Chem Unit3 Ans

8/12/2019 Chem Unit3 Ans

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UNIT-III

ELECTROCHEMISTRY

PART A

1. What is electrode potential?

The tendency of an electrode to lose or gain electrons, when it is in contactwith its own ions.

2. What is an electrochemical cell?

A device used to convert the chemical energy produced in a redox reaction

into electrical energy.

3. Write the Nernsts equation for the electrode reaction?

Mn+

(aq) + ne- M(s)

EMn+/M = E0

Mn+/M + 2.303 RT/ nF log [Mn+

(aq)]

4. Define a reference electrode?

The tendency of an electrode to lose electrons, when it is in contact with

solution of its own ions.

5. Why glass electrode cannot be used for solution of pH above 9.0?

At pH above 9.0, the ions of the solution affect the glass inter face and

render the electrode useless.

6. Glass electrode is preferred to quinhydrone electrode in measuring pH of a

solution. Give reason.

Glass electrode is simple, not easily oxidized and attains equilibrium

rapidly. It can safely be used up to pH of 10. On the other hand, quinhydrone

electrode can be used upto a pH of 8 only. Moreover, it cannot be used in solutions

containing redox system. Hence, use of glass electrode in pH measurement is

preferred over quinhydrone electrode.

7. Electrode potential of zinc is assigned a negative value; whereas that of

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copper a positive value.

Zn electrode is anodic wrt to SHE, so its electrode potential is assigned a

negative sign. On the other hand, Cu electrode is cathodic wrt to SHE, so its

electrode potential is assigned a positive sign.

8. What is galvanic cell or voltaic cell?

It is a simple device of producing electrical energy by chemical reaction, e.g.,

Daniel cell. Such a cell is also known as electrochemical cell.

ZnSO4 CuSO4

Zn --------> Zn+2+ 2e~ (oxidation)

Cu2+

+ 2e --------->Cu (reduction)

In the above cell Zn-electrode is anode and Cu-electrode is cathode.

9. What is an electrolytic cell?

It is a device used for converting electrical energy into chemical energy.

10. What do you mean by electrode potential (E)?

It is the tendency of an electrode in a half cell to lose or gain electrons when it is

in contact with the solution of its own ions.

11. What are reduction and oxidation potentials?

Reduction potential is the tendency of an electrode in a half cell to gain electrons

and oxidation potential is the tendency of an electrode in a half cell to lose electrons.

12. What do you mean by standard electrode potential (E)?

It is the electrode potential of a metal in contact with its ions when the

concentration of ions is 1 M (1 molar).




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13. What is a salt bridge?

It is an inverted U-tube containing an electrolyte (e.g., KC1, KNO3). It connects

(acts as a bridge) the solutions of the two half cells.

14. What is the E.M.F. of a cell?

It is defined as the potential difference between the two terminals of the cell

when no current is drawn from it.

15. Define normal hydrogen electrode.

It is a reference or standard reference electrode. Its electrode potential is taken

as zero at all temperatures. A normal hydrogen electrode generally consists of a Pt-

foil coated with platinum, dipped in solution having 1 M H+ ion concentration

and hydrogen gas at 1 atmospheric pressure constantly bubbled over it. It can be

represented as:

16. What do you mean by potentiometry?

Potentiometry is an electrochemical method of analysis based on

measuring the potential difference (e.m.f.) between two half cells, one of which

is an indicating electrode and the other is a reference electrode.

17. What is an indicating electrode?

It is an electrode in balance with an redox couple, the potential of which is

given by Nernst equation.

18. What are reference electrodes?

These are the electrodes whose potential is constant and independent

of the composition of the contacting solution.

The most widely used are saturated calomel electrodes (G = + 0.246 V) and

the silver-silver chloride electrode (e = + 0.222 V).

19. Define the electrochemical series?

When the standard reduction potentials of the electrodes are arranged in an




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increasing order, the series so obtained is known as electrochemical series.

20. How do you increase the value of reduction potential of an electrode?

By increasing the concentration of metal ions in contact with metal

electrode or increasing the temperature.

PART-B

1. How do you measure EMF of a cell?

The difference of potential which causes flow of current from an electrode of

higher potential to an electrode of lower potential is called electromotive force.

Measurement of EMF of a cell:

The emf of a cell can be measured using a potentiometer. This measurement is

based on poggendorffs compensation principle. In this method, the emf to be measured

is opposed by the emf of another cell or battery until the two emfs become equal and

there is no net flow of current in the circuit.The electrical assembly used is known as

potentiometer.

It consists of a uniform wire AC of high resistance. A storage battery of constant

emf is connected at the ends A and C by the wire.

The cell X, the EMF of which is to be determined is included in the circuit by

connecting the positive pole to A and the negative pole to the sliding contact J through a

galvanometer. The sliding contact is moved along the wire AB till no current flows

through the galvanometer.The distance AD is noted. The emf of the cell Ex isproportional to the distance AD,

i.e.,ExAD ------(1)

The cell X is replaced by a standard cell of known emf,E s. The position of the

sliding contact is readjusted by moving it the wire AB, as before ,till no current flows

through the galvanometer, i.e., the null point,is reached again at D as shown in the




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3. Discuss the construction of glass electrode.

It is an electrode, which produces current in response to a specific ion present in

the solution. This electrode is otherwise known as ion selective electrode.

Construction:

A glass membrane electrode consists of thin walled glass bulb containing AgCl

coated Ag electrode or platinum wire in 0.1M HCl. There is an equilibrium exist between

H+ ions of solution and Na

+ ions of glass. For a particular type of glass the electrode

potential depends on the concentration of H+ions. This can be represented as;

AgCl, Ag, HCl (0.1M)/Glass or Pt, HCl (0.1M)/Glass

If this electrode acts as anode , the electrode reaction will be;

H2 H+ + e

-

If this electrode acts as cathode , the electrode reaction will be;

H+ + e

- H2

The electrode potential can be written as;

E= Eg-0.0591 log[H+] -----------(oxidation potential)

E = Eg +0.0591pH

Determination of pH with the help of glass electrode: The electrode is




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dipped into the solution where pH is to be determined and is usually

combined with a reference electrode generally calomel electrodeto

form a complete cell. The potential of the glass electrode varies linearly with

pH .

`

Solution of unknown pH

Ecell= ER- EL

Ecell= Ecalomel - Eglass

Ecell= +0.2422- ( Eglass -0.0591log [H+]

Ecell = +0.2422- Eglass -0.0591pH

pH

=+.2422 - EG-Ecell/.0591

5. What is meant by EMF series? Explain the importance of EMF series.

The electrode potentials of various electrodes are arranged in the order of

increasing standard reduction potentials with respective to hydrogen scale.This

arrangement is known as electrochemical series

Importance of electrochemical series:

Saturated

calomel electrodeGlass

electrode




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Calculation of standard EMF of a cell:

The standard EMF of a cell can be calculated using the standard reduction

potentials of right hand side and left hand side electrodes from the emf series.

Ecell= ER- ELRelative ease of oxidation and reduction:

Higher the value of standard reduction potential, greater is the tendency to get

reduced. For example, in the electrochemical series, fluorine has high positive value of

reduction potential (+2.87 V) and is reduced. Lower the value of standard reduction

potential, greater is the tendency to get oxidized. In the electrochemical series, lithium

has the lower negative reduction potential value (-3.05V) and is oxidized.

Predicting spontaneity of a reaction:

The spontaneity of a reaction can be determined from the standard emf of the cell.

If the emf of a cell is positive, the reaction is spontaneous. If it is negative, the reaction is

non spontaneous.

Ecell= ER- EL= + ive (spontaneous)

Ecell= ER- EL= - ive (non spontaneous)

Displacement behaviour of hydrogen:

If zinc electrode is immersed in H2SO4 solution, it displaces hydrogen from acid

solution.

Zn + H2SO4 ZnSO4 +H2

If silver electrode is immersed in H2SO4 solution, it will not displace

hydrogen from acid solution. Because it has higher reduction potential value.

Ag + H2SO4 No reaction




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6. Derive Nernst equation for electrode potential.

Nernst equation for electrode potntial:

Consider the following redox reaction:

M

n+

+ ne

-

M

For such a redox reversible reaction, the free energy change (G) and its equilibrium

constant (K) are inter related as

G= - RTlnK + RTln [Product]/ [reactant]

G = G+ RTln [Product]/ [reactant] [ G

= - RTlnK ]

Where G=Standard free energy change

The above equation is known as Vant Hoff reaction isotherm. The decrease in free

energy in the above reversible reaction will produce electrical energy.

-G = nFE and G =- nFE

Where E is the electrode potential, E is the standard electrode potential, F = 96500

coulombs and n is the number of electrons transformed.

Comparing equations (1) and (2),it becomes

- nFE = -nFE + RTln [ M]/ [Mn+] (consider the reduction reaction)

- nFE = -nFE + RTln 1/ [M

n+] ([M] = 1)

Dividing eqn (3) by nF




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E = E

- RT/nF ln 1/ [Mn+

]

or

E = E

+ RT/nF ln [Mn+

]

E = E

+ 2.303 RT/nF log [Mn+

]

Where R =8.314 J/K/mole, F = 96500coulombs, T = 298K, the above equation becomes

E = E + .0591 /n log [Mn+

] This the Nernst equation for reduction potential

For oxidation potential

E = E - .0591 /n log [Mn+

]

Application:

i) It is used to calculate electrode potential of unknown metal.ii) It can be used to calculate the emf of a cell.

7. Write notes on potentiometric redox titrations.

Potentiometric Titrations:

A titration in which concentration change is followed by potential change is called

a potentiometric titrations. Two electrodes are required to measure the potential of the

test solution. The potential of one electrode should be constant while that of the other

should change with concentration of the test species. The former is called reference

electrode and the latter is called an indicator electrode which is different for different

types of titrations. For a redox titration (Fe

2+

Vs Cr

6+

), calomel electrode is used as areference electrode and platinum is used as an indicator electrode. The two electrodes are

connected to a potentiometer.




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Redox Titration:( Fe2+

Vs Cr6+

)

Titration of ferrous sulphate vs potassium dichromate is Fe2+

Vs Cr6+

.When

K2Cr2O7is added to ferrous sulphate in acid medium, ferrous ion is oxidized to ferric ion

hexavalent Cr is reduced to trivalent state. The potential of this system depends on ratioof ferrous to ferric.

trivalent Cr will be present. So the potential of this system will follow the concentration

ratio of hexavalent Cr to trivalent Cr. When the potential is plotted against the volume of

dichromate added, the following graph is obtained.




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The mid point of the curve is known as the end point. The cell is represented as

Pt, Hg, Hg2Cl2/ KCl(sat)//Fe2+

, Fe3+

(H+)/ Pt .

8. Write notes on conductivity Acid- Base Titration.

Strong Acid Vs Strong Base:

A known amount of acid (HCl) is taken in a beaker and alkali (NaOH) is taken in

the burette. A conductivity cell is introduced into the beaker solution. Initially the

conductivity of HCl is high, this is due to the presence of fast moving H+ions. As NaOH

is added gradually, conductance will be going on decreasing until the acid has been

completely neutralized. This is due to yhe replacement of fast moving H+ ions by slow

moving Na+ions. The point B indicates the complete neutralization of all H

+ions.




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HCl +NaOH NaCl + H2O

Further addition of NaOH will introduce fast moving OH- ions.Therefore the

conductance value will begin to increase. On plotting conductance against volume of

NaOH added, the two lines intersect at a point B which corresponds to the neutralization

point.

Advantages:

i) Conductometric titrations give more accurate end point.

ii) It can be used to titrate col,oured solutions where the colour change of the

indicator is not clear.

iii)It can be used for titratig dilute solutions.iv) There is no need to have keen observation near the end point, as it is

determined graphically.




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Problems

1. Calculate the emf of a cell at 25C, when the concentration of ZnSO4 and

CuSO4 are 0.001 M and 0.1 M, respectively. The standard potential of cell is

1.2 V.

SolutionCell is: Zn(s) |Zn

2+(0.001 M) ||Cu

2+(0.1 M) |Cu(s)

Ecell= Eo

cell + (0.0592 /2) log [Cu2+

]/ [Zn2+

]

= 1.2 + 0.0296 2

= 1.2 + 0.0592

= 1.2592 V

2. What is concentration of Ni2+ in the cell at 25C, if the emf is 0.601 V?

Ni(s) |Ni2+

(a = ?) ||Cu2+

(0.75 M) |Cu(s)

Given

Eo

Ni/Ni2+

= 0.25 V ; Eo

Cu2+

/Cu= 0.34 V

Solution

Ecell= Eo

cell + (0.0592 /2) log [Cu2+

]/ [Ni2+

]

0.601 = (0.34-(-0.25) + 0.0296 log [0.75/a]

0.011 = 0.0296 log 0.75/a

log 0.75/a = 0.011 / 0.0296

0.75/a = Antiliog 3.7162

a = 0.75 / 2,3529

= 0.3188 M.



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Chem Unit3 Ans

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