CBSE Class 12 Chemistry Notes: Surface Chemistry – Some Good Problems

Q.1       How is adsorption of gas related to its critical temperature?

Ans.     Higher is the critical temperature of a gas greater is the case of liquefaction, greater are the Vander wall’s forces of attraction hence greater is the adsorption.

Q.2       why adsorption is exothermic?

Ans.     When adsorption occurs, entropy decreases, ΔS is –ve, ΔG=ΔH-T ΔS and for this process to be spontaneous ΔG must be –ΔG. This can be only possible if ΔH is –ve thus the process is exothermic.

Q.3       Why it is essential to wash the precipitate with water before estimating it quantitatively?

Ans.     Some amount of estimating mixed to form the precipitate remains adsorbed on the surface of particles of precipitate. Hence it is essential to wash the precipitate with water to remove the stacking electrolytes or any other impurity before estimating it.

Q.4       Why is Ferric chloride preffered over potassium chloride in case of leading to Bleeding?

Ans.     Blood has negatively charged colloidal particles it is coagulated by +ve ions. As Fe+3 ions have a greater +ve charge than K+ ions. Coagulation with Fe+3 ions is Faster Hence they preferred.

Q.5       Why peptizing agent is added to convert the precipitate into Colloidal solution?

Ans.     Ions either (+ve or –ve) of peptizing agent (electrolyte)are adsorbed on the particles of precipitate. They repel and hit each other breaking the particles of the precipitate into colloidal size.

Q.6       why sky looks blue or sea water looks blue?

Ans.     Sky looks blue due to scattering of light by colloidal dust particles. As blue colour of white sunlight has minimum wavelength, it shows more intense scattering similarly blue colour of light is due to scattering of light by colloidal impurities insea.

Q.7       What is Hardy schulze’s rule?

Ans.     According to this rule the greater is the valancy of the coagulating or the Flocculating ions, greater is the power of it bring about the coagulation.

Q.8       Define peptization and peptizing agent?

Ans.     The process of converting a fresh precipitate into colloidal particles by shaking it with dispersion medium in presence of a small amount of suitable electrolyte is called peptization. The electrolyte added is called peptizing agent.

Q.9       Why are powdered substance more effective adsorbent than their crystalline forms?

Ans.     Powdered substances have the greater surface area, and surface area is directly proportional to adsorption. Thus powdered substance are effective adsorbent than their crystalline forms.

Q.10       What do you mean by the heat of adsorption?

Ans.       The amount of heat evolved when one mole of the gas is adsorbed on the adsorbent is called the heat of adsorption.

Q.11       Point out the differences between Adsorption and Absorption?

Ans.                    

Q12        How does a solid catalyst enhance the rate of combination of the gaseous molecules?

Ans.       As the gaseous molecules of the reactant are adsorbed on the surface of solid catalyst their concentration on surface increases and they lie close to each other. Greater the concentration faster is the reaction. Closeness of reactant molecules to each other also gives them a better chance to react. Moreover adsorption is an exothermic process. The heat released further increases the rate of reaction.

Q13        How does the emulsifying agent stabilize the emulsion?

Ans.       Emulsifying agent is concentrated at the interface (soap molecules RCOONa) acting as emulsifying agent are concentrated in the surface of the drop of the oil with R part in the oil and COO+Na+ part in the water. As a result interfacial tension between the oil and water gets decreases. Hence they are intermixed into each other to form a stable emulsion.

Q14     Explain the protective action of the lyophilic colloids?

Ans.     We know that the lyophobic sols are like those of metals [Au, Ag] are unstable and are easily precipitated by addition of electrolytes. However it is observed that the addition of certain lyophilic colloids like gums, soaps and gelatin to lyophobic colloids difficult to coagulate by addition of electrolyte the process is known as the protection and this type of lyophilic colloids are called the protective colloids.

Q15     Point out the differences between the lyophobic and lyophilic colloids?

Ans.  

Q16     Explain the terms:-

  1. Electrophoresis
  2. Coagulation

Ans.

  1. Electrophoresis:- The colloidal particles are charged particles thus on the application of electric field they starts moving towards the oppositely charged electrodes. This moment of charged particle is defined as the electro phoresis or catuphoresis.
  2. Coagulation:- Coagulation or precipitation is the process of aggregation together the colloidal particles so as to change them into larged sized particles which ultimately settles as a precipitate

Q.17    What are Zeolites or shape selective catalysts?

Ans.     Zeolites are alumino silicates having three dimensional honey comb like network  structure and containing water molecules of hydration to use them as the catalysts, they are heated so that their water of hydration is lost. As a result the cavities become vacant and they becomes porous the size of the pores is in the range 260 – 740 pm.    Thus the molecules having size smaller than pores can be absorbed here not bigger ones that’s why they are called shape selective catalysts. An important zeolite used in            petroleum industry is ZsM5.

Q.18    Define the given terms:-

  1. Peptization (peptizing agent)
  2. Autocatalysis

Ans.

i.            Peptization (Peptizing agent):- The process of converting a fresh precipitate into colloidal particles by shaking it with the dispersion medium in the presence of small amount of the electrolyte is known as the peptization. The small amount of the electrolyte so added is known as the peptizing agent.

ii.            Autocatalysis:-If one of the products acts as a catalyst in the reaction. Thenthe reaction or process is called the autocatalysis. For example in the reaction of oxalic acid with acidified KMnO4 solution, Mn+2 ions produced are acts as autocatalyst. The reaction speeds up with the time.

Q.19

i.            Which will be adsorbed on the surface of charcol and why – NH3 or CO2?

Ans.   i.                  NH3 has the higher critical temperature than CO2, NH3 is more easily liquefiable than           CO2, Hence NH3 has greater intermolecular forces of attraction and hence will be                       adsorbed more readily.

ii.            What causes the Brownian moment in the colloidal solution?

Ans.  ii.      Brownian movement (zig–zag moment) of the colloidal particles is due to hitting of              these particles by the molecules of the dispersion medium with different forces from              the different directions.

Q.20    What role does the adsorption plays in the heterogeneous catalysis?

Ans.     In the heterogeneous catalysis, generally the reactant are the gases where as catalyst is      a solid. The reactant molecules are adsorbed on the surface of solid catalyst by physical adsorption or chemisorptions. As a result the concentration on the surface increases and   hence the rate of reaction is also increases.
Alternatively one of the reactant molecule undergoes fragmentation on the surface of         the solid catalyst producing active species which reacts faster. Thus the product          molecules in the either case have no affinity for the solid catalysts and are distorted             making the surface free for fresh adsorption. This theory is called the adsorption           theory.

Four marks questions:-

Q.21    Explain the hetrogenous theory of catalysis?

Ans.     Please see the theory mechanism of heterogeneous catalysis

Q.22    Explain the following:-

  1. Ultra centrifugation
  2. Cause of the peptization

Ans.

  1. Ultra centrifugation:- This is a method of purification of colloids. In this method, the impure sol is taken in a tube which is placed in on attarcentrifuge. In this machine the tube is rotated at a very high speed. As a result the colloidal settles down at the bottom of the tube whereas the crystalloid and other soluble impurities remains in the solution. Thus this solution is decanted off and the colloidal particles are remained with the dispersion medium to give the pure colloidal sol.
  2. Cause of peptization:- As electrolyte is added to a freshly precipitated substance, the particles of the precipitate preferentially adsorbs one particular type of ions of the electrolyte. As a result they get dispersed due to electrostatic repulsions. This gives the particles of colloidal size. An example of peptization of freshly precipitated ferric hydroxide with ferric chloride solution.

            

 

Q.23    Explain the cleanising, action of soap?

  1. For answer please see the theory cleanising action of the soap

Q.24    Explain:-

i.            How colloids is applied in smoke precipitation

ii.            How the formation of delta takes place?

Ans.

i.            Smoke precipitation:-

  • Smoke particles are actually electrically charged colloidal particles of carbon in air. Precipitation of the smoke particles carried out by cotterell precipitator which is based upon the principle of electrophoresis.
  • Smoke is allowed to pass through a chamber having a number of metal plates attached to metal wire connected to a source of high potential. The charge particles of smoke get attracked by oppositely charged electrode and gets precipitated and hot air passes through chimney.

ii.            Formation of delta:-

  • River water contains the charged colloidal particles of clay band and many other materials.
  • Sea water is a big source of the varietly of electrolytes dissolved in it.
  • When river water comes in contact with sea water the electrolytes presents in sea water coagulate the suspended colloidal particles of river water.
  • Thus colloidal particles settle down at the bottom and level of riverbed rises as a result water adopts different course and delta is formed in due course of time.

Q.25    Explain the mechanism of the enzyme catalysis?

Ans.

  • The most accepted mechanism of the enzyme catalysed reaction is known as lock and key mechanism
  • Since enzymes are specific in their action there specificity is due to the presence of some specific regions called the active sites.
  • These active sites are associated with some functional group as –NH2, -COOH, -OH2, -SH which form weak bonds such as H – bonds with substrate molecules.
  • The shape of the active site of an enzyme is like a cavity such that only a specific substrate can fix into it. In the same way as a particular key can fit into a lock.
  • This specific binding substrate complex which accounts gor high specificity of enzyme catalysed reaction.
  • Once the proper orientation is achieved the substrate molecule react the form product in two steps as:-

Step I:- Formation of enzyme substrate complex

Step II:- Dissociation of enzyme substrate complex to form products:

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