Surface Chemistry

The two characteristics of Chemisorption are:

a) In Chemisorption which is highly specific in nature, the adsorbate and adsorbent get attached by chemical bonds which are either covalent or ionic in nature.

b) High activation energy is required and high temperature is also favourable.

c) Chemisorption increases with the increase in surface area which results in more number of active sites.

As Physisorption is Exothermic in nature, which means when gas gets adsorbed on the solid surface, Heat is evolved. So, according to Le-Chatelieres when the temperature is an increased reverse process (Desorption) will be favoured. So, Physisorption decreases with the increase of temperature.

Where x/m: Volume of gas adsorbed

T: Temperature.

As Adsorption is directly proportional to the available surface area and powdered form of a substance have a greater surface area than the crystalline form of the substance.

So, greater the surface area of the adsorbent more is the adsorption.

Hence, powdered substances are more effective adsorbents than their crystalline forms

It is important to remove CO (Carbon Monoxide) in the synthesis of ammonia as CO affects the activity of Iron catalyst which is required in Haber’s process.

Note: Haber’s process is a very important industrial process which is used to produce ammonia.

Ester hydrolysis is represented as:

Ester + Water→Acid + Alcohol

In this reaction the acid produced which is a product also acts as a catalyst and makes the reaction faster.

Such substances that act as catalysts in the same reaction in which they are obtained as products are known as Autocatalysts.

So, ester hydrolysis is slow in the beginning and becomes faster after some time as more acid is produced on the product side.

Desorption is a process in which substance (reactant + product) is released from the surface which is the opposite process of sorption.

The role of desorption in the process of catalysis is to make the surface of the solid catalyst-free for fresh adsorption of reactants on the solid surface for further reactions to take place.

According to Hardy-Schulze law

‘The greater the valence of the flocculating ion added, the greater is its power to cause precipitation.’

As this law takes into consideration of only the charge present on the ion and not the size of the ion. So when the size of the atom is considered, smaller the size of an atom more will be its polarising power.

So Hardy-Schulze can be modified in terms of the polarising power of the flocculating ion as ‘The greater the polarising power of the flocculating ion added, the greater is its power to cause precipitation.’

The precipitate which is obtained from a chemical reaction always contain some unwanted substances (eg ions, impurities) which get adsorbed onto the surface of the precipitate. Therefore, it becomes important to wash the precipitate before estimating it quantitatively so as to remove these unwanted adsorbed substances and obtain accurate results.

Adsorption- The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. In this substance accumulate at surface only and does not penetrate into the adsorbent (material on the surface of which adsorption takes place)

Ex.-If gases like O₂, H₂, NH₃ taken in a closed vessel containing charcoal then gases get adsorbed on the surface of charcoal and reduce the pressure.

Absorption- In this molecules get distributed uniformly throughout the material Instead of accumulating at the surface.

Ex- When gas like O₂ is mixed with water then O₂ get distributed throughout the solvent (Water).

Finely divided substances have large surface area compared to not divided substance. And the increase in surface area increases the adsorption. Hence Finely divided substances are more effective adsorbent.

Extent of adsorption of a gas on a solid is influenced by following factors- 1) Nature of adsorbate.

2) Nature of adsorbent.

3) Pressure.

4) Temperature.

Adsorption Isotherm- The variation in the amount of a gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by the curve . This curve is known as Adsorption Isotherm.

Freundlich adsorption isotherm- Freundlich gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent And pressure at a particular temperature.

x/m = k P^1/n_m

Increasing the adsorbing power of adsorbent is known as activation of adsorbent.it can be achieved by

1) Increase in surface area of adsorbent.

2) By dividing the adsorbent into small grains.

3) By making its surface rough.

Adsorption of reactants on the solid surface of the catalysts increases

the rate of reaction. Generally, the reactant is gas whereas catalyst is solid. Reactant molecules get adsorbed on the surface of the catalyst as a result concentration of reactant increases leads to increase in the rate of reaction. there are many important reactions is based on this

Ex1) manufacturing of ammonia using iron as a catalyst,

2) manufacture of H₂SO₄ By contact process.,

3) use of finely divided Nickel in the hydrogenation of oils.

_{When adsorbate (gas or liquid mostly) absorbed then their entropy getreduced i.e.Δ}S_{is negative. For every spontaneous process ΔG must be} negative. ΔG = ΔH - TΔS, as TΔS is positive (ΔS is negative) ΔH must be negative in order to make ΔG negative . Hence ΔH is negative that means the reaction is exothermic. Hence every adsorption reactions are exothermic.

Colloidal solutions get classified in 8 Types on the basis of physical state of the dispersed phase and dispersion medium. This are given in following table-

Temperature- Adsorption decreases with increase of temperature because it is an exothermic process, by using le chatelier's principle the reaction will proceed in backward direction.

Pressure- Adsorption increase with increase in pressure at constant temperature.

Lyophilic - lyophilic (liquid loving) this sols formed by direct mixing of substances like gum starch etc. with suitable liquid, called as lyophilic sols. This are called as reversible sol because if dispersion medium is separated from the dispersed phase it can be reconstructed just by mixing them. This sols are quite stable. Ex-Protein, Starch.

Lyophobic -lyophobic (liquid hating) this sols don't formed by direct mixing of dispersed phase and dispersion media they require special methods for preparation. Ex-Sols of metal sulphides .

This Lyophobic(Hydrophobic-water hating) sols gets easily coagulated. Because they are less stable. Their stability is only due to Charge If electrolyte is added to it opposite charge will attract and form coagulation easily.

Associated colloids- They formed by aggregation of large number of ions. instead of atoms or molecules in above two colloids. They have van der waals force directly proportional to concentration of ions. Ex- Soap Sol.

Enzymes are complex nitrogenous organic compounds which are produced by Living plants and animals. They are protein molecules of higher molecular masses. They are effective catalysts termed as “biochemical catalysts”

Mechanism of enzyme catalysis- Their are number of cavities present on the surface of colloidal particle of enzymes. They have specific shape and contains group like –COOH ,-SH, -OH. etc. they are active centres of enzyme particle molecules of reactant which have complimentary shape fit into this cavities, this formes activated complex Which then decompose to yield products. This reaction occur in 2 steps-

_{1) Enzyme(E) + Substrate (S)} → _ES

2) ES → Enzyme + Products

1) Colloidal solutions get classified in 8 Types on the basis of physical state of the dispersed phase and dispersion medium. This are given in following table-

2) Depending upon nature of Particles of dispersed phase it can be classified into 3 categories

a) Multimolecular Colloids-In this colloidal particle consist of aggregates of atoms having diameter less than 1nm.

b) Macromolecular colloids-The colloids in which Large particle aggregates and dissolved into suitable liquid.

c) Associated colloids – This colloids show Colloidal property at higher concentrations. This contains ions of colloidal size.

3) Depending upon interactions between dispersed phase and dispersion medium it is divided into 2 categories

a) Lyophilic Colloids- lyophilic (liquid loving) this colloids formed by direct mixing of substances like gum starch etc. with suitable liquid, called as lyophilic sols . This are called as reversible Colloids because if dispersion medium is separated from the dispersed phase it can be reconstructed just by mixing them. This sols are quite stable. Ex-Protein ,Starch.

b) Lyophobic colloids- lyophobic (liquid hating) this Colloids don't formed by direct mixing of dispersed phase and dispersion media they require special methods for preparation. Ex-Sols of metal sulphides .

(i) In this case Tyndall effect is observed i.e. scattering of light by the colloidal particles takes place and the path of light become visible.

(ii) The coagulation takes place in this case positive charge particles of

Fe(OH) gets attached with Cl^-. And ppt. of FeCl is obtained.

(iii) In this case coagulation will take place. After passing electric current through colloidal solution particle will get attracted towards oppositely charged electrodes. and lose their charge and coagulate.

Emulsions –The system which has Liquid as both dispersed phase and dispersion medium. There are two types of emulsions

1) Oil in Water-Ex-Milk

2) Water in oil-Ex-Butter.

Emulsifiers forms an interfacial film between the suspended particles and medium. It provide coating to every drop of suspended particle and prevent it from coagulating. And it remain suspended in medium.

Ex- Proteins, gums, alcohols, Lampblack etc.

Soap is sodium or potassium salts of fatty acids (long chain). Represented as RCOO^–Na⁺. When dissolved in Water it dissociates in the RCOO^– and Na⁺. In first ion there are two parts R and COO^–. R is hydrophobic tail. and COO^– Hydrophilic head. RCOO^– ions present at the surface with COO^– group inside and R group outside. Increasing concentration to critical micelle concentration anions are pulled inside the bulk and Form a spherical shape of hydrocarbon chain pointing towards the centre and COO^– pointing outside this are called as micelle.

Cleansing action of soap- Soap molecules form micelle around oil droplets. Soaps emulsifies the oil or grease droplets. and form emulsion after washing the cloth this emulsion easily gets removed. Hence Action of soap is due to emulsification and micelle formation.

Oxidation of sulphur dioxide into sulphur trioxide in the presence of

2) Formation of ammonia from Haber process.

3) Oxidation of ammonia into nitric acid in presence of platinum gauze in Ostwald's process

→ 4NH₃(g)+ 5O₂(g) Pt(s) 4NO +6H₂O

4) Hydrogenation of vegetable oils in presence of Nickel as catalyst

Activity - It is the ability of the catalyst to accelerate the Reaction. It mostly depends upon the Chemisorption strength.

Selectivity - It is an ability to direct reaction to yield of a particular product i.e., One catalyst cannot be a catalyst for other reactions.

Catalysis by zeolites is dependent on shape. Because zeolites are shape-selective catalysts. They are alumino silicates Which are microporous in nature. It has Honeycomb structure. That makes them shape selective. In zeolites, some si atoms are replaced by Al to form Al-O-Si network.

Reactants are very sensitive to the pore size of zeolites. Zeolites are used in petrochemical industry. Ex- ZSM-5 used to convert alcohol into gasoline.

The catalytic reaction that depends upon the pore structures of the catalysts and the size of reactant and product molecules is called SHAPE SELECTIVE CATALYSIS. Zeolites are good shape-selective catalysts.

(i) Electrophoresis: The movement of colloidal particles under an applied electric potential is called Electrophoresis. When an electric potential is applied to two platinum electrodes dipping in colloidal solutions the colloidal particle move towards the Oppositely charged electrodes.

(ii) Coagulation: The process of settling of colloidal particles is called coagulation. When the charge is removed from colloidal solution somehow, then particle start coagulation and settling due to the force of gravity.

(iii) Dialysis: It is a process of removing a dissolved substance from ac colloidal solution by means of diffusion through a suitable membrane. the animal membrane or parchment paper used as a membrane.

(iv) Tyndall Effect: When homogenous solution is placed in a dark and viewed in light beam then if it is viewed from sides it appears visible but if it is viewed from 90° it appears to be dark. This effect is known as Tyndall effect.

Uses of Emulsions-

1) It is used in making of medicines,

2) Cleansing action of soaps is based on this emulsion formation.

3) Digestion of fats in intestine takes place by the process of emulsification.

4) Antiseptics and disinfectant added to water form emulsion for cleaning.

Soaps contain hydrophobic and hydrophilic part when dissolved in the water they arrange themselves in such a way that They form a spherical structure having hydrophobic part towards the centre and hydrophilic part away from centre. This cluster is known as Micelle. Ex-Sodium stearate + Water

(CH₃ (CH₂)16COO-Na + H₂O)

i) Alcohol- a colloidal solution having alcohol as the dispersion medium and a solid substance as the dispersed phase. Ex- colloidal sol of cellulose nitrate in ethyl alcohol.

ii) Aerosol- a colloidal solution having gas as the dispersion medium and a solid substance as the dispersed phase. Ex-Smoke.

iii) Hydrosol- a colloidal solution having Water as the dispersion medium and a solid substance as the dispersed phase. Ex-Gold sol.

We can say that colloid is not a substance but a state of a substance Which is dependent on the size of particle colloidal state is intermediate between a true solution and a suspension. when a size of substance is between 1nm to1000nm it behaves as colloid otherwise not.