surface chemistry

SURFACE CHEMISTRY

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SURFACE CHEMISTRY
DEFINITIONS
Adsorption
It is a surface phenomenon. In this, concentration of any molecular species is greater at the surface than in the bulk. The phenomenon of accumulation of gas or liquid molecules on the surface of a solid or liquid is called adsorption.
The adsorption of gas on a solid is sometimes called occlusion.
Ex : Adsorption of ammonia (adsorbate) in charcoal (adsorbent)
Adsorption of water vapours (adsorbate) on silice gel (adsorbent)
Adsorption of dye (adsorbate) by charcoal (adsorbent)

Absorption
It is a bulk phenomenon. It is a process in which, a substance is not only retained on the surface but uniformly distributed throughout the body of the solid or liquid
Ex : Absorption of ammonia by water
Absorption of water vapours by only CaCl2
Absorption of water by sponge
Adsorbate
The substance on the surface of the solid is called the adsorbate.
Ex : Various gases (NH3, CO2, N2, O2 etc.)

Adsorbent
The substance to which the adsorbate is attached is called adsorbent.
Ex : Silica gel, charcoal, alumina gel, clay, Pd, Pt, Ni

Sorption
The process in which both adsorption and absorption takes place simultaneously.

Characteristics of adsorption
Adsorption is a physical phenomenon but accompanied by a chemical change.
It is a spontaneous process
It is a selective process
It is always accompanied by evolution of heat
It is specific
The rate of adsorption depends on temperature.
It depends on the nature of adsorbent and adsorbate.

Difference between adsorption and absorption
Adsorption
- Surface phenomenon
- Rapid process
- Equilibrium is attained easily
- Concentration of molecule are more on the surface and less in the bulk
- NH3 adsorbed on charcoal

Absorption
- Bulk Phenomenon
- Slow process
- Equilibrium is slowly , step by a step
- Distribution is uniform
- NH3 absorbed by water

Types of adsorption
Adsorption is classified into two types
1. Physical adsorption (or) vander waals adsorption (or) physisorption.
2. chemical adsorption (or) activated adsorption (or) chemisorption.

Physical adsorption (or) physisorption
It is the one in which the adsorbed molecules are held on the surface of the adsorbent by weak physical or Vander waal’s forces of attraction.
Ex. Adsorption of H2 (or) O2 on charcoal

Chemical adsorption (or) chemisorption
It is the one in which the adsorbed molecules are held on the surface of the adsorbent by chemical bonds (covalent bond (or) ionic bond)
Ex. Adsorption of H2 on NI.

Difference between physisorption and chemisorption

Physisorption
- Due to inter molecules vander waal’s forces
- Low heat of adsorption (o-40 K.cal/mol/)
- Easy and fast process
-Reversible
-Decreases with increase in temperature
-The rate of adsorption increase with increases of pressure and concentration
-Multi layer adsorption occurs
-Not specific
-No surface compound form action takes place.
-Equilibrium is established rapidly
-Involves very small activation energy

Chemisorption
-Due to story chemical bond formation.
- High heat of adsorption(40 – 400 k.cal/mol)
-Difficult and slow process
-Irreversible
-Increases with increases in temperature.
-The rate of adsorption decreases with increase of pressure and concentration.
-Monolayer adsorption occurs.
- Highly specific
-Surface compound formation takes place.
-Requires time
-Involves appreciable activate energy

ADSORPTION OF GASES ON SOLIDS

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ADSORPTION OF GASES ON SOLIDS
The magnitude of adsorption of gases by solids depends on the following factors.
1. Nature of the adsorbent
2. The surface area of the adsorbent
3. Nature of the gases
4. Pressure of the gas
5. Temperature of the gas
6. Reversible character of the adsorbed gases
7. Activation of the adsorbent
8. Enthalpy of adsorption

Factors infleunce adsorption of gases on solids
1. Nature of the adsorbent
The adsorption depends upon the type of adsorbent used. The excellent adsorbents are highly porous in nature. The larger the pores, the greater is the adsorption.
Ex : porous substances are silica gel, aluminia and charcoal.
The adsorption power of these substances can further be enhanced by a pores called activation. During activation, the adsorbent is heated in steam to about 1500°c. Heating drives out all impurities and leads to a lager free surface for adsorption.
Ex. Charcoal adsorbs 0.011 gms of CCl4 at 24°C.
Activated charcoal adsorbs 1.48 gm of at 24°C.

The surface area of adsorbent
The extent of adsorption depends on the surface area.
1. The greater the surface area, the greater is the adsorption.
2. Larger pores on the adsorbent, larger is the adsorption.
Ex. Charcoal and silica gel ( excellent adsorbents).

Nature of gases
The amount of gas adsorbed by a solid depends on the nature of the gas. Easily liquefiable gases like Hl, NH3, Cl2, SO2 etc., are adsorbed more easily then the permanent gases like H2, N2, O2 etc.,
This is due to the following reasons :

i. Critical temperature
The ease of liquifications depends on its critical temperature (i.e, the maximum temperature above which a gas cannot be liquefied). Thus if the critical temperature of the gas is more, it will be liquefied and adsorbed more readily.
Ex. Adsorption of various gases on 1g of activated charcoal.

ii. Vander waal’s forces
Easily liquefiable gases possess greater vander waal’s forces than permanent gases, so they are adsorbed more readily.

EFFECT OF TEMPERATURE,PRESSURE

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EFFECT OF TEMPERATURE

Physical adsorpotion
It occurs rapidly at lower temperature and decreases with increase in temperature.

Chemical adsorption
It increases with increase in temperature and then decreases with the increase of temperature, physical adsorption changes to chemical adsorption.
For ex. Nitrogen is physically adsorbed on iron at 190°c. But when the temperature is raised to 500°c, it is chemisorbed to from a nitride.

EFFECT OF PRESSURE
According to Le-chatlier’s principlel adsorption increases with the increase of pressure. Decreases of pressure causes desorption.

Reversible nature
Physical adsorption
It is a reversible process. The gas adsorbed on to a solid can be removed (desorbed) under reverse conditions of temperature and pressure.

Chemical adsorption
It is an irreversible process because a strong chemical bond is formed between adsorbent and adsorbate.

Heat of adsorption
It is defined as the energy liberated when 1g mole of gas is adsorbed on the solid surface.
In physical adsorption, gas molecules concentrate on the solid surface. Thus, it is similar to the condensation of a gas to liquid. So, adsorption is an exothermic process. The attraction between the gas molecules and solid surface are due to weak vander waal’s forces. Therefore, the heat of adsorption is very small (about 5kcal / mol) for physical adsorption.
In chemical adsorption, the attractive forces are due to the formation for true chemical bonds. Therefore, the heat of adsorption is large (20 to 100 kcal/mol)

Thickness of adsorbed layer of gas
Langmuir showed that at low pressure, the physically adsorbed gas forms only one thick molecular layer. However, above a certain pressure, multi-molecular thick layer is formed. In Chemistry ion, the adsorbed gas is one-molecule thick since chemical combination takes place with the adsorbent surface directly.

FACTORS INFLUENCING FACTORS ADSORPTION OF SOLUTES FROM SOLUTION

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Factors Influencing Adsorption of solutes from solutions

The amount of solute adsorbed by a solid from the solution depends on the following factors.

1. Effect of temperature
A rise in temperature increases the kinetic energies of solute particles and hence these particles leave the surfaces and thereby lowering the extent of adsorption.
Frendlich adsorption equation is found suitable to explain the effect of concentration.
If a graph is drawn between log x/m Vs log c, a straight line is obtained for small changes of concentrations.
i) when the solute is adsorbed by an adsorbent it is called positive adsorption.
ii) But of the solvent is taken up by the adsorbent it is called negative adsorption.

a. Negative adsorption
Adsorption of substance from the solution decrease with rise in temperature and decrease in concentration of solution. This type of adsorption is known as negative adsorption.
Ex. Adsorption of water
From the dilute solution of KCl, charcoal adsorbs water, thereby the concentration gets increased.

b. Positive adsorption
Adsorption of substance from solution increases with decreases in temperature and in concentration of solution. This type of adsorption is concentration of solution. This type of adsorption is known as positive adsorption.
Ex. Adsorption of salt (KCl)
Charcoal adsorbs KCl rather than water thereby salt concentration gets decreased.

i) The nature of the solute adsorbed
The extent of adsorption is usually greater when the molecular weight of the solute is high.

ii) Effect of surface area
If the surface area of the adsorbent is increased, adsorption also increases.

FREUNDLICH’S ADSORPTION ISOTHERM

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FREUNDLICH’S ADSORPTION ISOTHERM
The relationship between the magnitude of adsorption (x/m) and p can be expressed mathematically by an empirical equation known as freundlich adsorption isotherm.
x = KP1/xm
where,
x – amount of gas adsorbed
m – mob of adsorbent
P – equilibrium pressure
t,n – constants (depends on nature of adsorbate, adsorbent and T)

Derivations of Freundlich's adsorption isotherm

This equation may be derived from the result observed from the graph .
Thus
i) At low pressure adsorption increasesX x p
ii) At high pressure adsorption is almost constant X = constant
iii) At intermediate (normal) pressure adsorption depends on 0 to 1 power of pressure.
X x p 1/n (or) X = KP1/n m
where, n = whole number.
The above equation is called freundlich’s adsorption is other.

Testing of Freundlich adsorption isotherm
Taking log on both sides, the equation
x = KP 1/n becomes
log x = log k + 1 log p m/ n
on plotting log x/m Vs log p, a straight line is obtained with a scope of 1/n and intercepts log k.

Disadvantages or limitation of Freundlich's adsorption isotherm

1. Freundlich equation is purely empirical and has no theoretical basis.
2. The equation is valid only upto a certain pressure and invalid at higher pressure.
3. The constants K and n are not temperature independent, they vary with temperature.
4. Frendilich’s adsorption isotherm fails when the concentration of the adsorbate is very high.

LANGMUIR’S ADSORPTION ISOTHERM

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LANGMUIR’S ADSORPTION ISOTHERM
Langmuir derived an equation based on some theoretical considerations. The assumptions of Langmuir’s theory are,
1. Valencies at the surface of the adsorbent are not fully satisfied.
2. The adsorbed gas layer on the solid surface is only one molecule thick.
3. The surface of the solid is homogeneous, so the adsorbed layer is uniform all over the adsorbent.
4. There is no interaction between the adjacent adsorbed molecules.
5. The adsorbed gas molecules do not move around the surface.
6. The process of adsorption is a dynamic process, which consists of two opposite processes.

Condensation Process
It involves the condensation of the molecules of the gas on the surface of the solid.
Evaporation Process
It involves evaporation of the molecules of the adsorbate from the surface of the adsorbent.

Derivation of Langmuir Isotherm
According to Langmiur’s assumptions, when the gas molecules strike a solid surface, some of the molecules are adsorbed where some are desorbed. Thereby dynamic equilibrium is established between adsorption and desorption.
Consider an adsorbing surface of area 1 square cm exposed to a gas. Then

q - Fraction of the surface area covered by gas molecules
1- q - Fraction of the uncovered area (vacant area)
r - Pressure of the gas

The rate of adsorption = Ka (1-q)p – (1)
The rate of evaporation desorption of gas molecules = Kd q - (2)
Where K1 & K2 are proportionality constant for a given system.
At equilibrium :
Rate of adsorption = Rate of desorption
Ka(1-q)r = Kdq
Kar = Kdq+ Karq
Kar = q (Kd+ Kar)
q = Kar/Kd+Kar - (3)
Dividing the Eq. (1) by ‘ Kd’ it becomes
q = Ka/Kd) r/ 1+ (Ka/Kd) r
q = K r / 1+ K r
Where, Ka/Kd = b = equilibrium constant, called the adsorption coefficient.
But the amount of gas adsorbed per gram of the adsorbent (x)/m is proportional to the fraction (q) of the surface covered.
On comparing Eq (2) & (3) it becomes
x/k = k’kr /1+kr - (4)
where K’ = the new constant.
Equation (4) gives the relation between the amount of gas adsorbed to the pressure of the gas at constant T is known as Langmuir adsorption isotherm.
The above Eq. (4) may be rewritten as
1+kr = k’kr / x
1/k’k + kr / k’k = r/x - (5)
Equation (5) represent a straight line (i.e. y = c + mx) If the graph is plotted between r/x Vs r, we should get a straight line with slope k/k’k and the Intercept 1/k’k
This equatin is found valid in all cases.
Plot of r/x Vs r

Case (1) : At low pressure
If the pressure r is very low k/k’k , the r term is negligible.
i.e. 1/k’k >> k/k’k r
Hence Eq. (5) becomes
1/k’k = r/x (or) x = rk’k - (6)
x µ r
i.e. amount of adsorption per unit weight of adsorbent is directly proportional to the r at low pressure.
Case (ii) at high pressure.
If the pressure r is high 1/k’k term is negligilble,
i.e. k/k’k
Hence Eq (5) becomes
k/k’k r = r/x or x = k’ (constant)
x = k’p° - (7)
i.e. extend of adsorption is independent of pressure of the gas, because the surface becomes completely covered at high pressure.
Case (ii) At normal pressure
If the pressure r is normal (intermediate) the eq. (7) becomes
X = k’pn - (8)
Where n lies between 0 to 1
Equation (8) is called Frendliech’s adsorption isotherm.
Merits – Langmuir's adsorption isotherm holds good at low pressure.
Demerits - It fails at high pressure.

APPLICATIONS OF ADSORPTION

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Applications of Adsorption

The process of adsorption finds its application in following areas.
1. Silica and Alumina Gels
These are used as adsorbent for removing moisture and for controlling humidities of room.
2. Activated charcoal
a) It is used in gas masks, which adsorp all underivable (toxic) gases, while purified air passes through its poles.
b) It is also used for removing colouring matter from the sugar solution and the decoloration in vinegar.
3. Fuller’s Earth
It is used in large quantities for refining petroleum and vegetable oil since it adsorbs unwanted materials.
4. Mordants
These are used in dying industry, which adsorp the colouring matter without attaching to the fabrics.
5. Froth Flotation process
The sulphide ores are freed from silica and other earthy matter by froth – floatation process. (Oil adsorbs sulphilde ores only, but not others).
6. Ion – exchange resins
Softening of hardwater can be done based on the principle of competing adsorption using ion-exchange resins.

7. Arsenic poisoning
Colloidal ferric hydroxide is administered, which adsorbs arsenic poison and is removed from the 60 day by vomiting.

8. Chromatographic Analysis
Selective adsorption by alumina, magnesia, etc. can be used for separating different pigments and also mixtures of small quantities of organic substances with the help of adsorption chromatography.

9. Measurement of surface area
Surface area of powders and rough surfaces can be measured using adsorption measurements.

ROLE OF ADSORBENT IN CATALYTIC REACTIONS

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Role of adsorbent in Catalytic reactions

Adsorbent in Heterogeneous catalytic reaction
Adsorption plays a major role inmost of the industries. Various manufacturing industrial chemical process are heterogeneously catalyst reactions. The function of catalyst is to increase the rate of the reaction and it does not affect the equilibrium.

The reaction mechanism includes the following steps.
Step 1 : Adsorption of reactant molecules on the surface of the solid.
Step 2 : Formation of activated complex.
Step 3 : Decomposition of activated complex.
Step 4 : Desorption of products.

1. Synthesis of Ammonia (Haber’s Process)
For synthesis of ammonia iron is used as catalyst with K2O and Al2O3 as promoters. Promoters are the substances which increase the activity of a catalyst. These promoters change the lattice spacing and increase the peaks and cracks on the catalyst surface.
Fe + Promoter
N2 + 3H2 -----------> 2NH3

2. Gas Masks (Purification)
Gas masks are device, which contain activated charcoal as adsorbent. It has high adsorptive capacity and adsorbs all toxic gases immediately (in >0.1 sec) and reduces the concentration of toxic gases from 1000 ppm to 1 ppm. The function of gas mask is to purify the air for breathing by adsorbing the poisonous gases and vapours from the atmosphere.

3. In Oil Industry
For Saturating the unsaturated parts in vegetable oil, nickel catalyst is extensively used.
Vegetable Oil + H2 Ni Vegetable ghee.

Classification of Activated Carbon
Activated carbon is classified into two main classes based on their size and different adsorption capacities.

1. Granular Activated Carbon
These types of carbons have a diameter greater than 0.1 mm. It is used for adsorption of gases and vapours.
Carbon is classified into two main classes based on their size and different adsorption capacities.

2. Powdered Activated Carbon
These types of carbons have a diameter of less than 200 mesh. It is used for purification of liquids.

Activated of Carbon
During activation, charcoal is heated in stream or air atmosphere to about 1500°C. Heating drives out all the impurities as gases and these by leads to a layer surface for adsorption.

a. Direct Method
In this method, activation is carried out by heating carbon in steam or air at about 850°C. Heating drives out all the impurities sticking on the surface and blocking the capillary poles. This leads to a large surface area of carbon.

b. Briquetting Method
The powdered carbon is mixed with wood ear and caustic soda compressed into small briquettes and then heated in steam at 800°C.

c. Chemical Method
The wood is impregnated with a solution of Zinc Chloride or Phosphoric acid and then carbonized.

Role of Activated carbon in waste water treatment
The waste water contains number of compounds which are resistant to bio-degradation and persist in water even after primary and secondary treatments. These impurities are generally carcinogenic and toxic in nature
These compounds may be eliminated by passing over a bed of granulated activated carbon (GAC). These compounds present in waste water are adsorbed by charcoal and removed from water.

a. Treatment of waste water
1. Using a trickling filter method
The small quantities of dissolved organic hazardous waste can be removed by this method. The filter consists of a number rectangular or circular vessels. These vessels are packed with granular activated carbon (GAC). GAC has an enormous surface area (about 1000 m2/g) and is commonly used as adsorbent.

Process
The contaminated water is allowed to trickle down through the GAC packed in a series of vessels. As the trickled waste water starts percolating down wards through the granular activated carbon the hazardous organics in polluted water are adsorbed on the porous GAC matrix. The GAC filter after sometime gets clogged with the adsorbed contaminates and hence must be replaced or regenerated for further use.

Regeneration
When the charcoal becomes saturated, it is heated in vacuum to above 1500°C and the adsorbates are driver off.

Activated carbons used in other industries
1. Refining of sugar, glucose.
2. Removal of taste & colour of drinking water.
3. Removal of metals from waste water
4. Removal of impurities from pharmaceutical.

Role of activated carbon in air
Finally powdered activated carbon is an excellent adsorbent for the removal of toxic gaseous pollutants of impure air. It is used to remove impurities like NO2, SO2, CO and CO2 from impure air.

a. Treatment of polluted air
Using charcoal adsorption tube - In this technique, the polluted air is sucked by the tube and is passed through a glass wool. The pollutants in air are adsorbed on the granular activated charcoal.
The amount of pollutant adsorbed is proportional to the surface area of the adsorbent and the Physics, Chemical characteristics of the adsorbent. The temperature are pressure are maintained during the process. The GAC filter after sometime gets clogged with the adsorbed contaminates. The adsorbed pollutants can be leached with a suitable solvent such as carbon disulphide and the solution is analysed by gas chromatography.

ROLE OF ADSORBENTS IN ION–EXCHANGE ADSORPTION

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Role of adsorbents in Ion – exchange adsorption

Definition

Ion – exchange adsorption is defined as, “the process of releasing”, the one kind of ion and adsorbing another same kind of ion by an adsorbent”.

Classification of ion-exchangers

1. Cation exchange resin or cation exchanger or Decationizer resin.
2. Anion exchange resin or anion exchanger or Deanionizer resin.

1. Cation exchanger or Decationizer resin :
These resins are insoluble in water. When water containing salts like MgCl2, CaCl2 is passed through the cation exchanger, it exchanges cations with other cations present in the hardwater. The resins are made from styrene divi.,benzene,copolymer containing, acidic functional groups like sulphonic(-SO3H) acid. It is represented as RH+.

Definition :

It is defined as cross – linked polymer containing sulphonic group as integral part of resin and an equivalent amount of H+ ions.

Process :

When the solution, containing certain Ca2+, Mg+ is allowed to pass through it H+ ions are exchanged for cations present in solution.

2RH+ + Ca2+ --> R2 Ca2+ + 2H+
2RH+ + Mg2+ --> 2RMg2+ + 2H+
This process consists of desorption of H+ ions and adsorption of Ca2+, Mg2+ ions by the resin.

2. Anion exchanger or Deanionizer resin

When water is passed through anion exchange column, the resin is capable of exchanging their anions with other anions. The resin consists of quaternary ammonia hydroxide groups (-NR3OH) or amino group etc., on the benzene rings of styrene – divinyl benzene copolymer. This may be represented as R’OH_.

Definition :

Anion exchange is defined as “Cross-linked polymer containing amino group as integral part of resin and an equivalent amount of OH_ ions.

Process :

When the solution of another anion is allowed to pass through it, OH_ ions are exchanged by anion present in the solution.
R+ OH_ + Cl_ --> R+Cl_ + OH_

It also involves desorption of OH_ ions and adsorption of Cl_ ion by the resin.

Regeneration

When the cation exchange resin is exhausted it can be regenerated by passing a solution of dil. HCl.
R2 Ca 2+ 2H+ --> 2RH+ + Ca2+

When the anion exchange column is exhausted it can be regenerated by passing a solution of dil.NaOH.
R+ Cl_ + OH_ --> R+ OH_ + Cl_

Applications of Ion – exchange Adsorption

a. In this process not only hardness causing salts are removed, but also salts of trace metal from the water. E.g. Zeolite process and demineralization process etc.
b. Complex metal ions such as copper and nicked also can be removed from water used in industries.
c. Ion exchange technique can be applied for concentration of uranium in pitchblende ore.