenon and is an exothermic process. It depends on temperature and initially, the process will increase and will decrease gradually. Whereas, absorption is an endothermic process. Absorption is not affected by temperature and occurs at uniform rates. Substances penetrate the surface of the material. Absorption is spontaneous whereas adsorption is not spontaneous. Absorption is unique throughout the surface area and adsorption is not unique on the surface area. Molecule interaction is higher in absorption than adsorption.
Types of Adsorption
Adsorption is classified into two types further, physical adsorption and chemical adsorption. Adsorption, an exothermic process is an attractive force that takes place in-between the adsorbent and adsorbate, where heat is released. Moreover, the chemical and physical adsorption also depends upon the forces between adsorbent and adsorbate. In physical adsorption, gases get condensed to liquids depending on Van der Waals, or forces exist between adsorbates and solid absorbents. If the temperature is low and gas pressure is quite high, then solid can adsorb any gas without concerning chemical specifications. Whereas in chemical adsorption, the process depends upon the chemical forces acting upon the solid surfaces and gas absorbates. Chemical adsorption needs a higher temperature as compared to physical adsorption. Chemical adsorption involves energy activation, hence, it takes time and it is a slower process.
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Physical Adsorption and Chemical Adsorption: Difference
Adsorption Isotherm
Research on environmental protection and techniques of adsorption involves adsorption isotherm as it helps in the prediction of the adsorption capacity of the solid material. Furthermore, an adsorption isotherm is a graph that represents the amount of adsorbed adsorbate on the adsorbent surface keeping a constant temperature and changing pressure.
Application of Adsorption
Charcoal adsorbs the colouring molecules from the coloured sugar solution and decolorizes it.
Moisture is adsorbed by silica gel from the desiccators Alumina and silica gels remove moisture and control the humidity of offices and rooms because they act as adsorbents. In gas masks, activated charcoal is used as it adsorbs vapours, toxic gases to purify the breathing air. Heterogeneous catalysis is carried out through the adsorption process. Charcoal can be used as an absorbent to separate noble gases. The chromatographic analysis is based on the adsorption phenomenon. In syrups and cosmetics, stable emulsions are formed through adsorption. Drug adsorption kills germs.
FAQs on Adsorption: Meaning, Types, Examples, and Applications
1. What is adsorption as per the Class 12 syllabus?
Adsorption is a surface phenomenon where molecules of a substance (the adsorbate) accumulate on the surface of a solid or liquid (the adsorbent), rather than in its bulk. This results in a higher concentration of the adsorbate on the surface. A common example is charcoal adsorbing harmful gases from the air.
2. What is the fundamental difference between adsorption and absorption?
The key difference lies in where the substance accumulates. In adsorption, molecules stick to the surface only. In absorption, molecules penetrate and are distributed throughout the entire bulk of the material. Think of it like this: dust on a table is adsorption, while a sponge soaking up water is absorption.
3. What are the two main types of adsorption?
Adsorption is primarily classified into two types based on the forces involved:
- Physisorption (Physical Adsorption): This occurs due to weak van der Waals forces between the adsorbent and adsorbate. It is reversible, not very specific, and decreases with an increase in temperature.
- Chemisorption (Chemical Adsorption): This involves the formation of strong chemical bonds between the adsorbent and adsorbate molecules. It is irreversible, highly specific, and generally increases with temperature up to a certain point.
4. Why is adsorption an exothermic process?
Adsorption is exothermic (releases heat) because when molecules are adsorbed onto a surface, their freedom of movement is restricted. This leads to a decrease in the entropy (ΔS) of the system. According to the Gibbs-Helmholtz equation (ΔG = ΔH - TΔS), for a process to be spontaneous (ΔG < 0), the enthalpy change (ΔH) must be negative, as TΔS is already negative. Therefore, heat is released, making the process exothermic.
5. What factors affect the extent of adsorption of a gas on a solid?
Several factors influence the efficiency of adsorption:
- Nature of Adsorbate: Gases that can be easily liquefied (like NH₃, HCl, CO₂) are more readily adsorbed than permanent gases (like H₂, N₂, O₂).
- Surface Area of Adsorbent: Adsorption increases with a larger surface area. This is why porous or finely divided materials are effective adsorbents.
- Temperature: Physisorption decreases as temperature rises, while chemisorption may initially increase.
- Pressure: At a constant temperature, the extent of adsorption increases with an increase in the pressure of the gas.
6. How does temperature uniquely affect physisorption versus chemisorption?
Temperature has opposite initial effects on the two types of adsorption. In physisorption, which relies on weak forces, increasing temperature provides molecules with more kinetic energy, helping them overcome the attraction to the surface. Thus, physisorption always decreases as temperature rises. In chemisorption, which requires bond formation, an initial increase in temperature provides the necessary activation energy, so adsorption increases. However, at very high temperatures, the bonds may break, causing adsorption to decrease again.
7. What are some important real-world applications of adsorption?
Adsorption is used in many industrial and everyday processes, such as:
- Gas Masks: Activated charcoal adsorbs toxic gases and purifies the air.
- Water Purification: Alum and other materials adsorb impurities from water.
- Dehumidification: Silica gel packets adsorb moisture to keep products like electronics and leather goods dry.
- Heterogeneous Catalysis: The surface of a solid catalyst adsorbs reactants, increasing the reaction rate.
- Chromatography: It's a key principle in separating mixtures in a laboratory.
8. Why is a finely powdered substance a more effective adsorbent than its crystalline form?
A finely powdered substance is more effective because adsorption is directly proportional to the surface area. When a substance is ground into a fine powder, its total surface area for the same mass increases dramatically. This provides many more available sites for adsorbate molecules to attach to, thereby increasing the overall efficiency and rate of adsorption.
9. What is an adsorption isotherm?
An adsorption isotherm is a graph that represents the relationship between the amount of substance adsorbed by an adsorbent and the pressure (for gases) or concentration (for solutions) at a constant temperature. The two most common models described in the NCERT syllabus are the Freundlich and Langmuir adsorption isotherms, which help in understanding and quantifying adsorption behaviour.
