Surface Chemistry & Catalyst

In an adsorption process, which is spontaneous,

DS in negative, DH is also sufficiently negative, as the combination of these two factors

DG is negative

Table 1: Distinction between absorption and adsorption

Absorption	Adsorption
1. It is phenomenon as a result of which the species of onesubstance get distributed uniformly throughout the body of the othersubstance.	It is the phenomenon as a result of which thespecies of one substance get concentrated mainly at the surface of the othersubstance which adsorbs these.
2. The concentration of the same throughout the material.	The concentration on the surface of the adsorbentis different from that in the bulk.
3. It is a bulk phenomenon since it occurs throughout thebulk of the material.	It is only a surface phenomenon since it occursonly at the surface.
4. Absorption proceeds at uniform rate.	Adsorption is rapid in the beginning and its rateslowly decreases with the non-availability of the surface.

Table 2: Difference between Physisorption and Chemisorption

Property	Physisorption	Chemisorption
1. Natureof forces	Weak van der Waal's forcespresent.	Strong chemical forcespresent
2. Heat ofadsorption	Low; of the order of 20-40kJ/mol.	High; of the order of400kJ/mol
3. Temperature	Usually occurs at lowtemperature	Occurs at high temperature
4. Reversibility	Reversible	Irreversible
5. Specificity	It is not specific in nature,i.e., all gases are adsorbed on the surface of a solid to the same extent.	It is highly specific innature and occurs only when there is bond formation between adsorbent andadsorbate molecules.
6. Natureof molecular layers	Forms multi-molecular layers	Forms mono-molecular layer
7. Extentof adsorption	The extent of adsorption isdirectly related with the case of liquefaction of the gas.	There is no correlationbetween extent of adsorptiona nd the ease of liquefaction of the gas.
8. Activationenergy	It does not involveappreciable activation energy. In most cases, the activation energy required isalmost nil.	It generally requiresappreciable activation energy since a chemical reaction is to take place.
9. Stateof adsorbate	Same state as in the bulk.	State of adsorbate moleculesmay be quite different from that in the bulk.

Freundlich adsorption isotherm




x/m = K. P^1/n

or log x/m = log K + 1/2log P

Where x = mass of the gas

m = mass of adsorbent

P = pressure



Lanqmuir adsorption isotherm-




Rate of adsorption = K₁(1 - ) P

Rate of desorption = K₂

at equilibrium K = K₁/K₂ = constant



Adsorption isotherm for solution phase -

x/m = KC ^1/n

or log x/m = log K + 1/n log C.


From chemical kinetics -

log_e (Kp/Ka) = DE/RT

where K_p = Rate constant in pressure of catalyst


Ka = Rate constant in absence of catalyst.



If E_R and E_p represent the average energies of reactants and products. Then

G = E_R - E_P



General formula of Zeolites

M_x/n [(Alo₂) (Sio₂)_y] m.H₂O

Where M = Na⁺, K⁺, or Ca²⁺

n = valency of metal cation

m = molecules of water of crystallisation



The Colloidal state:

Thomas Grahma in 1862 coined the term colloids (Greek Kollo - glue, iods - like) to distinguish some amorphous glue-like substances from crystalloids which crystallize easily. He

distinguished these two substances on the basis of their diffusion through parchment membrane.

1. Crystalloids: The one having faster rate of diffusion.

2. Colloids: The one having slower rate of diffusion.

However, this classification has now historical importance and was rejected because some substances diffuse rapidly in water but slowly in alcohol medium (e.g. NaCl), whereas some

substances diffuse slowly in water but rapidly in alcohol medium (.e.g soaps). Therefore, a new term colloidal state was preferred.

It is therefore convenient to examine the colloidal state and not the colloids as such. Though Graham's concept of colloids is not fully valid, the term colloids has, however, been

retained.


Characteristics of Colloids

1. It is a particular state and not a class of compounds.

2. It is the state in which rate of diffusion through parchment membrane is slow.

3. It is heterogeneous state, i.e., number of phases 2, i.e, each particle is contained within its own boundaries which separate it form dispersion medium.

4. Thus a colloidal state must have at least two phases. The one phase dispersed in other is known as dispersed phase (D.P.) or internal phase whereas the other in which dispersions

are made is known as dispersion medium (D.M.) or external phase.


5. It is an intermediate state in between true solution state and suspension state, i.e., particle size of colloidal state (of dispersed phase) lies in between true solution state and suspension state.

True solution	Colloidal state	Suspension
Solution + Solvent	D.P. + D.M.	Precipitate + medium
Size: Solute < 1 × 10-7cm	DP = 10-5 to 10-7cm	Precipitate = 0.2 × 10-4 cm

6. Particles in a colloidal state are not detectable even under microscope; however, their images can be noticed in ultra microscope.

7. Colloidal particles do not settle down under the force of gravity even in keeping for long.

8. Colloidal particles always carry charge, +ve or -ve on dispersed phase particles.


Distinction in true solutions, colloids and suspensions

Table: Distinguishing features in suspension, colloids and true solution

Property	Suspension	Colloid	Solution
Particle size	> 10-5 cm or 103or 100 mm	10-7 to 10-5 cm or 10 to 103 or 1 mmto 100 mm	<10-7 cm or 10 or 1 mm
Visibility	Visible with naked eye	Images are visible under ultra microscope	Not visible with any of the optical means
Separation a) with filter paper b) with membranes	Possible Possible	Not possible Possible	Not possible Not possible
Diffusion	Does not diffuse	Diffuses very slowly	Diffuses readily
Setting or sedimentation	Settles under influence of gravity	Settle under centrifuge	Does not settle
Nature	Heterogeneous	Heterogeneous	Heterogeneous
Appearance	Opaque	Turbid	Clear

Table: Some examples of different class of Lyophobic Colloidal Systems

Internal phase ordispersed phase	External phase ordispersion medium	Colloidal name	Example
Solid	Solid	Solid Solution	Alloys, ruby glass, Gems orPrecious stones, Marbles, Optical and vision glasses.
	Liquid	Sol	Muddy water, gold sol,paints, cell fluids
	Gas	Aerosols	Smoke, particulate clouds,dust
Liquid	Solid	Gel	Alloys, ruby glass, Gems orPrecious stones, Marbles, Optical and vision glasses.
	Liquid	Emulsion	Muddy water, gold sol,paints, cell fluids
	Gas	Aerosols	Smoke, particulate clouds,dust
Gas	Solid	Solid form	Purnice stone, polystyrenefoam, foamed rubber.
	Liquid	Foam or froth	Froth, soap suds, whippedcream
	Gas	Homogenous system	Do not exits as colloids

Table: Lyophilic and Lyophobic Sols

Property	Lyophilic Sols	Lyophobic Sols
Nature	Dispersed phase has moreaffinity for dispersion medium	Dispersed phase has lessaffinity for dispersion medium.
Preparation	Need no special methods forpreparation. Adding of the material in solvent gives the sols. For exampleadding starch in water gives starch sol	Simple mixing does notproduce sols. Special methods (dispersion or condensation) are necessary.
Concentration	Sols have higherconcentration of dispersed phase.	Sols have lower concentrationof dispersed phase.
Stability	Thermodynamically more stablesystems	Thermodynamically less stablesystems. Coagulate with time.
Solvation	Dispersed phase particles arehighly solvated	Dispersed phase particles arenot solvated.
Viscosity	Viscosity of sols are oftenhigher than the viscosity of dispersion medium.	Almost same as that ofdispersion medium.
Surface tension	Surface tensions of sols aremuch lesser than the surface tensions of dispersion medium	Almost same as that ofdispersion medium.
Reversibility	Reversible with temperature.	Irreversible.
Charge	The charge on dispersed phaseparticles depends upon pH of medium.	Independent of pH of medium.
Tyndall effect	Less scattering of light.	More scattering of light.
Coagulation by electrolytes	Very high concentration ofsalt is required for phase separation.	A small amount of salt isrequired for their coagulation.

Protection of Colloids

i) Lyophobic sols are less stable than lyophilic sols. However, their stability may be increased on addition of lyophilic sols. This phenomenon of stabilizing lyophobic sols by the addition

of lyophilic colloids is known as protection. The lyophilic colloids used for this purpose are known as protective colloids.

ii) The protective colloids form a thin layer around the dispersed phase of lyophobic colloids and thus prevent them from coming close to each other and stabilize them.

iii) The protective character of various lyophilic substances is expressed in terms of gold number.

iv) Gold number of lyophilic is defined as the minimum amount of lyophilic colloid in milligrams which prevents the coagulation of 10 ml gold sol by 1 ml of 10% NaCl solution.

v) More is the gold number, lesser is the protective power of lyophilic colloid.

Substance------- Gold Number

Gelatin---------- 0.005 - 0.01

Haemoglobin ----0.03 - 0.07

Gum Arabic-----0.15 - 0.25

Sodium oleate --0.04 - 1

Starch ---------15 - 25


vi) Gelatin, protein, gums and other macromolecules etc. are popoular protective agents.

vii) A hydrophilic substance does not protect all hydrophobic colloids and protecting power varies for different lyophobic sols. Therefore, another term Congo robin number was

introduced to express protective power of lyophilics. Congo robin number is defined as the amount of protective colloid in milligrams which prevents colour change in 100 ml of 0.01%

Congo robin dye solution to which 0.16 equivalent of KCL are added.

viii) It has been found that protective action of hydrophilics is altogether different for gold and Congo robin sols.

Emulsions are colloidal dispersion of one liquid into another liquid (the two liquids should beimmiscible). Normally one of the two liquids is water.

Emulsions are broadly classified into two types:

i) Oil-in water (or o/W)


Table: Comparison of the properties of two types of emulsions

Characteristics	Water-in-oil emulsion	Oil-in-water emulsion
Dispersed phase	Water	Oil
Dispersion medium	Oil	Water
Appearance	Oily, opaque and translucent(permits light to pass through but diffusing it so that objects on the otherside are not clearly visible.	Watery, opaque and translucent.
Electrical conductivity	Very low	Appreciable
Viscosity	More than water	Little more than water.
Dilution test	More oil added is soluble butnot water.	More water added is solublebut no oil.
Spreading test	Spreads easily on oil surface	Spreads easily on watersurface.
Dye-test with oil soluble dyee.g., methylene blue. Examples	Dye is soluble giving abright colour. Butter, cold creams, benzene water, aniline-water etc.	Dye is practically insoluble,coloured droplets are seen. Milk, cream, face creams