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CBSE 12th Chemistry Board Exam 2020: Important Questions & Answers from Chapter 5 - Surface Chemistry

Check the important questions and answers for CBSE Class 12th Chemistry Board Examination 2020 from Chapter 5 - Surface Chemistry. These questions and answers are based on the latest CBSE Class 12th Syllabus prescribed by the CBSE Board. 

Mar 3, 2020 16:30 IST
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Surface Chemistry
Surface Chemistry

The CBSE Class 12th Chemistry Examination is scheduled on 7th March 2020. Therefore, for the ease of students, we have mentioned some important questions and answers which will prove to be helpful while preparing for the upcoming CBSE Class 12th Examination 2020.  In this article, we have listed important questions and answers from the chapter Surface Chemistry. 

Key Points to be mentioned while writing the answers to the below mentioned important questions: 

Question 1- Define: 

  1. Tyndall Effect
  2. Shape-selective catalysis
  3. Coagulation or precipitation
  4. Peptization
  5. Lyophilic colloids or Reversible sols

Answer: 1- Tyndall Effect: If a homogeneous solution placed in dark is observed in the direction of light, it appears clear and, if it is observed from a direction at right angles to the direction of the light beam, it appears perfectly dark. Colloidal solutions viewed in the same way may also appear reasonably clear or translucent by the transmitted light but they show a mild to strong opalescence when viewed at right angles to the passage of light, i.e., the path of the beam is illuminated by a bluish light. This effect was first observed by Faraday and is termed as Tyndall effect. The bright cone of the light is called Tyndall cone. Tyndall effect can be observed during the projection of the picture in the cinema hall due to scattering of light by dust and smoke particles present there. 

Tyndall effect can only be observed when:

(i) The diameter of the dispersed particles is not much smaller than the wavelength of the light used 

(ii) The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.

2- Shape-selective catalysis: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Example: Zeolites, as they have honeycomb-like structures. 

3- Coagulation or precipitation: The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity. This process of settling of colloidal particles is called coagulation or precipitation of the sol. 

Coagulation or precipitation of lyophobic sols can be carried out by:

a- Electrophoresis b- Mixing two oppositely charged sols c- Boiling d- Persistent dialysis e- The addition of electrolytes

4- Peptization: The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called the peptizing agent. This method is applied, generally, to convert a freshly prepared precipitate into a colloidal sol.

5- Lyophilic colloids: Lyophilic means liquid-loving Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols. An important characteristic of these sols is that if the dispersion medium is separated from the dispersed phase (say by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols. 

Question 2- Describe the change observed: 

(i) when a solution of NaCl is added to a sol. of hydrated ferric oxide.

(ii) when a beam of light is passed through a solution of NaCl and then through a sol. 

Answer: i) Coagulation or precipitation of sol. takes place.

(ii) Tyndall effect is observed in the sol.

Question 3- Describe: i) Oil dispersed in water (O/W type) (ii) Water dispersed in oil (W/O type)

Answer: i) Oil dispersed in water (O/W type): In this type, water acts as dispersion medium and oil acts as the dispersed phase. Example: milk and vanishing cream. In milk, liquid fat is dispersed in water. 

ii) Water dispersed in oil (W/O type):  In this type, oil acts as dispersion medium and water acts as a dispersed phase. Examples: butter and cream.

Question 4- Explain:

(a) The same substance can act both as colloids & crystalloids.

(b) Artificial rain is done by spraying salt over clouds. 

Answer:

(a) Sodium chloride acts as a colloid when dissolved in benzene and acts as a crystalloid when dissolved in water. 

(b) Artificial rain is done by spraying common salt over the clouds because it is an electrolyte and thus results in coagulation of water particles.

Question 5- State adsorption isotherm? What is Freundlich adsorption isotherm?

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

Freundlich adsorption isotherm: Freundlich, in the year 1909, gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation:

x / m = k.P1/n (n > 1)          

[x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature.]

Question 6- State the difference between macromolecular, multimolecular and associated colloids in tabular form with examples. 

Answer: 

Multimolecular Colloids

Macromolecular Colloids

Associated Colloids 

On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having the size in the colloidal range (diameter<1nm). The species thus formed are called multimolecular colloids.

Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids.

There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

Example: a gold sol may contain particles of various sizes having many atoms. 

Examples: Starch, cellulose, polystyrene, synthetic rubber, etc.

Example: Soap Sol. 


Question 7- The phenomenon of adsorption finds a number of applications. List a few important applications. 

Answer: (i) Production of high vacuum: The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very high vacuum.

(ii) Gas masks: The Gas mask is a device which consists of activated charcoal or mixture of adsorbents and is usually used for breathing in coal mines to adsorb poisonous gases. 

(iii) Control of humidity: Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.

(iv) Removal of colouring matter from solutions: Animal charcoal removes colours of solutions by adsorbing coloured impurities. 

(v) Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. There are many gaseous reactions of industrial importance involving solid catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact process and use of finely divided nickel in the hydrogenation of oils are excellent examples of heterogeneous catalysis.

(vi) Separation of inert gases: Due to the difference in the degree of adsorption of gases by charcoal, a mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures. 

(vii) In curing diseases: A number of drugs are used to kill germs by getting adsorbed on them. 

(viii) Froth floatation process: A low-grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method using pine oil and frothing agent.

(ix) Adsorption indicators: Surfaces of certain precipitates such as silver halides have the property of adsorbing some dyes like eosin, fluorescein, etc. and thereby producing a characteristic colour at the endpoint. 

(x) Chromatographic analysis: Chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical and industrial fields.

Question 8- Write observations for the following:

(i) when an electric current is passed through a sol

(ii) when a beam of light is passed through a sol

(iii) when an electrolyte (NaCl) is added to ferric hydroxide sol. 

Answer:

(i)  When an electric current is passed through a sol., electrophoresis occurs as +vly charged particles move towards the cathode and -vly charged particles move towards the anode. 

(ii) When a beam of strong light is passed through a colloidal solution, the Tyndall effect occurs. 

(iii) When an electrolyte (NaCl) is added to the ferric oxide sol., then coagulation will take place. 

Question 9- State the difference between Physisorption and Chemisorption. 

Answer: 

S.No.

Physisorption

Chemisorption

1.

If the accumulation of gas on the surface of a solid occurs on account of weak van der Waals’ forces, the adsorption is termed as physical adsorption or physisorption. 

If the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. 

2.

Not specific in nature.

Highly specific in nature.

3.

Reversible in nature.

Irreversible.

4.

It depends on the nature of the gas. More easily liquefiable gases are adsorbed readily.

It also depends on the nature of the gas. Gases which can react with the adsorbent show chemisorption.

5.

Enthalpy of adsorption is low (20-40 kJ mol–1).

Enthalpy of adsorption is high (80-240 kJ mol–1). 

6.

Low temperature is favourable for adsorption. It decreases with the increase in temperature. 

High temperature is favourable for adsorption. It increases with the increase in temperature.

7.

No appreciable activation energy is needed.

High activation energy is sometimes needed.

8. 

It depends on the surface area. It increases with an increase in surface area

It also depends on the surface area. It too increases with an increase in surface area.

9.

It results in multimolecular layers on the adsorbent surface under high pressure. 

It results in the unimolecular layer.

Question 10- Account for the following: 

1- Deltas are formed when seawater and river water meet. 

2- Leather gets hardened after tanning.

3- The necessity to remove CO when ammonia is prepared through Haber’s process.

4- Write the dispersion medium of milk along with its dispersed phase. 

Answer: 1- Deltas are formed when seawater and river water meet as the electrolytes which are present in seawater coagulate the colloidal solution of clay. In addition to this, river water is a negatively charged colloidal solution whereas seawater contains a number of electrolytes.

2-  Leather gets hardened after tanning due to the mutual coagulation. The +vly charged leather coagulates with the -vly charged particles of tanning. 

3- It is necessary to remove CO when ammonia is prepared by Haber’s process as CO acts as a poison for the catalyst. 

4- Dispersion medium: water

     Dispersed phase: liquid fat

Question 11- Define: 

  1. Lyophobic colloids or Irreversible sols
  2. Homogeneous catalysis
  3. Heterogeneous catalysis 
  4. Associated colloids (Micelles)
  5.  Aerosol

Answer: 1- Lyophobic colloids: Lyophobic means liquid-hating. Substances like metals, their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence, are not stable. Further, once precipitated, they do not give back the colloidal sol by the simple addition of the dispersion medium. Hence, these sols are also called irreversible sols. 

2- Homogeneous catalysis: When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis. Example: Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process. 

The reactants, sulphur dioxide and oxygen, and the catalyst, nitric oxide, are all in the same phase.

3- Heterogeneous catalysis: The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Example: Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.

The reactant is in the gaseous state while the catalyst is in the solid-state.

4- Associated colloids (Micelles): There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. Example: Soap Sol.

5- Aerosol: When a large mass of air containing dust particles, is cooled below its dewpoint, the moisture from the air condenses on the surfaces of these particles forming fine droplets. These droplets being colloidal in nature continue to float in the air in the form of mist or fog. Example: Clouds are aerosols having small droplets of water suspended in the air.

Question 12- Define: 

  1. Multimolecular colloids
  2. Macromolecular colloids
  3. Emulsions
  4. Aquasol or hydrosol
  5.  Desorption 

Answer: 1- Multimolecular colloids: On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having the size in the colloidal range (diameter<1nm). The species thus formed are called multimolecular colloids. Example, a gold sol may contain particles of various sizes having many atoms. 

2- Macromolecular colloids: Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples: Starch, cellulose, polystyrene, synthetic rubber, etc.

3- Emulsions: These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid. If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained and is called emulsion. 

There are two types of emulsions: 

(i) Oil dispersed in water (O/W type) and (ii) Water dispersed in oil (W/O type).

4- Aquasol or hydrosol: If the dispersion medium is water, the sol is called aquasol or hydrosol. Example: sol. of Starch. 

5- Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Question 13- Define: 

  1. Adsorption
  2. Absorption
  3. Sorption
  4.  Electrophoresis
  5.  Electroosmosis

Answer: 1- Adsorption: The substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent.

2- Absorption: The substance is uniformly distributed throughout the bulk of the solid. 

3- Sorption: Both adsorption and absorption can take place simultaneously also. The term sorption is used to describe both the processes.

4- Electrophoresis: When an electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged particles move towards the cathode while negatively charged particles move towards the anode. 

5-  Electroosmosis: When electrophoresis, i.e., movement of particles is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field.

Question 14- Define: 

  1. Dialysis
  2. Zeta potential
  3. Kraft temperature
  4. Brownian movement

Answer: 1- Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.

2- Zeta potential: The combination of the two layers of opposite charges around the colloidal particle is called a Helmholtz electrical double layer. The first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed the diffused layer. Since the separation of charge is a seat of potential, the charges of opposite signs on the fixed and diffused parts of the double layer result in a difference in potential between these layers. This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential. 

3- Kraft temperature: The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). 

4- Brownian movement: When colloidal solutions are viewed under a powerful ultramicroscope, the colloidal particles appear to be in a state of continuous zig-zag motion all over the field of view. It is known as Brownian movement. It is independent of the nature of the colloid but depends on the size of the particles and viscosity of the solution. Smaller the size and lesser the viscosity, faster is the motion. The Brownian movement has been explained to be due to the unbalanced bombardment of the particles by the molecules of the dispersion medium. The Brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sols.

The above-mentioned questions are based on NCERT Textbook, sample papers and previous year question papers. The students appearing for the CBSE Class 12th Examination 2020 are advised to go through the links mentioned below: 

CBSE Board Exam 2020: Check Important Questions of Class 12th Chemistry Subject

CBSE 12th Chemistry Board Exam 2020: Important Questions & Answers from Chapter 4 - Chemical Kinetics

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