# NCERT Solutions For Class 12 Chemistry Chapter 5 Surface Chemistry

NCERT solutions for class 12 chemistry chapter 5 Surface Chemistry - In this chapter, some important features of surface chemistry will be discussed like catalysis, adsorption and collides including emulsions and gels. CBSE NCERT solutions for class 12 chemistry chapter 5 Surface Chemistry explains answers with the help of a phenomena that occur at the surfaces or interfaces. Let's understand surface phenomena by an example. Have you noticed whenever you buy a new bag or pair of shoes there is a small packet of white colour filled with some grains in the package box. Do you know why these white packets are placed in the packaging boxes? The white packets contains grains of silica and the silica adsorbs moisture hence protecting the shoes or bag from moisture and various kinds of bacteria. The adsorption of water molecules on the surface of on the grains of silica is known as adsorption. You will read such interesting facts in this chapter and also you will get all the NCERT solutions for class 12 chemistry chapter 5 surface chemistry which are based on such facts or the others.

This chapter carries 4 marks in the CBSE Board exams. And this chapter contains 8 intext questions and 27 questions in the exercise. The NCERT solutions for class 12 chemistry chapter 5 Surface chemistry are prepared by chemistry experts and presented in a step-by-step manner. These solutions of NCERT class 12 chemistry chapter 5 will help you in your boards exams as well as in the competitive exams like JEE, BITSAT, VITEEE, etc. After completing the NCERT solutions for class 12 chemistry chapter 5 Surface Chemistry students will be able to describe the interfacial phenomenon and its significance, define adsorption, explain the mechanism of adsorption, also will be able to explain the factors controlling adsorption from gases and solutions on solids and adsorption results on the basis of Freundlich adsorption isotherms. In chemistry chapter 5 Surface chemistry, students will also able to classify emulsions and describe their properties and preparation. These NCERT solutions are very useful even if you want to prepare for some other subjecr or a class.

## Topics and Sub-topics of NCERT class 12 chemistry chapter 5 Surface Chemistry-

5.2 Catalysis

5.3 Colloids

5.4 Classification of Colloids

5.5 Emulsions

5.6 Colloids Around Us

## Find NCERT solutions for class 12 chemistry chapter 5 Surface Chemistry below:

Solutions to In Text Questions Ex 5.1  to 5.8

## Question 5.1   Write any two characteristics of Chemisorption.

Two characteristics of Chemisorption-

1. The bond formation between the solid(adsorbent) and the gas molecules is by chemical bonds like a covalent bond or ionic bond.

2. It is highly specific in nature and also irreversible.

Physisorption is an exothermic process:

$\underset{(Adsorbent)}{Solid}\:+\:\underset{(Adsorbate)}{Gas}\rightleftharpoons \:Gas adsorbed on solid+Heat$

So, therefore, according to Le-Chateliers principle, it decreases with increase in temperature.

Since adsorption is a surface phenomenon. So, the extent of adsorption directly depends on the surface area. Thus, the powdered substances more effective absorbents than their crystalline forms due to their large surface area which can adsorb more gas as compared to the crystalline form.

It is necessary to remove  $\dpi{80} \ce {CO}$  when ammonia is obtained in Haber's process because the $\dpi{80} \ce {CO}$ acts as a poison for iron catalyst so it can affect the activity of iron catalyst used in this process.

Ester hydrolysis-

$Ester(RCOOR) + water(H_{2}O) \rightarrow Acid(RCOOH) + Alcohol (ROH)$

Initially, the reaction is slow but after some time the reaction is fast because of the acid produced, act as a catalyst for this reaction. These type of reaction known as auto-catalysis.

In catalytic reaction, the reactant attached on the surface of the catalyst and form complex and after some procedure, it becomes the product of the reaction. We need to detach it from the surface of the catalyst to get the product and catalyst separately and hereby desorption process, we can get the product.

According to Hardy-Schulze law that ‘the higher the valence of the flocculating ion added, the higher is its power to cause precipitation. Here he does not about the size of the ion, which polarises the other oppositely charged ion. Hence the modified law should be  "the greater the polarising power of flocculating ion added, the greater is its power to cause precipitation."

The precipitate is formed from reactions so it contains some chemical or unwanted substances traces (like impurities, catalyst and reactant). For quantitative estimation of the product, we need to remove these additional substances which get adsorbed onto the surface of the required product and it gives us the wrong information about the product of the reaction. So to avoid this problem we wash the precipitate with the water (most of the impurities are water soluble).

NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry Exercises

Adsorption- It is a surface phenomenon in which the molecules or substances accumulate at the surface rather than in a bulk of a solid or liquid. The substance that gets adsorbed is called adsorbate and the material on which it is adsorbed is called adsorbent. In adsorption, the adsorbate does not penetrate the adsorbent surface it concentrates only at the surface of the solid and liquid. example-when we dip a chalk stick into the ink solution, only its surface becomes coloured. If we break the chalk stick, it will be found to be white from inside.

Absorption- It is a bulk phenomenon, in this process, the substance uniformly distributed throughout the bulk of the solid or liquid.example-Water vapours are absorbed by anhydrous calcium chloride.

Difference between physisorption and Chemisorption-

 Physisorption Chemisorption 1. Happens due to van der Waal forces. 2. It's not specific in nature and reversible in nature. 3. It depends on the nature of the gas. More liquefiable gas is adsorbed easily. 4. enthalpy of adsorption is low around 20-40 kJ/mol 5. Low temperature required 6. It depends on the surface area. Higher the area high in the process rate. 7. Results in multimolecular layers on the adsorbent surface. 1. Caused by a chemical bond 2. highly specific in nature and irreversible. 3. depend on the nature of the gas. those gases can react with the adsorbent show chemisorption. 4. Enthalpy if adsorption is high around 80-240 kJ/mol 5. High temperature is required. 6. High activation energy is needed sometimes 7. result is unimolecular

We know that adsorption is a surface phenomenon. The extent of adsorption depends on the surface area. So, Adsorption is directly proportional to the surface area of the adsorbent. More the surface area more is the amount of adsorbent adsorbed. So, therefore finely divided substance is more effective as an adsorbent due to its high surface area.

The factors which influence the adsorption of a gas on a solid-

• Surface area-the greater the surface area of the adsorbent higher is the amount of adsorbent to get absorbed.

• temperature- Adsorption is an exothermic process. So, physical adsorption readily at a lower temperature and decrease with increase in temperature.

• Pressure- On increasing pressure, the amount of adsorbate increased as the volume of gas decreased(Le-Chateliers principle)

• Nature of gas- If the gas is easily liquefiable ($NH_{3},HCl$) then it gets easily adsorbed on the adsorbent solid or liquid. Because these gases have high van der Waals force so their extent of adsorption is high as compare to $H_{2},O_{2}$.

The adsorption isotherm is a relation curve between the variation in the amount of gas adsorbed by the adsorbent with pressure at a constant temperature.

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).

• At high pressure-       $\frac{x}{m}=k.p^{0}$    [independent of P]

• At intermediate pressure    $\frac{x}{m}=k.p^{\frac{1}{n}}$ for n>1                                                                                          k and n are constant (depend on nature of gas) Here x = mass of gas adsorbed, and                                                                         m= mass of adsorbent at pressure p

On taking log both sides-

$\log\frac{x}{m}= \log(k)+\frac{1}{n}\log(p)$

by activation of adsorbent, it means increasing the adsorbing power of adsorbent. We can do it by following ways-

1. By increasing the surface area of the adsorbent by cutting adsorbent into small pieces or powdering it.

2. Some treatment can be also done to the adsorbent so that its power gets increased. example- charcoal is activated by heating it between 650 K and 1330 K in vacuum or air

Those catalytic process in which the reactant and catalyst are in a different phase is known as heterogeneous catalysis.

Role of adsorption in heterogeneous catalysis-

• Diffusion of reactants to the surface of the catalyst.

• Adsorption of reactant is on the surface of the catalyst.

• The chemical reaction can occur at the catalyst surface by intermediate formation between them.

• Desorption of the product of the reaction is also at the surface of the catalyst.

Question 5.8  Why is adsorption always exothermic ?

1. During adsorption, the residual forces of the surface always decrease. As a result, there is a decrease in surface energy, which emits as a heat.

2. $\Delta H$ Of adsorption is always negative. The freedom of movement of adsorbate (gas) molecule becomes restricted when the adsorbent adsorbs the adsorbate(gas). As a result, the entropy decreases ($\Delta S$ is negative). For the process to be spontaneous $\Delta G$ should be negative.$\Delta G=\Delta H-T\Delta S$ .Since $\Delta S$ is negative, $\Delta H$ has to be negative, so that $\Delta G$ is negative. Hence adsorption is always exothermic.

First criteria for classifying the colloids is the physical state of the dispersed phase and a dispersion medium. Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, there are eight types of colloidal systems -

 Dispersed phase Dispersion medium Type of colloid Examples solid solid solid sol some coloured gases and gemstones solid liquid sol Paints, cell fluids solid gas aerosol Smoke, dust liquid solid gel Cheese, jellies liquid liquid emulsion Milk, hair cream, butter liquid gas aerosol fog, mist, cloud, insecticide sprays gas solid solid sol pumice stone, foam rubber gas liquid foam froth, whipped cream, soap lather

Effect of temperature and pressure on the adsorption of gases on solids-

Temperature- According to Le-Chatelier’s principle the physical adsorption readily occurs at low temperature and adsorption rate is slow at high temperature.

Pressure-  As we increase the pressure the amount of gas adsorbed on adsorbent is increased as the volume of gas decreases. Since it is an exothermic process, the physical adsorption occurs fast at low temperature and decrease with increase in pressure.

(i)Lyophilic sols- These sols are solvent loving. In these types of sols, the dispersed phase has an affinity towards the dispersion medium(liquid).example- starch, gum and protein. The affinity is due to the ability to form a hydrogen bond with the dispersion medium(water) These sols are reversible in nature.

(ii)Lyophobic sols- These are water-hating. In these types of sols, the dispersed phase has no affinity towards the dispersion medium or the solvent. example- metals, their sulphides etc. on simply mixing they do not form colloidal solution. Their colloidal sols can be prepared only by some special methods. sol particles have some positive or negative charges. Irreversible in nature.

Multimolecular colloids- On dissolution, a large number of atoms or smaller molecules aggregate together to form species of size 1-1000 nm of colloidal range. This species termed as multimolecular collides. Example- gold sol and sulphur sol.

Macro-molecular colloids-  In suitable solvents, they form solutions in which the size of the macromolecules may or may not be in the colloidal range. Such systems are called macromolecular colloids.  Examples- starch, protein and cellulose.

Associated colloids- those substances, which behaves like normal strong electrolyte at low concentration, but exhibits colloidal behaviour at high concentration due to the formation of aggregates. And these are known as associated colloids also called micelle.

Enzymes are that substance which is produced by living plants and animals. These are the complex nitrogenous organic compound. Actually, it is a protein molecule of high molecular weight and forms colloidal solution on dissolving in water.

Brief mechanism of enzyme catalysis-

On the surface of enzymes, active centres are present of different shape and size and possess active groups like $-NH_{2},-COOH,-SH,-OH$ etc. The substrate which has complementary shape, attach into these active sites just like a key fits into a lock. As a result, an active complex formed, later on, which becomes a product.

step 1. Binding of an enzyme to the substrate to form complex ($E+S\rightarrow ES^{*}$)

step 2.To product formation, decomposition of the activated complex  ($ES^{*}\rightarrow E+P$)

Question 5.14(i)  How are colloids classified on the basis of

(i) physical states of components.

There are eight types of colloids whether the components are solids, liquids or gas. Here components mean dispersed phase and a dispersion medium.

 Dispersed phase Dispersion medium Type of colloid solid solid solid sol solid liquid sol solid gas aerosol liquid solid gel liquid liquid emulsion liquid gas aerosol gas solid solid sol gas liquid foam

Depending upon the type of particles of the dispersed phase, colloids are categories into-

1. Multimolecular colloids

2. Macro-molecular colloids

3. Associated  colloids

On the basis of the nature of the interaction between the dispersed phase and dispersion medium the colloids are divided into two categories;

(i)lyophilic sols(water-loving) and

(ii)lyophobic sols (water repelling)

(i) when a beam of light is passed the scattering of light is observed, which is known as Tyndall effect. The colloidal particles size, which is in the range of wavelength of light scatters them in all directions.

Question 5.15(ii)           Explain what is observed

(Ii) an electrolyte, $NaCl$  is added to hydrated ferric oxide sol.

The sol gets precipitated because of the neutralisation reaction. The ferric oxide sol is positive in nature and we get chloride ion from NaCl.

Question 5.15 (iii)         Explain what is observed

(iii) electric current is passed through a colloidal sol?

Under the influence of electric current, the colloidal particles start moving towards the oppositely charged electrode

The colloidal solution in which both the dispersed phase and dispersion medium are liquid in nature are called emulsions.

There are two types of emulsions-

(i) oil in water (O/W) -Oil is dispersed phase and water is dispersion medium (milk)

(ii) water in oil (W/O)- Water is the dispersed phase and oil is dispersion medium (cold  cream, butter etc.)

The emulsifying agents are also known as emulsifiers. For stabilisation of emulsion, they form an interfacial film between suspended particles and medium.

Example- for O/W gums, natural and synthetic soaps for W/O heavy salts of fatty acids, long chain alcohols

The cleansing action of soap is because of emulsification and micelle formation. Soaps are sodium or potassium salts of long chain fatty acids, represented as $RCOO^{-}Na^{+}$ (e.g. sodium stearate $CH_{3}(CH_{2})_{16}COO^{-}Na^{+}$). when dissolved into the water it dissociates into $RCOO^{-}$ and $Na^{+}$ ions. $(RCOO^{-}Na^{+}\rightleftharpoons RCOO^{-}+Na^{+})$. The long carbon chains are non-polar in nature (hydrophobic) and $COO^{-}$ are polar in nature (hydrophilic part).

When the soap is added to dirt containing water. The soap molecules surround the dirt particle where the hydrophobic part is attached to dirt and the hydrophilic part is pointing away from the dirt. This is called micelle formation. Therefore we can say that the non-polar part dissolved in dirt particle and the polar group dissolved in water. Now, as these micelles are negatively charged, they do not coalesce and a stable emulsion is formed.

Question 5.19    Give four examples of heterogeneous catalysis.

Four examples of heterogeneous catalysis-

1. oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.

$2SO_{2}(g)\overset{Pt(s)}{\rightarrow}2SO_{3}(g)$

2. the combination between dinitrogen and dihydrogen to form ammonia

$N_{2}(g)+3H_{2}(g)\overset{Fe(s)}{\rightarrow}2NH_{3}(g)$

3. Hydrogenation of vegetable oils in the presence of Ni  as a catalyst

$Veg. Oil(l)+H_{2}(g)\overset{Ni(s)}{\rightarrow}veg Ghee(s)$

4. Ostwald Process - oxidation of ammonia into nitric acid in the presence of Pt

$4NH_{3}(g)+5O_{2}(g)\overset{Pt(s)}{\rightarrow}4NO_{2}(g)+6H_{2}O$

(i)The activity of the catalyst is to increase the rate of a particular reaction. It depends on the strength of chemisorption to a large extent. Adsorption of reactants should be neither too high nor too weak. It should just be optimum so that the catalyst is staying active.

(ii)The selectivity of catalyst is the ability to direct the reaction to give a particular product when under the same reaction condition the products are possible also.

Example- Reaction between CO and $H_{2}$

$\\(1)CO(g)+3H_{2}(g)\overset{Ni}{\rightarrow}CH_{4}(g)H_{2}O(g)\\ (2)CO(g)+2H_{2}(g)\overset{Cu/ZnO-Cr_{2}O_{3}}{\rightarrow}CH_{3}OH(g)\\ (3)CO(g)+H_{2}(g)\overset{Cu}{\rightarrow}HCHO(g)$

Question 5.21     Describe some features of catalysis by zeolites.

Zeolites are good shape selective catalyst due to their honeycomb-like structures. They are alumino-silicates that are microporous in structures. They are the 3D network of silicates in which some atoms of silicon are replaced by aluminium giving $Al-O-Si$ framework.  Size and shape of reactant and product molecules, as well as pores and cavities of zeolites, are the reaction determining factors in zeolites.

Question 5.22  What is shape selective catalysis?

Shape-selective catalysis, in which the reaction mainly depends on the size of the reactants and product molecules and also the pore structure of the catalyst.

Ex. zeolites are good shape selective catalyst.

Question 5.23(i)    Explain the following terms:

(i) Electrophoresis

Electrophoresis-

The movement of colloidal particles under the influence of electric current is known as electrophoresis. The negatively charged particles move towards the cathode and positive charge particles move towards the anode.

Question 5.23(ii)        Explain the following terms:

(ii)  Coagulation

Coagulation- The settling down of the lyophobic sol due to the presence of charge on colloidal particles

Question 5.23(iii)        Explain the following terms:

(iii)  Dialysis

Dialysis-  The process of removing a dissolved substance from the colloidal solution by means of diffusion through a suitable membrane. The principle behind this is small molecules and ions can pass through the animal membrane.

Question 5.23(iv)         Explain the following terms:

(iv) Tyndall effect

Tyndall effect-  When a beam of light is passed through the colloidal solution the colloidal particles scatter the light and make the beam visible in colloidal solution. This effect is observed by Faraday and later on detail in studied by Tyndall and is known as the Tyndall effect.

Question 5.24       Give four uses of emulsions.

Four use of emulsions-

1. The cleansing action of soap

2. to make medicins by emulsification process

3. Digestion of fats in the intestine

4. Antiseptic and disinfectant when adding to water form emulsion.

When the long chain of fatty acids like soap and detergent is added to water the hydrophobic part of their chain is aligned towards the centre in such a manner that they form a spherical shape and their hydrophilic part is facing towards outside. This spherical like structure is known as a micelle.

Structure of sodium soap(Left side - hydrophobic part, Right side- hydrophilic part)

fig:- Micelle of that sodium soap.

Question 5.26(i)     Explain the terms with suitable examples:

(i) Alcosol

Alcosol-  A solution in which the dispersion medium is alcohol and the dispersed phase is a solid substance. examples- colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.

Question 5.26(ii)        Explain the terms with suitable examples:

(ii) Aerosol

Aerosol- A colloidal solution of the dispersion medium is a gas and the dispersed phase is a soil substance. example- fog.

Question 5.26(iii)    Explain the terms with suitable examples:

(iii)  Hydrosol

Hydrosol-  In THis type of colloidal solution the water is dispersion medium and a solid substance is a dispersed phase. example- starch sol and gold sol.

Common salt is behaving as a colloidal particle in benzene as a dispersion medium. It means the solute particles attain a size of the colloidal range of (1-1000 nm). So, we can say that the colloid is not a substance but a state of substance which depends on the particles size.

## NCERT solutions for class 12 chemistry

 Chapter 1 CBSE NCERT solutions for class 12 chapter 1 The Solid State Chapter 2 NCERT solutions for class 12 chemistry chapter 2 Solutions Chapter 3 Solutions of NCERT class 12 chemistry chapter 3 Electrochemistry Chapter 4 CBSE NCERT solutions for class 12 chemistry chapter 4 Chemical Kinetics Chapter 5 NCERT Solutions for class 12 chemistry chapter 5 Surface chemistry Chapter 6 NCERT solutions for class 12 chemistry General Principles and Processes of isolation of elements Chapter 7 CBSE NCERT solutions for class 12 chemistry chapter 7 The P-block elements Chapter 8 Solutions of NCERT class 12 chemistry chapter 8 The d and f block elements Chapter 9 NCERT solutions for class 12 chemistry chapter 9 Coordination compounds Chapter 10 Solutions of NCERT class 12 chemistry chapter 10 Haloalkanes and Haloarenes Chapter 11 CBSE NCERT solutions for class 12 chemistry Alcohols, Phenols, and Ethers Chapter 12 Solutions of NCERT class 12 chemistry chapter 12 Aldehydes, Ketones and Carboxylic Acids Chapter 13 NCERT solutions for class 12 chemistry chapter 13 Amines Chapter 14 CBSE NCERT solutions for class 12 chemistry chapter 14 Biomolecules Chapter 15 Solutions of NCERT class 12 chemistry chapter 15 Polymers Chapter 16 NCERT solutions for class 12 chemistry chapter 16 Chemistry in Everyday life

## NCERT solutions for class 12 subject wise

 Solutions of NCERT class 12 biology NCERT solutions for class 12 maths CBSE NCERT solutions for class 12 chemistry Solutions of NCERT class 12 physics

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