NCERT Solutions for Class 12 Chemistry Part 1 Chapter 5
Surface Chemistry Class 12
Chapter 5 Surface Chemistry Exercise Solutions
In text : Solutions of Questions on Page Number : 127
Write any two characteristics of Chemisorption.
1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbate.
2.
Why does physisorption decrease with the increase of temperature?
Why are powdered substances more effective adsorbents than their crystalline forms?
Why is it necessary to remove CO when ammonia is obtained by Haber's process?
Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?
Ester hydrolysis can be represented as:
What is the role of desorption in the process of catalysis?
What modification can you suggest in the Hardy-Schulze law?
Hardy-Schulze law states that 'the greater the valence of the flocculating ion added, the greater is its power to cause precipitation.'
Why is it essential to wash the precipitate with water before estimating it quantitatively?
Exercise : Solutions of Questions on Page Number : 146
Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
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.
What is the difference between physisorption and chemisorption?
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Physisorption |
Chemisorption |
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1. |
In this type of adsorption, the adsorbate is attached to the surface of the adsorbent with weak van der Waal's forces of attraction. |
In this type of adsorption, strong chemical bonds are formed between the adsorbate and the surface of the adsorbent. |
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2. |
No new compound is formed in the process. |
New compounds are formed at the surface of the adsorbent. |
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3. |
It is generally found to be reversible in nature. |
It is usually irreversible in nature. |
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4. |
Enthalpy of adsorption is low as weak van der Waal's forces of attraction are involved. The values lie in the range of 20-40 kJ mol- 1. |
Enthalpy of adsorption is high as chemical bonds are formed. The values lie in the range of 40-400 kJ mol- 1. |
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5. |
It is favoured by low temperature conditions. |
It is favoured by high temperature conditions. |
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6. |
It is an example of multi-layer adsorption |
It is an example of mono-layer adsorption. |
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Q3 :
Answer :
What are the factors which influence the adsorption of a gas on a solid?
There are various factors that affect the rate of adsorption of a gas on a solid surface.
Easily liquefiable gases such as NH3, HCl etc. are adsorbed to a great extent in comparison to gases such as H2, O2 etc. This is because Van der Waal's forces are stronger in easily liquefiable gases.
The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid surface.
Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.
What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
The plot between the extent of adsorption against the pressure of gas (P) at constant temperature (T) is called the 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 specific temperature.
From the given plot it is clear that at pressure PS, reaches the maximum valve. Ps is called the saturation pressure. Three cases arise from the graph now.
The plot is straight and sloping, indicating that the pressure in directly proportional to i.e.,
When pressure exceeds the saturated pressure, becomes independent of P values.
On plotting the graph between log and log P, a straight line is obtained with the slope equal to and the intercept equal to log k.
What do you understand by activation of adsorbent? How is it achieved?
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)
What role does adsorption play in heterogeneous catalysis?
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 affinity for the catalyst and are quickly desorbed, thereby making the surface free for other reactants.
Why is adsorption always exothermic?
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) ΔH 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., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.
ΔG = ΔH - TΔS
How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?
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.
Dispersed phase |
Dispersion medium |
Type of colloid |
Example |
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1. |
Solid |
Solid |
Solid Sol |
Gemstone |
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2. |
Solid |
Liquid |
Sol |
Paint |
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3. |
Solid |
Gas |
Aerosol |
Smoke |
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4. |
Liquid |
Solid |
Gel |
Cheese |
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5. |
Liquid |
Liquid |
Emulsion |
Milk |
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6. |
Liquid |
Gas |
Aerosol |
Fog |
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7. |
Gas |
Solid |
Solid foam |
Pumice stone |
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8. |
Gas |
Liquid |
Foam |
Froth |
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Discuss the effect of pressure and temperature on the adsorption of gases on solids.
Effect of pressure
Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.
What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?
(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.
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.
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?
(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)
What are enzymes? Write in brief the 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 fit into the cavities just like a key fits 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
How are colloids classified on the basis of
(ii) Nature of dispersion medium and
Answer :
Colloids can be classified 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:
Dispersion medium |
Name of sol |
Water |
Aquasol or hydrosol |
Alcohol |
Alcosol |
Benzene |
Benzosol |
Gases |
Aerosol |
(iii)
Explain what is observed
(ii) An electrolyte, NaCl is added to hydrated ferric oxide 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)
What are emulsions? What are their different types? Give example of each type.
The colloidal solution in which both the dispersed phase and dispersion medium are liquids is called an emulsion. There are two types of emulsions:
Here, oil is the dispersed phase while water is the dispersion medium. For example: milk, vanishing cream, etc.
What is demulsification? Name two demulsifiers.
Action of soap is due to emulsification and micelle formation. Comment.
The cleansing action of soap is due to emulsification 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 (non-polar) part.
Give four examples of heterogeneous catalysis.
(i) Oxidation of sulphur dioxide to form sulphur trioxide. In this reaction, Pt acts as a catalyst.
(ii) Formation of ammonia by the combination of dinitrogen and dihydrogen in the presence of finely divided iron.
This process is called the Haber's process.
(iii) Oswald's process: Oxidation of ammonia to nitric oxide in the presence of platinum.
(iv) Hydrogenation of vegetable oils in the presence of Ni.
What do you mean by activity and selectivity of catalysts?
(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.
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.
(ii)
Q21 :
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
What is shape selective catalysis?
Explain the following terms:
(iii) Dialysis (iv) Tyndall effect.
(i) Electrophoresis:
The movement of colloidal particles under the influence of an applied electric field 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.
The process of settling down of colloidal particles i.e., conversion of a colloid into a precipitate is called coagulation.
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.
Give four uses of emulsions.
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 emulsification.
(iii) Antiseptics and disinfectants when added to water form emulsions.
(iv)
Give four uses of emulsions.
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 emulsification.
(iii) Antiseptics and disinfectants when added to water form emulsions.
(iv)
Explain the terms with suitable examples:
Answer :
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.
A colloidal solution having a gas as the dispersion medium and a solid as the dispersed phase is called an aerosol.
For example: fog
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
Comment on the statement that "colloid is not a substance but a state of substance".
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.