Bihar Board - Class 12 Chemistry -Chapter 5: Surface Chemistry Long Answer Question

The long question answers for Chapter 5: Surface Chemistry of Class 12 Chemistry are provided here in English. These questions are based on the NCERT curriculum for Class 12. They are designed to help students gain a deep understanding of the concepts discussed in the chapter. These long question answers will be helpful not only for the Class 12 board exams but also for entrance exams like NEET. The questions and answers for Bihar Board Class 12 Chemistry, Chapter 5: Surface Chemistry, have been prepared by expert teachers at Vidyakul.

                              Long Question Answer

Question 1. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
Answer:  Adsorption isotherm : A graph between the amount of the gas adsorbed per gram of the adsorbent (x/m) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm.
The extent of adsorption is usually expressed as x/m ,

A relationship between the amount adsorbed (x/m) and the equilibrium pressure (P) can be obtained as follows :
At low values of P The graph is nearly straight and sloping.
xm∝ P or xm=constant P1

At high pressure, x/m becomes independent of the values of P . In this range of pressure,
xm∝ P or xm=constant P
In the intermediate range of pressure, x/m will depend on P raised to powers between 1 and 0 i.e. fraction. For a small range of pressure values, we can write :
xm∝ P1n or xm=K P1n

Where n= Positive integer
K= constant depends upon the nature of adsorbate and adsorbent at a particular temperature.
The factor 1n has values between 0 and 1.
This relationship was given by Freundlich and is known as Freundlich adsorption isotherm.

Question 2. 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:

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Question 3. What happens in the following activities and why?
(i) An electrolyte is added to a hydrated ferric oxide sol in water.
(ii) A beam of light is passed through a colloidal solution.
(iii) An electric current is passed through a colloidal solution.
Answer:
(i) When NaCl is added to the hydrated ferric oxide solution, then coagulation will take place. A negatively charged solution is obtained with absorption of OH- ion.

(ii) When electric current is passed through a colloidal solution, the positively charged particles move towards the cathode while negatively charged particles move towards anode where they lose their charge and get coagulated. The phenomenon is known as Electrophoresis.

(iii) When a beam of strong light is passed through a colloidal solution, scattering of light occurs by colloidal particles and the path of light becomes visible and the phenomenon is known as the Tyndall effect.

Question 4: Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.

Answer : The surface phenomenon of crowding up of molecules of a substance at the base surface rather than in the mass (bulk) of a solid or liquid is called Adsorption. Adsorbate is the substance that gets adsorbed and the adsorbent is the substance on whose surface the adsorption takes place. Here, the concentration of the adsorbate on the surface of the adsorbent increases. The concentration of the substance remains at the surface only in the case of adsorption. It does not penetrate through the surface to the bulk of the solid or liquid. For example, on dipping a chalk stick into an ink solution, there is only a color change on its surface and it will be found to be white from inside when the chalk stick is broken.

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. Explain how the phenomenon of adsorption finds application in each of the following processes :
(i) Production of vacuum
(ii) Heterogeneous catalysis
(iii) Froth Floatation process
Answer:
(i) Production of vacuum : In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which enters into annular space either due to glass imperfection or diffusion through glass is adsorbed and creates a vacuum.

(ii) Heterogeneous catalysis : If the catalyst is present in a different phase than that of the reactants, it is called a heterogeneous catalyst and this type of catalysis is called Heterogeneous catalysis.
Example : Manufacture of NH3 from N2 and H2 by Haber’s process using iron as catalyst

N2 (g)+3H2 (g) 2 NH3 (g)

Reactants are gaseous whereas catalysts are solid.

(iii) Froth Floatation process : When KCl is added to hydrated ferric oxide sol, then a negatively charged sol is obtained with absorption of OH- ion.

Question 6. What are emulsions? Describe different types of emulsions giving one example of each type. 
Answer:
Emulsion : The colloidal solution in which both dispersed phase and medium are in liquid state is called emulsion. This is a liquid-liquid colloidal system.
Different types of emulsions are :
Emulsions of oil in water in which oil is the dispersed phase and water is the dispersion medium.
Example : Milk is an emulsion of liquid fat dispersed in water.
Emulsions of water in oil in which water is the dispersed phase and oil is the dispersion medium. e.g. Cod liver oil is an emulsion of oil i.e. water is the dispersed phase and oil is the dispersion medium.
Two applications of emulsion are :

• The digestion of fats in the intestines takes place by the process of emulsification.

• Several oily drugs are prepared in the form of emulsion.

Question 7. Classify colloids where the dispersion medium is water. State their characteristics and write an example of each of these classes.
Answer: Colloids are classified into two types in the cases where dispersion medium is water. They are:
(i) Hydrophilic or Lyophilic colloids : Those substances, which when mixed with the dispersion medium, form directly the colloidal solution and are termed as hydrophilic colloids. They are reversible solutions, quite stable and cannot be easily precipitated.
Example : gum, gelatine, starch, rubber etc.

(ii) Hydrophobic or Lyophobic colloids : Those substances which do not form colloidal solution when simply mixed with the dispersion medium, are called hydrophobic colloids. They are irreversible solutions, unstable and can be easily precipitated. 

Example : Metals and their sulfides.

Question 8. (a) Adsorption of a gas on the surface of solid is generally accompanied by a decrease in entropy, still it is a spontaneous process. Explain.

(b) Some substances can act both as colloids ’ and crystalloids. Explain.

(c) What will be the charge on Agl colloidal particles when it is prepared by adding a small amount of AgNO3 solution to Kl solution in water ? What is responsible for the development of this charge ? 

Answer:

(a) We know G=H-TS for adsorption. H and S are negative and H>TS. Thus from this equation G is negative. Therefore, for adsorption H , G and S all are negative.

(b) A crystalloid can be found to behave as a colloid under a different set of conditions and vice-versa.

Example : NaCl behaves as a crystalloid when dissolved in water but behaves as a colloid when dissolved in benzene.

(c) When AgNO3 solution is added to Kl solution, the precipitated Agl adsorbs I- ions from the dispersion medium and negatively charged colloidal solution results.

Question 9. Explain the following:
(i) Deltas are formed when river and sea water meet.
(ii) Artificial rain is caused by spraying salt over clouds.
(iii) Physisorption is multilayered, while chemisorption is mono-layered.
Answer:
(i) River water is a colloidal solution of the day. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of deltas.

(ii) Clouds are colloidal dispersion of water particles in air carrying some charge over them.
It is possible to cause artificial rain by throwing electrified sand or spraying a sol carrying charge opposite to the one on clouds from an airplane. The colloidal water particles present in the clouds will get neutralized and as result they will come closer and grow in size to form bigger water drops and ultimately cause artificial rain.

(iii) In physical adsorption, layers of the gas can be adsorbed one over the other by van der Waals forces. Multimolecular layers are formed under high pressure. In chemical adsorption, chemical bonds can be formed only with the layer of molecules coming in direct contact with the surface of the adsorbent, hence this type of adsorption is mono-layered.

Question 10. What is an adsorption isotherm? Describe Freundlich adsorption isotherm. 

Answer: Adsorption isotherm : The variation of the amount of the gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve. This curve is termed as an adsorption isotherm at a particular temperature.

Freundlich adsorption isotherm : A mathematical equation which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature is called Freundlich adsorption isotherm.

xm=kp1n

log xm=log k + 1n log p

Question 11. What are the characteristics of the following colloids? Give one example of each.
(i) Multimolecular colloids
(ii) Lyophobic sols
(iii) Emulsions
Answer:
(i) Multimolecular colloids :

• They are formed by the aggregation of a large number of atoms or molecules which generally have diameter less than 1 nm, Example : sols of gold, sulphur etc.

• Their molecular masses are not very high.

• Their atoms or molecules are held together by weak Van der Waals’ forces.

(ii) Lyophobic sols : Liquid hating colloids in which there is no affinity between dispersed phase and dispersion medium.
Example: As2S3 sol. Fe(OH)3 sol . .

(iii) Emulsions : The colloidal solution in which both dispersed phase and medium are in liquid state.
Example : Milk, cream.

Question 12.
(a) How can we get the following colloidal solutions :
(i) Silver in water
(ii) Fe(OH)3 in water
(b) List two applications of adsorption.
Answer:
(a) (i) Silver in water : Colloidal sol of Ag in water (Reduction with dil Sn(Cl2).

2AgCl + SnCl2 2Ag + SnCl4

                                   Silver sol.

(ii) Fe(OH)3 in water : (By boiling with their chlorides):

FeCl3+3H2O Fe(OH)3+3HCl

                             Colloidal sol.

(b) Applications of adsorption :

• All gas devices contain suitable adsorbent so that poisonous gases present in the atmosphere are adsorbed and the air for breathing is purified.

• Sugar is decolourized by treating sugar solution with charcoal powder which adsorbs the undesirable colors present.

Question 13 . (a) In reference to Freundlich adsorption, isotherms write the expression for adsorption of gasses on solids in the form of an equation.

(b) Write an important characteristic of lyophilic sols.

(c) Based on the type of particles of dispersed phase, give one example of each associated colloid and multimolecular colloid. 

Answer: (a) The equation representing adsorption of gasses on solids is

xm=K P1n

where

xm = amount of gas adsorbed per unit mass of adsorbent

K= Constant

n= Positive integer

(b) Lyophilic sols are reversible and self stable.

(c) Associated colloid : Soap sol

Multimolecular colloid : Gold sol

Question 14. Giving appropriate examples, explain how the two types of processes of adsorption (physisorption and chemisorption) are influenced by the prevailing temperature, the surface area of adsorbent and the activation energy of the process? 

Answer: 

Question : 15. Give reasons for the following observations :

(i) Leather gets hardened after tanning.

(ii) Lyophilic sol is more stable than lyophobic sol.

(iii) It is necessary to remove CO when ammonia is prepared by Haber’s process. 

Answer:

(i) Due to mutual coagulation of leather by tanning. [Positively charged animal hyde (leather) with negatively charged colloidal particles of tannin].

(ii) Lyophilic sols are more stable because there is strong interaction between dispersed phase and dispersion medium.

(iii) Because CO acts as a poison for a catalyst.

Question 16. Write any three differences between Physisorption and Chemisorption.

Answer : 

Question 17.
(i) Differentiate between adsorption and absorption.
(ii) Out of MgCl2 and AlCl3 Which one is more effective in causing coagulation of negatively charged sol and why?
(iii) Out of sulfur sol and proteins, which one forms multimolecular colloids?
Answer:

Answer : (i) Differentiate between adsorption and absorption.

(ii) AlCl3 is more effective than MgCl2 in causing coagulation of negatively charged sol because coagulating power of an electrolyte is directly proportional to the valency of the active ion i.e., Al3+ > Mg2+ .

(iii) Sulphur sol forms multimolecular colloids.

Question 18. Write one difference between each of the following:

(i) Multimolecular colloid and Macromolecu- lar colloid

(ii) Sol and Gel

(iii) O/W emulsion and W/O emulsion 

Answer:

(i) Multimolecular colloid. A large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range.

Macromolecular colloid. Large sized molecules whose particle size lies in the colloidal range.

(ii) Sol is a colloidal system in which dispersed phase is a solid and dispersion medium is a liquid. Gel is a colloidal system in which dispersed phase is a liquid and dispersion medium is a solid.

(iii) In O/W emulsion, water acts as dispersion medium while in W/O emulsion oil acts as a dispersion medium.

Question 19. Write one difference in each of the following:
(a) Multimolecular colloid and Associated colloid
(b) Coagulation and Peptization
(c) Homogeneous catalysis and Hetero¬geneous catalysis.
Answer:
(a) Multimolecular colloid and Associated colloid. Multimolecular colloids are formed by the aggregation of a large number of atoms or molecules which generally have diameters less than 1 nm , eg., sols of gold, etc. while Associated colloids are formed by the aggregation of a large number of ions in concentrated solutions, e.g., micelles in soap.

(b) Coagulation and Peptization. Coagulation is a process of aggregating together the colloidal particles into large sized particles to form their precipitate while peptization is a process of converting fresh precipitate into colloidal particles by shaking it with the dispersion medium in the presence of a small amount of a suitable electrolyte.

(c) Homogeneous catalysis and Heterogeneous catalysis. Homogeneous catalysis is the ‘ phenomenon of changing the rate of reaction when catalyst has same phase as
the reactants while in heterogeneous catalysis, the catalyst has a different phase than that of the reactants.

Question 20. (a) Write the dispersed phase and dispersion medium of milk.

(b) Write one similarity between physisorption and chemisorption.

(c) Write the chemical method by which Fe(OH)3 sol is prepared from FeCl3. 

Answer:

(a) In milk dispersed phase is liquid fat and dispersion medium is water.

(b) Both physisorption and chemisorption increase with increase in pressure.

(c) Fe(OH)3 is prepared from FeCl by hydrolysis.

FeCl3 + 3H2O Fe(OH)3 (sol) +3HCl

Question 21. Define the following terms :

(i) Kraft temperature

(ii) Peptization

(iii) Electrokinetic potential 

Answer: (i) Kraft temperature : The formation of micelles takes place only above a particular temperature known as Kraft temperature (Tk).

(ii) Peptization : It is a process of converting a fresh precipitate into colloidal particles by shaking it with the dispersion medium in the presence of a small amount of a suitable electrolyte.

(iii) The potential difference between the fixed layer and the diffused layer is known as electrokinetic potential.

Question 22: What are the factors which influence the adsorption of a gas on a solid?

Answer : Rate of adsorption of a gas on a solid surface depends on various factors:

(1) Nature of the gas:

There is a strong Van der Waals force in easily liquefiable gasses hence, easily liquefiable gasses such as NH3 , HCl etc. are adsorbed to a great extent in comparison to gasses such as H2 , O2 etc.

(2) Surface area of the solid

The adsorption rate of a gas on the solid surface increases with the increase in the surface area of the adsorbent.

(3) Effect of pressure

Adsorption increases with an increase in pressure and, as a result adsorption is a reversible process and is accompanied by a decrease 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 23: How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

Answer : One of the criteria of classifying colloids is the physical state of the dispersed phase and dispersion medium. Depending upon the type of the dispersed phase and dispersion medium (solid, liquid, or gas), there can be eight types of colloidal systems.

Question 24: Action of soap is due to emulsification and micelle formation. Comment.

Answer :  Emulsification and micelle formation determines the cleansing action of soap. Sodium and potassium salts of long-chain fatty acids ( R-COO- Na+ ) is the basic mixture of a soap. Sodium is attached to the end of the molecule, which is polar in nature, while the alkyl-end is non- polar. Thus, a soap molecule contains both polar and nonpolar part. i.e. hydrophilic and hydrophobic part, respectively.

Micelle is formed when water containing dirt is mixed with soap, the molecules of soap surround the dirt particles in such a manner that the dirt molecule and the hydrophobic part get attached and the hydrophilic parts point away from the dirt molecule. Thus, the non-polar group dissolves in the dirt particle while the polar group dissolves in the dirt particle. Now, a stable emulsion is formed as these micelles are negatively charged and they do not coalesce.

Question 25: What do you understand by activation of adsorbent? How is it achieved?

Answer : Increasing the adsorbing power of the adsorbent is known as activating an adsorbent.

Adsorbents are activated by the following ways:

(i) The surface area of the adsorbent can be increased. Breaking it into smaller pieces or powdering it, is one way of doing it.

(ii)  Activation of the adsorbent can be done by some specific treatments. For example, wood charcoal is activated is done by heating it between 650 K and 1330 K in vacuum or air. It expels all the gases absorbed or adsorbed and thus, creates a space for adsorption of gases.