The branch of chemistry in which studies related to the phenomena occurring at the surface or interface of a solid or liquid are called surface chemistry. The boundary separating the two phases is called the surface or surface. A hyphen (-) or a slash (/) is used to separate two phases. For example, the interface between a solid and a gas is written as solid-gas or solid/gas.
The surface between the different phases is only up to the thickness of a few molecules, but its area depends on the size of particles of macro phases. Some common phenomena like catalysis, crystallization, corrosion etc. can be explained by surface chemistry. Surface chemicals are used in industry, analysis work, etc. In this chapter, we will learn about adsorption, catalysis and colloidal state.
When a solid substance is placed in contact with a liquid or gas, more liquid or gas accumulates on the surface of solid than the bulk, this process is called adsorption. Adsorbate The chemical species that is adsorbed on a surface is called an adsorbent. It is also called adsorbed.
Adsorbent – The solid or liquid substance on whose surface adsorption takes place is called adsorbent.
Adsorption In Various Mechanisms
(1) If charcoal powder is put in a closed vessel filled with gases like O₂, N₂, CO, Cl₂, NH₃, SO₂, etc., then the pressure of gas decreases. Because some part of gas is adsorbed by charcoal.
(2) To make the sugar solution colorless, it is passed over the layers of animal charcoal. The adsorption of color is done by animal charcoal.
(3) Silica gel is used to remove moisture from the air and dry it.
(4) The color of organic dyes such as methylene blue can also be made colorless by animal charcoal.
Desorption :- The process of removal of an adsorbed substance from the surface of adsorbent is called desorption.
In the process of absorption, the molecules of one substance are distributed equally throughout the other substance. That is, the particles of adsorbent move inside the adsorbent, then this process is called absorption. Absorption of water vapor by anhydrous CaCl, absorption of water by sponge are examples of absorption.
Adsorption vs Absorption
- Adsorption is a completely different phenomenon from absorption.
- In adsorption, due to unbalanced internal attraction, the adsorbed substance is attracted to the surface of solid or liquid and its concentration increases on the surface as compared to the rest of macro, whereas in absorption the substance moves from the surface to the gross liquid or solid and around the inside. is evenly dispersed.
- Both these processes can be demonstrated by the following example.
- Dip chalk in a container filled with ink. On observation, it is found that the color of ink gets adsorbed on the surface of chalk while the solvent (water) of ink goes deep into the chalk due to absorption. The surface appears colored when the chalk is broken, while the chalk inside appears white but wet with solvent (water).
Cause And Mechanism Of Adsorption
There is no resultant force of attraction on the particles present in the bulk of adsorbent as shown in the figure, but due to unbalanced forces of attraction on the particles present on the surface, a resultant force of attraction acts inwards.
Due to this unbalanced internal attraction, when other particles of gas or liquid come in contact with the particles present on the surface of liquid or solid, they tend to collect on the surface.
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Types of Point Defects In Crystals Solids
Therefore, on the surface of a solid or liquid, the process of attracting and accepting the molecules of another substance is called adsorption. Or the change of concentration on the surface, which is produced by the process of surface forces, is called adsorption. This being a one page process, it happens at a rapid rate. Especially when the adsorbent is in a fine powdered state. Because the surface area of the adsorbent is more in this state.
Charcoal, silica gels, alumina gels, finely divided metals, etc. act as good adsorbents.
The process of adsorption can also be represented by thermodynamics. Due to adsorption, the surface energy of surface decreases, energy residual forces decrease, that is, this reduction is released as heat. Hence adsorption is an exothermic process. That is, in this process the value of ΔH is –ve.
As the gas is adsorbed on the surface, the movement of its molecules is reduced. That is, the entropy of gas decreases. That is, the value of 1ΔS is -ve.
Based on the Gibbs-Helmotz equation ΔG = ΔH – TΔS, the value of ΔG for a spontaneous process should be -ve. The value of ΔG can be -ve when the value of ΔH is sufficient –ve because the value of TΔS becomes +ve. As adsorption increases. ΔH becomes less negative and finally ΔH becomes equal to TΔS. In this case ΔG = 0. And the equilibrium is established. In equilibrium, the rate of adsorption and adsorption becomes equal.
If in a process both absorption and adsorption take place simultaneously, then that process is called Sorption. In Sorption, the adsorbed substance is distributed evenly in the bulk of adsorbent solid, but its concentration is high on the surface. For example, when H2 gas is exposed to charcoal or Pd, it is first adsorbed. Then some amount of hydrogen diffuses towards the inside of charcoal or Pd.
Positive & Negative Adsorption
Positive adsorption: When the concentration of adsorbent is more than the concentration in the bulk at the surface of adsorbent, then this adsorption is called positive (+ve) adsorption. Negative adsorption- When the concentration of adsorbent is less than the concentration in the macro on the surface of adsorbent, then this adsorption is called negative (–ve) adsorption.
In some liquid solutions, it has been found that the concentration of solute is lower than that of bulk at the surface of solution, thus this example shows negative adsorption.
Types of Adsorption
On the basis of forces of attraction between the adsorbent and the molecules of adsorbent, there are two types of adsorption.
(1) Physical Adsorption
When the particles of adsorbent are bound to the surface of adsorbent by physical forces such as vanderball forces, it is called physical adsorption. Vanderwaal forces are weak forces, so they can be easily removed by increasing the temperature or decreasing the pressure.
Symptoms Of Physical Adsorption
(i) Lack of specificity
Physical adsorption is not of specific nature, i.e. any gas can be adsorbed on the surface of any adsorbent. Because wanderball forces are extensive.
(ii) The Nature Of Adsorbent Or The Nature Of Gas
Easily liquefiable gases such as SO₂, NH₃, HCI, CO₂ etc. get quickly adsorbed on the surface of adsorbent. Whereas ordinary gas H₂, O₂, N₂ is less adsorbed. It is clear from the following table that the adsorption capacity of SO₂ is higher than that of N₂. (As the critical temperature of gases increases, their adsorption capacity increases.) A rapidly liquefied gas or a gas that dissolves quickly in water will be adsorbed quickly. Critical temperature is related to intermolecular attraction.
|Gas||Adsorbed Volume||Critical Temperature (K)|
Increases In Critical Temperature → Increases In Adsorption
Increases In Ability To Liquefy Easily → Increases In Adsorption
(iii) Nature & Surface Area Of Adsorbent
It is often observed that the same gas at the same temperature is adsorbed in different amounts on different solids. That is, adsorption depends on the nature of adsorbent. As compared to hard and non-porous materials, the adsorption is more in porous and fine powdered solids such as charcoal, silica gel, rane nickel. When porous or in the form of fine powder, the surface area of the solid increases, due to which the amount of adsorption also increases. For this reason, fine powder of charcoal is used in gas masks. The adsorption capacities of various metals in the form of fine powder are in the following order:
Colloidal Pd > Pd > Pt > Au> Ni
The surface area per gram of adsorbent is called the specific area.
adsorption c surface area
(iv) Enthalpy of Adsorption
Physical adsorption is an exothermic process. But the value of enthalpy of adsorption is very low (20 to 40 kJmol-1). The change in enthalpy caused by the adsorption of 1 mole of gas on the surface of a metal is called the enthalpy of adsorption.
(v) Reversible Nature
Physical adsorption is a reversible process. Therefore, equilibrium is achieved quickly.
Solid + Gas = Gas/Solid + Energy
By increasing the pressure at the equilibrium, the equilibrium is shifted in the forward direction, that is, the amount of adsorption increases. On lowering the pressure, the gas is adsorbed.
(vi) Effect of Temperature
Adsorption is an exothermic process. That is, heat is produced as a result of adsorption.
Gas + Solid = Gas/Solid + Heat
In the above equilibrium, if the temperature is increased according to the law of Lashaliye, then the equilibrium shifts in the backward direction. That is, the adsorbed gas starts coming out and the process of desorption starts. Hence the adsorption is reduced.
(2) Chemical Adsorption or Chemisorption
When the gas molecules are bound by chemical bonds on the surface of adsorbent, then this adsorption is called chemical adsorption. Chemical bonds can be ionic or covalent in nature, high activation energy is required for chemical adsorption. In this type of adsorption, a chemical compound is formed on the surface of adsorbent, hence it is irreversible. With increasing temperature, chemical adsorption increases. Chemical adsorption is also called activated adsorption or Lagmure adsorption.
Characteristics Of Chemical Adsorption Or Chemisorption
(1) Highly Specific – Chemotherapy is very specific. This is possible only when there is a possibility of formation of a chemical bond between the adsorbent and the adsorbent. For example, the adsorption of hydrogen on the surface of metals like Pd, Pt occurs due to their formation of hydrides.
(2) Irreversibility – The nature of chemical absorption is irreversible as a chemical compound is formed in this type of absorption.
(3) Enthalpy Of Adsorption – Chemical bonds are formed in chemical adsorption, so their enthalpy of adsorption or molar heat of adsorption is high. Generally its value is 80-240kJ mol-1.
(4) Surface Area – Similar to physical adsorption, due to increase in the surface area of the adsorbent, the amount of chemical adsorption also increases.
(5) Effect On Temperature – On increasing the temperature, chemisorption first increases to some extent and then decreases. At low temperature, the gas is first physically adsorbed, but on increasing the temperature, the physical adsorption gets converted into chemisorption. For example, on the surface of nickel metal, hydrogen gas is adsorbed by Vanderwaal forces, but with increasing temperature, the hydrogen molecules dissociate and adsorb to nickel hydride.
Differences Between Physical Adsorption And Chemical Adsorption
|Property||Physical Adsorption||Chemical Adsorption|
|Nature||There is a weak Vander Waal force between the adsorbent and the adsorbent.||There is a chemical reaction between the adsorbent and the adsorbent. Hence a strong chemical bond is formed.|
|Specification||It doesn’t have a specific nature.||It have a specific nature.|
|Reversibility||It is remarkable.||It is irreversible.|
|Nature of Adsorbent||The amount of adsorption is related to the ease of liquefaction of gas. That is why easily liquefied se (NH₃, CO₂) adsorbents are quickly adsorbed.||Only that gas will be adsorbed, which forms a chemical compound with the adsorbent.|
|Thickness of Adsorption Layer||It has a multimolecular surface.||A molecular surface is formed.|
|Effect of Pressure||Increasing the pressure will increase the amount of adsorption, hence is proportional to the pressure.||There is no direct effect of increasing the pressure.|
|Effect of Heat||This is rapid at low temperatures, but decreases at high temperatures.||The adsorption increases with the increase of adsorption temperature.|
|Heat Of Adsorption||The value of this heat is low. (20-40 kJ mol⁻¹)||The value of adsorption heat is high. (80 to 240 ki mol⁻¹)|
|Activation Energy||The value of activation energy is low. Because no chemical bond is formed between the adsorbent.||The value of its activation energy is comparatively high.|
|Adsorbent Surface Area||As the surface area of the adsorbent increases, the amount of adsorbent increases as the free valencies increase.||This also increases as the surface area of the adsorbent increases.|
Activation of Adsorbent
Increasing the adsorption capacity of adsorbent is called activation of adsorbent.
This is done in the following three ways.
(a) Their surface is made rough to make metallic adsorption active. For this, its surface is made rough by depositing a layer of fine particles of metal by mechanical method or by any chemical reaction or by any other process.
(b) To make the adsorbent active, it is divided into very small pieces, due to which its surface area increases and the adsorption capacity increases.
(c) To make some adsorbents active, they are heated in superheated vapor or vacuum at high temperature (626-1273K) to remove the already absorbed gases (air) on their surface. To activate charcoal, it is heated in supersaturated vapor
Adsorption of Gases on Solids
The adsorption of gases on a metal surface or on charcoal or on silica gel is physical adsorption. Almost all solids adsorb some kind of gases, charcoal toxic gases, transition metals like Fe, Ni, Pt, Pd etc. gases like H₂, N₂, O₂ etc. are adsorbed. The adsorption of gases on the surface of a solid depends on the following factors.
(1) Nature of Gas
Easily liquefiable gases like CO₂, HCl, NH₃, Cl, etc. have more adsorption but stable gases like H₂, N₂, O₂ etc. have less adsorption. The gases which have high critical temperature liquefy easily.
Increase in critical temperature = increase in absorption
Increase in ability to liquefy easily = increase in absorption
(2) Nature and Surface Area of Adsorbent
It is often observed that the same gas is absorbed in different amounts by different solids at the same temperature. That is, adsorption depends on the nature of adsorbent. Porous and finely powdered solids like charcoal, silica gel, ranenickel, finely divided metals etc. adsorb more than hard and porous materials. In this stage the surface area of the adsorbent becomes more, due to which the amount of adsorption increases. Fine powder of charcoal is used in gas masks.
The adsorption capacity of different metals in the form of fine powder is in the following order.
Colloidal Pd > Pd > Pt > Au > Ni
Specific Area – The surface area per gram of adsorbent is called specific area.
(3) Effect of Pressure
The amount of adsorption of gases increases with increase in pressure at constant temperature.
The amount of adsorption of gases increases rapidly with increasing pressure at low temperature, but this sequence does not apply at high temperature. For the relation between pressure and variation in adsorption of gases at different constant temperatures, the adsorption of N2 on 1 gram of charcoal is shown-
(4) Effect of Temperature
Adsorption is an exothermic process, which has the following equilibrium.
Gas (Adsorbent) + Solid (Adsorbent) = Gas Adsorbed On Solid + Heat
In this equilibrium there are two opposite processes – condensation (or adsorption) of gas on the surface of solid and evaporation (adsorption) of gas molecules from the surface of solid. Condensation (adsorption) is an exothermic process, while evaporation (adsorption) is an endothermic process. According to La Chatelier’s law, adsorption will decrease with increase in temperature (i.e. reverse reaction will occur) whereas it will increase with decrease in temperature.
As mentioned earlier, the degree of adsorption depends on the pressure.
The relationship between the amount of adsorbed gas at constant temperature and the pressure of gas at equilibrium is called adsorption isotherm. The amount of adsorption is usually represented by.
Here x = Amount Of Adsorbent (Mass)
m = Amount Of Adsorbent.
Adsorption Isotherm :- The relationship between the pressure P of adsorbed gas and the amount (x/m) of adsorbed gas at constant temperature is called adsorption isotherm.
It is clear from the figure that the amount of adsorbed gas
The value of K increases with increasing pressure (P) and the amount of adsorption is maximum at P pressure. Here Ps is called saturation pressure and this state is called saturation state because in this state the amount of adsorbed gas is equal to the amount of adsorbed gas. That is why the amount of adsorption remains constant at this pressure and there is no effect on increasing the pressure.
Freundlich has mathematically explained the isotherm curve of a gas given in the figure, hence it is called Freundlich isotherm curve. Freundlich made the following observations to explain the curve.
(i) At low pressure
The graph is in the form of a straight line, which shows that (x/m) pressure is proportional to P—
It Means x/m ∝ P
x/m = K
Here K = Constant
(ii) At High Pressure
The graph has become completely horizontal, which means that there is no effect of pressure on it. It is shown as follows-
x/m ∝ P⁰
Or x/m = KP⁰
P0 = 1
Therefore x/m = K
(iii) At Medium Pressure
The value of x/m depends on the exponent of pressure P, which ranges from zero to one.
x/m ∝ P¹/ⁿ
x/m = KP¹/ⁿ …..(3)
n and K are constants, whose value depends on the adsorbent and the nature of adsorbent.
This relation was first given by Friendlik. That is why it is also called the normal adsorption isotherm equation.
eq. (3) Taking logarithm of both sides of
log(x/m) = log K + (1/n) log P
If a graph is plotted between log(x/m) and log P, then a straight line is obtained (figure). The slope of this line will be equal to (1/n) and hence the intercept will be equal to log K.
The validity of Freundlich isotherm curve can also be proved by the graph between log(x/m) and log P. If the graph between log(x/m) and log P is a straight line, then the Freundlich isotherm curve is proved. not otherwise.
Adsorption From Solutions
Solids also adsorb dissolved substances in solutions.
(a) When a solution of acetic acid is stirred by adding charcoal, its concentration decreases because some amount of CH3COOH is adsorbed by the charcoal.
(b) The solution of litmus becomes colorless when it is shaken with charcoal.
(c) When Mg(OH)₂ (white) is precipitated with magneson (blue colour) reagent, it acquires a blue color because Mg(OH)₂ adsorbs the blue color of magneson reagent from the precipitate solution. takes it.
(d) Animal charcoal or bone charcoal is used to make impure solution of sugar colorless. In the process of adsorption from the solution phase also the following observation have been done-
(1) The extent of adsorption depends on the concentration of solute present in the solution.
(2) The extent of adsorption decreases with increasing temperature.
(3) The extent of adsorption increases as the surface area of the adsorbent increases.
(4) The extent of adsorption depends on the nature of both the adsorbent and the adsorbent.
The Fyndlik isotherm equation is roughly (approximately) applicable to adsorption in the solution phase as well, but the concentration of solution (C) is taken instead of pressure P.
Therefore x/m = KC¹/ⁿ
Here C = equilibrium concentration of solution
x/m = is the amount of adsorption.
log (x/m) = log K + (1/n)log C
If a graph is drawn between log (x/m) and log C, a straight line is obtained here as in gases.
The values of n and K can be found from the above graph. The above equation can be verified by adsorption of solutions of different concentrations of acetic acid with charcoal.
Applications of Adsorption
The phenomenon of adsorption has many applications in our life, industries, medical field etc. Important applications ofse are depicted below.
Creating high vacuum
Most of air is removed from the vessel in which it is to be vacuumed first by connecting it to the vacuum pump. After this, charcoal is poured into it to remove the remaining air from the vessel. Charcoal absorbs the remaining air of vessel and a high degree of vacuum is obtained.
Workers in mines (especially in coal mines) wear gas masks, which contain activated charcoal. Which, while breathing, adsorbs the toxic gases and only pure air passes through the pores of mask.
Some use silica gel or aluminum gel for the dehydration of gases. Both these types of gels adsorb moisture (water vapour).
Decolourisation of Solutions
Fuller’s Earth and animal charcoal are used to decolorize solutions of petroleum, vegetable oils, sugars, etc., which adsorb the colored substance from the solution.
The activation of a catalyst in industrial processes is due to the adsorption of reactants on the catalyst surface. As a result of adsorption, the concentration of reactants on the surface of catalyst increases and the reaction rate increases. The use of Fe catalyst in the preparation of ammonia by the Haber method, the use of Pt in the preparation of H2SO4 by the contact method, the use of finely divided nickel in the hydrogenation of oils, etc. are examples of heterogeneous catalysis.
Separation of Inert gases
Coconut charcoal is used as an adsorbent at different temperatures to separate the inert gases (noble gases like He, Ne, Ar etc.) present in the air.
When the ointment or lotion is applied, the bacteria present in the wounds get adsorbed on them and die.
Froth Flotation Process
By froth flotation method, undesirable substances present in sulphide ores like silica, clay etc. are separated. For this purpose pine oil or turpentine oil is used as a foam holder.
On the surface of some precipitates such as eosin on silver halide, Mg(OH)2 precipitates by adsorption of magneson indicate the completion of reaction and give the characteristic color at the end point.
Characteristic analysis is based on the different adsorption tendency of different components of a mixture on the adsorbent.
Permutit Method In permutite method, permutite (zeolite) adsorbs Ca⁺² and Mg⁺² ions present in hard water.
Concentrations Of Many Substances
It is also used to concentrate many substances. For example, when a mixture of CH₃COOH and water is passed through charcoal, the charcoal adsorbs CH₃COOH, whereas water does not. The adsorbed can be recovered from CH₃COOH on charcoal.