CBSE Class 12 Chemistry Surface Chemistry Assignment Set B

Very Short Answer Type Questions

Question. Physisorption is reversible while chemisorption is irreversible. Why ? 
Answer: Physisorption takes place (any one) with the help of non-covalent bonding between an adsorbate and an adsorbent; it makes the process reversible. Chemisorption, on the other hand, takes place with the help of covalent bonding; it makes the process irreversible.

Question. Define the following term : Adsorption
Answer: Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid resulting in higher concentration of the molecules on the surface.

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 type of forces are responsible for the occurrence of physisorption?
Answer: The forces operating in these cases are weak van der Wals’ forces.

Question. Define the following term : Sorption
Answer: The term sorption is used to describe both the processes adsorption and absorption.

Question. Of physisorption or chemisorption, which has a higher enthalpy of adsorption?
Answer: Chemisorption has higher enthalpy of adsorption.

Question. Out of NH₃ and CO₂, which gas will be adsorbed more readily on the surface of activated charcoal and why?
Answer: NH₃ gas will be adsorbed more readily on the surface because it has higher critical temperature than CO₂ gas. Due to the greater attraction of the gas molecules on the surface of the adsorbent, greater will be the adsorption.

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. Write two applications of adsorption.
Answer: Applications of adsorption :
(i) Deionisation of water
(ii) In chromatographic analysis.

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. 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. 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. 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. 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. Write the main reason for the stability of colloidal sols.
Answer: The main reason for the stability of colloids is the electrostatic stabilisation i.e., equal and same type of charge on the colloidal particles which causes repulsion between them and prevents the coagulation of the sol.

Question. Out of BaCl₂ and KCl, which one is more effective in causing coagulation of a negatively charged colloidal sol? Give reason.
Answer: BaCl₂ is more effective in causing coagulation of negatively charged colloidal sol. Because greater the valency of the coagulating ion, greater is its power to bring about coagulation.

Question. Write the dispersed phase and dispersion medium of butter.
Answer: Dispersed phase : Liquid
Dispersion medium : Solid

Question. Give one example each of lyophobic sol and lyophilic sol.
Answer: A colloidal sol in which dispersed phase and dispersion medium attract each other is called lyophilic colloid. e.g., gum. A colloidal sol in which dispersed phase and dispersion medium repel each other is called lyophobic colloid. e.g., gold solution.

Question. What are the dispersed phase and dispersion medium in milk?
Answer: Liquid fat is the dispersed phase and water is the dispersion medium.

Question. Name of the temperature above which the formation of micelles takes place.
Answer: The formation of micelles takes place only above a particular temperature called Kraft temperature (T_k).

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

Question. Which aerosol depletes ozone layer?
Answer: CFC (Chlorofluorocarbon)

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

Question. Define peptization.l
Answer: Peptization is the process of conversion of a precipitate into colloidal state in the presence of some electrolyte.

Question. How can a colloidal solution and true solution of the same colour be distinguished from each other.
Answer: When a powerful beam of light is passed through true and colloidal solutions each kept in a glass vessel then, colloidal solution exhibits tyndall effect whereas true solution does not.

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 causes brownian movement in a colloidal solution?
Answer: The continuous rapid zig-zag motion of the colloidal particles in the dispersion medium is called Brownian movement. Unbalanced bombardment of the particles of dispersed phase by molecules of dispersion medium causes Brownian motion.
This stabilises the sol.

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. 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. Write the dispersed phase and dispersion medium of the following colloidal systems :
(i) Smoke
(ii) Milk
Answer: (i) Dispersed phase of smoke = Solid
Dispersion medium of smoke = Gas
(ii) Dispersed phase of milk = Liquid
Dispersion medium of milk = Water (liquid)

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.

Short Answer Type Questions

Question. In reference to surface chemistry, Define dialysis.
Answer: Dialysis : It is the process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through membrane into the water and pure colloidal solution is left behind.

Question. Give one example each of sol and gel.
Answer: 

Question. What is the difference between lyophobic sol and lyophilic sol ?
Answer: Lyophilic sols : 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. Lyophobic sols : The colloidal solution in which particles of the dispersed phase have no or very little affinity for dispersion medium. These are irreversible in nature i.e., once precipitated, they have little tendency to get back into the colloidal form on simply adding dispersion medium e.g., As₂S₃ solution. Lyophobic sols need stabilising agents for their preservation.

Question. Explain the following :
Artificial rain is caused by spraying salt over clouds.
Answer: Clouds are colloidal dispersion of water particles in air. These water particles carry some charge over them. On spraying oppositely charged colloidal dust or sand particles over a cloud from an aeroplane, the colloidal water particles present in the cloud will get neutralized and as a result they will come closer and will grow in size to form bigger water drops and ultimately will coagulate or precipitate causing artificial rain.

Question. Give reasons for the following observations :
A delta is formed at the meeting point of sea water and river water
Answer: The two types of emulsions are :
(i) Oil-in-water type in which small droplets of an oil are dispersed in water.
Example : Milk, cod liver oil.
(ii) Water-in-oil type in which water droplets are dispersed in an oil medium.
Example : Butter.

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.

Question. In reference to Freundlich adsorption isotherm write the expression for adsorption of gases on solids in the form of an equation. 
Answer: 

Question. What is the effect of temperature on chemisorption?
Answer: Effect of temperature : Chemisorption is an exothermic process. Hence, according to Le Chatelier principle, rate of adsorption decreases with rise in temperature. Rate of adsorption is low at lower temperature because of high energy of activation hence, with increasing temperature rate of adsorption

Question. Physisorption is multi-layered, while chemisorption is mono-layered.
Answer: Physical adsorption occurs due to intermolecular attractive forces between the adsorbate and adsorbent. If the size of the adsorbent pores is close to the size of adsorbate molecules, multilayer adsorption takes place, i.e., adsorption takes place until all the pores are filled with adsorbate molecules, whereas in chemisorption chemical bonds are formed between adsorbate and adsorbent molecules. Therefore, it is monolayered.

Question. Why is a finely divided substance more effective as an adsorbent? 
Answer: A finely divided substance is more effective as adsorbent because
(i) It has more surface area so more adsorption occurs.
(ii) e number of active sites (active centres) becomes more and the extent of adsorption increases.

Question. What are physisorption and chemisorption?
Answer: Physisorption : The phenomenon in which adsorbate and adsorbent are held by van der Waals’ forces.
It is reversible in nature. e.g., setting a layer of dust particles on the furniture.
Chemisorption : The phenomenon in which adsorbate and adsorbent are held by chemical bonds.
It is irreversible in nature. e.g., painting on a furniture.

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.

Question. Which complex ion is formed when undecomposed AgBr is washed with hypo solution in photography ?
Answer: The developed film is immersed in sodium thiosulphate (hypo) solution which removes unchanged silver bromide as a complex ion.
This is known as fixing.
AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr
After fixing, the film is not sensitive to light.

Question. Explain how the phenomenon of adsorption find application in the following processes :
(i) Production of vaccum
(ii) Heterogeneous catalysis.
Answer: (i) Production of Vacuum : Adsorption can be applied to create condition of high vacuum. Vessel which has already been exhausted by vacuum pump is connected to a bulb containing charcoal. The remaining traces of air inspite of low pressure are adsorbed by the charcoal almost completely.
(ii) Role of adsorption in heterogeneous catalysis The reactant molecules in gaseous state or in solutions are adsorbed on the surface of the solid catalyst by physisorption or chemisorption. As result, the concentration of the reactant molecules on the surface increases and hence, the rate of reaction increases.

Question. (i) Out of MgCl₂ and AlCl₃, which one is more effective in causing coagulation of negatively charged sol and why ?
(ii) Out of sulphur sol and proteins, which one forms multimolecular colloids?
Answer: (i) According to Hardy-Schulze rule, for negatively charged sol greater the valency of positive ion added to it, greater is its coagulation power. In AlCl₃, Al has +3 charge which is more than Mg with +2 charge in MgCl₂. us, AlCl₃ is more effective in causing coagulation of negatively charged sol.
(ii) Proteins are macromolecules which cannot form multimolecular colloids while sulphur sol have smaller S8 molecules which can form multimolecular colloids.

Question. Give reasons for the following observations :
(i) Leather gets hardened after tanning.
(ii) Lyophilic sol is more stable than lyophobic sol.
Answer: (i) Animal hides are colloidal in nature. When a hide, which has positively charged particles is soaked in tannin, containing negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather.
(ii) Lyophilic sol is more stable than lyophobic sol because It is highly hydrated in the solution.

Question. (i) Based on type of particles of dispersed phase, give one example each of associated colloid and multimolecular colloid
(ii) Write an important characteristic of lyophilic sols.
Answer: (i) Associated colloid : Soap
Multimolecular colloid : Sulphur sol
(ii) Lyophilic sols are reversible sols. These are quite stable and cannot be easily precipitated.

Question. How do the size of particles of adsorbent, pressure of gas and temperature influence the extent of adsorption.
Answer: Size of adsorbent particles : Smaller the size of adsorbent particles, larger is the surface area and hence, higher is the adsorption.
Pressure : Increase in pressure forces gas molecules to come closer to the surface of adsorbent leading to increase in the amount of adsorption.
Temperature : Adsorption is an exothermic reaction hence is favoured at lower temperature, at higher temperature the K.E. of adsorbate is high and hence extent of adsorption is low.

Question. 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: Effect of temperature : Physisorption decreases with increase of temperature and chemisorption first increases then decreases with increase of temperature.
Surface area : Greater the surface area, greater is the physisorption and chemisorption.
Activation energy : In physisorption, no appreciable activation energy is needed. In chemisorption, sometimes high activation energy is needed.

Question. Explain what is observed when :
(i) A beam of light is passed through a colloidal solution.
(ii) NaCl solution is added to hydrated ferric oxide sol.
(iii) Electric current is passed through a colloidal solution.
Answer: (i) Scattering of light by the colloidal particles takes place and the path of light becomes visible (Tyndall effect).
(ii) The positively charged colloidal particles of
ferric hydroxide sol get coagulated by the oppositely charged Cl^– ions provided by NaCl.
(iii) On passing electric current through a sol, colloidal particles start moving towards oppositely charged electrodes where they lose their charge and get coagulated (electrophoresis).

Question. What is meant by coagulation of a colloidal solution? Describe briefly any three methods by which coagulation of lyophobic sols can be carried out.
Answer: The process of setting of colloidal particles is called coagulation of the sol. It is also known as precipitation. Following are the three methods by which coagulation of lyophobic sols can be carried out :
(i) Electrophoresis: In this process, the colloidal
particles move towards oppositely charged electrodes and get discharged resulting in coagulation.
(ii) Mixing of two oppositely charged sols: When equal proportions of oppositely charged sols are mixed, they neutralise each other resulting in coagulation.
(iii) Dialysis: By this method, electrolytes present in sol are removed completely and colloid becomes unstable resulting in coagulation.

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. How are the following colloidal solutions prepared?
(a) Sulphur in water (b) Gold in water
Answer: (i) Sulphur sol is prepared by the oxidation of H₂S with SO₂.

Question. (i) Same substances can act both as colloids and crystalloids. Explain
(ii) What will be the charge on AgI colloidal particles when it is prepared by adding small amount of AgNO₃ solution to KI solution in water? What is responsible for the development of this charge ?
Answer: (i) The same substance can act as both colloid and crystalloid. It depends on the size of the particles.
When the size of the particles lies between 1 to 1000 nm, it behaves as a colloid. If particle size is less than 1 nm, it exists as a true solution and behave like a crystalloid.
(ii) When AgNO₃ solution is added to aqueous KI solution, a negatively charged sol of Agl is formed.
This is due to selective adsorption of l– ions from the dispersion medium.

Question. Define the following terms giving an example of each :
(i) Emulsion
(ii) Hydrosol
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) A hydrosol : It is a colloidal solution of a solid in water as the dispersion medium. e.g., starch solution.

Question. Distinguish between micelles and colloidal particles. Give one example of each.
Answer: Micelles : When small particles (ions) of an electrolyte molecule form the aggregate particles which behave like colloidal particles, these aggregated particles are known as micelles. 
Examples : Soap and detergents.
Colloidal particles : Colloidal particles have an enormous surface area per unit mass as a result of their small size. Its size ranges between 1 nm to 100 nm.
e.g., sulphur sol.

Question. What is the difference between oil/water (O/W) type and water/oil (W/O) type emulsions?
Given an example of each type. 
Answer: The two types of emulsions are :
(i) Oil-in-water type in which small droplets of an oil are dispersed in water.
Example : Milk, cod liver oil.
(ii) Water-in-oil type in which water droplets are dispersed in an oil medium.
Example : Butter.

Long Answer Type Questions

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

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. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.

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

Freundlich adsorption isotherm : It is an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.

where x is the mass of 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 the particular temperature. These curves indicate that on increasing temperature, physical adsorption decreases at a fixed pressure.
Taking log in Eq. (i), gives
log(x/m) = log k + (1/n) log P
The validity of Freundlich isotherm can be verified by plotting x/m on y-axis and log p on x-axis.
If it comes to be a straight line, the Freundlich isotherm is valid.

 Question. Name the two groups into which phenomenon of catalysis can be divided. Give an example of each group with the chemical equation involved. 
Answer: Catalysis is divided into following two groups.
Homogeneous catalysis : When reactants and the catalysts are in the same phase i.e., liquid or gas, the catalysis is known as homogeneous catalysis.

Heterogeneous catalysis: When reactants and the catalysts are in different phases, the catalysis is known as heterogeneous catalysis. In most cases, the catalyst is solid, while reactants are either liquid or gases.
Here, the catalyst is usually a metal or an oxide in finely divided form e.g.,

 Question. Define the following term :
Electrophoresis
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.

 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.