Electrochemistry

Class 12 > Chemistry > NCERT-I > Electrochemistry

Electrochemistry

Book Solutions

1

In the order of their reactivity, i.e how they displace each other from their salt solutions, allign the metals in decreasing order. Cu, Fe, Al, Zn and Mg.

Answer

According to their reactivity, the given metals replace the others from their salt solutions in the said order: Mg : Al : Zn : Fe : Cu

Exercise

2

Standard electrode potentials given as, Mg2+/Mg = −2.37 V, Hg2+/Hg = 0.79V, Cr3+/Cr = − 0.74V, Ag+/Ag = 0.80V, K+/K = −2.93V. In the order of increasing of reducing power arrange the given metals accordingly.

Answer

The reducing power increases with the lowering of reduction potential. In order of given standard electrode potential (increasing order): K+/K < Mg2+/Mg < Cr3+/Cr < Hg2+/Hg < Ag+/Ag.

Thus, in the order of reducing power, we can arrange the given metals as: Ag< Hg < Cr < Mg < K.

Exercise

3

Represent the galvanic cell in which the following reaction takes place.

Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)

Also find :

(i) The negatively charged electrode ?

(ii) Current carriers in the cell.

(iii) At each electrode, the individual reaction.

Answer

Exercise

4

With the following reactions given, find the standard cell potentials of galvanic cells  with given reactions.

(i) 2Cr(s) + 3Cd2+(aq) → 2Cr3+(aq) + 3Cd(s)

(ii) Fe2+(aq) + Ag+(aq) → Fe3+(aq) + Ag(s)

Calculate the ∆rGθ and equilibrium constant of the reactions.

Answer


Exercise

5

Write the Nernst equation and emf of the following cells at 298 K:

(i) Mg(s) | Mg2+(0.001M) || Cu2+(0.0001 M) | Cu(s)

(ii) Fe(s) | Fe2+(0.001M) || H+(1M)|H2(g)(1bar) | Pt(s)

(iii) Sn(s) | Sn2+(0.050 M) || H+(0.020 M) | H2(g) (1 bar) | Pt(s)

(iv) Pt(s) | Br2(l) | Br−(0.010 M) || H+(0.030 M) | H2(g) (1 bar) | Pt(s).

Answer


Exercise

6

In the button cells widely used in watches and other devices the following reaction takes place:

Zn(s) + Ag2O(s) + H2O(l) → Zn2+(aq) + 2Ag(s) + 2OH−(aq)

Determine ΔrGΘ and E0 for the reaction.

Answer

Exercise

7

For the solution of an electrolyte describe its conductivity and molar conductivity. Also put some light on how they vary with concentration.

Answer

Conductivity of a solution is defined as the conductance of a solution of 1 cm in length and area of cross – section 1 sq. cm. Specific conductance is the inverse of resistivity and it is represented by the symbol κ. If ρ is resistivity, then we can write:

k=1/ρ

At any given concentration, the conductivity of a solution is defined as the unit volume of solution kept between two platinum electrodes with the unit area of cross- section at a distance of unit length.

G=ka/l=k×1=k [Since a = 1 , l = 1]

When concentration decreases there will a decrease in Conductivity. It is applicable for both weak and strong electrolyte. This is because the number of ions per unit volume that carry the current in a solution decreases with a decrease in concentration.

Molar conductivity –

Molar conductivity of a solution at a given concentration is the conductance of volume V of a solution containing 1 mole of the electrolyte kept between two electrodes with the area of cross-section A and distance of unit length.

Λm=kA/l

Now, l = 1 and A = V (volume containing 1 mole of the electrolyte).

Λm=kV

Molar conductivity increases with a decrease in concentration. This is because the total volume V of the solution containing one mole of the electrolyte increases on dilution. The variation of Λm with √c for strong and weak electrolytes is shown in the following plot :


Exercise

8

The conductivity of 0.20 M solution of KCl at 298 K is 0.0248 Scm−1. Calculate its molar conductivity.

 

Answer

Given, κ = 0.0248 S cm−1

c = 20 M

Molar conductivity, Λm=k×1000/c 

=0.0248×1000/0.2

= 124 Scm2mol-1

Exercise

9

Considering the case of a conductivity cell having 0.001 M KCl solution at 298 K is 1500 Ω. If given, conductivity of 0.001M KCl solution at 298 K is 0.146 × 10−3 S, find the cell constant?

Answer

Given,

Conductivity, k = 0.146 × 10−3 S cm−1

Resistance, R = 1500 Ω

Cell constant = k × R

= 0.146 × 10−3 × 1500

= 0.219 cm−1

Exercise

10

The conductivity of NaCl at 298 K has been found at different concentrations and the results are given below:

Concentration/M            0.001     0.010     0.020     0.050     0.100

102 × k/S m−1                      1.237     11.85     23.15     55.53     106.74

Calculate Λm  for all concentrations and draw a plot between Λm  and c1⁄2. Find the value of Λ0m

Answer



Exercise

11

Find the molar conductivity of acetic acid if its conductivity is given to be 0.00241 M . Also, if the value of Λ0m is given to be 390.5 S cm2 mol−1, calculate its dissociation constant?

Answer

Exercise

12

How much charge is required for the following reductions of 1 mol of :

(i) Al3+ to Al.

(ii) Cu2+ to Cu.

(iii) MnO4-  to Mn2+.

Answer

(i) Al3++ 3e− → Al

Required charge = 3 F

= 3 × 96487 C

= 289461 C

(ii) Cu2+ +2e− → Cu

Required charge = 2 F

= 2 × 96487 C

= 192974 C

(iii) MnO4- → Mn2+

i.e Mn7++ 5e− → Mn2+

Required charge = 5 F

= 5 × 96487 C

= 482435 C


Exercise

13

In the terms of Faraday, how much electricity is required to produce :

(i) From molten CaCl2, 20.0 g of Ca.

(ii) From molten Al2O3, 40.0 g of Al.

Answer

Exercise

14

Calculate the amount of electricity required for the oxidation of 1 mol of the following in coulombs : (i) H2O to O2. (ii)FeO to Fe2O3.

Answer

Exercise

15

For 20 minutes, a current of 5 A is applied to between platinum electrodes to electrolyze a solution of Ni(NO3)2. Find the amount of Ni deposited at the cathode?

Answer

Exercise

16

Solutions of 3 electrolytic cells are ZnSO4, AgNO3 and CuSO4,cells are connected in series. Of the cells, A,B,C respectively, after passing a steady current of 1.5 amperes , 1.45 g of silver was found deposited at the cathode of cell B. How much time did the current flow? What amount of zinc and copper were deposited?

Answer


Exercise

17

Using the standard electrode potentials given in Table 3.1, predict if the reaction between

the following is feasible:

(i)Fe3+(aq) and I−(aq)

(ii) Ag+ (aq) and Cu(s)

(iii) Fe3+ (aq) and Br− (aq)

(iv) Ag(s) and Fe3+ (aq)

(v) Br2 (aq) and Fe2+ (aq).

Answer


Exercise

18

Predict the products of electrolysis in each of the following :

(i) An aqueous solution of AgNO3 with silver electrodes.

(ii) An aqueous solution of AgNO3with platinum electrodes.

(iii) A dilute solution of H2SO4with platinum electrodes.

(iv) An aqueous solution of CuCl2 with platinum electrodes.

Answer

Exercise

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