Surface Chemistry Class 12th Chemistry Part I CBSE Solution

Class 12th Chemistry Part I CBSE Solution
Intext Questions Pg-127
  1. Write any two characteristics of Chemisorption.
  2. Why does physisorption decrease with the increase of temperature?…
  3. Why are powdered substances more effective adsorbents than their crystalline forms?…
Intext Questions Pg-133
  1. In Haber’s process, hydrogen is obtained by reacting methane with steam in presence of NiO…
  2. Why is the ester hydrolysis slow in the beginning and becomes faster after some time?…
  3. What is the role of desorption in the process of catalysis.
Intext Questions Pg-145
  1. What modification can you suggest in the Hardy Schulze law?
  2. Why is it essential to wash the precipitate with water before estimating it…
Exercises
  1. Distinguish between the meaning of the terms adsorption and absorption. give one example…
  2. What is the difference between Physisorption and Chemisorption?
  3. | Give reasons why a finely divided substance is more effective as an adsorbent?…
  4. What are the factors which influence the adsorption of a gas on a solid?…
  5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.…
  6. What do you understand by activation of adsorbent? How is it achieved?…
  7. What role does adsorption play in heterogeneous catalysis ?
  8. Why is adsorption is always exothermic ?
  9. How are the colloidal solutions classified on the basis of physical states of the…
  10. Discuss the effect of pressure and temperature on the adsorption of gases on solids?…
  11. What are lyophilic and lyophobic sols ? give one example of each type. Why are hydrophobic…
  12. What is the difference between multimolecular and macromolecular colloids ? Give one…
  13. What are enzymes? Write in brief the mechanism of enzyme catalysis ?…
  14. How are the colloids formed classified on the basis of 1) Physical states of components 2)…
  15. Explain what is observed (i) When a beam of light is passed through a colloidal sol (ii)…
  16. What are the emulsions ? What are their different types ? give one example of each type.…
  17. How do emulsifiers stabilize the emulsion? name two emulsifiers ?…
  18. Action of soap is due to emulsification and micelle formation. Comment ?…
  19. Give four examples of heterogeneous catalysis.
  20. What do you mean by activity and selectivity of catalysts?
  21. Describe some features of catalysis by zeolites.
  22. What is shape selective catalysis?
  23. Explain the following terms (i) Electrophoresis, (ii) Coagulation (iii) Dialysis (iv)…
  24. Give four uses of emulsions.
  25. What are micelles? Give an example of micelles system.
  26. Explain the terms with suitable examples. (i) alcohol, (ii) aerosol (iii) Hydrosol.…
  27. Comment on the statement That “colloid is not a substance but a state of substance”.…

Intext Questions Pg-127
Question 1.

Write any two characteristics of Chemisorption.


Answer:

The two characteristics of Chemisorption are:


a) In Chemisorption which is highly specific in nature, the adsorbate and adsorbent get attached by chemical bonds which are either covalent or ionic in nature.


b) High activation energy is required and high temperature is also favourable.


c) Chemisorption increases with the increase in surface area which results in more number of active sites.



Question 2.

Why does physisorption decrease with the increase of temperature?


Answer:

As Physisorption is Exothermic in nature, which means when gas gets adsorbed on the solid surface, Heat is evolved. So, according to Le-Chatelieres when the temperature is an increased reverse process (Desorption) will be favoured. So, Physisorption decreases with the increase of temperature.



Where x/m: Volume of gas adsorbed


T: Temperature.



Question 3.

Why are powdered substances more effective adsorbents than their crystalline forms?


Answer:

As Adsorption is directly proportional to the available surface area and powdered form of a substance have a greater surface area than the crystalline form of the substance.


So, greater the surface area of the adsorbent more is the adsorption.


Hence, powdered substances are more effective adsorbents than their crystalline forms




Intext Questions Pg-133
Question 1.

In Haber’s process, hydrogen is obtained by reacting methane with steam in presence of NiO as a catalyst. The process is known as steam reforming. Why is it necessary to remove CO when ammonia is obtained by Haber’s process?


Answer:

It is important to remove CO (Carbon Monoxide) in the synthesis of ammonia as CO affects the activity of Iron catalyst which is required in Haber’s process.


Note: Haber’s process is a very important industrial process which is used to produce ammonia.



Question 2.

Why is the ester hydrolysis slow in the beginning and becomes faster after some time?


Answer:

Ester hydrolysis is represented as:


Ester + WaterAcid + Alcohol


In this reaction the acid produced which is a product also acts as a catalyst and makes the reaction faster.


Such substances that act as catalysts in the same reaction in which they are obtained as products are known as Autocatalysts.


So, ester hydrolysis is slow in the beginning and becomes faster after some time as more acid is produced on the product side.



Question 3.

What is the role of desorption in the process of catalysis.


Answer:

Desorption is a process in which substance (reactant + product) is released from the surface which is the opposite process of sorption.


The role of desorption in the process of catalysis is to make the surface of the solid catalyst-free for fresh adsorption of reactants on the solid surface for further reactions to take place.




Intext Questions Pg-145
Question 1.

What modification can you suggest in the Hardy Schulze law?


Answer:

According to Hardy-Schulze law


‘The greater the valence of the flocculating ion added, the greater is its power to cause precipitation.’


As this law takes into consideration of only the charge present on the ion and not the size of the ion. So when the size of the atom is considered, smaller the size of an atom more will be its polarising power.


So Hardy-Schulze can be modified in terms of the polarising power of the flocculating ion as ‘The greater the polarising power of the flocculating ion added, the greater is its power to cause precipitation.’



Question 2.

Why is it essential to wash the precipitate with water before estimating it quantitatively?


Answer:

The precipitate which is obtained from a chemical reaction always contain some unwanted substances (eg ions, impurities) which get adsorbed onto the surface of the precipitate. Therefore, it becomes important to wash the precipitate before estimating it quantitatively so as to remove these unwanted adsorbed substances and obtain accurate results.




Exercises
Question 1.

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


Answer:

Adsorption- The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. In this substance accumulate at surface only and does not penetrate into the adsorbent (material on the surface of which adsorption takes place)


Ex.-If gases like O2, H2, NH3 taken in a closed vessel containing charcoal then gases get adsorbed on the surface of charcoal and reduce the pressure.


Absorption- In this molecules get distributed uniformly throughout the material Instead of accumulating at the surface.


Ex- When gas like O2 is mixed with water then O2 get distributed throughout the solvent (Water).



Question 2.

What is the difference between Physisorption and Chemisorption?


Answer:







Question 3.

| Give reasons why a finely divided substance is more effective as an adsorbent?


Answer:

Finely divided substances have large surface area compared to not divided substance. And the increase in surface area increases the adsorption. Hence Finely divided substances are more effective adsorbent.



Question 4.

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


Answer:

Extent of adsorption of a gas on a solid is influenced by following factors- 1) Nature of adsorbate.


2) Nature of adsorbent.


3) Pressure.


4) Temperature.



Question 5.

What is an adsorption isotherm? Describe Freundlich adsorption isotherm.


Answer:

Adsorption Isotherm- The variation in the amount of a gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by the curve . This curve is known as Adsorption Isotherm.


Freundlich adsorption isotherm- Freundlich gave an empirical relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent And pressure at a particular temperature.


x/m = k P1/nm





Where x is the mass of gas adsorbed on mass m of the adsorbent pressure P. k and n are constants depend on nature of adsorbent. This relationship is expressed in the form of curves where the mass of the gas adsorbed per gram of adsorbent is plotted against pressure.




Question 6.

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


Answer:

Increasing the adsorbing power of adsorbent is known as activation of adsorbent.it can be achieved by


1) Increase in surface area of adsorbent.


2) By dividing the adsorbent into small grains.


3) By making its surface rough.



Question 7.

What role does adsorption play in heterogeneous catalysis ?


Answer:

Adsorption of reactants on the solid surface of the catalysts increases


the rate of reaction. Generally, the reactant is gas whereas catalyst is solid. Reactant molecules get adsorbed on the surface of the catalyst as a result concentration of reactant increases leads to increase in the rate of reaction. there are many important reactions is based on this


Ex1) manufacturing of ammonia using iron as a catalyst,


2) manufacture of H2SO4 By contact process.,


3) use of finely divided Nickel in the hydrogenation of oils.



Question 8.

Why is adsorption is always exothermic ?


Answer:

When adsorbate (gas or liquid mostly) absorbed then their entropy getreduced i.e.ΔSis negative. For every spontaneous process ΔG must be negative. ΔG = ΔH - TΔS, as TΔS is positive (ΔS is negative) ΔH must be negative in order to make ΔG negative . Hence ΔH is negative that means the reaction is exothermic. Hence every adsorption reactions are exothermic.



Question 9.

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


Answer:

Colloidal solutions get classified in 8 Types on the basis of physical state of the dispersed phase and dispersion medium. This are given in following table-




Question 10.

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


Answer:

Temperature- Adsorption decreases with increase of temperature because it is an exothermic process, by using le chatelier's principle the reaction will proceed in backward direction.


Pressure- Adsorption increase with increase in pressure at constant temperature.







Question 11.

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


Answer:

Lyophilic - lyophilic (liquid loving) this sols formed by direct mixing of substances like gum starch etc. with suitable liquid, called as lyophilic sols. This are called as reversible sol because if dispersion medium is separated from the dispersed phase it can be reconstructed just by mixing them. This sols are quite stable. Ex-Protein, Starch.


Lyophobic -lyophobic (liquid hating) this sols don't formed by direct mixing of dispersed phase and dispersion media they require special methods for preparation. Ex-Sols of metal sulphides .


This Lyophobic(Hydrophobic-water hating) sols gets easily coagulated. Because they are less stable. Their stability is only due to Charge If electrolyte is added to it opposite charge will attract and form coagulation easily.



Question 12.

What is the difference between multimolecular and macromolecular colloids ? Give one example of each .How are associated colloids different from this two types of colloids ?


Answer:



Associated colloids- They formed by aggregation of large number of ions. instead of atoms or molecules in above two colloids. They have van der waals force directly proportional to concentration of ions. Ex- Soap Sol.



Question 13.

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


Answer:

Enzymes are complex nitrogenous organic compounds which are produced by Living plants and animals. They are protein molecules of higher molecular masses. They are effective catalysts termed as “biochemical catalysts”


Mechanism of enzyme catalysis- Their are number of cavities present on the surface of colloidal particle of enzymes. They have specific shape and contains group like –COOH ,-SH, -OH. etc. they are active centres of enzyme particle molecules of reactant which have complimentary shape fit into this cavities, this formes activated complex Which then decompose to yield products. This reaction occur in 2 steps-


1) Enzyme(E) + Substrate (S) → ES


2) ES → Enzyme + Products



Question 14.

How are the colloids formed classified on the basis of

1) Physical states of components

2) Nature of dispersed phase

3) Interaction between Dispersed phase and dispersion medium.


Answer:

1) Colloidal solutions get classified in 8 Types on the basis of physical state of the dispersed phase and dispersion medium. This are given in following table-







2) Depending upon nature of Particles of dispersed phase it can be classified into 3 categories


a) Multimolecular Colloids-In this colloidal particle consist of aggregates of atoms having diameter less than 1nm.


b) Macromolecular colloids-The colloids in which Large particle aggregates and dissolved into suitable liquid.


c) Associated colloids – This colloids show Colloidal property at higher concentrations. This contains ions of colloidal size.


3) Depending upon interactions between dispersed phase and dispersion medium it is divided into 2 categories


a) Lyophilic Colloids- lyophilic (liquid loving) this colloids formed by direct mixing of substances like gum starch etc. with suitable liquid, called as lyophilic sols . This are called as reversible Colloids because if dispersion medium is separated from the dispersed phase it can be reconstructed just by mixing them. This sols are quite stable. Ex-Protein ,Starch.


b) Lyophobic colloids- lyophobic (liquid hating) this Colloids don't formed by direct mixing of dispersed phase and dispersion media they require special methods for preparation. Ex-Sols of metal sulphides .




Question 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) In this case Tyndall effect is observed i.e. scattering of light by the colloidal particles takes place and the path of light become visible.


(ii) The coagulation takes place in this case positive charge particles of


Fe(OH) gets attached with Cl-. And ppt. of FeCl is obtained.


(iii) In this case coagulation will take place. After passing electric current through colloidal solution particle will get attracted towards oppositely charged electrodes. and lose their charge and coagulate.



Question 16.

What are the emulsions ? What are their different types ? give one example of each type.


Answer:

Emulsions –The system which has Liquid as both dispersed phase and dispersion medium. There are two types of emulsions


1) Oil in Water-Ex-Milk


2) Water in oil-Ex-Butter.



Question 17.

How do emulsifiers stabilize the emulsion? name two emulsifiers ?


Answer:

Emulsifiers forms an interfacial film between the suspended particles and medium. It provide coating to every drop of suspended particle and prevent it from coagulating. And it remain suspended in medium.


Ex- Proteins, gums, alcohols, Lampblack etc.



Question 18.

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


Answer:

Soap is sodium or potassium salts of fatty acids (long chain). Represented as RCOONa+. When dissolved in Water it dissociates in the RCOO and Na+. In first ion there are two parts R and COO. R is hydrophobic tail. and COO Hydrophilic head. RCOO ions present at the surface with COO group inside and R group outside. Increasing concentration to critical micelle concentration anions are pulled inside the bulk and Form a spherical shape of hydrocarbon chain pointing towards the centre and COO pointing outside this are called as micelle.


Cleansing action of soap- Soap molecules form micelle around oil droplets. Soaps emulsifies the oil or grease droplets. and form emulsion after washing the cloth this emulsion easily gets removed. Hence Action of soap is due to emulsification and micelle formation.



Question 19.

Give four examples of heterogeneous catalysis.


Answer:

Oxidation of sulphur dioxide into sulphur trioxide in the presence of



2) Formation of ammonia from Haber process.




3) Oxidation of ammonia into nitric acid in presence of platinum gauze in Ostwald's process


→ 4NH3(g)+ 5O2(g) Pt(s) 4NO +6H2O


4) Hydrogenation of vegetable oils in presence of Nickel as catalyst





Vegetable oil(l)+H2(g) Ni → Vegetable ghee (s)




Question 20.

What do you mean by activity and selectivity of catalysts?


Answer:

Activity - It is the ability of the catalyst to accelerate the Reaction. It mostly depends upon the Chemisorption strength.


Selectivity - It is an ability to direct reaction to yield of a particular product i.e., One catalyst cannot be a catalyst for other reactions.



Question 21.

Describe some features of catalysis by zeolites.


Answer:

Catalysis by zeolites is dependent on shape. Because zeolites are shape-selective catalysts. They are alumino silicates Which are microporous in nature. It has Honeycomb structure. That makes them shape selective. In zeolites, some si atoms are replaced by Al to form Al-O-Si network.


Reactants are very sensitive to the pore size of zeolites. Zeolites are used in petrochemical industry. Ex- ZSM-5 used to convert alcohol into gasoline.



Question 22.

What is shape selective catalysis?


Answer:

The catalytic reaction that depends upon the pore structures of the catalysts and the size of reactant and product molecules is called SHAPE SELECTIVE CATALYSIS. Zeolites are good shape-selective catalysts.



Question 23.

Explain the following terms (i) Electrophoresis, (ii) Coagulation (iii) Dialysis (iv) Tyndall effect.


Answer:

(i) Electrophoresis: The movement of colloidal particles under an applied electric potential is called Electrophoresis. When an electric potential is applied to two platinum electrodes dipping in colloidal solutions the colloidal particle move towards the Oppositely charged electrodes.


(ii) Coagulation: The process of settling of colloidal particles is called coagulation. When the charge is removed from colloidal solution somehow, then particle start coagulation and settling due to the force of gravity.


(iii) Dialysis: It is a process of removing a dissolved substance from ac colloidal solution by means of diffusion through a suitable membrane. the animal membrane or parchment paper used as a membrane.


(iv) Tyndall Effect: When homogenous solution is placed in a dark and viewed in light beam then if it is viewed from sides it appears visible but if it is viewed from 90° it appears to be dark. This effect is known as Tyndall effect.



Question 24.

Give four uses of emulsions.


Answer:

Uses of Emulsions-


1) It is used in making of medicines,


2) Cleansing action of soaps is based on this emulsion formation.


3) Digestion of fats in intestine takes place by the process of emulsification.


4) Antiseptics and disinfectant added to water form emulsion for cleaning.



Question 25.

What are micelles? Give an example of micelles system.


Answer:

Soaps contain hydrophobic and hydrophilic part when dissolved in the water they arrange themselves in such a way that They form a spherical structure having hydrophobic part towards the centre and hydrophilic part away from centre. This cluster is known as Micelle. Ex-Sodium stearate + Water


(CH3 (CH2)16COO-Na + H2O)




Question 26.

Explain the terms with suitable examples.

(i) alcohol, (ii) aerosol (iii) Hydrosol.


Answer:

i) Alcohol- a colloidal solution having alcohol as the dispersion medium and a solid substance as the dispersed phase. Ex- colloidal sol of cellulose nitrate in ethyl alcohol.


ii) Aerosol- a colloidal solution having gas as the dispersion medium and a solid substance as the dispersed phase. Ex-Smoke.


iii) Hydrosol- a colloidal solution having Water as the dispersion medium and a solid substance as the dispersed phase. Ex-Gold sol.



Question 27.

Comment on the statement That “colloid is not a substance but a state of substance”.


Answer:

We can say that colloid is not a substance but a state of a substance Which is dependent on the size of particle colloidal state is intermediate between a true solution and a suspension. when a size of substance is between 1nm to1000nm it behaves as colloid otherwise not.


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