Q 5.1 Distinguish between the meaning of the meaning of the terms adsorption and absorption. Give one example of each.
Q 5.2 What is the difference between physical adsorption and chemisorption?
Q 5.4 What are the factors which influence the adsorption of a gas on a solid?
Ans. Following factors affect adsorption of a gas on a solid: -
(i) Nature and Surface area of the adsorbent: - Greater the surface area of the adsorbent, greater is the volume of the gas adsorbed.
(ii) Nature of the gas being adsorbed: - Higher the critical temperature of a gas, greater is the amount of that gas adsorbed ( a gas which is more easily liquefiable or is more soluble in water is more readily adsorbed).
(iii) Temperature: - Adsorption decreases with increase in temperature and vice versa.
(iv) Pressure: - At constant temperature, the adsorption of a gas increases with increase of pressure.
(v) Activation of the solid adsorbent: - Increasing the surface area of the adsorbent. It can be done by following ways;
a. By making the surface of the adsorbent rough: by mechanical rubbing or by chemical action
b. By subdividing the adsorbent into smaller pieces or grains
c. 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.
Freundlich adsorption isotherm:
Q 5.6 What do you understand by activation of adsorbent? How is it achieved?
Ans.
Q 5.6 What do you understand by activation of adsorbent? How is it achieved?
Ans. Activation means increasing the adsorbing power of an adsorbent. This is done by increasing 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: 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: 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.
c. By removing the gases already adsorbed: charcoal is activated by heating in superheated steam or in vacuum at a temperature between 623K to 1273K.
Q 5.9 How are the colloidal solutions classified, on the basis of the physical states of the dispersed phase and dispersion medium?
Ans. 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 homogenous mixture and not a colloidal system.
The most common are sols (solids in liquids), gels (liquids in solids) and emulsions (liquids in liquids).
Dispersion medium ...............Nature of the sol
Water ....................................Aquasol or Hydrosol
Alcohol...................................Alcosol
Benzene.................................Benzosol
Gases.....................................Aersool
Q 5.10 Discuss the effect of pressure and temperature on the adsorption of gases by solids.
Ans. Effect of Temperature: - Adsorption decreases with increase of temperature and vice versa. Adsorption involves true equilibrium. By applying Le Chatelier's principle it can be seen that increase of temperature decreases the adsorption.
Gas (adsorbate) + Solid (Adsorbent) ⇌ Gas adsorbed on solid + heat
Effect of Pressure: - At constant temperature, the adsorption of a gas increases with increase of pressure. At low temperature, the adsorption of a gas increase very rapidly as the pressure is increased from small values.
Q 5.13 What are enzymes? Write in brief mechanism of enzyme catalysis.
Ans. Enzymes are bio-chemical catalysts. Chemically all enzymes are globular proteins.
The most accepted mechanism of enzyme catalysed reaction is known as Lock and Key mechanism. Presence of -NH2, -COOH, -OH, -SH etc. groups create active sites on Enzymes to make them specific in their action. Shape of active site is like a cavity such that only a specific substrate can fit into it like Lock Key. Enzyme catalysed reactions take place in two steps as follows:
Step 1: Formation of enzyme-substrate complex: E (enzyme) +S (substrate) ⇌ ES (Enzyme-substrate complex)
Step 2: Dissociation of enzyme-substrate complex to form products: ES → [EP] → E + P (Product)
[EP] is (Enzyme Product dissociation)
Q 5.14 How are colloids classified on the basis of
(i) physical states of the components
(ii) nature of the dispersion medium
(iii) interaction between dispersed phase and dispersion medium.
Ans. Colloids are classified in three different ways on the basis of :
(i) Physical state of dispersed phase and dispersion medium: - Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight type of colloidal systems are possible. A gas mixed with another gas forms a homogenous mixture and not a colloidal system.
(ii) Based on nature of interactions between dispersed phase and 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.
(iii) Based on the type of particles of the dispersed phase (Multimolecular, Macromolecular, and associated colloids): - Depending upon how the different substances may have size in the range of the colloids the various types of colloids or colloidal dispersions may be divided into the following three categories.
Q 5.16 What are emulsions? What are their different types? Give one example of each type.
Ans. Emulsion: It is a colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids. (The two liquids involved are otherwise immiscible).
Type of emulsions: if the term oil is used for any liquid which is immicible with water and is capable of forming an emulsion with water, then the various types of emulsions may be classified into two types.
i. Emulsion of oil in water (o/w) in which oil is the dispersed phase and water is the dispersion medium. Milk is an emulsion of liquid fat dispersed in water. Another well known example is that of vanishing cream.
ii Emulsion of water in oil (w/o) in which water is dispersed phase and oil is dispersion medium. Cod liver oil is an emulsion of water in oil i.e. water is the dispersed phase and oil is the dispersion medium. Two more common examples of this butter and cream.
Q 5.18 Action of soap is due to emulsification and micelle formation. Comment.
Ans. Soaps are sodium or potassium salts of higher fatty acids e.g. sodium palmitate (C15H32COONa), and sodium stearate (C17H35COONa). A molecule of soap consists of two parts, the hydrocarbon part (C15H31 - and C17H35 -) which is soluble in oil and the polar group (COO- Na+) which is soluble in water
Soap is RCOO- Na+ and in soap alkyl chain i.e. R is Hydrophobic part (soluble in oil)and carboxylic part COO- Na+ is Hydrophilic part (soluble in water)
If a drop of oil is surrounded by soap solution, the R- part of the soap remains in the oil and the COO- Na+ part remains in water. Soap molecules are concentrated over the surface of the drop of oil. As a result, the interfacial tension between oil and water decreases and hence they are intermixed into each other to form the emulsion. Thus, micelles, are formed.
Q 5.19 Give four examples of heterogenous catalysis.
Ans. Heterogeneous Catalysis: If the catalyst is present in a different phase than that of the reactants, it is called a heterogeneous catalyst and this type of catalysis is called heterogenous catalyst.
The catalyst in heterogenous catalysis is generally solid and the reactants are mostly gases and sometimes liquid. In heterogenous catalysis, the reaction starts at the surface of the solid catalyst. That is why it is also known as surface catalysis.
(i) Manufacture of ammonia from N2 and H2 by Haber's process using iron as catalyst
N2(g) + 3H2 (g) → Fe(s)→ 2NH3 (g)
Here, reactants are gaseous whereas catalyst is solid.
(ii) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of Cu, ZnO and Cr2O3 as catalyst.
CO(g) + 2H2(g) → Cu(s),ZnO (s),Cr2O3 (s)→ CH3OH
Here, reactants are gaseous and catalyst is a mixture solids.
(iii) Manufacture of sulphuric acid by oxidation of SO2 to SO3 using platinised asbestos or V2O5 as catalyst.
SO2(g) + O2(g) → Pt(s)→ 2SO3(g)
(iv) Oxidation of CO and unburnt petrol (hydrocarbons) to CO2 and reduction of NO to N2 in the automobile exhaust gases by the catalytic converter (consisting of transition metals and their oxides) fitted in the automobile exhaust system.
2CO + O2 → Catalyst→ 2CO2
2NO → Catalyst→ N2 + O2
Hydrocarbons (unburnt petrol) → Catalyst (O2)→ CO2 + H2O
Q 5.20 What do you understand by activity and selectivity of catalysis?
Ans.
Q 5.22 What is shape selective catalysis?
Ans. Catalysis by zeolites is called shape selective catalysis.
Q 5.23 Explain the following terms:
(i) Electrophoresis
(ii) Coagulation
(iii) Dialysis
(iv) Tyndall effect
Ans.
i. Electrophoresis or Cataphoresis: The existence of the electrical charge (positive or negative) can be shown by the process of 'Electrophoresis' also called 'Cataphoresis' which involves the movement of colloidal particles towards one or the other electrode when placed under the influence of an electric field.
ii. Coagulation:
Coagulation or Flocculation or precipitation is a process of aggregating together the colloidal particles so as to change them into large sized particles which ultimately settle as a precipitate. Coagulation is brought about by the addition of electrolytes. When an electrolyte is added to a colloidal solution, the particles of the sol take up the ions which are oppositely charged and thus get neutralised. The ion responsible for neutralisation of charge on the colloidal particles is called the coagulating ion or flocculating ion. The neutral particles then start accumulating to form particles of larger size which settle down.
iii. Dialysis: - The process of separating the particles of colloids from those of crystalloids by diffusion of the mixture through a parchment or animal membrane is known as dialysis. The separation of crystalloids from the colloids is based upon the principle that the particles of the crystalloids pass through parchment paper or cellophane membrane whereas those of the colloids do not. The apparatus used is called a dialyser. A cellophane sheet or parchment paper is turned into a bag with a funnel tied in the mouth of the bag for addition of the impure sol. The impure sol is filled into the bag which is then suspended into a vessel containing distilled water. After some time, all the crystalloids in solution pass out leaving the colloidal sol behind in the bag. The distilled water is renewed frequently to avoid accumulation of the crystalloids as otherwise they may start diffusing back into the bag. This process quickened if an electric field is applied around the membrane (then the process called Electro-dialysis). The most important application of dialysis is in the purification of blood in the artificial kidney machine. Dialysis membrane permits small particles of the excess ions and waste products to pass through whereas colloid - sized particles such as haemoglobin do not pass through the membrane.
Electro - dialyser
iv. Tyndal Effect: - If a strong converging beam of light is passed through a colloidal solution placed in a dark room, the path of beam gets illuminated with a bluish light when viewed at right angles to the direction of the passage of light. The path of the light becomes visible due to scattering of light by the colloidal particles. The phenomenon was observed by Tyndall in 1869 and is called Tyndall effect. Illuminated path of beam is called Tyndall cone. The phenomenon is also observed when a beam of light is projected in a cinema hall and it becomes visible due to the scattering by colloidal dust particles in the air of the room.
Q 5.24 Give four uses of emulsions.
Ans. Use of emulsions:
a. In the metallurgical processes, the concentration of ore by froth floatation process is based upon the treatment of the powdered ore with oil emulsion. The valuable particles of the ore form foam which comes to the surface and is skimmed off.
b. Asphalt emulsified in water is used for building roads without the necessity of melting the asphalt.
c. Milk which is an important constituent of our diet is an emulsion of liquid fats in water.
d. Several oily drugs are prepared in the form of emulsions.
e. Cleansing action of ordinary soap for washing clothes, crockery, etc. is based upon the formation of oil-in-water emulsion.
Q 5.25 What are micelles? Give an example of a micellar system.
Ans. 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. The formation of micelles takes place only above a particular temperature called Kraft Temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC).
The most common example of associated colloids is that of surface active agents such as soaps and synthetic detergents. For soaps, the CMC is 10-4 to 10-3 mol/L. Each micelle contains at least 100 molecules.
