1.Adsorption is a surface phenomenon.Explain?

Whenever, any two phases come in contact, they create an interface. The constituent particles of either of the two phases tend to accumulate at the interface. This leads to a relatively higher concentration of one of the phases at the interface. This is called adsorption. Thus, adsorption may be defined as follows:

"The process in which molecular,  atomic or ionic species of one substance get accumulated at the surface of another is called adsorption."

Thus, as a result of adsorption, the concentration of the adsorbed substance is more at the surface ( or interface) than in the bulk.

2.Explain the terms (i) adsorbate (ii) adsorbent  (iii) desorption (iv) absorption (v) sorption.

(i)Adsorbate :  The substance which gets adsorbed on any surface is called adsorbate. For example, if a gas gets adsorbed onto the surface of a solid, then the gas is termed as the adsorbate.

(ii)Adsorbent  : The substance on the surface of which adsorption takes place is called adsorbent. Adsorbent may be a solid or a liquid. Metal powders, powdered charcoal, animal charcoal, silica powder,etc are the commonly used adsorbents. Activated charcoal, silica ( precipitated), alumina etc., are very effective adsorbents due to their large surface area.

(iii) Desorption  : The removal of the adsorbed substance from a surface is called desorption. Heating and lowering of pressure favour desorption.

(iv) Absorption   : When the molecules of a substance penetrate uniformly into another, the phenomenon is called absorption. Absorption is not limited to the surface only.

Difference between adsorption and absorption is shown in the Figure.

(v) Sorption   :  In some cases, adsorption and absorption occur simultaneously and it is often not very easy to determine how much of a substance is absorbed, and how much is adsorbed. It is for this reason that a non- commital term sorption is frequently used in such cases.

3.Give three differences below absorption and adsorption?

4.Explain types of adsorptions.

Depending upon the nature of forces acting between the adsorbent and the adsorbate, adsorption process can be classified into the following types.

(a) Physical adsorption           (b) Chemical adsorption

(a) Physical adsorption : If the adsorbate is held on a surface by the weak forces such as, van der Waal's type, then the adsorption is called van der Waals' adsorption, or physical adsorption.

Examples   :  (i) Adsorption of gases on animal charcoal is a physical adsorption.

                     (ii) Adsorption of water vapour on silica gel is a physical adsorption.

(b) Chemical adsorption  : When the adsorbate is held to the surface of an adsorbent by forces nearly as strong as a chemical bond, the adsorption is called chemical adsorption.

Sometimes, the adsorbed material ( the adsorbate) slowly penetrates into the bulk of the adsorbent. This leads to the absorption of the adsorbate into the bulk of the adsorbent. Thus, adsorption is accompanied by absorption . Such a situation is represented by a general term sorption. So, where there is no clear cut case of adsorption, we generally use the terms, physisorption ( physical adsorption associated with absorption), and chemisorption ( chemical adsorption associated with absorption).

Adsorption is an exothermic process, that is heat is evolved during adsorption. The enthalpy change during the adsorption of one mole of an adsorbate on adsorbent is called enthalpy of adsorption. Enthalpy of adsorption is also termed as heat of adsorption.

The enthalpy of adsorption for physical adsorption is low, generally 20 - 40 kJ mol^-1, while that for chemical adsorption is high, generally of the order 50 - 400 kJ mol^-1.

5.What are the applications of the adsorption of gases on solids?

(i) For creating partial vacuum : The phenomenon of adsorption of gases on solids particularly at low temperature has been used for creating partial vacuum. The vessel to be evacuated is connected to a   narrow container containing activated animal charcoal dipped into liquid nitrogen. The extent of adsorption of gases on solids is very high at low temperature. So, the gases present in the vessel are adsorbed by the animal charcoal, and partial vacuum is created.

(ii) In gas masks   : Gas masks are used for preventing the inhalation of poisonous gases. The common form of gas masks contain a layer of activated charcoal. The inhaled air must pass through this layer of activated charcoal. Any poisonous gas, such as chlorine, oxides of sulphur, carbon monoxide, etc., if present in the air get adsorbed on the activated charcoal. Thus, the inhaled air is free from such toxic/poisonous gases.

(iii) In heterogeneous catalysis : Most heterogeneous catalytic reactions proceed through the adsorption of gaseous reactants on the solid catalyst. For example,

 (a) Finely powdered nickel is used for the hydrogenation of oils.

 (b)Finely divided vanadium pentoxide (V₂O₅) is used in the Contact Process for the manufacture of sulphuric acid.

 (c) Pt, Pd are used in many industrial processes as catalyst.

(iv) For desiccation or dehumidification   : Certain substances have a strong tendency, to adsorb water. These substances can be used to reduce/ remove water vapour present in the air. Silica gel is used for dehumidification in electronic equipments.

6.What are the applications of adsorption from solutions?

The phenomenon of adsorption from solutions has been used for many industrial / domestic applications. Some main application are described below.

(i) In dyeing  :   Many dyes get adsorbed on the cloth either directly or by the use of mordants.

(ii)For the removal of colouring and odouring matter:The principle of adsorption from solutions is also used for the removal of undesirable colouring and odouring matter from the solutions or liquids. Activated charcoal is used for removing colouring matter from solutions of sugar and other organic liquids such as oils etc. Fuller's earth can also be used for removing undesirable colouring/ odouring matter from liquids/solutions.

(iii) For recovery from industrial wastes/ mixtures : Components of a mixture can be separated  by making use of the differences in the adsorption power of the various components of any mixture.

Adsorption provides an efficient method for the concentration of the substances present in very dilute solutions. Several valuable recoveries are made from the industrial wastes by employing the principle of adsorption.

(iv)For softening of water  : Hard water can be softened by using ion exchange resins. The use of these ion exchange resins is based on selective adsorption from the solutions.

(v) Chromatographic analysis : The chromatographic technique is based on the principle of adsorption. This technique is used for the identification/separation of various components of a mixture/solution.

(vi) Use of adsorption indicators :  Many dyes such as, fluorescein can be used as adsorption indicator in silver nitrate precipitation titrations. At the completion of the titration,the ions produced by the dye in the solution get adsorbed on to the precipitates of silver halide, and undergo a colour change.

7.Name and explain two industrial processes in which heterogeneous catalysts are employed?

(i) Manufacture of ammonia by Haber's process   :  Combination of nitrogen and hydrogen in the presence of iron and molybdenum ( as promoter) to give ammonia,

This reaction forms the basis of the Haber's process for the manufacture of ammonia.

(ii) Manufacture of nitric acid by Ostwald's process  :  In this process, ammonia is oxidised to nitric oxide (NO) by air in the presence of Pt catalyst at 800ºC to give nitric oxide.

8.List out the general characteristics of enzyme reactions.

Some main characteristics of enzyme reactions are given below.

(i) High specificity  : Reactions catalysed by enzymes are high specific, that is, each reaction is catalysed by a particular enzyme only. For example, finely divided metals like platinum can catalyse a large number of different heterogeneous gas reactions, but an enzyme like invertase, which can break up disaccharide sucrose ( cane sugar ) into two monosaccharide molecules, glucose and fructose, is powerless to break up another closely related disaccharide maltose. For the latter purpose another enzyme maltase is required.

The enzyme urease catalyses the hydrolysis of urea.

(ii)Proportionality of the rate with concentration  :    In the case of enzyme catalysis, the rate of the reaction is much more dependent on the amount of the catalyst ( enzyme) than it is in the case of the other types of catalysis.

(iii) Optimum temperature :   Ordinary catalytic reactions, like any reaction, go faster with rise of temperature and this applies to enzyme reactions too. But in the case of an enzyme reaction, rise in temperature quite often causes a loss of activity of the enzyme. Thus, there is always a narrowly defined temperature range at which enzyme activity is the maximum. This temperature, which gives the best results is called the optimum temperature. For most enzyme-activated reactions this is around 35ºC - 37ºC, which corresponds to the body temperature of most animals.

(iv) Poisoning and activation: Enzyme reactions are far more sensitive to catalytic poisons than any other types of catalytic reactions. The enzyme poisons are biological poisons too. Examples are, HCN, H₂S, CS₂,etc.

The activators or promotors for enzyme reactions are called co-enzymes and for every enzyme there is a co-enzyme. If the co-enzyme is separated from the enzyme, the activity of the enzyme is paralysed.

(v) Inactivation by electrolytes : The activity of an enzyme gets affected adversely by the addition of small quantities of electrolytes. The electrolytes inactivate the enzyme. This indicates colloidal nature of the enzyme as electrolytes cause the coagulation of colloids.

(vi) Action of ultraviolet light : Ultraviolet light destroys the delicate enzyme molecule, thus rendering it inactive.

9.Write the mechanism of enzyme catalysis?

Enzymes accelerate reactions by providing a reaction pathway which involves lesser activation energy.  Fischer suggested the lock and key hypothesis to explain the action of enzymes in enzyme - catalysed reactions. This hypothesis is still acceptable but in the modified way as induced fit hypothesis. These theories assume the existence of active sites or cavities on the surface of the enzyme molecules.

The enzyme - catalysed reactions are considered to proceed through a number of steps as described below :

Step 1  :      Binding of the enzyme to the substrate to form the enzyme-substrate complex,

Step 2  :    Formation of product on the surface of the enzyme.

                                    [ES] → EP

Step 3  :     Release of the product from the enzyme

                                 EP → E + P

where, E is the enzyme, S is the substrate, ES is the enzyme-substrate complex, and P is the product.

The representation of a substrate molecule binding to an enzyme is shown below in the figure .

The reactant molecules (substrate) bind to an active site on the surface of the enzyme. The active site of a given enzyme is so shaped that only its specific substrate could fit into it  just as only a particular key opens a particular lock. This specific binding leading to the formation of an enzyme-substrate complex is responsible for the high specificity of the enzymatic reactions. In this complex, the substrate is placed in the right orientation to facilitate a given reaction. This lowers the activation energy of the reaction and therefore, increases the rate of reaction. For example, when urea is hydrolysed in the presence of H⁺, the activation energy is 104 kJ  mol^-1, while when urea is hydrolysed by urease, the activation energy is only 29 kJ mol^-1.

At higher substrate concentration, the enzyme reactions are zero order reactions, while at very low concentration of substrate, the enzyme catalysed reactions are first order reactions.

Enzymes have very useful applications in industry. Chief among these are the use of several carbohydrases in fermentation processes to manufacture beer and wine, in food industry for producing sweet syrups from corn starch, and in cheese production.

10.Write about intermediate compound formation theory .

According to this theory, the desired reaction is brought about by a path involving the formation of an unstable intermediate compound, followed by its decomposition into the desired end-products with the regeneration of the catalyst. The intermediate compound may be formed in either of two ways :

In many cases, the intermediate compounds postulated to be formed are known compounds and often their presence is detected.

11.What is meant by selectivity of  catalyst ?

The ability of a catalyst to direct a reaction to yield a particular product is called its selectivity towards that product. For example, the reaction between CO(g) and H₂(g) gives different products with different catalyst.

12.What are the steps involved in mechanism of surface reactions

Heterogeneous catalytic reactions generally proceed via adsorption of reactants on the surface of the catalyst. Mechanism of such surface reactions may be explained in terms of diffusion theory of catalysis. This theory postulates the following sequence for gaseous reactions on a solid surface.

Step (1)  :Diffusion of the reactants to the surface.

Step (2)   : Adsorption of the reactant molecules onto the surface.

Step (3)   : Actual chemical reaction on the surface.

Step (4)    :Desorption of the products from the surface.

Step (5)    :Diffusion of the products away from the surface.