Class 12 - Chemistry - Surface Chemistry

Question 5.1.

Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.

Answer:

Adsorption is a surface phenomenon of accumulation of molecules of a substance at the surface rather than in the bulk of a solid or liquid.

The substance that gets adsorbed is called the ‘adsorbate’ and the substance on whose surface the adsorption takes place is called the ‘adsorbent’.

Here, the concentration of the adsorbate on the surface of the adsorbent increases.

In adsorption, the substance gets concentrated at the surface only.

It does not penetrate through the surface to the bulk of the solid or liquid.

For example, when we dip a chalk stick into an ink solution, only its surface becomes coloured.

If we break the chalk stick, it will be found to be white from inside.

On the other hand, the process of absorption is a bulk phenomenon.

In absorption, the substance gets uniformly distributed throughout the bulk of the solid or liquid.

 

 

Question 5.2.

What is the difference between physisorption and chemisorption?

 Answer:

 

Physisorption

Chemisorption

1.

Here adsorbate is held on the surface of adsorbent by van der Waals force.

Here molecules of adsorbate and adsorbent are held by chemical bonds.

2.

Enthalpy of adsorption is comparatively low. i.e., in the order of 20 kjmol-1.

Enthalpy of adsorption is high. i.e., in the order of 200 kjmol-1.

3.

It is reversible

It is irreversible

4.

It is usually takes place at low temperature and decreases with increasing temperature.

It takes place at relatively high temperature.

5.

It is related to the case of liquefication of the gas. Easily liquefiable gases are more adsorbed. For example NH3, HCl etc. are more adsorbed than permanent gases like He, O2, N2 etc.

It is not related.

6.

Multi molecular layer of adsorbate occurs at the surface.

It forms only monomolecular layers.

7.

It does not require any activation energy.

It requires high activation energy.

8.

High pressure is favourable. Decrease of pressure causes desorption.

High pressure is favourable. Decrease of pressure does not cause desorption.

9.

It is not specific. i.e., all gases are adsorbed by an adsorbent.

It is highly specific.

 

 

 

Question 5.3.

Give reason why a finely divided substance is more effective as an adsorbent.

Answer:

Adsorption is a surface phenomenon. The extent of adsorption depends on the surface area.

Therefore, adsorption is directly proportional to the surface area.

Increase in the surface area of the adsorbent, increases the total amount of gas adsorbed.

A finely divided substance like nickel, platinum & porous substances like charcoal, silica gel provide large surface area.

Both physisorption and chemisorption increase with an increase in the surface area.

Hence, a finely divided substance behaves as a good adsorbent.

 

 

Question 5.4.

What are the factors which influence the adsorption of a gas on a solid?

Answer:

There are various factors that affect the rate of adsorption of a gas on a solid surface.

(1) Nature of the gas:

Easily liquefiable gases such as NH3, HCl etc. are adsorbed to a great extent in comparison to gases such as H2, O2 etc.

This is because Van der Waal’s forces are stronger in easily liquefiable gases.

(2) Surface area of the solid

The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid surface.

(3) Effect of pressure

Adsorption is a reversible process and is accompanied by a decrease in pressure.

Therefore, adsorption increases with an increase in pressure.

(4) Effect of temperature

Adsorption is an exothermic process.

Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

 

 

Question 5.5.

What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

Answer:

The plot between the extents of adsorption(x/m) against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

Class_12_Chemistry_Surface_Chemistry_Adsorption_Isotherm

Freundlich adsorption isotherm:

Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas

adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.

From the given plot it is clear that at pressure PS, (x/m) reaches the maximum valve. Ps is called the saturation pressure.

Three cases arise from the graph now.

Case I- At low pressure:

The plot is straight and sloping, indicating that the pressure in directly proportional to (x/m) i.e. (x/m) ∝ P

Or (x/m) = k P (k=constant)

Case II- At high pressure:

When pressure exceeds the saturated pressure (x/m), becomes independent of P values.

(x/m) ∝ P0

(x/m) = k P0

Case III- At intermediate pressure:

At intermediate pressure, (x/m) depends on P raised to the powers between 0 and 1.

This relationship is known as the Freundlich adsorption isotherm.

(x/m) ∝ P (1/n)

(x/m) = k P (1/n) (n>1)

Now taking log

log(x/m) = log k + (1/n) log P

On plotting the graph between log(x/m) and log P, a straight line is obtained with the slope equal to (1/n) and the intercept equal to log k.

 

 Class_12_Chemistry_Surface_Chemistry_Graph1

 

Question 5.6.

What do you understand by activation of adsorbent? How is it achieved?

Answer:

Activation of adsorbent can be achieved by following (a) Increase the surface area of adsorbent.

This can be achieved by making adsorbent into smaller pieces. (ii) Charcoal is activated by heating it between 650 K and 1330 K in vacuum or air.

It expels all the gases absorbed or adsorbed and therefore, creates a space for adsorption of gases.

 

 

Question 5.7.

What role does adsorption play in heterogeneous catalysis?

Answer:

Heterogeneous catalysis:

 A catalytic process in which the catalyst and the reactants are present in different phases is known as a heterogeneous catalysis.

This heterogeneous catalytic action can be explained in terms of the adsorption theory. The mechanism of catalysis involves the following steps:

  • Adsorption of reactant molecules on the catalyst surface.
  • Occurrence of a chemical reaction through the formation of an intermediate
  • De-sorption of products from the catalyst surface
  • Diffusion of products away from the catalyst surface.
  • In this process, the reactants are usually present in the gaseous state and the catalyst is present in the solid state.
  • Gaseous molecules are then adsorbed on the surface of the catalyst.
  • As the concentration of reactants on the surface of the catalyst increases, the rate of reaction also increases.
  • In such reactions, the products have very less affinity for the catalyst and are quickly desorbed, thereby making the surface free for other reactants.

 

 

Question 5.8.

Why is adsorption always exothermic?

Answer:

Adsorption is always exothermic. This statement can be explained in two ways.

(i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent.

This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

(ii) ∆H of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is

restricted leading to a decrease in the entropy of the gas i.e., ∆S is negative.

Now for a process to be spontaneous, ∆G should be negative.

∆G = ∆H − T∆S

Since ∆S is negative, ∆H has to be negative to make ∆G negative. Hence, adsorption is always exothermic.

 

 

Question 5.9.

How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

Answer:

Depending upon the physical state of dispersed phase and dispersion medium whether these are solids, liquids or gases, eight types of colloidal systems are possible.

Dispersed Phase

Dispersion Medium

Colloidal System

Examples

Liquid

Gas

Aerosol of liquids

Fogs, Clouds, mists

 

Solid

Gas

Aerosol of solids

Smoke, haze

Gas

Liquid

Foam or Froth

Soap Lather, Lemonade Froth

Soda Water

Whipped cream

Liquid

Liquid

Emulsions

Milk, medicines

Solid

Liquid

Sols

Starch in water

Proteins, Gold Sol, Ink

 

Gas

Solid

Solid Foam

Pumice stone,

Rubber,

Foam Rubber

Liquid

Solid

Gels

Cheese, Butter,

Jelly,

Curd

Solid

Solid

Solid sols(coloured glass)

Ruby glass, some gem stones and

alloys

Question 5.10.

Discuss the effect of pressure and temperature on the adsorption of gases on solids.

 

 

Answer:

Effect of pressure

Adsorption is a reversible process and is accompanied by a decrease in pressure.

Therefore, adsorption increases with an increase in pressure.

Effect of temperature

Adsorption is an exothermic process.

Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

 

 

Question 5.11.

What are lyophilic and lyophobic sols? Give one example of each type. Why hydrophobic sols are easily coagulated?

Answer:

Lyophilic Sols:-

The word meaning of lyophilic means ‘liquid-loving’ or ‘solvent- attracting’.

This means that in this colloidal solution there is a strong attraction between the dispersed phase and dispersion medium, i.e.,

the dispersed phase has great affinity for the dispersion medium that results in the extensive solvation of the colloidal particles.

In such solids, the dispersed phase does not easily precipitate and the sols are quite stable. These sols are reversible in nature.

The dispersed phase obtained by the evaporation can be easily converted to the sol state by simply agitating it with the dispersion medium.

Additional stabilisers are not required during their preparation.

If water is used as the dispersion medium, lyophilic sols are called hydrophilic sols.

Starch, gum, gelatine, egg albumin etc. are examples of lyophilic sols.

Starch Sol

Starch forms lyophilic sol when water is used as the dispersion medium.

The formation of sol is accelerated by heating. Starch sol can be prepared by heating it and water at 100 °C.

It is quite stable and is not affected by the presence of any electrolytic impurity.

Lyophobic Sols

The word lyophobic means ‘liquid-hating’. That means in these sols, there is little or no interaction between the

dispersed phase and the dispersion medium i.e., dispersed phase has little affinity for dispersion medium.

These sols are easily precipitated by the addition of small amounts of electrolyte, by heating or by shaking;

therefore these sols are relatively less stable than lyophilic sols.

They need stabilising agents for their preparation.

If water is used as the dispersion medium, lyophobic sols are called hydrophobic sols.

Examples of lyophobic sols include sols of metals and their insoluble compounds like sulphides and oxides.

Ferric Hydroxide Sol

Ferric hydroxide forms lyophobic sols on treatment with water.

Ferric hydroxide sol is prepared by the hydrolysis of ferric chloride with boiling distilled water.

The reaction takes place is as follows.

FeCl3 (aq) + 3H2O (l) --> Fe (OH) 3(s) + 3HCl (aq)

                            (Boil)  Red Sol          

The hydrolysis reaction produces insoluble ferric hydroxide particles

which undergo agglomeration to yield bigger particles of colloidal dimensions.

These particles absorb Fe3+ ions preferentially from the solution to give positive charge to the sol particles.

Stability of sol is due to the charge on the sol particles.

Hydrochloric acid produced during hydrolysis must be removed from the sol because it destabilizes the sol.

HCl can be removed from the sol by dialysis process otherwise sol will not be stable.

Now, the stability of hydrophilic sols depends on two things- the presence of a charge and the salvation of colloidal particles.

On the other hand, the stability of hydrophobic sols is only because of the presence of a charge.

Therefore, the latter are much less stable than the former.

If the charge of hydrophobic sols is removed (by addition of electrolytes), then the particles present in them come closer and form aggregates,

leading to precipitation.

 

 

Question 5.12.

What is the difference between multimolecular and macromolecular colloids?

Give one example of each. How are associated colloids different from these two types of colloids?

Answer:

(i) In multi-molecular colloids, the colloidal particles are an aggregate of atoms or small molecules with a diameter of less than 1 nm.

The molecules in the aggregate are held together by van der Waal’s forces of attraction.

Examples of such colloids include gold sol and sulphur sol.

 

(ii)     In macro-molecular colloids, the colloidal particles are large molecules having colloidal dimensions.

These particles have a high molecular mass. When these particles are dissolved in a liquid, sol is obtained.

For example: starch, nylon, cellulose, etc.

(iii)    Certain substances tend to behave like normal electrolytes at lower concentrations.

However, at higher concentrations, these substances behave as colloidal solutions due to the formation of aggregated particles.

Such colloids are called aggregated colloids.

 

 

Question 5.13.

What are enzymes? Write in brief the mechanism of enzyme catalysis.

Answer:

Enzymes are basically protein molecules of high molecular masses.

These form colloidal solutions when dissolved in water.

These are complex, nitrogenous organic compounds produced by living plants and animals.

Enzymes are also called ‘biochemical catalysts’.

Mechanism of enzyme catalysis:

Class_12_Chemistry_Surface_Chemistry_Mechanism_Of_Enzyme_Catalysis

On the surface of the enzymes, various cavities are present with characteristic shapes.

These cavities possess active groups such as −NH2, −COOH, etc.

The reactant molecules having a complementary shape fit into the cavities just like a key fits into a lock.

This leads to the formation of an activated complex. This complex then decomposes to give the product.

Hence,

Step 1: E + S → ES+

(Activated complex)

Step 2: ES+ → E + P

  

Question 5.14.

How are colloids classified on the basis of:-

(i) Physical states of components

(ii) Nature of dispersion medium and

(iii) Interaction between dispersed phase and dispersion medium?

Answer:

(i) On the basis of the physical state of the components (by components we mean the dispersed phase and dispersion medium).

Depending on whether the components are solids, liquids, or gases, we can have eight types of colloids.

(ii) On the basis of the dispersion medium, sols can be divided as:

Dispersion Medium

Name of sol

Water

Aquasol or hydrosol

Alcohol

Alcohol

Benzene

Benzosol

Gases

Aerosol

 

(iii) On the basis of the nature of the interaction between the dispersed phase and dispersion medium,

the colloids can be classified as lyophilic (solvent attracting) and lyophobic (solvent repelling).

 

 

Question 5.15.

Explain what is observed

(i) when a beam of light is passed through a colloidal sol.

(ii) an electrolyte, NaCl is added to hydrated ferric oxide sol.

(iii) electric current is passed through a colloidal sol?

Answer:

(i)      When a beam of light is passed through a colloidal solution, then scattering of light is observed.

This is known as the Tyndall effect. This scattering of light illuminates the path of the beam in the colloidal solution.

 

(ii)     When NaCl is added to ferric oxide sol, it dissociates to give Na+ and Cl-ions.

Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of negatively charged Cl- ions.

 

(iii)    The colloidal particles are charged and carry either a positive or negative charge.

The dispersion medium carries an equal and opposite charge.

This makes the whole system neutral.

Under the influence of an electric current, the colloidal particles move towards the oppositely charged electrode.

When they come in contact with the electrode, they lose their charge and coagulate.

 

 

Question 5.16.

What are emulsions? What are their different types? Give example of each type.

Answer:

The colloidal solution in which both the dispersed phase and dispersion medium are liquids is called an emulsion.

There are two types of emulsions: (a) Oil in water type: Here, oil is the dispersed phase while water is the dispersion medium.

For example: milk, vanishing cream, etc. (b) Water in oil type: Here, water is the dispersed phase while oil is the dispersion medium.

For example: cold cream, butter, etc.

 

 

Question 5.17.

What is demulsification? Name two demulsifiers.

 

Answer:

The process of decomposition of an emulsion into its constituent liquids is called demulsification.

Examples of demulsifiers are surfactants, ethylene oxide, etc.

 

 

Question 5.18.

Action of soap is due to emulsification and micelle formation. Comment.

Answer:

The cleansing action of soap is due to emulsification and micelle formation.

Soaps are basically sodium and potassium salts of long chain fatty acids, R-COO-Na+.

The end of the molecule to which the sodium is attached is polar in nature, while the alkyl-end is non- polar.

Thus, a soap molecule contains a hydrophilic (polar) and a hydrophobic (nonpolar) part.

When soap is added to water containing dirt, the soap molecules surround the dirt particles in such a manner

that their hydrophobic parts get attached to the dirt molecule and the hydrophilic parts point away from the dirt molecule.

This is known as micelle formation.

Thus, we can say that the polar group dissolves in water while the non-polar group dissolves in the dirt particle.

Now, as these micelles are negatively charged, they do not coalesce and a stable emulsion is formed.

 

 

Question 5.19.

Give four examples of heterogeneous catalysis?

 

Answer:

(i) Oxidation of sulphur dioxide to form sulphur trioxide. In this reaction, Pt acts as a catalyst.

2SO2 (g) --> 2SO3 (g)

         (Pt(s))

(ii) Formation of ammonia by the combination of dinitrogen and dihydrogen in the presence of finely divided iron.

N2 (g) + 3H2 (g)  --> 2NH3 (g)

                      (Fe(s))

This process is called the Haber’s process.

(iii) Oswald’s process: Oxidation of ammonia to nitric oxide in the presence of platinum.

4NH3 (g) + 5O2 (g) --> 4NO (g) + 6H2O (g)

                          (Pt(s))

(iv) Hydrogenation of vegetable oils in the presence of Ni.

Vegetable oil (l) + H2 (g) --> Vegetable ghee(s)

                                  (Ni(s))

 

Question 5.20.

What do you mean by activity and selectivity of catalysts?

Answer:

(a) Activity of a catalyst:

The activity of a catalyst is its ability to increase the rate of a particular reaction.

Chemisorption is the main factor in deciding the activity of a catalyst.

The adsorption of reactants on the catalyst surface should be neither too strong nor too weak. It should just be strong enough to make the catalyst active.

(b) Selectivity of the catalyst:

The ability of the catalyst to direct a reaction to yield a particular product is referred to as the selectivity of the catalyst.

For example, by using different catalysts, we can get different products for the reaction between H2 and CO.

  • CO(g) + 3H2(g) --> CH4(g) + H2O(g) (In presence of Ni)
  • CO(g) +2H2(g) --> CH3OH(g) ( In presence of Cu/ZnO-CrO3)
  • CO(g) + H2(g) --> HCHO(g) (IN presence of Cu)

 

 

Question 5.21.

Describe some features of catalysis by zeolites.

Answer:

Features of catalysis by Zeolites:

  • Zeolites are hydrated alumino silicates which have a three dimensional network structure containing water molecules in their pores.
  • To use them as catalysts, they are heated so that water of hydration present in the pores is lost and the pores become vacant.
  • The size of the pores varies from 260 to 740 pm.
  • Thus, only those molecules can be adsorbed in these pores and catalysed whose size is small enough to enter these pores.
  • Hence, they act as molecular sieves or shape selective catalysts. An important catalyst used in petroleum industries is ZSM-5.
  • It converts alcohols into petrol by first dehydrating them to form a mixture of hydrocarbons.

Alcohols à Hydrocarbons (In presence of ZSM-5/Dehydration)

 

 

Question 5.22.

What is shape selective catalysis?

Answer:

A catalytic reaction which depends upon the pore structure of the catalyst and on the size

of the reactant and the product molecules is called shape-selective catalysis.

For example, catalysis by zeolites is a shape-selective catalysis.

The pore size present in the zeolites ranges from 260-740 pm.

Thus, molecules having a pore size more than this cannot enter the zeolite and undergo the reaction.

 

 

Question 5.23.

Explain the following terms:

(i) Electrophoresis (ii) Coagulation (iii) Dialysis (iv) Tyndall effect.

Answer:

  • Electrophoresis: The movement of colloidal particles under the influence of an applied electric field is known as electrophoresis.
  • Positively charged particles move to the cathode, while negatively charged particles move towards the anode.
  • As the particles reach oppositely charged electrodes, they become neutral and get coagulated.
  • Coagulation: The process of settling down of colloidal particles i.e., conversion of a colloid into a precipitate is called coagulation.
  • Dialysis: - The process of removing a dissolved substance from a colloidal solution by the means of diffusion through a membrane is known as dialysis.
  • This process is based on the principle that ions and small molecules can pass through animal membranes unlike colloidal particles.
  • Tyndall effect: When a beam of light is allowed to pass through a colloidal solution, it becomes visible like a column of light.
  • This is known as the Tyndall effect. This phenomenon takes place as particles of colloidal dimensions scatter light in all directions.

 

 

Question 5.24.

Give four uses of emulsions?

Answer:

Following are the Uses of Emulsions:-

(i) Cleansing action of soaps is based on the formation of emulsions.

(ii) Digestion of fats in intestines takes place by the process of emulsification.

(iii) Antiseptics and disinfectants when added to water form emulsions.

(iv) The process of emulsification is used to make medicines.

 

 

Question 5.25.

What are micelles? Give an example of a micellers system.

 Answer:

Micelle formation is done by substances such as soaps and detergents when dissolved in water.

The molecules of such substances contain a hydrophobic and a hydrophilic part.

When present in water, these substances arrange themselves in spherical structures in such a manner

that their hydrophobic parts are present towards the centre, while the hydrophilic parts are pointing towards the outside (as shown in the given figure).

This is known as micelle formation.

 

 Class_12_Chemistry_Surface_Chemistry_Micelle_Formation_By_Soaps

 

Question 5.26.

Explain the terms with suitable examples:

(i) Alcosol (ii) Aerosol (iii) Hydrosol.

Answer:

(i) Alcosol:

A colloidal solution having alcohol as the dispersion medium and a solid substance as the dispersed phase is called an alcosol.

For example: colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.

(ii) Aerosol:

A colloidal solution having a gas as the dispersion medium and a solid as the dispersed phase is called an aerosol.

For example: fog

(iii) Hydrosol

A colloidal solution having water as the dispersion medium and a solid as the dispersed phase is called a hydrosol.

For example: starch sol or gold sol

 

 

Question 5.27.

Comment on the statement that “colloid is not a substance but a state of

substance”.

Answer:

A colloid is not a substance but a state of the substance which is dependent on the size of the particle.

A colloid is formed when the size of the solute particle lies between 1 nm and 1000 nm. For example:

E.g., NaCl in water behaves as a crystalloid while in benzene, behaves as a colloid (called associated colloid).

It is the size of the particles which matters i.e., the state in which the substance exist.

This shows that a substance can be brought into a colloidal state by different methods.

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