In this article, we shall study some applications of adsorption.

Adsorption Indicators:

The phenomenon of adsorption is used to detect the endpoints of precipitation titrations. In such titrations, dyestuffs like eosin, fluorescein, alizarin red etc, are used as adsorption indicators. At the endpoint of the titration anions of indicator adsorb on precipitate and colour change of the precipitate takes place.

When a known volume of KBr (in a conical flask) is titrated against AgNO₃ (in burette) using eosin as adsorption indicator, AgBr a white precipitate is formed.

AgNO₃ + KBr  →  Ag⁺Br^– +  KNO₃

There is no colour change of the precipitate as long as Br^– ions are present in the solution. Before endpoint AgBr precipitate is in contact with unreacted KBr and therefore it will adsorb Br^– ions and negatively charged (AgBr)Br^– are formed. The negatively charged precipitate will repel anions of eosin which are pink in colour. Colour of precipitate remains unchanged.

When all Br^– ions are consumed i.e. entire KBr is converted into AgBr, at this stage, an excess drop of AgNO₃ results into adsorption of Ag⁺ ions and (AgBr) Ag⁺ are formed. These particles immediately adsorb the coloured anions of the indicator eosin and colour of the precipitate changes to pink.  This is the end of the titration.

Adsorption Chromatography:

Different constituents of a mixture can be separated by adsorption chromatography technique. The chromatographic technique is based on the fact that, the different constituents of the mixture will adsorb to a different extent on the adsorbent due to their varying adsorption affinity. This is called as preferential or selective or differential adsorption.

In chromatography technique, the mixture to be separated is first dissolved in a suitable solvent like acetone, benzene, ether etc. The solution is then allowed to pass through a glass tube containing adsorbent like silica gel, alumina, cellulose, resins etc. called adsorbing column. The different constituents are adsorbed preferentially at different layers (bands).  The most readily adsorbed constituents form the upper most band while the least readily adsorbed constituents form the lowermost band.  In this way, all constituents are separated making different bands.  A pure solvent is then poured, different bands are dissolved and collected separately in the form of a solution.  This is called elution.

For e.g. in column chromatography, a glass tube is filled with a slurry of alumina and water. a solution of a mixture containing ions of Fe⁺⁺⁺, Cu⁺⁺, Co⁺⁺ etc. is added to the top of the column and there is a phenomenon of differential adsorption.

These adsorbed ions are then washed (Elution) with HCl solution (Eluent). Now Co⁺⁺ de-adsorb first and are collected in one receiver Then Cu⁺⁺ are de-adsorbed and finally Fe⁺⁺⁺ are de-adsorbed and collected in separate receivers. Thus we can conclude that the ions which are readily adsorbed are least readily or reluctantly de-adsorbed.

Chromatography is extensively used in the separation of metallic salts from their mixture using ion exchange resins.

Water Purification:

Using charcoal:

Naturally, occurring water is impure.  It is contaminated with soluble and insoluble impurities.  When impure muddy water is passed through a bed of activated charcoal.  Many impurities like vegetable and colouring matter get adsorbed on charcoal and water is purified.

Charcoal has a double action. Its porous nature acts as a filter for removing insoluble impurities in impure water. It acts as adsorbent and adsorbs dissolved impurities from impure water.

Using alum:

Impurities in water can be removed by adding alum.  Alum is a good coagulating agent, so the colloidal impurities precipitate easily. Alum forms a gelatinous precipitate of positively charged colloidal  AI(OH)3 which is good adsorbent.  It absorbs impurities and colouring matter particularly negatively charged particles and by mutual coagulation. Both settle down and supernatant water becomes clear.

Using ion-exchange resins :

De-ionisation of water by ion-exchange resins is also considered to be adsorption phenomenon. The cation exchange resins adsorb cations like Ca⁺⁺, Mg⁺⁺ and exchange H⁺ ions. The anion exchange resin adsorbs anions like Cl^–, SO₄^— and exchange OH^– ions. Thus all the ions except H⁺ and OH^– are removed from the water and the pure water called de-ionised water is obtained.

Science > Chemistry > Surface Chemistry > Applications of Adsorption

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In this article, we shall discuss very important application of the phenomenon of adsorption, called catalysis. Remaining applications are discussed in the next article.

Catalyst:

A catalyst is defined as a substance which when added to the reacting system increases the rate of the reaction without itself being consumed in the reaction.

e.g. Thermal decomposition of KClO₃ is a very slow process. But this decomposition can be carried out even at a lower temperature by heating KClO₃ with MnO₂ powder. Here MnO₂ acts as a catalyst.

Catalysis:  

Finely divided metals are used as a catalyst in many gaseous reactions. The catalytic action can be explained as follows. In heterogeneous reactions, catalyst acts as adsorbent and the reactants act as the adsorbate.

In catalytic reactions of gases, the reacting gas molecules are adsorbed on the surface of metal catalysts.  Thus concen­tration of reacting gas molecules increases due to the accumulation of it in a smaller region on the surface of the catalyst.  Since according to the law of mass action the rate of chemical reaction is proportional to the concentration of the reactants, the reaction will proceed faster at the surface of the adsorbent.

Similarly adsorption results into weakening of interatomic bonds in the reactant molecules which results in easier rupture of the bonds and into higher activity of reactants

Adsorption is an exothermic phenomenon. Heat evolved during the adsorption helps in exciting adsorbed molecules of reactants. Thus the overall rate of chemical reaction increases.

Examples:

• In the synthesis of ammonia by Haber’s process, finely divided iron is used as a catalyst.
• Finely divided nickel is used as a catalyst in the hydrogenation of oils.
• In preparation of sulphur trioxide from sulphur dioxide, vanadium pentoxide is used as a catalyst.

Characteristics of Catalyst:

• It seems that the catalyst does not take part in the reaction but actually it forms a complex with reactant/ reactants which further regenerates into product/ products and catalyst. Thus
• Reactant / Reactants → Catalyst  Complex
• Catalyst Complex  →  Product / Products + Catalyst
• Thus catalyst is recovered at the end of the reaction.
• In reversible reactions, the catalyst increases the rate of both the forward reaction and the backward reaction. Thus equilibrium is not influenced by the presence of a catalyst.
• An extremely small quantity of catalyst causes a considerable increase in the rate of reaction.
• The activation energy of catalysed reaction is always lower than that of the same reaction when it is uncatalysed.

• A catalyst does not affect the energies of reactants and products. Hence the heat of reaction is the same for catalysed and uncatalysed reaction.
• The substances which inhibit the catalytic activity are called catalytic poisons. In case of conversion of SO₂ into SO₃ catalytic activity of platinum is totally destroyed by the presence of small traces of arsenic due to the formation of platinum arsenide. Thus, in this case, arsenic is catalytic poison.
• A catalyst increases the rate of reaction but they don’t initiate the reaction.

Inhibition or Retardation of a Reaction:

A substance that decreases the rate of a chemical reaction is called an inhibitor. The phenomenon in which the rate of a chemical reaction is reduced is called inhibition or retardation. For Example:

• Chloroform reacts with atmospheric air to form poisonous carbonyl chloride. Thus the use of chloroform as an anaesthetic is dangerous to life. To avoid this reaction or to reduce the rate of the reaction 2% ethanol is added to chloroform. Thus ethanol acts as an inhibitor.
• Hydrogen peroxide decomposes itself and thereby reduces in strength. This decomposition can be retarded by adding dilute acid or glycerol to it. Thus the dilute acid or glycerol acts as an inhibitor.

Classification of Catalysis:

On the Basis of the phases of catalyst and reaction mixture:

Homogeneous catalysis:

A homogeneous catalysis is one in which the catalyst and the reactants exist in the same phase. A homogeneous catalyst dissolves in the gas phase or solution and acts uniformly throughout.

Examples:

Other Examples:

Characteristics of Homogeneous Catalysis:

• The catalyst and the reactants form a single phase.
• The catalyst dissolves into the gas phase or solution.
• The reaction occurs in the gas phase or liquid phase.
• The catalyst is often involved in the chemical reaction.
• The catalyst cannot be easily separated from the products of the reaction.
• The rate of reaction does not depend on the surface area of the catalyst.
• These reactions are generally faster than heterogeneous catalysis.

Heterogeneous catalysis:

A catalyst which exists in a different phase from the reactants is known as a heterogeneous catalyst and the catalysis known as heterogeneous catalysis. Generally, the Heterogenous catalysts are in a solid state, while the reactants are in the liquid or gaseous state.

Examples:

Characteristics of Heterogeneous Catalysis:

• The catalyst and the reactants form different phases.
• The catalyst does not dissolve into reacting mixture.
• The reaction does not occur in the gas phase or liquid phase but takes place on the surface of the catalyst.
• The catalyst is not involved in the chemical reaction. It absorbs the reactants on its surface.
• The catalyst can be easily separated from the products of the reaction.
• The rate of reaction depends on the surface area of the catalyst.
• These reactions are generally slower than homogeneous catalysis.

Steps Involved in Heterogeneous Catalysis:

• The reactant molecules diffuse to the surface of the solid catalyst.
• Reactants molecules are adsorbed on the surface of the catalyst by chemical bonding between surface molecules and the reactant molecules.
• The reactants are converted into products on the surface of the catalyst.
• The product molecules leave the catalyst surface. i.e. they are desorbed.
• The product molecules then diffuse into the gaseous phase.

Catalytic Activity:

The activity of catalyst depends on the strength of chemical adsorption. When a solid catalyst is highly covered by the adsorbate, the chemisorption is said to be strong and the catalyst is then active. If this chemisorption is too strong the adsorbate molecules become motionless on the surface, thus the activity of reacting substance decreases. Thus very strong chemisorption weakens the activity of the catalyst. Thus the adsorbate should get adsorbed strongly but not so strong that their activity reduces.

The metals which lie close to the middle of d block of the periodic table are the most active catalyst.

Catalytic Selectivity:

Different catalysts with same reacting mixture give different product. Selectivity of catalyst is its tendency to catalyse the reaction to form particular products.

Enzyme Catalysis:

Enzymes are the biological homogeneous catalyst. They are large protein molecules. They have large complex structure. Enzymes are very efficient catalysts under very mild conditions in comparison with another type of catalysts. This is because the reduction in activation energy is much greater than that of other types of catalysts.

The enzymes are highly specific in their action. They catalyse only a single reaction of a single compound.For example, the enzyme amylase catalyses the conversion of starch into glucose but don’t have any effect on cellulose.

Science > Chemistry > Surface Chemistry > Application of Adsorption: Catalysis

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In last article, we have studied the meaning of terms like adsorption, absorption, adsorbate, adsorbent. In this article, we shall study the factors affecting adsorption.

The Extent of Adsorption:

The quantity of an adsorbate that is adsorbed per unit mass of adsorbent is called extent of adsorption.

Thus,   a = x/m

Where a = Extent of adsorption

x = The mass of gas adsorbed

m = Mass of adsorbent

The Factors Affecting Adsorption:

Temperature:

Adsorption is an exothermic process, Hence according to Le Chatelier’s principle at given pressure low temperature favours adsorption. If the temperature is increased, adsorbate molecules get removed from the adsorbent and this process is called as desorption.

Thus, adsorption is inversely proportional to the temperature. This is true for physical adsorption. In chemical adsorption due to the high energy of activation, the extent of adsorption increases initially and decreases as the temperature increases further.

Pressure:

When a gas is adsorbed on the surface of adsorbent there is a decrease in the volume of adsorbent. Thus, by Le Chatelier’s principle If the temperature is kept constant, the quantity of gas adsorbed by metal adsorbent increases with the increase in pressure.  Adsorption is directly proportional to the pressure of a gas over a limited range of pressure.

Graphical representation:

At low pressure:

at low pressure   1/ n = 1

∴  x/m ∝  P

Thus at low pressure, the extent of adsorption is directly proportional to the pressure and hence for low pressure, the graph is a straight line.

At medium pressure:

At medium pressure  0 < 1/ n <  1

∴  x/m ∝  P^1/n

Thus at medium pressure, the extent of adsorption increases less rapidly with increase in the pressure hence for medium pressure the graph is a curve.

At high pressure :

At high pressure  1/ n = 0

∴  x/m = constant

Thus at high pressure, the extent of adsorption is independent of pressure hence for high pressure, the graph is parallel to pressure axis.

Nature of Adsorbent:

Since adsorption is a surface phenomenon, adsorption increases with the increase in the surface area of the adsorbent. More finely divided or rougher the surface of adsorbent, the greater will be the surface area and hence the greater will be the adsorption. Metal catalysts in the finely divided form, colloidal form, rough surfaces, activated adsorbent provides more surface area.

Chemisorption is preferential and more specific. A gas will be chemisorbed on a given solid only if it can provide large surface area. For e.g. Hydrogen gas is adsorbed by nickel but not by iron. The chemical nature of the adsorbent should be such so that it can cause chemisorption of adsorbate on the adsorbent.

Nature of Adsorbate:

In case of adsorption of gases by solids, it has been found that more easily liquefiable and highly water-soluble gases are adsorbed more readily due to greater van der Walls forces. Hence ammonia, hydrogen chloride, chlorine and sulphur dioxide are more adsorbed than hydrogen, nitrogen, oxygen.

In physical adsorption, the extent of adsorption depends on the boiling point of the gases. Gases are adsorbed on solids more readily than liquids.

Concentration of Adsorbate:

When liquid is adsorbed on solid, at higher concentration of adsorbate, the extent of adsorption is greater, provides that the temperature is kept constant. The concentration of adsorbate has a similar type of effect as that of pressure.

Adsorption Isotherm:

An adsorption isotherm is a relation between the extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) and the equilibrium pressure or concentration at a constant temperature. Actually, it is a curve obtained by plotting extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) against the equilibrium pressure or concentration at a constant temperature.

Freundlich Adsorption Isotherm:

Adsorption isotherm is a curve obtained by plotting extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) against the equilibrium pressure or concentration at a constant temperature.

An empirical equation for the variation of gas adsorption with pressure at a constant temperature over a limited range of pressure which was put forwarded by Freundlich and is known as “Freundlich adsorption isotherm.” Mathematically the equation is,

Where x  = mass of adsorbate (gas) adsorbed.

m = mass of adsorbent (solid).

P = equilibrium pressure of adsorbate.

C = equilibrium concentration of adsorbate in the solution.

In this equation ‘K’ and ‘n’ are constant and ‘n’ is less than one.

At low pressure, the extent of adsorption is directly proportional to the pressure and hence for low pressure, the graph is a straight line. At medium pressure, the extent of adsorption increases less rapidly with increase in the pressure hence for medium pressure the graph is a curve. At high pressure, the extent of adsorption is independent of pressure hence for high pressure the graph is parallel to pressure axis. i.e.   x/m = constant. Thus Freundlich adsorption isotherm holds good only at moderate pressure.

By taking log of both the side of isotherm,

The form of the equation is y = mx + c. On plotting (log x/m) against (log P), a straight line graph is ob­tained which indicates the validity of the equation.  Thus the slope of the graph = 1/n and its y-intercept is log k. From this graph values of k and n can be found out.

Science > Chemistry > Surface Chemistry > Factors Affecting Adsorption

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Adsorption is defined as the phenomenon in which there is the accumulation of one substance on the surface of the other substance. It can also be defined as the change in concentration at the interfacial layer between two phases of the system due to surface forces. OR It can also be defined as the change in concentration at the interfa­cial layer between two phases of the system due to surface forces.

Examples: 

If animal charcoal is shaken with a diluted solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. If the ink is shaken with charcoal then the intensity of colour decreases.  This is due to the adsorption of ink molecules on the charcoal.

Explanation:

The molecules below the surface of the substance i.e. in the bulk (Molecule A) are equally attracted by other molecules from all sides but molecules on the surface (Molecule B)  are subjected to an unbalanced attraction from molecules in the bulk. These unbalanced forces try to drag the molecules inside. So as to satisfy these unbalanced residual forces surface molecules tend to attract and retain particles or molecules of other substances on the surface with which they come in contact.

When a solid surface is exposed to a gas or liquid, the molecules from the gas or liquid accumulate or concentrate at the surface.  This process is called as adsorption.

Since the surface molecules are responsible for adsorption, it is a surface phenomenon and its extent depends on the surface area of the absorbent. Adsorption may occur when two heterogeneous phases are in contact with each other.

Terms Used in Surface Chemistry:

Adsorbent:

The substance whose surface adsorbs the gas or solute molecules from solution is called as an adsorbent.

e.g.  If animal charcoal is shaken with a dilute solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. Thus charcoal is adsorbent.

Adsorbate:

The substance which gets adsorbed on the surface of solid or a liquid is called as adsorbate or adsorbed phase.

e.g. If animal charcoal is shaken with a dilute solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. Thus acetic acid is adsorbate.

Heat of Adsorption:

Adsorption is an exothermic process. The heat evolved per mole of adsorbate is called heat of adsorption.

Desorption:

The removal of an adsorbed substance from the surface is known as desorption. It is an endothermic process. i.e. Increase in temperature increases the rate of desorption.

Absorption:

Absorption is a phenomenon in which a substance penetrates through the surface and gets distributed uniformly throughout the body or bulk of another substance.  e.g. 1. Water (absorbate)  is absorbed by the sponge (absorbent).2.  Ammonia gas is absorbed in water.

Difference Between Adsorption and Absorption:

Characteristics of Adsorption:

• It is a surface phenomenon.
• It takes place due to the presence of residual surface forces.
• It is dependent upon temperature and  pressure.
• It is affected by the surface area of adsorbent.
• It is in an exothermic process.
• It is a reversible process and a state of dynamic equilibrium is attained.
• Example: Accumulation of acetic acid molecules on the surface by charcoal.

Characteristics of Absorption:

• It is a bulk phenomenon.
• It takes place due to the porous nature of the substance.
• It is independent of temperature and pressure.
• It is not affected by the surface area of the absorbent.
• It is neither exothermic nor endothermic process.
• It is not a reversible process and a state of static equilibrium may be reached.
• Example: Absorption of water by a sponge.

Experiment to Demonstrate Adsorption:

Take about 100 ml of 0.1 N acetic acid solution in a beaker. Add about 5 grams of activated and powdered charcoal to the solution. Stir the solution containing charcoal and keep it standing for 30 minutes. Filter the solution and find the strength of this solution (filtrate) by titration against 0.1 N NaOH.

It is observed that the concentration of an acetic acid solution is decreased as acetic acid is adsorbed by charcoal. This experi­ment proves that the molecules of acetic acid must be adsorbed by activated charcoal. It is a liquid-solid system in which acetic acid is adsorbate and charcoal is adsorbent.

This is reversible as if the charcoal in above experiment is boiled with water, the adsorbed acetic acid molecules will be desorbed.

Types of Adsorptions:

It is found that the forces operative in adsorption are not the same in all cases.  Depending upon the forces which hold the particles or molecules of adsorbate on the surface of adsorbent there are two types of adsorption namely a) Physical adsorption and b) Chemical adsorption or chemisorption.

Physical Adsorption:

The adsorption in which molecules of adsorbate are held on the surface of adsorbent by Van der Waals forces or weak physical forces is called as physical adsorption or Van der Waals adsorption or physisorption.

Examples:

• Accumulation of ammonia gas on the surface of activated charcoal.
• Accumulation of hydrogen gas on the surface by platinized platinum.
• Accumulation of acetic acid on surface by charcoal.

Characteristics of Physical Adsorption:

• In this type, the adsorbed molecules are held on the surface of adsorbent by weak Van der Waals forces.
• As the Van der Waals forces or physical forces are not specific in character, it is said to be general in character.
• Van der Waals forces or physical forces are weak therefore it is reversible and it has a low heat of adsorption.
• Van der Waals forces or physical forces may operate between adsorbed molecules and the other non-adsorbed molecules this makes it multilayer in character.
• The energy of activation is low.
• It takes place at low temperature.
• It adsorption is a fast process.

Chemical Adsorption:

The adsorption in which molecules of adsorbate are held on the surface of adsorbent by chemical forces (chemical bonds) is called as chemical adsorption or chemisorption.

Examples:

• Accumulation of hydrogen gas on nickel.
• Accumulation of oxygen or carbon monoxide on the surface of tungsten.

Characteristics of Chemical Adsorption:

• In this type, the adsorbed molecules are held on the surface of adsorbent by strong chemical bonding forces.
• As the chemical bonds are highly specific in character, it is said to be specific in character.
• Chemical bonds are strong therefore it is irreversible and it has a high heat of adsorption.
• For it to take place, there must be a direct contact between adsorbate and adsorbent molecules, therefore, it results in monolayer formation of adsorbate on the surface of the adsorbent.
• The energy of activation is high.
• It takes place at high temperature.
• It is a slow process.

Science > Chemistry > Surface Chemistry > Adsorption

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