How Temperature, Pressure, and Nature of Adsorbent Influence 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.
FAQs on Key Factors Affecting the Extent of Adsorption
1. What are the primary factors that affect the extent of adsorption?
The extent of adsorption is influenced by several key factors. The five main factors are:
- Nature of the Adsorbate (Gas): Gases that can be easily liquefied, meaning they have higher critical temperatures, are adsorbed more readily.
- Nature of the Adsorbent: The type and properties of the solid material determine its capacity to adsorb substances.
- Surface Area of the Adsorbent: Adsorption increases as the surface area per unit mass of the adsorbent increases.
- Temperature: Physical adsorption is an exothermic process, so it decreases with an increase in temperature.
- Pressure of the Gas: For the adsorption of a gas on a solid, the extent of adsorption increases with an increase in pressure at a constant temperature.
2. How does increasing the surface area of an adsorbent enhance the extent of adsorption?
Adsorption is a surface phenomenon, meaning it occurs only at the surface of the adsorbent. By increasing the surface area, more sites become available for the adsorbate particles (gas or solute) to bind to. For example, a solid lump will have far less surface area than the same mass of that solid ground into a fine powder. This is why highly porous or finely divided materials like activated charcoal and silica gel are excellent adsorbents—they possess a very large surface area for their mass.
3. What specific factors influence the adsorption of gases on solid surfaces?
When considering the adsorption of gases on solids, two factors are particularly important:
- Critical Temperature of the Gas: Gases with stronger intermolecular forces and higher critical temperatures (like NH₃, HCl, SO₂) are easier to liquefy and are therefore adsorbed to a greater extent than gases with low critical temperatures (like H₂, O₂, N₂).
- Pressure: At a constant temperature, increasing the pressure of the gas increases the rate at which gas molecules strike the adsorbent surface, leading to a higher extent of adsorption until an equilibrium or saturation point is reached.
4. Why does the extent of physical adsorption generally decrease with an increase in temperature?
Physical adsorption (physisorption) is an exothermic process, meaning it releases heat when the adsorbate molecules bind to the adsorbent surface. According to Le Chatelier's principle, if a change is applied to a system in equilibrium, the system will shift to counteract that change. When temperature is increased, the equilibrium between the adsorbed gas and the gas in the bulk phase shifts in the direction that absorbs heat, which is the reverse (endothermic) process of desorption. Therefore, a lower temperature favours physical adsorption.
5. How does the effect of temperature on chemisorption differ from its effect on physisorption?
The effect of temperature on chemisorption is different from physisorption. While physisorption always decreases with increasing temperature, chemisorption often shows an initial increase. This is because chemisorption involves the formation of a chemical bond between the adsorbate and adsorbent, which requires a certain amount of activation energy. Increasing the temperature initially provides the necessary energy to overcome this barrier, thus increasing the rate of adsorption. However, at very high temperatures, the chemical bonds may become unstable and start to break, causing the extent of adsorption to decrease again.
6. What is the difference between the extent of adsorption and the rate of adsorption?
The extent of adsorption refers to the amount of adsorbate that is bound to the surface of the adsorbent at equilibrium. It is a measure of the total capacity of the adsorbent under specific conditions of temperature and pressure. In contrast, the rate of adsorption refers to how quickly this equilibrium is reached. The rate is typically fast at the beginning when the surface is empty and slows down as the surface becomes covered with adsorbate molecules.
7. In the context of adsorption from a solution, what role does pH play?
In adsorption from a solution, pH is a critical factor because it can influence both the surface of the adsorbent and the adsorbate molecules. The pH of the solution can alter the surface charge of the adsorbent. For example, at low pH, a surface might become positively charged, making it more effective at adsorbing negatively charged ions (anions). Conversely, at high pH, the surface might become negatively charged, favouring the adsorption of cations. The pH also affects the degree of ionization of the adsorbate, further influencing the electrostatic interactions between the two.
