NCERT Solutions for Class 12 Chemistry Surface Chemistry [Old Book]

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Chemistry
Chapter : 5
Chapter Name : Surface Chemistry

Q5.1 Write any two characteristics of Chemisorption.

Answer. 1. Chemisorption is highly speci c in nature. It occurs only if there is a possibility of
chemical bonding between the adsorbent and the adsorbate.
2. Like physisorption, chemisorption also increases with an increase in the surface area of the
adsorbent.

Page : 129 , Block Name : Intext Questions

Q5.2 Why does physisorption decrease with the increase of temperature?

Answer. Physisorption is exothermic in nature. Therefore, in accordance with Le-Chatelier's
principle, it decreases with an increase in temperature. This means that physisorption occurs
more readily at a lower temperature.

Page : 129 , Block Name : Intext Questions

Q5.3 Why are powdered substances more effective adsorbents than their crystalline forms?

Answer. Powdered substances are more effective adsorbents than their crystalline forms because
when a substance is powdered, its surface area increases and physisorption is directly proportional
to the surface area of the adsorbent.

Page : 129 , Block Name : Intext Questions

Q5.4 In Haber’s process, hydrogen is obtained by reacting methane with steam in presence of NiO
as 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 in the synthesis of ammonia as CO adversely affects the
activity of the iron catalyst, used in Haber's process.

Page : 135 , Block Name : Intext Questions

Q5.5 Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?

Answer. Ester hydrolysis can be represented as:
Ester + Water − > Acid + Alcohol
The acid produced in the reaction acts as a catalyst and makes the reaction faster.
Substances that act as catalysts in the same reaction in which they are obtained as products are
known as auto catalysts.

Page : 135 , Block Name : Intext Questions

Q5.6 What is the role of desorption in the process of catalysis.

Answer. The role of desorption in the process of catalysis is to make the surface of the solid
catalyst free for the fresh adsorption of the reactants on the surface.

Page : 135 , Block Name : Intext Questions

Q5.7 What modi cation can you suggest in the Hardy Schulze law?

Answer. Hardy-Schulze law states that 'the greater the valence of the occulating ion added, the
greater is its power to cause precipitation.'
This law takes into consideration only the charge carried by an ion, not its size. The smaller the
size of an ion, the more will be its polarising power. Thus, Hardy-Schulze law can be modi ed in
terms of the polarising power of the occulating ion. Thus, the modi ed Hardy-Schulze law can be
stated as the greater the polarising power of the occulating ion added,the greater is its power to
cause precipitation.'

Page : 147 , Block Name : Intext Questions

Q5.8 Why is it essential to wash the precipitate with water before estimating it quantitatively?

Answer. When a substance gets precipitated, some ions that combine to form the precipitate get
adsorbed on the surface of the precipitate. Therefore, it becomes important to wash the
precipitate before estimating it quantitatively in order to remove these adsorbed ions or other
such impurities.

Page : 147 , Block Name : Intext Questions

Q5.1 Distinguish between the meaning of the terms adsorption and absorption.Give one example
of each.

Answer. Adsorption is a surface phenomenon of accumulation of molecules of a substance at the
surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed is called the
'adsorbate' and the substance on whose surface the adsorption takes place is called the 'adsorbent'.
Here, the concentration of the adsorbate on the surface of the adsorbent increases. In adsorption,
the substance gets concentrated at the surface only. It does not penetrate through the surface to
the bulk of the solid or liquid. For example, when we dip a chalk stick into an ink solution, only its

surface becomes coloured. If we break the chalk stick, it will be found to be white from inside. On
the other hand, the process of absorption is a bulk phenomenon. In absorption, the substance gets
uniformly distributed throughout the bulk of the solid or liquid.

Q5.2 What is the difference between physisorption and chemisorption?

Answer.

Q5.3 Give reason why a nely divided substance is more effective as an adsorbent.

Answer. Adsorption is a surface phenomenon. Therefore, adsorption is directly proportional to the
surface area. A nely divided substance has a large surface area. Both physisorption and
chemisorption increase with an increase in the surface area. Hence, a nely divided substance
behaves as a good adsorbent.

Q5.4 What are the factors which in uence the adsorption of a gas on a solid?

Answer. There are various factors that affect the rate of adsorption of a gas on a solid surface.
(1) Nature of the gas:
Easily lique able gases such as NH3, HCI etc. are adsorbed to a great extent in comparison to
gases such as H2, 02 etc. This is because Van der Waal's forces are stronger in easily lique able
gases.
(2) Surface area of the solid
The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid
surface.
(3) Effect of pressure
Adsorption is a reversible process and is accompanied by a decrease in pressure.
Therefore, adsorption increases with an increase in pressure.
(4) Effect of temperature
Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier's principle, the
magnitude of adsorption decreases with an increase in temperature.

Q5.5 What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

Answer.

The plot between the extent of adsorption ( against the pressure of gas (P) at constant
x
)
m

temperature (T) is called the adsorption isotherm.
Freundlich adsorption isotherm:
Freundlich adsorption isotherm gives an empirical relationship between the quantity of
gas adsorbed by the unit mass of solid adsorbent and pressure at a speci c temperature.
From the given plot it is clear that at pressure P reaches the maximum valve. P is called the
S, m
x
S

saturation pressure. Three cases arise from the graph now.
Case I- At low pressure:
The plot is straight and sloping, indicating that the pressure in directly proportional to
x x
αP
m i.e., m

x

m
= kP ( k is a constant)
Case II- At high pressure:
When pressure exceeds the saturated pressure, becomes independent of P values.
x

m
x ∘
aP
m

x ∘
= kP
m

Case III- At intermediate pressure:
At intermediate pressure, depends on P raised to the powers between O and 1. This relationship
x

m

is known as the Freundlich adsorption isotherm.
1
x
αP n
m
1
x
= kP n n > 1
m

Now, taking log:
x 1
log = log k + log P
m n

On plotting the graph between log ( and log P, a straight line is obtained with the slope equal
x
)
m

to and the intercept equal to log k.
1

n

Q5.6 What do you understand by activation of adsorbent? How is it achieved?

Answer. By activating an adsorbent, we tend to increase the adsorbing power of the
adsorbent.Some ways to activate an adsorbent are:
(i) By increasing the surface area of the adsorbent. This can be done by breaking it into
smaller pieces or powdering it.

(ii) Some speci c treatments can also lead to the activation of the adsorbent. For example, wood
charcoal is activated by heating it between 650 K and 1330 K in vacuum or air. It expels all the
gases absorbed or adsorbed and thus, creates a space for adsorption of Gases.

Q5.7 What role does adsorption play in heterogeneous catalysis?

Answer. Heterogeneous catalysis:
A catalytic process in which the catalyst and the reactants are present in different phases is known
as a heterogeneous catalysis. This heterogeneous catalytic action can be explained in terms of the
adsorption theory. The mechanism of catalysis involves the following steps:
(i) Adsorption of reactant molecules on the catalyst surface.

(ii) Occurrence of a chemical reaction through the formation of an intermediate.

(iii) De-sorption of products from the catalyst surface

(iv) Diffusion of products away from the catalyst surface.
In this process, the reactants are usually present in the gaseous state and the catalyst is present in
the solid state. Gaseous molecules are then adsorbed on the surface of the catalyst. As the
concentration of reactants on the surface of the catalyst increases, the rate of reaction also
increases. In such reactions, the products have very less af nity for the catalyst and are quickly
desorbed, thereby making the surface free for other reactants.

Q5.8 Why is adsorption always exothermic?

Answer. Adsorption is always exothermic. This statement can be explained in two ways.
(i) Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This
causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always
exothermic.

(ii) AH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is
restricted leading to a decrease in the entropy of the gas i.e., AS is negative. Now for a process to
be spontaneous, AG should be negative.
AG = AH - TAS
Since AS is negative, AH has to be negative to make AG negative. Hence, adsorption is always
exothermic.

Q5.9 How are the colloidal solutions classi ed on the basis of physical states of the dispersed
phase and dispersion medium?

Answer. One criterion for classifying colloids is the physical state of the dispersed phase and
dispersion medium. Depending upon the type of the dispersed phase and dispersion medium
(solid, liquid, or gas), there can be eight types of colloidal systems.

Q5.10 Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Answer. Effect of pressure
Adsorption is a reversible process and is accompanied by a decrease in pressure.
Therefore, adsorption increases with an increase in pressure.

Effect of temperature
Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier's principle, the
magnitude of adsorption decreases with an increase in temperature.

Q5.11 What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic
sols easily coagulated?

Answer. (i) Lyophilic sols:
Colloidal sols that are formed by mixing substances such as gum, gelatin, starch, etc. with a
suitable liquid (dispersion medium) are called lyophilic sols. These sols are reversible in nature
i.e., if two constituents of the sol are separated by any means (such as evaporation), then the sol
can be prepared again by simply mixing the dispersion medium with the dispersion phase and
shaking the mixture.

(ii) Lyophobic sols:
When substances such as metals and their sulphides etc. are mixed with the dispersion
medium, they do not form colloidal sols. Their colloidal sols can be prepared only by special
methods. Such sols are called lyophobic sols. These sols are irreversible in nature. For example:
sols of metals.

Now, the stability of hydrophilic sols depends on two things- the presence of a charge and the
salvation of colloidal particles. On the other hand, the stability of hydrophobic sols is only because
of the presence of a charge. Therefore, the latter are much less stable than the former. If the
charge of hydrophobic sols is removed (by addition of electrolytes), then the particles present in
them come closer and form aggregates, leading to precipitation.

Q5.12 What is the difference between multimolecular and macromolecular colloids?Give one
example of each. How are associated colloids different from these two types of colloids?

Answer. (i) In multi-molecular colloids, the colloidal particles are an aggregate of atoms or small
molecules with a diameter of less than 1 nm. The molecules in the aggregate are held together by
van der Waal's forces of attraction. Examples of such colloids include gold sol and sulphur sol.

(ii) In macro-molecular colloids, the colloidal particles are large molecules having colloidal
dimensions. These particles have a high molecular mass. When these particles are dissolved in a
liquid, sol is obtained. For example: starch, nylon, cellulose, etc.

(iii) Certain substances tend to behave like normal electrolytes at lower concentrations.
However, at higher concentrations, these substances behave as colloidal solutions due to the
formation of aggregated particles. Such colloids are called aggregated colloids.

Q5.13 What are enzymes ? Write in brief the mechanism of enzyme catalysis.

Answer. Enzymes are basically protein molecules of high molecular masses. These form colloidal
solutions when dissolved in water. These are complex, nitrogenous organic compounds produced
by living plants and animals. Enzymes are also called 'biochemical catalysts'.

Mechanism of enzyme catalysis:
On the surface of the enzymes, various cavities are present with characteristic shapes.
These cavities possess active groups such as -NH2, -COOH, etc. The reactant molecules having a

complementary shape t into the cavities just like a key ts into a lock. This leads to the formation
of an activated complex. This complex then decomposes to give the product.
Hence,
step 1: E + S → ES+
(Activated complex)
step 2: ES+ → E+P

Q5.14 How are colloids classi ed on the basis of
(i)physical states of components
(ii)nature of dispersed phase and
(iii)interaction between dispersed phase and dispersion medium?

Answer. Colloids can be classi ed on various bases:
(i) On the basis of the physical state of the components (by components we mean the
dispersed phase and dispersion medium). Depending on whether the components are
solids, liquids, or gases, we can have eight types of colloids.

(ii) On the basis of the dispersion medium, sols can be divided as:

(iii) On the basis of the nature of the interaction between the dispersed phase and dispersion
medium, the colloids can be classi ed as lyophilic (solvent attracting) and lyophobic (solvent
repelling).

Q5.15 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) When a beam of light is passed through a colloidal solution, then scattering of light is
observed. This is known as the Tyndall effect. This scattering of light illuminates the path of the
beam in the colloidal solution.

(ii) When NaCl is added to ferric oxide sol, it dissociates to give Na+ and Cl- ions.
Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of
negatively charged Cl- ions.

(iii) The colloidal particles are charged and carry either a positive or negative charge.
The dispersion medium carries an equal and opposite charge. This makes the whole system
neutral. Under the in uence of an electric current, the colloidal particles move towards the
oppositely charged electrode. When they come in contact with the electrode, they lose their
charge and coagulate.

Q5.16 What are emulsions? What are their different types? Give example of each type.

Answer. The colloidal solution in which both the dispersed phase and dispersion medium are
liquids is called an emulsion.
There are two types of emulsions:

(a) Oil in water type:
Here, oil is the dispersed phase while water is the dispersion medium. For example: milk,
vanishing cream, etc.
(b) Water in oil type:
Here, water is the dispersed phase while oil is the dispersion medium. For example: cold cream,
butter, etc.

Q5.17 How do emulsi res stabilise emulsion? Name two emulsi ers.

Answer. The process of decomposition of an emulsion into its constituent liquids is called
demulsi cation. Examples of demulsi ers are surfactants, ethylene oxide, etc.

Q5.18 Action of soap is due to emulsi cation and micelle formation. Comment.

Answer. The cleansing action of soap is due to emulsi cation and micelle formation. Soaps are
basically sodium and potassium salts of long chain fatty acids, R-COO-Na+. The end of the
molecule to which the sodium is attached is polar in nature, while the alkyl-end is non-polar.
Thus, a soap molecule contains a hydrophilic (polar) and a hydrophobic (nonpolar) part. When
soap is added to water containing dirt, the soap molecules surround the dirt particles in such a
manner that their hydrophobic parts get attached to the dirt molecule and the hydrophilic parts
point away from the dirt molecule. This is known as micelle formation.Thus, we can say that the
polar group dissolves in water while the non-polar group dissolves in the dirt particle. Now, as
these micelles are negatively charged, they do not coalesce and a stable emulsion is formed.

Q5.19 Give four examples of heterogeneous catalysis.

Answer. (i) Oxidation of sulphur dioxide to form sulphur trioxide. In this reaction, Pt acts as a
catalyst.
Pt(s)

2SO2(g) ⟶ 2SO3(g)

(ii) Formation of ammonia by the combination of dinitrogen and dihydrogen in the presence of
nely divide iron.
Fe(s)

N2(g) + 3H2(g) ⟶ 2NH3(g)

This process is called the Haber’s process.

(iii) Oswald's process: Oxidation Of ammonia to nitric oxide in the presence Of platinum.
Pt(s)

4NH3(g) + 5O2(g) ⟶ 4NO(g) + 6H2 O(g)

(iv) Hydrogenation of vegetable oils in the presence of Ni.
Ni(s)

Vegetable oil + H2(g) ⟶ vegetable ghee
(l) (s)

Q5.20 What do you mean by activity and selectivity of catalysts?

Answer. (a) Activity of a catalyst:
The activity of a catalyst is its ability to increase the rate of a particular reaction.
Chemisorption is the main factor in deciding the activity of a catalyst. The adsorption of
reactants on the catalyst surface should be neither too strong nor too weak. It should just be
strong enough to make the catalyst active.

(b) Selectivity of the catalyst:
The ability of the catalyst to direct a reaction to yield a particular product is referred to as the
selectivity of the catalyst. For example, by using different catalysts, we can get different products
for the reaction between H2 and CO.
Ni
(i) CO (g)
+ 3H2(g) ⟶ CH4(g) + H2 O(g)

(ii) CO (g) + 2H2(g)

Cu

(iii) CO (g)
+ H2(g) ⟶ HCHO(g)

Q5.21 Describe some features of catalysis by zeolites.

Answer. Zeolites are alumino-silicates that are micro-porous in nature. Zeolites have a
honeycomb-like structure, which makes them shape-selective catalysts. They have an extended
3D-network of silicates in which some silicon atoms are replaced by aluminium atoms, giving
them an A1—O-Si framework. The reactions taking place in zeolites are very sensitive to the pores
and cavity size of the zeolites. Zeolites are commonly used in the petrochemical industry.

Q5.22 What is shape selective catalysis?

Answer. A catalytic reaction which depends upon the pore structure of the catalyst and on the size
of the reactant and the product molecules is called shape-selective catalysis.
For example, catalysis by zeolites is a shape-selective catalysis. The pore size present in the
zeolites ranges from 260-740 pm. Thus, molecules having a pore size more than this cannot enter
the zeolite and undergo the reaction.

Q5.23 Explain the following terms:
(i)Electrophoresis
(ii)Coagulation
(iii)Dialysis
(iv)Tyndall effect.

Answer.(i) Electrophoresis:
The movement of colloidal particles under the in uence of an applied electric eld is known as
electrophoresis. Positively charged particles move to the cathode, while negatively charged
particles move towards the anode. As the particles reach oppositely charged electrodes, they
become neutral and get coagulated.
(ii) Coagulation:
The process of settling down of colloidal particles i.e., conversion of a colloid into a precipitate is
called coagulation.
(iii) Dialysis:
The process of removing a dissolved substance from a colloidal solution by the means of diffusion
through a membrane is known as dialysis. This process is based on the principle that ions and
small molecules can pass through animal membranes unlike colloidal particles.
(iv) Tyndall effect:
When a beam of light is allowed to pass through a colloidal solution, it becomes visible like a
column of light. This is known as the Tyndall effect. This phenomenon takes place as particles of
colloidal dimensions scatter light in all directions.

Q5.24 Give four uses of emulsions.

Answer. Four uses of emulsions:
(i) Cleansing action of soaps is based on the formation of emulsions.
(ii) Digestion of fats in intestines takes place by the process of emulsi cation.
(iii) Antiseptics and disinfectants when added to water form emulsions.
(iv) The process of emulsi cation is used to make medicines.

Q5.25 What are micelles? Give an example of a micellers system.

Answer. Micelle formation is done by substances such as soaps and detergents when dissolved in
water. The molecules of such substances contain a hydrophobic and a hydrophilic part. When
present in water, these substances arrange themselves in spherical structures in such a manner
that their hydrophobic parts are present towards the centre, while the hydrophilic parts are
pointing towards the outside (as shown in the given gure). This is known as micelle formation.

Q5.26 Explain the terms with suitable examples:
(i)Alcosol
(ii)Aerosol
(iii)Hydroso

Answer. (i) Alcosol:
A colloidal solution having alcohol as the dispersion medium and a solid substance as the
dispersed phase is called an alcosol.
For example: colloidal sol of cellulose nitrate in ethyl alcohol is an alcosol.

(ii) Aerosol:
A colloidal solution having a gas as the dispersion medium and a solid as the dispersed phase is
called an aerosol.
For example: fog
(iii) Hydrosol
A colloidal solution having water as the dispersion medium and a solid as the dispersed phase is
called a hydrosol.
For example: starch sol or gold sol

Q5.27 Comment on the statement that “colloid is not a substance but a state of substance”.

Answer.Common salt (a typical crystalloid in an aqueous medium) behaves as a colloid in a
benzene medium. Hence, we can say that a colloidal substance does not represent a separate class
of substances. When the size of the solute particle lies between 1 nm and 1000 nm, it behaves as a
colloid.
Hence, we can say that colloid is not a substance but a state of the substance which is dependent
on the size of the particle. A colloidal state is intermediate between a true solution and a
suspension.