NCERT 12 Chemistry Surface chemistry Chapter 5 exercises 2

Q – 5.1. Distinguish between the meaning of the terms adsorption and adsorption. Give one example of each.
Ans.
Adsorption –
1. It is a surface phenomenon, i.e., it occurs only at the surface of the adsorbent.
2. In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk.
3. Its rate is high in the beginning and then decreases till equilibrium is attained.
Absorption –
It is a bulk phenomenon, i.e., occurs throughout the body of the material.
In this phenomenon, the concentration is same throughout the material.
Its rate remains same throughout the process.
Q – 5.2. What is the difference between physical adsorption and chemisorption?
Ans.
Physical –
The forces operating in these case are weak van der Walls forces.
The heats of adsorption are low, viz., about 20 – 40 kJ mol-1
No compound formation takes place in these cases.
The process is reversible, i.e., desorption of the gas occurs by increasing the temperature or decreasing the pressure.
It does not require any a activation energy.
This type of adsorption usually takes place at low temperature and decreases with increase of temperature.
Chemisorption –
The forces operating in these cases are similar to those of a chemical bond.
The heats of adsorption are high, viz., about 40-400 kJ mol-1
Surface compounds are formed.
The process is irreversible. Efforts to free the adsorbed gas does compound.
It requires activation energy.
This type of adsorption first increase of temperature. The effect is called activated adsorption.
Q – 5.3. Give reason why a finely divided substance is more effective as an adsorbent?
Ans. Finely divided substance has large surface area and hence greater adsorption.
Q – 5.4. What are the factors which influence the adsorption of a gas on a solid?
Ans. almost all solids adsorb gases to some extent. However, the exact amount of a gas adsorbed depends upon a number of factors, as briefly explained below:
Nature and Surface area of the adsorbent- It is observed that the same gas adsorbed to different extents by different solids at the same temperature. Further, as may be expected, the greater the Surface area of the adsorbent, greater is the volume of the gas adsorbed. It is for this reason that substance like charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas. For the same reason, finely divided substance have larger adsorption power than when they are present in the compact form.
Nature of the gas being adsorbed – Different gases are adsorbed to different extents by the same adsorbent at the same temperature.
Temperature – Studying the adsorption of any particular gas by some particular adsorbent, it is observed that the adsorption decreases with increase of temperature and vice versa.
Pressure – At constant temperature, the adsorption of a gas increases with increase of pressure. It is observed that at low temperature, the adsorption of a gas increases very rapidly as the pressure is increased from small values.
Activation of the solid adsorbent – It means increasing the adsorbing the surface area (or the specific area) of the adsorbent which can be achieved in any of the following ways:
By making the surface of the adsorbent rough,
By subdividing the adsorbent into smaller pieces or grains.
By removing the gases already adsorbed.
Q – 5.5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
Ans. Adsorption isotherm. It represents the variation of the mass of the gas adsorbed per gram of the adsorbent with pressure at constant temperature.
Freudlich adsorption isotherm. The extent of adsorption on a given surface generally increases with increase in pressure (for gases) and concentration (for solution) at constant temperature. At low temperature, the physical adsorption of a gas increases very rapidly as the pressure rises. When the temperatures is high the increase in adsorption is relatively less.
To understand the effect of pressure on adsorption of a gas, we consider adsorption as an equilibrium process. When the adsorbent and the adsorbate are enclosed in a closed vessel, after an initial decrease in the pressure of the gas, gas pressure as well as the amount of gas adsorbed reach constant values. This is because after the equilibrium is attained, rate of adsorption becomes equal to the rate of desorption.
Q – 5.6. What do you understand by activation of adsorbent? How is it achieved?
Ans. Activation of the solid adsorbent – It means increasing the adsorbing the surface area (or the specific area) of the adsorbent which can be achieved in any of the following ways:
(a) By making the surface of the adsorbent rough, e.g., by mechanical rubbing or by chemical action or by depositing finely dispersed metals on the surface of the adsorbent by electroplating.
(b) By subdividing the adsorbent into smaller pieces or grains. No doubt, this method increases the surface area but it has a practical limitation, that is, if the adsorbent is broken into too fine particles that it becomes almost powder, then the penetration of the gas becomes difficult and this will obstruct adsorption.
By removing the gases already adsorbed. e.g., charcoal is activated by heating in superheated steam or in vacuum at a temperature between 623 to 1273 K.
Q – 5.7. What role does adsorption play in heterogeneous catalysis?
Ans. In heterogeneous catalysis, generally the reactants are gaseous whereas catalyst is a solid. The reactant molecules are adsorbed on the surface of the solid catalyst by physical adsorption or chemisorption. As a result, the concentration of the reactant molecules on the surface increases and hence the rate of reaction undergoes increases. Alternatively, one of the reactant molecules undergoes fragmentation on the surface of the solid catalyst producing active species which react faster. The product molecules in either case have no affinity for the solid catalyst and are desorbed making the surface free for fresh adsorption. This theory is called adsorption theory.
Q – 5.8. Why is adsorption always exothermic?
Ans. When a gas is adsorbed on the surface of a solid, its entropy decreases, i.e., ∆S is – ve. Now, ∆G = ∆H – T∆S and for the process to be spontaneous, free energy change, ∆G, must be negative. As T∆S is – ve, i.g., – T∆S is + ve, therefore, ∆G can be – ve only if ∆H is – ve. Hence, adsorption is always exothermic.
Q – 5.9. How are the colloidal solutions classified on the basis of Physical states of the dispersed phase and dispersion medium?
Ans. Classification of colloids –
Colloids are classified in three different ways as follows:
Based on physical state of dispersed phase and dispersion medium.
Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal systems are possible. A gas mixed with another gas forms a homogeneous mixture and not a colloidal system.
Based on nature of interaction between dispersed phase dispersion medium.
On this basis, colloidal sols are divided into two categories, namely, lyophilic and lyophobic. If water is the dispersion medium, the terms used are hydrophilic and hydrophobic.
Based on the type of particles of the dispersed phase – (Multimolecular, Macromolecular and Associated Colloids)
Q – 5.10. Discuss the effect of pressure and temperature on the adsorption of gases by solids.
Ans. Temperature – Studying the adsorption of any particular gas by some particular adsorbent, it is observed that the adsorption decreases with increase of temperature and vice versa. For example, one gram of charcoal adsorbs about 10 mL of N2 at 273 K, 20 mL at 244 K and 45 mL at 195 K. the decrease of adsorption with increase of temperature may be explained as follows:
Like any other equilibrium, adsorption is a process involving a true equilibrium. The two opposing processes involved are condensation (i.e., adsorption) of the gas molecules on the surface of the solid and evaporation (i.e., desorption) of the gas molecules from the surface of the solid into the gaseous phase. Moreover, the processes of condensation (or adsorption) is exothermic so that equilibrium may be represented as:
chemistry chapter 5 ex 2 ques 10
Applying Le Chatelier’s principal, it can be seen that increase of temperature decreases the adsorption and vice versa.
Pressure – At constant temperature, the adsorption of a gas increases with increase of pressure. It is observed that at low temperature, the adsorption of a gas increases very rapidly as the pressure is increased from small values. The variation of adsorption with pressure at different constant temperature is shown in Fig 1 for the adsorption of N2 gas by 1 g of charcoal.
Q – 5.11. What are lyophilic and lyophobic sols? Give one example of each type. Why is hydrophobic sol easily coagulated?
Ans. 1. Lyophilic colloids. Substances like gum, gelatine, starch, rubber etc. which when mixed with a suitable liquid as the dispersion medium directly from the colloidal sol are called lyophilic and sols thus obtained are called lyophilic sols.
2. Lyophobic colloids. Substance like metals, their sulphides etc. when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods (as discussed later). Such substance are called lyophobic and the formed by them are called lyophobic sols.
The stability of hydrophobic sol is only due to the presence of charge on the colloidal particles. If charge is removed, i.e., by addition of suitable electrolytes, the particles will come nearer to each other to from aggregate, i.e., they will coagulate and settle sown. On the other hand, the stability of hydrophilic sol is due to charge as well as solvation of the colloidal particles.
Q – 5.12. What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids and multimolecuular and macromolecular colloids?
Ans. Comparison of some important characteristics of Multimolecular and Macromolecular colloids and associated colloids
Multimolecular colloids
They are formed by the aggregation of a large number of atoms or molecules which generally have diameters less than 1 nm, e.g., sols of gold, Sulphur, etc.
Their molecular masses are not very high.
Their atoms or molecular masses are held together by weak van der Waals forces.
Macromolecular colloids
They are not molecules of large size, e.g., polymers like rubber, nylon, starch, proteins, etc.
They have high molecular masses.
Due to long chain, the van der Waals forces holding them are comparatively stronger.
Associated colloids
They are formed by of aggregation of a large number of ions in concentrated solution, e.g., soap sol.
Their molecular masses are generally high.
Higher is the concentration, greater are the van der Waals forces.
Q – 5.13. What are enzymes? Write in brief mechanism of enzyme catalysis.
Ans. All biological reaction are catalyzed by special catalysts called enzymes. Thus, enzymes are defined as bio- chemical cataysts. Chemically, all enzymes are globular proteins (which are complex nitrogenous compounds) with high molar mass ranging from 15,000 to 1,000,000 g mol-1 and form colloidal solution in water.
Every biological reaction requires a different kind of enzymes, Since there is a large number of such biological reactions, therefore, there is a large number of enzymes, functioning in a living systems. A typical cell, on the average, contains about 3000 different kinds of enzymes, each catalyzing a different reaction.
Q – 5.14. How are colloids classified on the basis of (i) Physical states of the components (ii) interaction between dispersed phase and dispersion medium.

(i)

Ans. Based on physical state of dispersed phase and dispersion medium.
Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal systems are possible. A gas mixed with another gas forms a homogeneous mixture and not a colloidal system.
Based on nature of interaction between dispersed phase dispersion medium.
On this basis, colloidal sols are divided into two categories, namely, lyophilic and lyophobic. If water is the dispersion medium, the terms used are hydrophilic and hydrophobic.
Q – 5.15. Explain what is observed (i) when a beam of light is passed through a colloidal solution (ii) an electrolyte, NaCl is added to ferric hydroxide sol (or hydrated ferric oxide sol in water) (iii) electric current is passed through a colloidal sol.
Ans. (i) Scattering of light by the colloidal particles takes place of light becomes visible (Tyndall effect).
(ii) The positively charged colloidal particles of Fe(OH)3 get coagulated by the oppositely charged Ci- ions provided by NaCl.
On passing an electric current, colloidal particles move towards the oppositely charged electrode where they lose their charge and get coagulated.
Q – 5.16. What are emulsion? What are their different types? Give one example of each type.
Ans. An emulsion is a colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids.
Types of Emulsions. If the term “oil” is used any liquid which is immiscible with water and is capable of forming an emulsion with water.
Emulsion of oil-in-water(o/w) in which oil is the dispersed phase and water is the dispersion medium.
For example, milk is an emulsion of liquid fat dispersed in water. Another well known example is that of vanishing cream.
Emulsion of water-in-oil (w/o) in which water is the dispersed phase and oil is the dispersion medium.
Q – 5.17. What is demulsification? Name two demulsifies.
Ans. The process of separation of the constituent liquids of an emulsion is called demulsification. Demulsification can be done by centrifuging or boiling.
Q – 5.18. Give four examples of heterogeneous catalysis.
Ans. (i) Manufacture of ammonia from N2 and H2 by using iron as catalyst (Habes’s process)
chemistry chapter 5 ex 2 ques 18
Here, reactants are gaseous whereas catalyst is solid.
Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of Cu, ZnO and Cr2O3, as catalyst.
chemistry chapter 5 ex 2 ques 18.1
Here, again reactants are gaseous and Catalyst is a mixture of solids
Manufacture of sulphuric acid by the oxidation of SO2 to SO3 using platinized asbestos or V2O5 as catalyst (contact process)
chemistry chapter 5 ex 2 ques 18.2
In the manufacture of nitric acid, oxidation of ammonia to nitric oxide using platinum as catalyst (Ostwald process).
Q – 5.19. what do you understand by activity and selectivity of catalysts?
Ans. (1) Activity – by activity of the catalyst we mean its capacity to increase the speed of the chemical reaction. In certain cases, the extent to speed of the reaction is increased is as high as 1010 times. Combination of H2 and O2 in the presence of platinum (catalyst) to form water with explosive violence is an excellent example of catalytic activity. In the absence of the catalyst, platinum, H2 and O2 do not combine and can be stored as such for an indefinite period.
chemistry chapter 5 ex 2 ques 19
The activity depends upon the extent of chemisorption. The adsorption should be reasonably strong but not so strong that the adsorbed molecules become immobile and no space is available for other reactants to get adsorbed.
(2) Selectivity – By selectivity of a catalyst we mean its ability to direct the reaction to form particular products excluding others. For example,
(i) CO and H2 react to from different products in presence of different catalysts as follows:
chemistry chapter 5 ex 2 ques 19.1
Q – 5.20. Describe some features of catalysis by zeolites.
Ans. Features of catalysis by zeolites. (i) Zeolites are hydrated alumino-silicates which have a three-dimensional network structure containing water molecules in their pores.
(ii) To use them as catalysts, they are heated so that water of hydration present in the pores is lost and the pores become vacant.
(iii) The size of the pores varies from 260 to 740 pm. Thus, only those molecules can be adsorbed in these pores and catalyzed whose size is small enough to enter these pores. Hence, they act as molecular sieves or shape selective catalysts.
An important catalyst used in petroleum industry is ZSM-5 (Zeolite sieve of molecular porority 5). It converts alcohols into petrol by first dehydrating them to from a mixture of hydrocarbons.
chemistry chapter 5 ex 2 ques 20
Q.5.21. What are shape selective catalysis ?
Ans. Catlaysis by zeolites is called shape selective catalysis. For defer to, if a catalyst increases (accelerates) the speed of a reaction, it is called a positive catalyst and the phenomenon is called positive catalysis. On the other hand, if a catalyst decreases (retards) the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalysis.
Q.5.22. Give four uses of emulsions.
Ans. Applications of emulsions. Emulsions are useful in a number of ways. A few of the applications are given below :
(i) In the metallurgical processes , the concentration of ore by froth floatation process is bases upon the treatment of the powdered ore with oil emulsion. The valuable particles of the ore from based which comes to the surface and is skimmed off.
(ii) Asphalt emulsified in water is used for building roads without the necessity of melting the asphalt.
(iii) Milk which is an important constituent of our diet is an emulsion of liquid fats in water.
(iv) Several oily drugs are prepared in the form of emulsions.
(v) Certain disinfectants such as Dettol and Lysol give emulsions of oil-in-water type on mixing with water.
(vi) The cleansing action of ordinary soap for washing clothes, crockery, etc. is based upon the formation of oil-in-water emulsion.
(vii) The digestion of fats in the intestines takes place by the process of emulsification. A small amount of the fat reacts with alkaline solution present in the intestines to form a sodium soap. This soap causes the emulsification of the rest of the fat thereby making the function of the digestive enzymes easier in carrying out the metabolic processes.
Q.5.23. What are micelles ? Give an example of a miceller system.
Ans. Micelles. The substances which when dissolved in a medium at low concentrations behave as normal, strong electrolytes but at higher concentrations exhibit colloidal state properties due to the formation of aggregated particles are called associated colloids. The aggregated particles thus formed are called micelles.
Q.5.24. Explain the following terms with suitable examples (i) Aerosol (ii) Aerosol and (iii) Hydrosol
Ans. (i) Alcosol. It is a colloidal dispersion having alcohol as the dispersion medium, e.g., collodion (a colloidal sol of cellulose nitrate in ethyl alcohol). (ii) Aerosol. It is a colloidal dispersion of a liquid in a gas, e.g., fog.
(iii) Hydrosol. It is a colloidal sol of a solid in water as the dispersion medium, e.g., starch sol or gold sol.
Q.5.25. Comment on the statement that ‘’Colloid is not a substance but state of a substance.’’
Ans. The given statement is true. This is because the same substance may exist as a colloid under certain conditions and as a crystalloid under certain other conditions. For example, Na Cl in water behaves as a crystalloid while in benzene, it behaves as a colloid. Similarly, dilute soap solution behave like a crystalloid while concentrated solution behaves as a colloid (called associated colloid). It is the sizeof the particles which matters, i.e., the state in which the substance exists. If the size of the particles lies in the range 1 nm to 1000 nm, it is in the colloidal state.
Updated: October 2, 2020 — 5:51 pm

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