Unit – I Electrochemistry Part – A Questions & Answers

[Pages:7]Unit ? I

Electrochemistry

Part ? A Questions & Answers

1. Differentiate between electrolytic cells and galvanic cells? (Nov. 2005), (May 2003)

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? (Nov.2010), (May 2011), (May 2005), (Nov. 2002), ((Apr. c1996)

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. (Oct. 1996), (May 2005)

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. (Nov. 2004), (Jan.2005)

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? (Nov. 2012) 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) 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 - Ecell = + 0.2422 - 1.0025

E?Zn = - 0.7603 volt 7. Why can glass electrode not be used for a solution of high alkalinity? (May. 2005)

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.(Nov. 2010), (May 2010), (Nov. 2006)

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. (Nov. 2005)

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

follows

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

- G? = 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 2.303RT pH

nF

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. (Nov. 1994)

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

11. How will you predict the spontaneity of any redox system using emf? (May 2010)

Spontaneity of redox reaction can be predicted from the emf (E?) value of the complete cell reaction. (i) If the E? of the cell is positive, the reaction is spontaneous. (ii) If the E? of the cell is negative, the reaction is not feasible.

In general, an element having lower reduction potential can displace another metal having higher reduction potential from its salt solution spontaneously.

12. Distinguish between electrochemical series and galvanic series. (May 2004)

Sl.N Electrochemical series

Galvanic series

o.

1

When the various electrodes (metals) The metal electrodes are placed in sea

are arranged in the order of their water under std. conditions and the

increasing values of standard reduction electrode potentials are measured.

potential on the hydrogen scale, then When these electrode potentials are

the arrangement is called arranged in the decreasing order, a

electrochemical series.

serious produced known as galvanic

series.

2

This series comprises of metals and The series comprises metals and alloys

non-metals.

13. What is an ion selective electrode? (June 2009) Ion-selective electrodes are the electrodes having the ability to respond only to a

particular ion, and develop potential, ignoring the other ions in a mixture totally.

Example: Glass electrode

14. What are the conditions for an electrochemical cell to act as a standard cell? (June 2009)

The conditions for an electrochemical cell to act as a standard cell are (i) The e.m.f of the cell is reproductive. (ii) The temperature-coefficient of e.m.f (change in e.m.f with temperature) should be very low.

15. What is a secondary reference electrode? Give one example with its electrode potential value. (Jan. 2006)

Calomel electrode consists of a glass tube containing mercury at the bottom over which mercurous chloride is placed. The remaining portion of the tube is filled with a

saturated solution of KCl. The bottom of the tube is sealed with a platinum wire. The side tube is used for making electrical contact with a salt bridge. The electrode potential of the calomel electrode is + 0.2422 V.

It is represented as, Hg , Hg2Cl2 (s) , KCl (satd. solution); E? = 0.2422 V

16. Give two limitations of H2 electrode. (Nov.2009)

It requires hydrogen gas and is difficult to set up and transport.

It requires considerable volume of test solution. The solution may poison the surface of the platinum electrode. The potential of the electrode is altered by changes in barometric

pressure

17. Can we use a nickel spatula to stir a solution of copper sulphate? Given that E0Ni2+/Ni= + 0.0025 & E0Cu2+/Cu= + 0.34V. (Nov. 2003)

No. Since nickel has negative reduction potential, it has the tendency to go into

ions in solution.

18. A zinc rod is placed in 0.1M ZnSO4 solution at 2980K. Write the electrode

reaction and calculate the potential of the electrode. E0 Zn2+/Zn = - 0.76V. (Nov. 2004) Electrode reaction is, Zn2+ +2e- Zn

E E o 2 .3 0 3 R T lo g [M n ] nF

E -0.76 0.0591 nF

Log [ Zn2+ ]

0.0591 E -0.76

nF

Log [ 0.1 ]

= - 0.76 ? 0.03 V = - 0.79V.

19. Zinc reacts with dilute H2SO4 to give hydrogen but silver does not. Explain given that

E0 Ag+/Ag = + 0.080V and E0 Zn2+/Zn = - 0.76V. Since zinc has negative reduction potential and is placed above hydrogen

in electrochemical series, it liberates hydrogen. But silver has positive reduction potential and is placed below hydrogen in

emf series. So, Ag does not liberate hydrogen.

20. Calculate the reduction potential of lead electrode in conduct with a solution of 0.018MPb2+ ions (std. reduction potential [E0] = - 0.13V. (Nov.2009)

Nernst equation: E = E0 + 0.0591log [Pb2+]

Given:

2 E0 = - 0.13V; [Pb2+] = 0.018M; n= 2

E = -0.13-0.02955 log (0.018) = - 0.13- 0.05156 E = - 0.1815V

21. What are the advantages of conductometric titrations? (May 2011)

(i) It gives more accurate end point.

(ii) It is used in the case of "coloured solutions", where colour change of the indicator is not clear.

(iii) It is also used for the analysis of dilute solutions and weak acids.

(iv) Since the end point is detected graphically, no keen observation is necessary near the end point.

22. Why does glass electrode not be used for a solution of high alkalinity?

If the pH of the solution is above 9, the glass membrane of the electrode is attacked by hydroxyl ions. It leads to the alteration in the relationship between pH and glass electrode potential.

23. What is Conductometric titration? A method of estimation of strength or the endpoint with the measurement of change in conductance of the solution is called conductometric titration. The conductance depends on the number and mobility of the ions.

24. What are the conditions for a cell to be reversible? A cell will be reversible if it satisfies the following conditions,

i) If Eappl = Ecell, no chemical reaction takes place

ii) If Eappl > Ecell , chemical reaction will take place in reverse direction

25. What are the factors which control the potential of a Daniel cell?

The rate of the above reactions depend on i. The nature of the metal. ii. The temperature.

iii. The concentration of metal ions in solution.

UNIT I (PART B) Electrochemistry

1. Describe SHE and SCE. Explain its use in the determination of single electrode potential.

2. How emf of an unknown cell is determined by poggendorff's compensation method? 3. Derive Nernst equation and mention its applications. 4. What are reversible and irreversible cells? Illustrate with examples. 5. What is electro chemical series? Give its application with suitable example. 6. Give an account on conductometric titrations. 7. What is potentiometric titration? Give an account on potentiometric redox titration. 8. Give an account on potentiometric Precipitation titration. 9. Describe the determination of pH of a solution using glass electrode. 10. Briefly discuss the ion selective electrodes. 11. A zinc rod is placed in 0.1 M solution of zinc sulphate at 25OC. Assuming that the

salt is dissociated to the extent of 95% at this dilution. Calculate the potential of the electrode at this temperature. EO= -0.76V. 12. Calculate EMF of the cell at 25OC. Zn/Zn2+ (0.1) //Ag+ (0.1M) / Ag E cell at 25OC=1.56V.

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