Solutions-Class-11-Science-Chemistry-Chapter-11-Adsorption and Colloids-Maharashtra Board

(b) 10^-9 m and 10^-6 m

(c) lyophilic sol

(c) promotor

(b) gel

Physical adsorption or physisorption involves van der Waals forces.

In chemical adsorption or chemisorption a compound is formed.

Freundlich adsorption isotherm is represented as : $\frac{x}{m}=kp^{1/n}$

where : x = mass of adsorbate

m = mass of adsorbent

p = equilibrium pressure

k and n = constants.

Inhibition : The phenomenon of retarding a rate of a reaction in the presence of an inhibitor is called inhibition.

Electrophoresis : The movement of colloidal particles under an applied electric potential is called electrophoresis.

Catalysis : The phenomenon of increasing the rate of a chemical reaction with the help of a catalyst is known as catalysis.

(a) Adsorption :The accumulation of a substance on the surface of another substance due to unsatisfied or unbalanced attractive forces on the surface is called adsorption.

(b) When letters are written on black board by chalks, the particles of the chalks are adsorbed on the surface of the board. Hence students can read black board.

• It is a surface phenomenon.
• Adsorbate and adsorbent involve intermolecular forces like van der Waals forces, chemical bonds, etc.
• It is exothermic.
• There are two types of adsorption namely, Physical adsorption and Chemical adsorption.
• It has day to day life and industrial applications, in catalysis, etc.

Tyndall effect : “The phenomenon of scattering of light by colloidal particles and making path of light visible through the dispersion is referred as Tyndall effect.”

• When a light passes through a true solution, containing solute particles of size less than 1 nm, the path of the light is not visible.
• If the light is passed through a colloidal solution, the path of the light through the dispersion medium becomes visible due to the scattering of the light. This is the optical property of the colloids.
• The bright cone of the light observed is called Tyndall cone.

Importance of Tyndall effect :

• It is useful in determining number of particles in colloidal system and the particle size therein.
• It is used to distinguish between colloidal dispersion and true solution.

Brownian motion : The random ceaseless motion of the colloidal or disperse phase particles taking place in all directions over a largo area is called Brownian motion.

Cause of Brownian motion :

• Brownian motion occurs when colloidal particles are constantly bombarded by dispersion medium molecules, gaining kinetic energy through collisions.
• These particles' Brownian motion is slower than medium molecules due to their heavier nature, as they are heavier than the medium's molecules.

There are two types of emulsions

(i) Emulsion of oil in water (o/w type) : An emulsion in which dispersed phase is oil anddispersion medium is water is called emulsion of oil in water.

For example : milk, vanishing cream, paint etc. Milk consists of particles of fat dispersed in water.

(ii) Emulsion of water in oil (w/o type): An emulsion in which dispersed phase is water and dispersion medium is oil is called emulsion of water in oil.

For example, codliver oil consists of particles of water dispersed in oil. Some other examples of this type include butter, cream, etc.

Hardy-Schulze rule : The ability of the flocculating ion to precipitate is often inversely proportional to its valence. This is referred to as the Hardy-Schulze rule.

(i) Ions of the added electrolyte neutralize oppositely charged colloidal particles in the colloidal solution and cause the precipitation.

(ii) The precipitation power of an electrolyte increases with the increase in the valence of an anion or a cation of the electrolyte.

For example, Na+ < Mg⁺ < Al²⁺ and Cl^— < SO₄^2— < PO₄^3—

• The lyophilic colloids require large amount of electrolyte for their coagulation, because the lyophilic particles are surrounded by a layer of medium through which the penetration of ions becomes more difficult.
• The higher concentration of the added electrolyte removes this layer of medium and ions neutralise the charges of colloidal particles and coagulation takes place.
• Lyophobic sols require less amounts of the electrolyte of coagulation.

Electrophoresis : The movement of colloidal particles under an applied electric potential is called electrophoresis.

Electrophoresis set up :

• Above figure shows U tube set up in which two platinum electrodes are dipped in a colloidal solution.
• When electric potential is applied across two electrodes, collodial particles move towards one or other electrode.
• Positively charged particles move towards cathode while negatively charged particles migrate to anode and get deposited on the respective electrode.

Applications :

• To decide the electrical charge on the colloidal particles.
• To separate colloidal particles and obtain a pure true solution.
• Since the components of colloidal particles in a mixture migrate with different rates, they can be separated.
• The rate of migration of colloidal particles can be measured.

Finely divided small size particles provide a large surface area compared to a large particle. Hence adsorption is to a greater extent. This increases the catalytic activity to a greater extent.

Adsorption Isotherm : The relationship between the amount of a substance adsorbed per unit mass of an adsorbent and the equilibrium pressure (in case of gas) or concentration (in case of solution) at a given constant temperature is called an adsorption isotherm.

Animal charcoal adsorbs the colouring matter in aqueous solution of raw sugar on its surface. Hence, the solution of sugar becomes colourless.

• When a light passes through a true solution, containing solute particles of size less than 1 nm, the path of the light is not visible.
• But if the light is passed through a colloidal solution, the path of the light through the dispersion medium becomes visible due to the scattering of the light. This is the optical property of the colloids.

The adsorption of gases on solids depends on the following factors :

• Nature of adsorbate (gas)
• Nature of adsorbent
• Surface area of adsorbent
• Temperature of surface
• Pressure of an adsorbing gas.

(i) Catalysis: The solid catalysts are used in many industrial manufacturing processes.

(ii) Gas masks: It is a device which consists of activated charcoal or mixture of adsorbents. It is used for breathing in coal mines to avoid inhaling of the poisonous

gases.

(iii) Control of humidity : Silica and alumina gels are good adsorbents of moisture.

(iv) Production of high vacuum : Lowering of temperature at a given pressure, increases the rate of adsorption of gases on charcoal powder. By using this principle, high vacuum can be attained by adsorption.

Bredig’s Arc method :

• This process involves vaporization as well as condenstion.
• In this method, electric arc is struck between electrodes of metal immersed in the dispersion medium.
• The intense heat produced vapourises the metal which then condenses to form particles of colloidal sol.
• Colloidal sols of metals such as gold, silver, platinum can be prepared by this method.

Emulsions : A colloidal system in which one liquid is dispersed in another immiscible

liquid is called an emulsion.

• There are liquid-liquid colloidal system in which both liquids are completely or partially immiscible.

There are two types of emulsions

(i) Emulsion of oil in water (o/w type) : An emulsion in which dispersed phase is oil anddispersion medium is water is called emulsion of oil in water.

For example : milk, vanishing cream, paint etc. Milk consists of particles of fat dispersed in water.

(ii) Emulsion of water in oil (w/o type): An emulsion in which dispersed phase is water and dispersion medium is oil is called emulsion of water in oil.

For example, codliver oil consists of particles of water dispersed in oil. Some other examples of this type include butter, cream, etc.

Finely divided small size particles provide a large surface area compared to a large particle. Hence adsorption is to a greater extent. This increases the catalytic activity to a greater extent.

Graphical Representation of Freundlich equation :

(1) When x/m is plotted against P, a smooth parabolic curve is obtained.

(i) At low pressure : x/m increases linearly with the increase in pressure,

Hence x/m ∝ P¹.

(ii) At high pressure : Adsorption is independent of P.

Hence x/m ∝ P⁰.

(m) At moderate pressure : x/m varies exponentially.

x/m ∝ P^1/n where 0 < 1/n < 1.

(2) Taking the logarithm to the base 10 of both sides of Freundlich’s isotherm (equation),

log₁₀ $\frac{x}{m}$ = $\frac{1}{n}$ log₁₀ C + log₁₀ k

This is an equation of a straight line (y = mx + c).

If  log₁₀ $\frac{x}{m}$  is plotted against log₁₀ P, a straight line is obtained.

The slope of this line is $\frac{1}{n}$ and the intercept on log₁₀ $\frac{x}{m}$  axis is log₁₀ k

For a solution we can write,

log₁₀ $\frac{x}{m}$ = $\frac{1}{n}$log₁₀ C + log₁₀ k

• The adsorption phenomenon is caused by unbalanced forces on the surface of a bulk involving London-dispersion forces or van der Waals forces. These are short range forces.
• There is a tendency for a liquid to decrease the surface tension and free energy and this leads to adsorption phenomenon.

(a)

Nature of adsorbate:

• All solids adsorb gases to some extent. It is observed that gases having high critical temperature liquefy easily and can be readily adsorbed.
• The gases such as SO₂, Cl₂, NH₃ which are easily liquefiable are adsorbed to a larger extent as compared to gases such as N₂, O₂, H₂, etc. which are difficult to liquify.
• Thus, the amount of gas adsorbed by a solid depends on the nature of the adsorbate gas i.e., whether it is easily liquefiable or not.

Nature of adsorbent:

• Substances that provide large surface area for a given mass are effective as adsorbents and adsorb appreciable volumes of gases. e.g. Silica gel and charcoal are effective adsorbents due to their porous nature.

(b) Surface area of an adsorbent : Adsorption is a surface phenomenon, hence as the surface area increases, the extent of adsorption increases.

For example, finely divided substances like charcoal powder, rough metal surfaces, colloidal substances, etc. are good adsorbents.

(i) Separation of inert gases : In a mixture of noble gases, different gases adsorb to different extent. Due to selective adsorption principle, gases can be

separated on coconut charcoal.

(ii) Froth floatation process : A low grade sulfide ore is concentrated by separating it from silica and other earthy matter using  pine oil as frothing agent. Hydrophobic pine oil preferentially wets (adsorbs on) sulfide ore which is taken up in the froth.

Micelle formation in a soap solution :

• Soap has a long hydrophobic hydrocarbon chain, called tail, attached to hydrophilic ionic group carboxylate, called head.
• In water, the soap molecules arrange themselves to form spherical particles that are called micelles. See above Fig.
• In each micelle, the hydrophobic tails of soap molecules point to the centre and the hydrophilic heads lie on the surface of the sphere.
• As a result of this, soap dispersion in water becomes stable.