Electro-Chemistry Conceptual Questions Answers - 1

Unit – I

Electrochemistry

Part – A Questions & Answers

These are Engineering "Conceptual Questions and Numerical Question Answer on "ELECTROCHEMISTRY". Practice for examination.

1. Differentiate between electrolytic cells and galvanic cells? 

Ans.

S.No	electrolytic cells	Electrochemical cell
1	Electrical  energy  is  converted  into	Electrochemical cell is  the one, in which
	chemical energy.	chemical energy is converted into electrical
		energy.
2	The anode carries positive charge	The anode carries negative charge
3	The cathode carries negative charge	The cathode carries positive charge
4	Example:  Electrolysis,  electroplating,	Example: Daniel cell
	etc.,

2. What is single electrode potential?

Ans. It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in contact with a solution of its own salt.

3. Define EMF of a cell. 

Ans. Electromotive force is defined as, "the difference of potential which causes flow of current from one electrode of higher potential to the other electrode of lower potential.

Thus,  the  emf  of  a  galvanic  cell  can  be  calculated  using  the  following relationship.

E°      = E°        − E°

cell          right           left

4. Distinguish between reversible and irreversible cells giving one example for each.

Ans.

Sl.No	Reversible Cells				Irreversible Cells
1	A  cell  which  obeys  the  three				Cells which do not obey the conditions
	conditions	of	thermodynamic		of  thermodynamic  reversibility  are
	reversibility	is	called	reversible	called irreversible cells.
	cell.
2	Cell  reaction  is  reversed  when				The  cell  reaction  is  not  completely
	external potential greater than cell				reversed.
	potential is applied.
3	Daniel  cell,	secondary		batteries	Zinc – silver cell, Dry cell (Primary
	(rechargeable batteries).				Cells)

5.  What is the role of salt bridge in galvanic cells? 

Ans. Salt bridge provides electrical contact between two compartments of the galvanic cell.

           It also helps in maintaining electrical neutrality by allowing the migration of ions.

         
  

            It is used to reduce LJP (liquid junction potential)

6.  Suggest a method to determine the electrode potential of zinc. (May. 2005)

  

Ans. The saturated calomel electrode is coupled with another Zn electrode, the potential of which is 

to be determined. Since the reduction potential of the coupled Zn electrode is less than E° of calomel 

electrode (+ 0.2422 V), the calomel electrode will act as cathode and the reaction is

^Ecell ^{= E°}right ^{− E°}left

^Ecell ^{= E°}cal ^{− E°}Zn

^E°Zn ^{= E°}cal ^{− E}cell

= + 0.2422 − 1.0025

E°_Zn = − 0.7603 volt

7. Why can glass electrode not be used for a solution of high alkalinity? 

Ans. The glass electrode can be used in solutions only with pH range of 0 to 10. However above the 

pH 12 (high alkalinity), cations of the solution affect the glass and make the electrode useless.

8. What are reference electrodes? Give two examples.

Ans. The electrode potential is found out by coupling the electrode with another reference electrode, 

the potential of which is known or arbitrarily fixed as zero.

Primary  reference  electrode:  standard  hydrogen electrode.

Secondary reference electrode: calomel electrode.

9.  Mention any two application of EMF measurement. 

Ans. 1.  Determination of standard free energy change and equilibrium constant.

   

 (i) The standard free energy change of a reaction can be calculated as

    

      follows

−  ΔG° = nFE°

(ii)   The equilibrium constant of a reaction can be calculated as follows. E° = Standard emf of the 

cell; K = Equilibrium constant

2.  Determination of pH by using a standard hydrogen electrode.

A hydrogen electrode is introduced into the solution, pH of which is to be determined. It

is then coupled with a standard hydrogen electrode through the salt bridge and the emf

of the cell is measured. If E is the emf of the cell,

E =  ( 2.303 RT / nF  ) x ( pH )

From the above equation the hydrogen ion concentration or the pH of the 

solution can be calculated.

10. Write down the formulation of the standard hydrogen electrode. (OR)

What is a primary reference electrode? Give one example with its electrode potential value. 

Hydrogen electrode consists of platinum foil, that is connected to a platinum wire and sealed in a glass tube. Hydrogen gas is passed through the side arm of the glass tube. This electrode, when dipped in a 1N HCl and hydrogen gas at 1 atmospheric pressure is passed forms a standard hydrogen electrode. The electrode potential of SHE is zero at all temperatures.

It is represented as,

Pt , H₂ (1 atm)/H⁺ (1 M); E° = 0 V