Electrochemistry - Questions and Answers

1. What are electrochemical cells ?

Ans. It is an electrochemical device, which converts electrical energy into chemical energy (electrolytic cells) or chemical energy into electrical energy (galvanic cells).

2. Differentiate Electrolytic and Galvanic cells.

Ans. Electrolytic cells – An electrochemical device which converts electrical energy into chemical energy, for example, electroplating of copper on a substrate on electrolyzing copper sulphate solution.

Galvanic cells – An electrochemical device which converts chemical energy into electrical energy, for example, Daniel cell supplies electrical energy of 1.1 volts due to the chemical reactions taking place in it.

3. What are Galvanic / Voltaic cells ?

Ans. Galvanic cell is an electrochemical device in which chemical reaction taking place is the cause, giving some amount of electrical energy, for example, Daniel cell supplies electrical energy of 1.1 volts due to the redox reactions taking place in it. The net reaction is,

Zn²⁺ + Cu²⁺ ⇌ Cu²⁺ + Zn²⁺

4. What are conductors and insulators?

Ans. Conductors are the substances, which allow the electric current to pass through them. Example : metals, graphite, fused salts, aqueous solutions of acids etc.

Insulators (non-conductors) are the substances, which do not allow the electric current to pass through them. Example : rubber, paper, wood, diamond etc.

5. Write difference between metallic and electrolytic conductors.

S.no	Metallic conductors	Electrolytic conductors
1.	Passage of current is due to movement of electrons.	Passage of current is due to movement of ions.
2.	No chemical change.	Chemical change take place.
3.	Conductance decreases with increase in temperature	Conductance increases with increase in temperature

6. Define redox reaction.

Ans. The chemical reactions in which both oxidation and reduction occur simultaneously are called redox reaction.

7. Define electrode potential.

Ans. Measure of tendency of an metallic electrode to lose or gain electrons when it is in contact with its own ions in solution is called electrode potential.

8. Define EMF. Mention its applications.

Ans. EMF may be defined as the difference in potential which causes the flow of current from an electrode having higher potential to an electrode having lower potential.

Applications :

Determination of solubility of a sparingly soluble salt.
Determination of valency of an ion.
Determination of pH of a solution by a reversible electrode.
Potentiometric titrations.

9. Write Nernst equation and explain the terms in it.

Ans. For a simple reduction reaction,

M_n+ + ne^– ⇌ M

the Nernst equation can be given as, $\begin{gathered} \mathrm{E}=\mathrm{E}^{\circ}+-2.303 \mathrm{RT} \\ \mathrm{nF} \\ \text { or simply } \\ \mathrm{E}=\mathrm{E}^{\circ}+------\log \left[\mathrm{M}_{\mathrm{n}+}\right] \\ 0.0592 \mathrm{RT}\left[\mathrm{M}_{\mathrm{n}+}\right] \end{gathered}$

Where, E = Electrode Potential at the given concentration

E^° = Standard Electrode Potential
R = 8.314 J/K/ mole
T = Temperature (generally 298K )
n = number of electron transfer involved in the reaction
[M]= Concentration / activity of metal and
[M_n+ ]= Concentration of the metal ion solution

10. What are reversible (secondary) and irreversible (primary) cells ?

Ans. Secondary or reversible cells – They are electrochemical cells, which obey the condition of thermodynamic reversibility. The cell reaction of such cells can be reversed on supplying an emf from external source slightly higher than the cell voltage. Eg. : Daniel cell, Lead acid storage batteries.

Primary or irreversible cells – They are electrochemical cells, which do not obey the condition of thermodynamic reversibility. The cell reaction of such cells cannot be reversed on supplying an emf slightly higher than the cell voltage. Eg. : Dry cells or Lechlanche cells, Zinc – Silver cells.

11. Define single electrode potential and standard electrode potential.

Ans. The measure of tendency of a metal to lose or gain electrons when it is placed in a solution of its own metal ion is called electrode potential or single electrode potential.

At standard conditions, the value of single electrode potential is known as standard electrode potential. (or) The measure of tendency of a metal to lose or gain electrons when it is placed in a solution of its own metal ion of 1.

12. What is reference electrode ? Give examples.

Ans. The electrodes which are used to measure the electrode potential of an unknown electrode on combining with it and whose potential is known and arbitrarily fixed are known as reference electrodes. That is, the relative potential of one electrode is measured with reference to another of known potential.

Eg. : Standard hydrogen electrode, Calomel electrode, Silver – Silver Chloride electrode etc.

13. Represent the following (i) NHE (ii) SCE (iii) reversible cell (iv) An irreversible cell

(i) NHE : Pt, H₂ (1 atm)/H₊(1M)
(ii) SCE : Hg/Hg₂Cl₂,KCl(Sat)
(iii) Rev. cell : Zn/ZnSO₄//CuSO₄/Cu²⁺
(iv) Irreve. cell : Zn/H₂SO₄/Ag

14. What is emf series ? Give its significance.

Ans. The arrangement of various metals in the increasing order of their standard electrode potentials is called electrochemical series. From this series one can

Predict the relative tendency of a metal to undergo oxidation or reduction
Predict the spontaneity of a reaction
find preferable displacement of one metal ion over the other in solution
Calculate the standard emf of a cell
Determine the equilibrium constant of a reaction.

15. What are redox electrodes. Give suitable example.

Ans. A redox system is one in which a reduction – oxidation reaction is taking place on an inert electrode.

eg. : The reduction of to taking place over Platinum inert electrode.

16. Describe the construction and working of Daniel cell.

Ans. Daniel cell is an example of galvanic cell in which. Zinc rod is dipped in 1M solution of , and copper rod is diped 1M solution of ZnSO₄ . The two solutions are separated by a porous spot. (The solutions can seep through the pot). The two metal rods are connected through a wire The electrode reactions are,
At Anode : Zn→Zn²⁺+2e^–
At Cathode : Cu²⁺+2e^–→Cu
Cell potential volts = 1.1volts

17. What is Helmholtz electrical double layer ?

Ans. When a metal is immersed in a solution of its own salt, either positive or negative charge is developed on metal surface which in turn attracts either negative or positive ions from solution. This results a formation of layer around the electrode and is known as Helmholtz electrical double layer.

18. What is electromotive force ?

Ans. The difference in potential which causes flow of current from one electrode (of higher reduction potential) to another electrode (of lower reduction potential) is called Electromotive force (E.M.F) of a cell, It is measured in volts

i.e., E_cell = E_cathode – E_anode
(or) E_cell = E_right – E_left

19. Give the difference between electrolytic cells and electrochemical cells.

Ans. Difference between electrolytic cells and electrochemical cells.

	Electrolytic cell	Electrochemical cell
(i)	It is a device in which chemical reactions are brought about by a passage of electric current.	It is a device in which chemical reaction is utilized to get electrical energy.
(ii)	In this type of cell negative electrode is cathode and positive electrode is anode.	In this type of cell negative electrode is anode and positive electrode is cathode.
(iii)	(e.g.,) Electrolysis of an aqueous solution of HCl into H2 and Cl2	(e.g.,) Daniel cell, fuel cell etc.