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(1) Enzymes are complex nitrogenous substances these are actually protein molecules of higher molecular mass. (2) Enzymes catalyse numerous reactions, especially those connected with natural processes.   (3) Numerous reactions occur in the bodies of animals and plants to maintain the life process. These reactions are catalysed by enzymes. The enzymes are thus, termed as bio-chemical catalysts and the phenomenon is known as bio-chemical catalysis. (4) Nitrogenase an enzyme present in bacteria on the root nodules of leguminous plants such as peas and beans, catalyses the conversion of atmospheric \[{{N}_{2}}\] to \[N{{H}_{3}}\]. (5) In the human body, the enzyme carbonic anhydrase catalyses the reaction of \[C{{O}_{2}}\] with \[{{H}_{2}}O\],      \[C{{O}_{2}}(aq)+{{H}_{2}}O(l)\]\[{{H}^{+}}(aq.)+HCO_{3}^{-}(aq.)\] The forward reaction occurs when the blood takes up \[C{{O}_{2}}\]in the tissues, and the reverse reaction occurs when the blood releases \[C{{O}_{2}}\] in lungs. (6) In manufacturing of ethyl alcohol (i) \[{{C}_{12}}{{H}_{22}}{{O}_{11}}(l)+{{H}_{2}}O(l)\xrightarrow{\text{Invertase}}\]\[\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,+\underset{\text{Fructose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,\]     \[{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)\xrightarrow{\text{Zymase}}2{{C}_{2}}{{H}_{5}}OH(l)+2C{{O}_{2}}(l)\] (ii) Starch (l)\[\xrightarrow{\text{Diastase}}\text{Maltose }(l)\]  Maltose \[\xrightarrow{\text{Maltase}}\text{Glucose}\xrightarrow{\text{Zyamase}}\text{Alcohol}\]

The following are the characteristics which are common to must of catalytic reactions. (1) A catalyst remains unchanged in mass and chemical composition at the end of the reaction. (2) A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions (i) For example, in the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyses \[{{10}^{8}}\]litres of hydrogen peroxide. (ii) In Friedel craft’s reaction, anhydrous aluminium chloride is required in relatively large amount to the extent of 30% of the mass of benzene,  \[{{C}_{6}}{{H}_{6}}+{{C}_{2}}{{H}_{5}}Cl\xrightarrow{AlC{{l}_{3}}}{{C}_{6}}{{H}_{5}}{{C}_{2}}{{H}_{5}}+HCl\] (3) The catalyst can not initiate the reaction: The function of the catalyst is to alter the speed of the reaction rather than to start it. (4) The catalyst is generally specific in nature: A substance, which acts as a catalyst for a particular reaction, fails to catalyse the other reaction , different catalysts for the same reactant may for different products. Examples :              (5) The catalyst can not change the position of equilibrium : The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant. (6) Catalytic promoters : Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators. (i) For example, in Haber’s process for the synthesis of ammonia, traces of molybdenum increases the activity of finely divided iron which acts as a catalyst. (ii) In the manufacture of methyl alcohol from water gas \[(CO+{{H}_{2}})\], chromic oxide \[(C{{r}_{2}}{{O}_{3}})\] is used as a promoter with the catalyst zinc oxide \[(ZnO)\]. (7) Catalytic poisons : Substances which destroy the activity of the catalyst  by their presence are known as catalytic poisons. (i) For example, the presence of traces of arsenious oxide \[(A{{s}_{2}}{{O}_{3}})\] in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid. (ii) The activity of iron catalyst is destroyed by the presence of \[{{H}_{2}}S\] or \[CO\] in the synthesis of ammonia by Haber’s process. (iii) The platinum catalyst used in the oxidation of hydrogen is poisoned by \[CO\]. (8) Change of temperature alters the rate of catalytic reaction as it does for the same reaction in absence of catalyst : By increasing the temperature, there is an increase in the catalytic power of a catalyst but after a certain temperature its power begins to decrease. A catalyst has thus, a particular temperature at which its catalytic activity is maximum. This temperature is termed as optimum temperature. (9) A positive catalyst lowers the activation energy (i) According to the collision theory, a reaction occurs on account of effective collisions between the reacting molecules. (ii) For effective collision, it is necessary that the molecules must possess a minimum amount of energy known as activation energy (Ea). (iii) After the collision molecules form an activated complex which dissociate more...

Catalytic reactions can be broadly divided into the following types, (1) Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis. (i) In the lead chamber process \[2S{{O}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{NO(g)}2S{{O}_{3}}(g)\]                                                 (ii) In the hydrolysis of ester                                           \[C{{H}_{3}}COOC{{H}_{3}}(l)+{{H}_{2}}O(l)\xrightarrow{HCl(l)}\]                                                    \[C{{H}_{3}}COOH(l)+C{{H}_{3}}OH(l)\] (iii) In the hydrolysis of sugar \[\underset{\text{(Sucrose solution)}}{\mathop{{{C}_{12}}{{H}_{22}}{{O}_{11}}(l)}}\,+{{H}_{2}}O(l)\xrightarrow{{{H}_{2}}S{{O}_{4}}(l)}\]                                                \[\underset{\text{(Glucose solution)}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,+\underset{\text{(Fructose solution)}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,\] (2) Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below. (i) In contact process for \[{{H}_{2}}S{{O}_{4}}\] \[2S{{O}_{2}}(g)+{{O}_{2}}(g)\underset{Br{{V}_{2}}{{O}_{5}}}{\mathop{\xrightarrow{Pt(s)}}}\,2S{{O}_{3}}(g)\] (ii) In Haber’s process for \[N{{H}_{3}}\] \[{{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow{Fe(s)}2N{{H}_{3}}(g)\]   (iii) In Ostwald’s process for \[HN{{O}_{3}}\] \[4N{{H}_{3}}(g)+5{{O}_{2}}(g)\xrightarrow{Pt(s)}4NO(g)+6{{H}_{2}}O(g)\]  (3) Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below. (i) Decomposition of \[KCl{{O}_{3}}\] \[2KCl{{O}_{3}}(s)\underset{{{270}^{o}}C}{\mathop{\xrightarrow{Mn{{O}_{2}}(s)}}}\,2KCl(s)+3{{O}_{2}}(g)\] (ii) Oxidation of \[S{{O}_{2}}\] \[2S{{O}_{2}}(g)+{{O}_{2}}(g)\underset{or\,Pt(s)}{\mathop{\xrightarrow{{{V}_{2}}{{O}_{5}}(s)}}}\,2S{{O}_{3}}(g)\] (iii) Decon’s process \[4HCl(g)+{{O}_{2}}(g)\underset{{{450}^{o}}C}{\mathop{\xrightarrow{CuC{{l}_{2}}(s)}}}\,2C{{l}_{2}}(g)+2{{H}_{2}}O(g)\]                       (4) Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as follows. (i) Oxidation of sodium sulphite \[2N{{a}_{2}}S{{O}_{3}}(s)+{{O}_{2}}(g)\xrightarrow{Alcohol(l)}2N{{a}_{2}}S{{O}_{4}}(s)\]                                            (ii) Oxidation of benzaldehyde \[2{{C}_{6}}{{H}_{5}}CHO(l)+{{O}_{2}}(g)\underset{amine(l)}{\mathop{\xrightarrow{Diphenyl}}}\,2{{C}_{6}}{{H}_{5}}COOH(l)\]  (iii) Tetra ethyl lead (TEL) is added to petrol to retard the ignition of petrol vapours on compression in an internal combustion engine and thus minimise the knocking effect. (5) Auto-catalysis : In certain reactions, one of the product acts as a catalyst. In the initial stages the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis. Some examples are as follows, (i) The rate of oxidation of oxalic acid by acidified potassium permanganate increases as the reaction progresses. This acceleration is due to the presence of \[M{{n}^{2+}}\] ions which are formed during reaction. Thus \[M{{n}^{2+}}\]ions act as auto-catalyst. \[5{{H}_{2}}{{C}_{2}}{{O}_{4}}+2KMn{{O}_{4}}+3{{H}_{2}}S{{O}_{4}}\to 2MnS{{O}_{4}}+{{K}_{2}}S{{O}_{4}}\]                                                                 \[+10C{{O}_{2}}+8{{H}_{2}}O\] (ii) When nitric acid is poured on copper, the reaction is very slow in the beginning, gradually the reaction becomes faster due to the formation of nitrous acid during the reaction which acts as an auto-catalyst. (6) Induced catalysis : When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis. Some examples are as follows,   (i) Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. The oxidation of sodium sulphite, thus, induces the oxidation of sodium arsenite. (ii) The reduction of mercuric chloride \[(HgC{{l}_{2}})\]with oxalic acid is very slow, but potassium permanganate is reduced readily with oxalic acid. If, however, oxalic acid is added to a mixture of potassium permanganate more...

“Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.” Berzelius (1836) introduced the term catalysis and catalyst. Ostwald (1895) redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as catalysis.”   

The phenomenon of adsorption finds a number of applications. Important applications are given as follows. (1) Production of high vacuum (2) In Gas masks : This apparatus is used to adsorb poisonous gases (e.g. \[C{{l}_{2}},\,CO,\,\]oxide of sulphur etc.) and thus purify the air for breathing. (3) For desiccation or dehumidification : These substances can be used to reduce/remove water vapours or moisture present in the air. Silica gel and alumina are used for dehumidification in electronic equipment. (4) Removel of colouring matter from solution : (i) Animal charcoal removes colours of solutions by adsorbing coloured impurities. (ii) Animal charcoal is used as decolouriser in the manufacture of cane sugar. (5) Heterogeneous catalysis : Mostly heterogeneous catalytic reactions proceed through the adsorption of gaseous reactants on solid catalyst. For example, (i) Finely powdered nickel is used for the hydrogenation of oils. (ii) Finely divided vanadium pentaoxide \[({{V}_{2}}{{O}_{5}})\] is used in the contact process for the manufacture of sulphuric acid. (6) Separation of inert gases : Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low temperatures. (7) Softening of hard water (i) The hard water is made to pass through a column packed with zeolite (sodium aluminium silicate)               (ii) Ca­­­++, Mg++ ions which are responsible for hardness, get adsorbed on zeolite, exchanging sodium ions. \[N{{a}_{2}}A{{l}_{2}}S{{i}_{2}}{{O}_{8}}+CaC{{l}_{2}}\xrightarrow{{}}CaA{{l}_{2}}S{{i}_{2}}{{O}_{8}}+2NaCl\] (iii) The exhausted zeolite is regenerated with 10% of sodium chloride solution. \[CaA{{l}_{2}}S{{i}_{2}}{{O}_{8}}+2NaCl\xrightarrow{{}}N{{a}_{2}}A{{l}_{2}}S{{i}_{2}}{{O}_{8}}+CaC{{l}_{2}}\] (8) De-ionisation of water (i) Water can be de-ionised by removing all dissolved salts with the help of cation and anion-exchanger resin. (ii) Cation-exchanger is an organic synthetic resin such as polystyrene-containing a macroanion \[(R-SO_{3}^{-}\,\text{etc}.)\]which has adsorbed H+ ions.   (iii) A resin containing a basic group \[({{R}_{3}}N_{{}}^{+}\,\text{etc}\text{.})\] which has adsorbed \[O{{H}^{-}}\] ions acts as anion exchanger. (9) In curing diseases : A number of drugs are adsorbed  on the germs and kill them or these are adsorbed on the tissues and heat them. (10) Cleaning agents : Soap and detergents get adsorbed on the interface and thus reduce the surface tension between dirt and cloth, subsequently the dirt is removed from the cloth. (11) Froth floatation process   A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method. (12) In adsorption indicators Surface of certain precipitates such as silver halide, have the property of adsorbing some dyes like eosin, fluorescein etc. (13) Chromatographic analysis The phenomenon of adsorption has given an excellent technique of analysis known as chromatographic analysis. (14) In dyeing : Many dyes get adsorbed on the cloth either directly or by the use of mordants.

(1) The process of adsorption can take place from solutions also. (2) In any solution, there are two (or more) components ; solute and solvent. The solute may be present in the molecular or ionic form.                 (3) The extent of adsorption from solution depends upon the concentration of the solute in the solution, and can be expressed by the Freundlich isotherm. (4) The Freundlich adsorption isotherm for the adsorption from solution is, \[\frac{x}{m}=k{{c}^{\frac{1}{n}}}\] where, x is the mass of the solute adsorbed, m is the mass of the solid adsorbent, c is the equilibrium concentration of the solute in the solution, n is a  constant having value greater than one, k is the proportionality constant, (The value of k depends upon the nature of solid, its particle size, temperature, and the nature of solute and solvent etc.) (5)  The plot of x/m  against c is similar to that Freundlich adsorption isotherm. The above equations may be written in the following form, \[\log \frac{x}{m}=\log k+\frac{1}{n}\log c\] where c, is the equilibrium concentration of the solute in the solution.

A mathematical equation, which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature, is called adsorption isotherms. The extent of adsorption is expressed as mass of the adsorbate adsorbed on one unit mass of the adsorbent. Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent, then \[\text{Extent}\,\text{of}\,\text{adsorption}\,=\frac{x}{m}\] Various adsorption isotherms are commonly employed in describing the adsorption data. (1) Freundlich adsorption isotherm (i) Freundlich adsorption isotherm is obeyed by the adsorptions where the adsorbate forms a monomolecular layer on the surface of the adsorbent. \[\frac{x}{m}=k{{p}^{\frac{1}{n}}}\] (Freundlich adsorption isotherm) or \[\log \frac{x}{m}=\log k+\frac{1}{n}\log p\] where x is the weight of the gas adsorbed by m gm  of the adsorbent at a pressure p, thus x/m represents the amount of gas adsorbed by the adsorbents per gm  (unit mass), k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure. (ii) At low pressure, the extent of adsorption varies linearly with pressure. \[\frac{x}{m}\propto p'\] (iii) At high pressure, it becomes independent of pressure. \[\frac{x}{m}\propto {{p}^{0}}\] (iv) At moderate pressure \[\frac{x}{m}\] depends upon pressure raised to powers \[\frac{x}{m}\propto {{p}^{\frac{1}{n}}}\]   (2) The Langmuir - adsorption isotherms  (i) One of the drawbacks of Freundlich adsorption isotherm is that it fails at high pressure of the gas. Irving Langmuir in 1916 derived a simple adsorption isotherm, on theoretical considerations based on kinetic theory of gases. This is named as Langmuir adsorption isotherm. (a) Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed gas. (b) Adsorption consists of two opposing processes, namely Condensation of the gas molecules on the solid surface and  Evaporation (desorption) of the gas molecules from the surface back into the gaseous phase.             (c) The rate of condensation depends upon the uncovered (bare) surface of the adsorbent available for condensation. Naturally, at start when whole of the surface is uncovered the rate of condensation is very high and as the surface is covered more and more, the rate of condensation progressively decreases. On the contrary, the rate of evaporation depends upon the covered surface and hence increases as more and more of the surface is covered ultimately an equilibrium will be set up at a stage when the rate of condensation becomes equal to the rate of evaporation (adsorption equilibrium). (d) The rate of condensation also depends upon the pressure of the gas since according the kinetic theory of gases, the number of molecules striking per unit area is proportional to the pressure. Mathematically, \[\frac{x}{m}=\frac{ap}{1+bp}\], where a and b are constants and their value depends upon the nature of gas (adsorbate), nature of the solid adsorbent and the temperature. Their more...

  Adsorption can be classified into two categories as described below, (1) Depending upon the concentration :  In adsorption the concentration of one substance is different at the surface of the other substance as compared to adjoining bulk or interior phase. (i) Positive adsorption : If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk phase then it is called positive adsorption. Example : When a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption. (ii) Negative adsorption : If the concentration of the adsorbate is less than its concentration in the bulk then it is called negative adsorption. Example : When a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption. (2) Depending upon the nature of force existing between adsorbate molecule and adsorbent (i) Physical adsorption : If the forces of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or Vander Waal’s adsorption. It can be easily reversed by heating or decreasing the pressure. (ii) Chemical adsorption : If the forces of attraction existing between adsorbate particles and adsorbent are almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also called as chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed.   Comparison between physisorption and chemisorption

Physisorption  (Vander Waal's adsorption)	Chemisorption  (Langmuir adsorption)
Low heat of adsorption usually in range of 20-40 kJ/mol	High heat of adsorption in the range of 50-400 kJ/mol
Force of attraction are Vander Waal's forces.	Forces of attraction are chemical bond forces.
It is reversible	It is irreversible
It is usually takes place at low temperature and decreases with increasing temperature.	It takes place at high temperature.
It is related to the case of liquefication of the gas.	It is not related.
It forms multimolecular layers.	It forms monomolecular layers.
It does not require any activation energy.	It requires high activation energy.
High pressure is favourable. Decrease of pressure causes desorption	High pressure is favourable. Decrease of pressure does not cause desorption.
It is not very specific.	It is highly specific.

  Factors which affect the extent of adsorption : The following are the factors which affect the adsorption, (1) more...

(1) Definition : The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in to higher concentration of the molecules on the surface is called adsorption. (2) Causes of adsorption : Unbalanced forces of attraction or free valencies which is present at the solid or liquid surface, have the property to attract and retain  the molecules of a gas or a dissolved substance on to their surfaces with which they come in contact. Example : Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water, Some basic terms used in adsorption

Interface : Any surface is a plane which separates any two phases in contact with each other. The plane which separates any two phase is generally called an interface between the two phases.	Adsorbate and Adsorbent : The substance which gets adsorbed on any surface is called adsorbate for example, if a gas gets adsorbed on to the surface of a solid, then the gas is termed as the adsorbate. The substance on the surface of which adsorption takes place is called adsorbent.
Desorption :  The removal of the adsorbed substance from a surface is called desorption.	Absorption :  When the molecules of a substance are uniformly distributed throughout the body of a solid or liquid. This phenomenon is called absorption.
Sorption :  The phenomenon in which adsorption and absorption occur simultaneously is called sorption. Mc. Bain introduced a general term sorption describeing both the processes, however adsorption is instantaneous i.e. a fast process while absorption is a slow process.	Occlusion :  When adsorption of gases occur on the surface of metals this is called occlusion.

   (3) Difference between adsorption and absorption

Adsorption	Absorption
It is a surface phenomenon.	It concerns with the whole mass of the absorbent.
In it, the substance is only retained on the surface and does not go into the bulk or interior of the solid or liquid.	It implies that a substance is uniformly distributed, through the body of the solid or liquid.
In it the concentration of the adsorbed molecules is always greater at the free phase.	In it the concentration is low.
It is rapid in the beginning and slows down near the equilibrium.	It occurs at the uniform rate.
Examples : (i) Water vapours adsorbed by silica gel. (ii) NH3 is adsorbed by charcoal.	Examples : (i) Water vapours absorbed by anhydrous CaCl2 (ii) NH3 is absorbed in water forming NH4OH

  (4) Surface forces : Only the surface more...

One and the same substance may act simultaneously as an oxidising agent and as a reducing agent with the result that a part of it gets oxidised to a higher state and rest of it is reduced to lower state of oxidation. Such a reaction, in which a substance undergoes simultaneous oxidation and reduction is called disproportionation and the substance is said to disproportionate.            Following are the some examples of disproportionation,