NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry are part of NCERT Solutions for Class 12 Chemistry. Here we have given NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry.
|Chapter Name||Surface Chemistry|
|Number of Questions Solved||35|
NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry
NCERT IN-TEXT QUESTIONS
Why are substances like platinum and palladium often used for carrying out the electrolysis of aqueous solutions ?
The metals like platinum and palladium are used as inert electrodes for carrying out the process of electrolysis because these are not attacked by the ions involved in the process.
Why does physisorption decrease with increase in temperature ?
Physisorption or physical adsorption of a gas on the surface of a solid is exothermic in nature.
When temperature is increased, the equilibrium gets shifted in the backward direction to neutralise the effect of increase in temperature. Consequently, physisorption decreases with the increase in temperature.
Why are powdered substances more effective as adsorbents than their crystalline forms ?
The powdered form of the adsorbent has greater surface area as compared to the crystalline form. This will be therefore, more effective as adsorbent.
Why is it necessary to remove CO when ammonia is obtained by Haber’s process ?
Carbon monoxide (CO) acts as a poison for the catalyst iron as well as promoter molybdenum which are used in the Haber’s process. Moreover, it is likely to combine with iron to form iron carbonyl Fe(CO)5. Therefore, it is necessary to remove it from the reaction mixture by suitable means.
Why is ester hydrolysis slow in the beginning and becomes fast after sometime ?
In ester hydrolysis, an acid and alcohol are formed as the products. For example,
Acid will release H+ ions in solution which act as catalyst (auto-catalysis) for the reaction. That is why, the hydrolysis is slow in the beginning and becomes faster later on.
What is the role of desorption in the process of catalysis ?
In the heterogeneous catalysis carried over metal surface, adsorbed reactant species combine to form the products. They have to be desorbed from the surface so that more of reactants may be accommodated on the surface of the catalyst. Therefore, desorption has a specific role to play in the process of catalysis.
What modification can you suggest for Hardy-Schulze Law ?
According to Hardy-Schulze Law, the ions carrying charge opposite to the charge on sol particles neutralise their charge and thus cause their coagulation or precipitation. The law takes into account the charge carried by the ion and not its size. Smaller the size of the ion, more will be its polarising power. Thus, the law should be modified in terms of the polarising power of the flocculating ion or the ion causing the precipitation. The modified form of the law states that “Greater the polarising power of the flocculating ion added, greater is its power to cause precipitation.”
Why is it essential to wash a precipitate with water before estimating it quantitatively ?
Precipitates are generally formed in the ionic reactions. Some ions of the reactants may be adsorbed or may stick on the surface of the particles of the precipitate. These can be removed by washing the precipitate repeatedly with water. In case these ions are not removed, they may introduce some error in weighing when the precipitate is estimated quantitatively.
Distinguish between the meaning of terms adsorption and absorption. Give one example in each case.
The phenomenon of accumulation of the molecules of a substance on a solid or liquid surface resulting in the increased concentration of the molecules on the surface is called adsorption. In absorption, the substance is uniformly distributed throughout the bulk of the solution. A distinction can be made by taking an example of water vapours. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel.
|1.||It is the phenomenon as a result of which the species of one substance get distributed uniformly throughout the body of the other substance.||It is the phenomenon as a result of which the species of one substance get concentrated mainly on the surface of the other substance which adsorbs these.|
|2.||The concentration is the same throughout the material.||The concentration on the surface of the adsorbent is different from that in the bulk.|
|3.||It is a bulk phenomenon since it occurs throughout the material.||It is only a surface phenomenon since it occurs only on the surface.|
|4.||Absorption proceeds at uniform rate.||Adsorption is rapid in the beginning and its rate slowly decreases|
What is the difference between physical adsorption and chemisorption ?
|Weak van der Waals’ forces present.||Strong chemical bond forces present.|
|Low; of the order of 20-40 kJ/mol.||High; of the order of 80-240 kJ/mol.|
|Usually occurs at low temperature.||Occurs at high temperature.|
|It is not specific in nature, i.e. all gases may be adsorbed on the surface of a solid.||It is highly specific in nature and occurs only when there is bond formation between adsorbent and adsorbate molecules.|
|Forms multi-molecular layers under high pressure.||Forms mono-molecular layer.|
|The extent of adsorption is directly related with the ease of liquefaction of the gas.||There is no correlation between extent of adsorption and the ease of liquefaction of the gas.|
|It does not involve appreciable activation energy. In most cases, the activation energy required is almost nil.||It generally requires appreciable activation energy since a chemical reaction is to take place.|
|Same state as in the bulk.||May be quite different from that in the bulk.|
Why is a finely divided substance more effective as an adsorbent ?
A finely divided substance (adsorbent) has more adsorbing power due to the following reasons :
(i) The surface area increases and the adsorbate particles get better opportunity to be adsorbed.
(ii) The number of active sites, also called active centres become more and the extent of adsorption increases.
What are the factors which influence the adsorption of a gas on a solid ?
The adsorption of a gas on a solid surface is an example of physical adsorption. It is influenced by the following factors :
(i) Nature of the adsorbate
(ii) Nature of the adsorbent
(iii) Surface area of the adsorbent
(iv) Activation of the adsorbent
(v) Effect of pressure
(vi) Effect of temperature.
Factors affecting adsorption of a gas on solids are :
Nature of the adsorbate: The same gas is adsorbed to different extents by different solids at the same temperature. Also, greater the surface area of the adsorbent, more is the gas adsorbed.
Nature of the adsorbent: Different gases are adsorbed to different extents by different solids at the same temperature. Higher the critical temperature of the gas, greater is its amount adsorbed.
Surface area of the adsorbent: Surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. Greater the surface area, higher would be the adsorption therefore, porous or powdered adsorbents are used.
Activation of adsorbent: It means increasing the adsorbing power of an adsorbent by increasing its surface area. It is done by :
- making the adsorbent’s surface rough
- removing gases already adsorbed
- subdividing the adsorbent into smaller pieces.
Pressure: At constant temperature, the adsorption of gas increases with pressure.
Temperature: Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.
What is adsorption isotherm ? Distinguish between Freundlich adsorption isotherm and Langmuir adsorption isotherm.
Adsorption isotherm represents the variation of the amount of the gas adsorbed and the corresponding pressure at a certain temperature. The mathematical forms of the two adsorption isotherms are :
The main points of distinction in the two adsorption isotherms are :
(i) Freundlich Adsorption isotherm is applicable to all types of adsorption whereas Langmuir Adsorption isotherm is applicable mainly to chemical adsorption or chemisorption.
(ii) Freundlich adsorption isotherm fails at high pressure of the gas whereas Langmuir Adsorption isotherm can be applied under all pressures.
What do you understand by activation of adsorbent ? How is it achieved ?
Activation of adsorbent implies increasing its adsorbing power. This is achieved by increasing the surface area and also the number of pores and vacant sites (active centres) per unit area. The activation can be achieved in a number of ways.
What role does adsorption play in heterogeneous catalysis ?
Heterogeneous catalysis is generally carried on the surface of the finely divided metals of the transition series. Due to the availability of large surface area, the reacting species get adsorbed on the surface either by physical adsorption or by chemisorption. The adsorbed species get opportunity to mutually combine to form the products which are released or desorbed from the surface so as to accommodate more reacting species.
(a) Diffusion of the reactants on the surface of the catalyst.
(b) Some association between the catalyst surface and the reactants i. e., adsorption.
(c) Occurrence of the chemical reactions on the catalyst surface.
(d) Dissociation of the reaction products from the catalyst surface i.e., desorption.
(e) Diffusion of the products from the catalyst surface.
Give two chemical methods for the preparation of colloids.
These are formed in two ways :
- Condensation methods
- Dispersion methods.
Condensation methods: The particles of the dispersed phase are very small in size. They have to be condensed suitably to be of colloidal size.
A colloidal solution of sulphur is obtained when H2S gas is bubbled through the solution of oxidising agent
like bromine water, sulphur dioxide, dilute HNO3 etc.
Dispersion methods: In these methods, bigger particles of a substance (suspension) are broken into smaller particles of colloidal dimensions. The substance whose colloidal solution is to be prepared, is first ground to coarse particles. It is then mixed with the dispersion medium to get a suspension
How are colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium ?
There are in all eight types of colloidal solutions.
Discuss the effect of pressure and temperature on the adsorption of gases on solids.
- Adsorption decreases with an increase in, temperature because it is an exothermic process and according to Le Chatelier’s principle the reaction will proceed in backward direction with increase in temperature.
- At a constant temperature, adsorption increases with pressure.
What are lyophilic and lyophobic sols ? Give one example in each case.
Lyophilic colloids: The colloidal solution in which the particles of the dispersed phase have a great affinity (or love) for the dispersion medium, are called lyophilic colloids. Such solutions are easily formed the moment the dispersed phase and the dispersion medium come in direct contact. e.g., sols of gum, gelatin, starch, etc.
Lyophobic colloids: The colloidal solutions in which the particles of the dispersed phase have no affinity or love, rather have hatred for the dispersion medium, are called lyophobic colloids. The solutions of metals like Ag and Au, hydroxides like Al(OH)3 and Fe(OH)3 and metal sulphides like As2S3 are examples of lyophobic colloids.
What is the difference between muitimolecular and macromolecular colloids ? Give one example of each. How are associated colloids different from these two types of colloids ? (C.B.S.E. 2008, 2009, 2010)
Difference between muitimolecular and macromolecular colloids
The main points of distinction are listed.
|Muitimolecular colloids||Macromolecular colloids|
1.The particle size is less than that of colloidal range (< 103 pm)
2. They exist as aggregate of smaller particles.
3. These are mostly lyophobic colloids.
1. The particle size falls in the colloidal range (103 to 106 pm).
2. These are already macro molecular in nature.
3. These are mostly lyophilic colloids.
Colloidal sol of sulphur (Sg) is an example of muitimolecular colloid while colloidal sol of starch represents macromolecular colloid.
Associated colloids also called micelles, are generally electrolytes. They exist as ions at low concentration. However, above a particular concentration called critical micellear concentration (CMC) and above a particular temperature called Kraft temperature (Tk), these get associated and exhibit colloidal behaviour. Soap is a common example of associated colloids.
Multimolecular colloids: In these colloids, the individual particles consist of an aggregate of atoms or small molecules with molecular size less than 103 pm. For example, gold sol consists of particles of various sizes having several atoms. Similarly, a sulphur sol consists of particles each having eight sulphur atoms (Sg). In these colloids, the particles are held by van der Waals’ forces.
Macromolecular colloids: In this type, the particles of the dispersed phase are sufficiently big in size (macro) to be of colloidal dimensions. These are normally polymers. A few naturally occurring macromolecules are starch, cellulose and proteins. The examples of artificial macromolecules are those of polythene, nylon, polystyrene, plastics etc.
What are enzymes ? Write in brief the mechanism of enzyme catalysis.
Enzymes are the complex nitrogeneous compounds which are produced by living plants and animals. In fact, these are proteins produced by living systems and catalyse certain biological reactions. These are therefore, often known as bio-chemical catalysts and this phenomenon is known as bio-chemical catalysis.
The rate of enzyme-catalysed reaction which is initially of first order changes to zero order as the concentration of substrate species on the catalyst surface increases.
Two models have been proposed by bio-chemists to explain the mechanism of enzyme catalysed reactions. These are briefly discussed.
How are colloids classified on the basis of :
(a) physical states of components
(b) nature of dispersion medium
(c) interaction between dispersed phase and dispersion medium ?
(a) Based on physical states of components. Based on physical states of components i.e., dispersed phase and dispersion medium, there are eight types of colloidal solutions.
(b) Nature of dispersion medium. The dispersion medium can be either gas, liquid or solid. Based upon its nature, the colloids or colloidal solutions are of three types.
- Aerosols: Air or gases act as the dispersion medium
- Liquid sols: Liquids like water, alcohol or benzene act as the dispersion medium.
- Solid sols: Solid acts as the dispersion medium.
(c) Interaction between dispersed phase and dispersion medium. Colloidal solutions are classified into two types. These are lyophilic and lyophobic sols.
(i) Lyophilic colloids: The colloidal solution in which the particles of the dispersed phase have a great affinity (or love) for the dispersion medium, are called lyophilic colloids. Such solutions are easily formed the moment the dispersed phase and the dispersion medium come in direct contact. e.g., sols of gum, gelatin, starch, etc.
(ii) Lyophobic colloids: The colloidal solutions in which the particles of the dispersed phase have no affinity or love, rather have hatred for the dispersion medium, are called lyophobic colloids. The solutions of metals like Ag and Au, hydroxides like Al(OH)3 and Fe(OH)3 and metal sulphides like As2S3 are examples of lyophobic colloids.
Explain what is observed when :
(i) a beam of light is passed through colloidal sol
(ii) an electrolyte NaCl is added to ferric hydroxide sol
(iii) electric current is passed through a colloidal sol ? (C.B.S.E. Delhi 2008, 2009; Outside Delhi, 2010, 2011)
(i) The path of light as well as the colloidal particles become visible because of Tyndall effect. When a beam of light is passed through a true solution, no scattering of light takes place. When the same beam of light is passed through a colloidal solution, scattering of light takes place. The visibility of dust particles in a semi darkened room, when a beam of sunlight enters or when a beam of light is thrown from a projector, are familiar examples of this scattering.
(ii) The positively charged sol particles of ferric hydroxide get their charge neutralised by interacting with negative chloride ions (CF). As a result, they get coagulated.
(iii) The colloidal particles move towards the oppositely charged electrode and this phenomenon is called electrophoresis.
What are emulsions ? What are their different types ? Give example of each type.
(a) Oil-in-water emulsion (O/W type). In this case, the dispersed phase is oil while the dispersion medium is water. Milk is a common example in which liquid fats are dispersed in water. Similarly, if a few drops of nitrobenzene (oil) is added to water, an emulsion results. Vanishing cream is another example of this type.
(b) Water-in-oil emulsion (W/O type). In this type of emulsions, the dispersed phase is water while the dispersion medium is oil. Butter is an example of water in oil emulsion in which water is dispersed in oil. Cod liver oil and cold cream are the other examples of these emulsions.
What is demulsification ? Name two demulsifiers.
Demulsification is the process of decomposition of an emulsion back to the constituent liquids. It can be achieved by centrifugation or by boiling.
Action of soap is due to demulsification and micelle formation. Comment.
Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO– Na+ (e.g., sodium stearate CH3(CH2)]16 COO– Na+, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO– and Na+ ions. The RCOO– ions, however, consist of two parts – a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO– (also called polar-ionic ‘head’), which is hydrophilic (water loving).
The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the centre of the sphere with COO– part remaining outward on the surface of the sphere. An aggregate thus formed is known as ‘ionic micelle’.
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 of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles
Give four examples of heterogeneous catalysts.
(i) The combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron acting as catalyst. This is known as Haber’s process.
(ii) Formation of sulphur trioxide by the oxidation of sulphur dioxide in the presence of platinum catalyst is the basis of the manufacture of sulphuric acid in Contact process.
(iii) Oxidation of ammonia into nitric oxide in the presence of platinum catalyst is employed for the commercial preparation of nitric acid in Ostwald process.
(iv) In the hydrogenation of vegetable oils (unsaturated in nature) resulting in solid fats (saturated in nature), hydrogen gas is passed through the oil in the presence of nickel catalyst at about 473 K.
What do you mean by activity and selectivity of catalysts ?
(a) Activity : The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed reasonably strongly on to the catalyst to become active. But adsorption must not be so strong that they are immobilised. It is observed that maximum activity is shown by elements of groups 7 – 9 of the periodic table
2H2 + O2 2H2O
(b) Selectivity: The selectivity of a calatyst is its ability to yield a particular product in the reaction e.g.,
Thus, a selective catalyst can act as a catalyst in one reaction and may fail to catalyse another reaction.
Describe some features of catalysis by zeolites.
(a) Zeolites are hydrated aluminosilicates which have a three dimensional network structure containing water molecules in their pores.
(b) The pores are made vacant by heating before catalysis.
(c) The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules and also on the pores and cavities in them, e.g., ZSM-5 converts alcohols to hydrocarbons by dehydrating them.
What are shape selective catalysts ?
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. Zeolites are good shape-selective catalysts because of their honeycomb-like structures. They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al-O-Si framework. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well as synthesised for catalytic selectivity.
Explain the following terms :
(v) Tyndall effect.
(i) Peptisation: The process of converting a freshly prepared precipitate into colloidal form by the addition of a suitable electrolyte in small amount. Normally a freshly prepared precipitate is preferred because the particles are not so firmly attached to each other and can be easily disintegrated. Please note the electrolyte should not be added in excess because in that case, the oppositely charged ions not involved in peptisation, may neutralise the charge on the colloidal particles.
(ii) Electrophoresis: When electric potential is applied across two platinum electrodes dipped in a colloidal solution, the colloidal particles move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis.
(iii) Coagulation: The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity. The process of settling down of colloidal particles is called coagulation.
(iv) Dialysis: It is the process of removing dissolved substances from a colloidal solution by means of diffusion through a suitable membrane. Since particles (ions or smaller molecules) in a true solution can pass through animal membrane (bladder) or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis. The apparatus used for this purpose is called dialyser. 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 outer water and pure colloidal solution is left behind.
(v) Tyndall effect: When a beam of light is passed through colloidal particles, its path becomes clearly visible and is known as Tyndall effect. It is due to scattering of light by colloidal particles. The bright cone of the light is called Tyndall cone.
Give four uses of emulsions.
1. Some of the medicines are effective as emulsions.
2. Paints are emulsions which are used in our daily life.
3. Soaps and detergents act as cleansing agents, action of which is based on emulsification.
4. Photographic films are coated with emulsion of AgBr on its surface.
What are micelles ? Give an example of micelles system.
Micelles are substances that behave as normal strong electrolytes at low concentration but at high concentrations behave as colloids due to formation of aggregates. They are also called associated colloids, e.g., soaps and detergents. They can form ions and may contain 100 or more molecules to form a micelle.
Explain the following terms with suitable examples. (C.B.S.E. Delhi 2009)
(i) Gel. Colloidal solution of liquid in a solid, e.g., butter.
(ii) Aerosol. Colloidal solution of liquid in a gas e.g., fog.
(iii) Hydrosol. Colloidal solution of solid in water, e.g., gold sol.
Comment on the statement that colloid is not a substance but a state of substance.
This statement is true because the same substance in one solvent may behave as a colloid while in the other as crystalloid. For example, sodium chloride behaves as crystalloid in water but as colloid in benzene solvent. This means that the colloid is not a substance. It depends upon the particle size of the dispersed phase which varies from 103 to 106 pm.
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