Surface Chemistry  Class 12 Chemistry Exam Questions

Exam Questions Class 12

Please refer to the below Surface Chemistry  important questions for Class 12 Chemistry. These questions and answers have been prepared as per the latest NCERT Book for Class 12 Chemistry. Students should go through chapter wise Class 12 Chemistry Important Questions designed as per the latest examination pattern issued by CBSE.

Very Short Answer Questions :

Question. Describe ‘electrophoresis’ briefly.   
Answer : The movement of colloidal particles under an applied electric potential is called electrophoresis.  Positively charged colloidal particles move towards the cathode, while negatively charged particles move towards the anode.

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. Adsorption of a gas on surface of solid is generally accompanied by a decrease in entropy, still it is a spontaneous process. Explain.  
Answer : For the process to be spontaneous ΔG must be negative.
As ΔS is negative, ΔG can be negative only if ΔH is negative and greater than TΔS.

Question. To which colloidal system does milk belong ?  
Answer : Emulsion

Question. Name the two types of adsorption phenomenon.  
Answer : The two types of adsorption phenomenon are chemisorption or chemical adsorption and physisorption or physical adsorption.

Question. Give an example of ‘shape-selective catalyst’.  
Answer : Zeolites (ZSM – 5) are good shape selective catalysts which convert alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

Question. What happens when gelatin is added to gold solution?   
Answer : Gold solution which is lyophobic solution starts behaving like a lyophilic colloid when gelatin is added to it.

Question. Why does physisorption decrease with the increase of temperature ?   
Answer : Since, adsorption is exothermic and according to Le-chatelier’s principle, low temperature is favourable for physical adsorption hence, physisorption decreases with increase in temperature. Solid + Gas  Gas adsorbed on solid + heat

Question. How is a sol different from an emulsion ?  
Answer : Sol is a type of colloid in which the dispersed phase is solid and the dispersion medium is a liquid.
Examples include mud, milk of magnesia.
Emulsion is a type of colloid in which the dispersed phase is liquid and dispersion medium is also a liquid.
Examples include milk, face cream etc.

Question. What are emulsions? Name an emulsion in which water is a dispersed phase.  
Answer : (i) An emulsion : It is a colloidal system when both the dispersed phase and the dispersion medium are in the liquid state. e.g., milk.
(ii) Butter is an emulsion in water acts a dispersed phase and oil acts as the dispersion medium.

Question. In chemisorption why x/m initially increases and then decreases with rise in temperature?  
Answer : Effect of temperature : Chemisorption is an exothermic process. Hence, according to Le Chatelier principle, rate of adsorption decreases with rise in temperature.

Question. Define the term ‘Tyndall effect’.  
Answer : When a beam of light is passed through a colloidal solution and viewed perpendicular to the path of incident light, the path of beam is illuminated by a bluish light. This phenomenon is called Tyndall effect. This is due to the fact that colloidal particles scatter light in all the directions in space.

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. What type of colloid is formed when a liquid is dispersed in a solid? Give an example. 
Answer : When a liquid is dispersed in a solid, ‘gel’ is formed, e.g., butter.

Question. Give reasons for the following observation :
It is necessary to remove CO when ammonia is prepared by Haber’s process.   
Answer : CO is a catalytic poison. It reacts with iron to form iron carbonyl thus inhibiting the activity of catalyst.

Question. Why is adsorption always exothermic?  
Answer : In adsorption, there is always a decrease in residual unbalanced forces on the surface. This results in decrease in surface energy which appears as heat. Hence, adsorption is unconditionally an exothermic process.

Question. What are biocatalysts ? Give an example.  
Answer : Enzymes are termed as biocatalysts as they help in catalysis of biological reactions.
For example, Inversion of cane sugar with the help of invertase enzyme.

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. Explain the following terms giving a suitable example : Emulsification   
Answer : Emulsification : The process of making emulsion is known as emulsification. To stabilise an emulsion, an emulsifying agent or emulsifier is added. Soaps and detergents are most frequently used emulsifiers.

Question. Give one example each of ‘oil in water’ and ‘water in oil’ emulsion.   
Answer : Oil in water emulsion : Milk
Water in oil emulsion : Butter

Question. Define the following term :
Shape selective catalysis  
Answer : 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.

Question. Based on the type of dispersed phase, what type of colloid is micelles.  
Answer : Associated colloids

Question. Differentiate between adsorption and absorption.  
Answer : Adsorption is a surface phenomenon. In this process the adsorbate is concentrated on the surface of the adsorbent and does not penetrate into the bulk whereas, absorption of a substance takes place throughout the bulk of the material. In adsorption, concentration of adsorbate is high on the surface of adsorbent, while during absorption concentration is uniform throughout. e.g., water vapour is adsorbed by silica gel whereas absorbed by anhydrous calcium carbide.

Short Answer Questions :

Question. What are emulsions? What are their different types? Give one example of each type.  
Answer : (i) An emulsion : It is a colloidal system when both the dispersed phase and the dispersion medium are in the liquid state. e.g., milk.
Types of emulsions :
Oil dispersed in water e.g., milk.
Water dispersed in oil e.g., butter.

Question. Write one difference between each of the following :
(i) Multimolecular colloid and macromolecular colloid
(ii) Sol and gel 
Answer : 

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. Define the following terms :
(i) Tyndall effect
(ii) Electrophoresis   
Answer : (i) When a beam of light is passed through a colloidal solution and viewed perpendicular to the path of incident light, the path of beam is illuminated by a bluish light. This phenomenon is called Tyndall effect. This is due to the fact that colloidal particles scatter light in all the directions in space.

Surface Chemistry  Class 12 Chemistry Exam Questions

(ii) The movement of colloidal particles under an applied electric potential is called electrophoresis.  Positively charged colloidal particles move towards the cathode, while negatively charged particles move towards the anode.

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. Explain the cleaning action of soap. Why do soaps not work in hard water?   
Answer : The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part is in the oil droplet and hydrophilic part interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus, soap helps in emulsification and washing away of oils and fats. Yhe negatively charged sheath around the globules prevents them from coming together and forming aggregates.

Surface Chemistry  Class 12 Chemistry Exam Questions

Hard water contains calcium and magnesium ions. These ions form insoluble calcium and magnesium salts when sodium or potassium soaps are dissolved in hard water. These insoluble soaps separate as scum in water and are useless as cleansing agent.

Question. Distinguish between multimolecular, macromolecular and associated colloids. Give one example of each.   
Answer : 

Surface Chemistry  Class 12 Chemistry Exam Questions

Associated colloids : The substances which at low concentration, behave as normal strong electrolytes but at higher concentration exhibit colloidal behaviour due to the formation of aggregated particles, are known as associated colloids.
These are also known as micelles. The formation of micelles takes place only above a particular temperature, called the Kraft temperature and above a particular concentration, called the Critical Micelle Concentration (CMC), e.g., surface active agents such as soaps and synthetic detergents.

Question. Define the following :
(i) Peptization
(ii) Reversible sols   
Answer : (i) Peptization is the process of conversion of a precipitate into colloidal state in the presence of some electrolyte.
(ii) Reversible sols Lyophilic colloids are also known as reversible sols. These sols are directly formed by mixing substances like gum, gelatin, starch etc with a suitable liquid. These sols are stable and cannot be easily coagulated.

Question. Define the following terms :
(i) Brownian movements
(ii) Peptization
(iii) Multimolecular colloids   
Answer : (i) Brownian movement : When the colloidal particles are observed under the ultramicroscope, the particles are seen to be in constant motion in zig-zag path.
This zig-zag motion of dispersed phase particles is called Brownian movement.
Importance : Avogadro’s number can be calculated with the help of Brownian movement.
(ii) Peptization is the process of conversion of a precipitate into colloidal state in the presence of some electrolyte.
(iii) Multimolecular colloids : A colloid in which large number of small molecules combine to form a particle of colloidal size is called multimolecular colloid e.g., sulphur sol.

Question. Define the following terms :
(i) Peptization
(ii) Sol   
Answer : (i) Peptization is the process of conversion of a precipitate into colloidal state in the presence of some electrolyte.
(ii) Sol is a type of colloid in which dispersed phase is solid and dispersion medium is liquid e.g., paints

Question. What are lyophilic and lyophobic sols?
Give one example of each type. Which one of these two types of sols is easily coagulated and why?  
Answer : The colloidal solution in which particles of the dispersed phase have a strong affinity for the dispersion medium. 
These colloidal sols, even if precipitated, change back to the colloid form simply by adding dispersion medium. So, lyophilic sols are reversible in nature. e.g., glue, starch, rubber, etc.
Hydrophobic solutions get easily coagulated on the addition of small amount of electrolyte or by heating or even shaking as they are not stable.

Question. Explain the following terms :
(i) Tyndall effect
(ii) Coagulation  
Answer : (i) Sol is a type of colloid in which the dispersed phase is solid and the dispersion medium is a liquid.
Examples include mud, milk of magnesia.
Emulsion is a type of colloid in which the dispersed phase is liquid and dispersion medium is also a liquid.
Examples include milk, face cream etc.
(ii) Coagulation The process of aggregating together the colloidal particles into large sized particle which ultimately settle down under the force of gravity as a precipitate is called coagulation.

Question. What is an ‘adsorption isotherm’ ?  
Answer : Adsorption isotherm : It is the variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature.

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. Classify colloids where the dispersion medium is water. State their characteristics and write an example of each of these classes.  
Answer : (i) Sol: When solids is dispersed in water, it is called sol, e.g., gold sol starch sol.
(ii) Emulsion: When liquid is dispersed in water, it is called emulsion, e.g., milk.
(iii) Foam : When gas is dispersed in water, it is called foam or froth, e.g., soap lather, whipped cream.

Question. (i) What is the difference between a colloidal solution and an emulsion? Give one example of each.
(ii) What are emulsifiers ?   
Answer : (i) Colloidal solution : These are the solutions in which the diameter of dispersed phase particles may range from 1 to 1000 nm. These are intermediate of true solutions and suspensions. The colloidal particles do not settle down under the force of gravity even on long standing. A colloid is a heterogeneous system, e.g., gold sol, sulphur sol, soap, etc. Emulsions are one of the types of colloidal system, in which both the dispersed phase and dispersion medium are liquids, e.g., milk.
(ii) Emulsifiers : The substances which are added to stabilise the emulsion are called emulsifiers. e.g., various kinds of soaps, lyophilic colloids (proteins, gum etc.).

Question. What is the difference between multimolecular and macromolecular colloids? Give one example of each.   
Answer :

Surface Chemistry  Class 12 Chemistry Exam Questions

Question. What is meant by coagulation of a colloidal solution? Name any method by which coagulation of lyophobic sols can be carried out.  
Answer : Coagulation : The process of aggregating together the colloidal particles into large sized particle which ultimately settle down under the force of gravity as a precipitate is called coagulation.
Coagulation of lyophobic sol can be carried out by adding electrolyte.

Question. Explain the following terms giving one example for each.
(i) Micelles
(ii) Aerosol  
Answer : (i) Aggregated particles of associated colloids at high concentration are called micelles. e.g., soaps.
(ii) Colloid of a liquid in a gas is called aerosol e.g., fog, sprays etc.