Electrochemistry Class 12th Solved Questions



Topic - Electrolysis and Faraday's laws

 1. What is meant by Faraday constant ?    

 Ans. Faraday constant, F = 96500 coulombs. It is that quantity of electricity which deposits one gram equivalent of the substance or it is the charge carried by one mole of electrons.


2. How many faradays of electric charge is required to  liberate 5600 cm3 of oxygen at STP ?

Ans 
 


 Thus, 1 mole of 02, i.e., 22400 cm3 require 4 faradays of charge.
5600 cm3 of 02 will require charge =  4 x 5600 = 1 faraday


 3. What is the effect of temperature on the electrical conduction of (i) metallic conductor (ii) electrolytic  conductor ?

  Ans. With increase of temperature, the electrical conduction of metals decreases whereas that of electrolytes increases..

 4. How do metallic and ionic substances differ in conducting electricity ?                  

Ans. In metallic substances, flow of electricity is due to flow of electrons while in ionic substances in the aqueous solution or molten state, the flow of electricity is due to flow of ions.

ILL Conductance and specific, equivalent and molar conductivities                                                      

 5. What are the units of molar conductivity ?


 Ans. Ohm-1 cm2 mo1-1 or S cm2 mol-1.  

 6. Give the relationship between molar conductivity and specific conductivity.                                       

                                      
   Ans. A = K X 1000\M where K is specific conductivity in S cm -1 and M is the molar concentration in mol L-1,  Am is in S cm2 mot-1
                                
  or Am  = K\M where K is S 111-1, M is in mol m-3, AM is in S m2 mol-1.


7. What is the relationship between specific conductance and equivalent conductance ? 


 Ans.  K x 1000\N , where K is specific conductance in S cm-1, N is normality in g eq, L-1, Aeg is in  S    cm (g.   

       

 8. Express the relation among conductivity of the solution in the cell, the cell constant and resistance solution in the cell. 


Ans. Conductivity (K) = 1\Resistance (R) X Cell constant .

 9. Give the relationship between equivalent and molar conductance ofa given solution ?


Ans. Am = K =   1000\ Molarity   and A eq = K x  1000\ Noormality  ; Am\Aeq  = K X  Noormality  \ Molarity

10. What is the direction of flow of conventional current in a galvanic cell ?


Ans. Conventional current flows from cathode to anode. It is opposite to the direction of flow of electrons.

11. What flows in the internal circuit of a galvanic cell ?


Ans. ions. 

IV.] Electrode potential, cell potential, electrochemical series and its applications


12. Why is it not possible to measure the single electrode potential ? (J & K Board 2012)


Ans. Oxidation or reduction cannot take place alone. Moreover, it is a relative tendency and can be measured with respect to a reference electrode only.

13. Given that the standard electrode potentials (E°) of metals are :
on K*/K = — 2-93 V, Ag*/Ag = 0:80 V, Cu’/Cu = 0-34 V, Mg/Mg = - 2.37 V,
Cr3+/Cr = — 0-74 V, Fe**/Fe = - 0-44 V
Arrange these metals in an increasing order of their reducing power. (CBSE 2010)


Ans, Higher the oxidation potential more easily it is oxidised and hence stronger is the reducing power,
Therefore order is Ag < Cu < Fe < Mg < K.

14. Define electrochemical series.or Write two applications of electrochemical series. (UP Board 2012)


Ans. The arrangement of various electrodes in the decreasing or increasing order of their standard reduction potentials is called electrochemical series.

or

Two applications of electrochemical series are |
(i) To compare oxidizing or reducing powers of different elements,
(ii) To predict the spontaneity of any redox reaction.

 15. Can we store copper sulphate in iron vessel ? Why ?
 

Ans. No, because iron is more reactive than copper. :

 16. Why does zinc react with dilute sulphuric acid but copper does not ? (Jharkhand Board 2010)


Ans. Zinc lies above hydrogen whereas copper lies below hydrogen in the electrochemical series. Hence, zine is more reactive than H* whereas copper is less reactive, Therefore, zinc reacts with dilute HSO, but copper cannot liberate H, gas by reaction with dilute H,8Q,.

 17. Give an example of a secondary cell. (HP Board 2013)


Ans. Lead storage battery.

18. Write the name of a cell used in small watches. (Raj. Board 201%)


Ans. Mercury cell (Rubon-Mallory cell).

Corrosion |


 19. What is the overall electrochemical reaction taking place in rusting ?


Ans. 2 Fe (s) + O, (g) + 4 H* (aq) —> 2 Fe* (ag) + 2 H,0 ()

 20. Out of zinc and tin which one protects iron better even after cracks and why ?


Ans. Zinc protects better because oxidation potential of zinc is greater but that of tin is less than that of iron.

21. Define corrosion. What is the chemical formula of rust ?


Ans. Corrosion is the slow eating away of the surface of the metal due to attack of atmospheric gases. The formula of rust is Fe,O,. xH,O.

 22. Why is chromium used for coating iron ?


Ans. Chromium is a non-corroding metal which forms a protective layer on iron. 

 23. What would happen if the protective tin coating over an iron bucket is broken in some places ?


Ans. In such a case, iron corrodes faster than it does in the absence of tin as oxidation potential of Fe is greater than that of Sn.

 24. Which type of metal can be used in cathodic protection of iron against rusting ?


Ans. A metal which is more electropositive than iron such as Al, Zn, Mg.

 25. Rusting of iron is quicker in saline water than in ordinary water. Give reason. 


Ans. In saline water, the presence of Na* and CI ions increases the conductance of the solution in contact with the metal surface. This accelerates the formation of Fe** ion and hence that of rust, Fe,O,

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