Electrochemistry

Previous Years' CBSE Board Questions

2.1 Electrochemical Cells

MCQ

An electrochemical cell behaves like an electrolytic cell when

1. <math>E_{ceil} = E_{external}</math> (b) <math>E_{ceil} = 0</math>

1. E_external > E_cell
2. E_external < E_cell

(2020) R

SAI (2 marks)

2 Give two points of differences between electrochemical and electrolytic cells.

(2/5, 2020) R

1. Define electrochemical cell. What happens if external potential applied becomes greater than E_cell of electrochemical cell?

(Al 2019, 2/5, Al 2016) [II]

2.2 Galvanic Cells

MCQ.

1. The correct cell to represent the following reaction IS

<math>Zn + 2Ag^{+} \longrightarrow Zn^{2+} + 2Ag</math>

1. 2Ag | Ag⁺ || Zn | Zn²⁺ (b) Ag⁺ | Ag || Zn²⁺ | Zn

(2023)

1. <math>Ag | Ag^{+} | | Zn | Zn^{2+}</math> (d) <math>Zn | Zn^{2+} | | Ag^{+} | Ag</math>

(2023)

1. If the standard electrode potential of an electrode is greater than zero, then we can infer that its
2. reduced form is more stable compared to hydrogen gas
3. oxidised form is more stable compared to hydrogen gas
4. reduced and oxidised forms are equally stable.
5. reduced form is less stable than the hydrogen gas. (2020) 1

2.3 Nernst Equation

MCQ

1. <math>\Delta G</math> and <math>E_{cell}^o</math> for a spontaneous reaction will be
2. positive, negative (b) negative, negative
3. negative, positive (d) positive, positive.

(2/3,Al 2014) An

1. <math>Ag_{(aa)}^+ + e^- \longrightarrow Ag_{(s)}</math>; <math>E^0 = +0.80 \text{ V}</math> <math>Fe_{(a)}^{2+} + 2e^- \longrightarrow Fe_{(s)}; \quad E^\circ = -0.44 \text{ V}</math> Find the E cell for : <math>Fe_{(s)} + 2Ag^{+}_{(aa)} \longrightarrow Fe^{2+}_{(aa)} + 2Ag_{(s)}</math>
2. 1.6 V (b) -1.16 V (c) 2.04 V (d) 1.24 V

(2023)

Consider the following standard electrode potential

values <math>Sn^{2+}_{(aa)} + 2e^{-} \rightarrow Sn_{(s)}</math> <math>E^{\circ} = -0.14 \text{ V}</math>

<math>Fe^{3+}_{(aa)} + e^{-} \rightarrow Fe^{2+}_{(aa)} E^{o} = +0.77 V</math>

What is the cell reaction and potential for the

spontaneous reaction that occurs?

(a) <math>2Fe^{2+}_{(aq)} + Sn^{2+}_{(aq)} \rightarrow 2Fe^{3+}_{(aq)} + Sn_{(s)}</math>;

<math>E^{\circ} = -0.91 \text{ V}</math>

(b) <math>2Fe^{3+}_{(aq)} + Sn_{(s)} \rightarrow 2Fe^{2+}_{(aq)} + Sn^{2+}_{(aq)}</math>;

<math>E^{\circ} = +0.91 \text{ V}</math>

(c) <math>2Fe^{2+}_{(aq)} + Sn^{2+}_{(aq)} \rightarrow 2Fe^{3+}_{(aq)} + Sn_{(s)}</math>;

<math>F^{\circ} = +0.91 \text{ V}</math>

(d) <math>2Fe^{3+}_{(aa)} + Sn_{(s)} \rightarrow 2Fe^{2+}_{(aa)} + Sn^{2+}_{(aa)}</math>;

<math>E^{\circ} = +1.68 \text{ V}</math>

(2023)

(2 marks)

Calculate the emf of the following cell at 298 K:

Given <math>E_{cell}^o = 0.44 \text{ V}</math>. <math>Fe_{(s)} | Fe^{2+}(0.01 M)| | H^{+}_{(1M)} | H_{2(g)} (1 bar), Pt_{(s)}</math> (2023)

Calculate Δ,G° for the cell reaction at 25°C:

Zn | Zn2+ | Cd2+ | Cd

Given that: <math>E^{\circ}_{Zn^{2+}/Zn} = -0.76 \text{ V}</math>,

<math>E^{\circ}_{Cd^{2+}/Cd} = 0/.40 \text{ V}, 1 \text{ F} = 96500 \text{ C mol}^{-1}</math>

Write the Nernst equation for the following cell

reaction: <math>Zn_{(s)} + Cu_{(aq)}^{2+} \rightarrow Zn_{(aq)}^{2+} + Cu_{(s)}</math>

How will the Ecell be affected when concentration of

(i) Cu2+ ions is increased and

(ii) Zn2+ ions is increased? (Term II, 2021-22)

1. For an electrochemical cell <math>Mg_{(s)} + Ag^{+}_{(aa)} \longrightarrow Ag_{(s)} + Mg^{2+}_{(aa)}</math> give the cell representation. Also write the Nernst equation for the above cell at 25°C. (2020)
2. Calculate the emf of the following cell at 25°C: <math>Al_{(s)} |Al^{3+} (0.001 M)| |Ni^{2+} (0.1 M)| Ni_{(s)}</math> Given: <math>E_{(Ni^{2+}/Ni)}^{\circ} = -0.25 \text{ V}</math> E(A)3+/AI) = -1-66 V <math>[\log 2 = 0.3010, \log 3 = 0.4771]</math> (2019) Ev

Calculate Δ,G° for the reaction:

<math>Mg_{(s)} + Cu_{(aa)}^{2+} \longrightarrow Mg_{(aa)}^{2+} + Cu_{(s)}</math>

Given <math>E_{cell}^o = +2.71 \text{ V}</math>, <math>1 \text{ F} = 96500 \text{ C mol}^{-1}</math>

Equilibrium constant <math>(K_c)</math> for the given cell reaction is

1. Calculate Ecell- <math>A_{(s)} + B_{(aq)}^{2+} = A_{(aq)}^{2+} + B_{(s)}</math> (2/3, Foreign 2014)

(3 marks)

Write the Nernst equation and calculate the emf of

the following cell at 298 K:

<math>Zn | Zn^{2+} (0.001 M) | H^{+} (0.01 M) | H_{2(g)} (1 bar) | Pt_{(s)}</math> Given: <math>E_{Zn}^{\circ}^{2}+/Z_{n} = -0.76 \text{ V}, E_{H}^{\circ}+/H_{2} = 0.00 \text{ V}, [log 10 = 1]</math> (Term II, 2021-22, Foreign 2015) [67]

Calculate Δ,G° and log K, for the following cell: <math>Ni_{(s)} + 2Ag^{+}_{(aa)} \rightarrow Ni^{2+}_{(aa)} + 2Ag_{(s)}</math> Given that <math>E_{cell}^{\circ} = 1.05 \text{ V}</math>, <math>1F = 96,500 \text{ C mol}^{-1}</math> (Term II, 2021-22) OR Calculate the maximum work and logK_c for the given reaction at 298 K: <math>Ni_{(s)} + 2Ag^{+}_{(aa)} \rightleftharpoons Ni^{2+}_{(aa)} + 2Ag_{(s)}</math> Given: E_Ni²⁺/Ni = -0.25 V, E_Ag⁺/Ag = + 0.80 V 1F = 96500 C mol-1 (2020) OR For the cell reaction, <math>Ni_{(s)} |Ni_{(aa)}^{2+}||Ag_{(aa)}^{+}|Ag_{(s)}^{-}|</math> Calculate the equilibrium constant at 25°C. How much maximum work would be obtained by operation of this cell? <math>E_{(Ni^{2+}/Ni)}^{\circ} = -0.25 \text{ V} \text{ and } E_{Ag^{+}/Ag}^{\circ} = 0.80 \text{ V}</math>

(3/5, Delhi 2015C)

1. Calculate the e.m.f. of the following cell at 298K: <math>Fe_{(s)}|Fe^{2+}(0.001 M)||H^{+}(0.01 M)|H_{2(g)}(1 bar)|Pt_{(s)}</math> Given that <math>E_{cell}^o = +0.44 \text{ V}</math> <math>[\log 2 = 0.3010, \log 3 = 0.4771, \log 10 = 1]</math> (Term II, 2021-22) (Ev OR Calculate emf of the following cell at 25°C: <math>Fe|Fe^{2+}(0.001 M)||H^{+}(0.01 M)|H_{2(g)}(1 bar)|Pt_{(g)}</math> <math>E^{\circ}_{(Fe^{2+}/Fe)} = -0.44 \text{ V}, E^{\circ}_{(H^{+}/H_{2})} = 0.00 \text{ V}</math> (Delhi 2015)

Calculate ΔG° for the reaction, <math>Zn_{(s)} + Cu^{2+}_{(aa)} \rightarrow Zn^{2+}_{(aa)} + Cu_{(s)}</math> Given: <math>E^{\circ}</math> for <math>Zn^{2+}/Zn = -0.76 \text{ V}</math> and <math>E^{\circ}</math> for <math>Cu^{2+}/Cu = +0.34 \text{ V}</math> <math>R = 8.314 \text{ J K}^{-1} \text{ mol}^{-1}</math>, <math>1 F = 96500 \text{ C mol}^{-1}</math>. (3/5, 2020)

Calculate e.m.f of the following cell: <math>Zn_{(s)} | Zn^{2+}(0.1 M) | | Ag^{+}(0.01 M) | Ag_{(s)}</math> Given: <math>E_{Zn^{2+}/Zn} = -0.76 \text{ V}, E_{Ag^{+}/Ag} = +0.80 \text{ V}</math> [Given: <math>log 10 = 1</math>] (3/5, 2020)

1. Calculate <math>\Delta_r G^\circ</math> and <math>\log K_c</math> for the following reaction. <math>Cd_{(aa)}^{2+} + Zn_{(s)} \longrightarrow Zn_{(aa)}^{2+} + Cd_{(s)}</math> Given: <math>E_{Cd}^{2}+_{ACd} = -0.403 \text{ V}</math>; <math>E_{Zn}^{2}-_{AZn} = -0.763 \text{ V}</math> (AI 2019) (Ap)
2. Zinc rod is dipped in 0.01 M solution of zinc sulphate when temperature is 298 K. Calculate the electrode potential of zinc. (Given: <math>E_{Z_0}^{\circ}^{2+}/Z_0 = = -0.76 \text{ V}</math>; log 10 = 1) (2019C) [1]
3. Write the cell reaction and calculate the e.m.f. of the following cell at 298 K. <math>Sn_{(s)} | Sn^{2+} (0.004 M) | H^{+}(0.020 M) | H_{2(g)} (1 bar) | Pt_{(s)}</math> (Given: <math>E_{Sn^{2+}/Sn}^{o} = -0.14 \text{ V}</math>) (3/5, 2018)

For the reaction,

<math>2AgCl_{(s)} + H_{2(g)} (1 atm) \rightarrow 2Ag_{(s)} + 2H^{+}(0.1 M)</math>

+ 2Cl*(0.1 M)

<math>\Delta G^{\circ} = -43600 \text{ J at } 25 \,^{\circ}\text{C}.</math>

Calculate the e.m.f. of the cell. (<math>log 10^{-n} = -n</math>)

(3/5, 2018)

Calculate e.m.f. of the following cell at 298 K.

<math>2Cr_{(s)} + 3Fe^{2+}(0.1M) \rightarrow 2Cr^{3+}(0.01M) + 3Fe_{(s)}</math>

Given: <math>E_{(Cr^{3+}/Cr)}^{\circ} = -0.74 \text{ V}, E_{(Fe^{2+}/Fe)}^{\circ} = -0.44 \text{ V}</math>

(Delhi 2016)

Calculate E°_cell for the following reaction at 298 K.

<math>2Al_{(s)} + 3Cu^{2+}(0.01M) \rightarrow 2Al^{3+}(0.01M) + 3Cu_{(s)}</math>

Given: <math>E_{rell} = 1.98 \text{ V}</math> (3/5,Al 2016)

Calculate the standard cell potential of the galvanic

cell in which the following reaction takes place:

<math>Fe_{(aa)}^{2+} + Ag_{(aa)}^{+} \rightarrow Fe_{(aa)}^{3+} + Ag_{(s)}</math>

Calculate the <math>\Delta_rG^\circ</math> and equilibrium constant of the

reaction. <math>(E_{Ag^{+}/Ag}^{*} = 0.80 \text{ V}; E_{Fe^{3+}/Fe^{2+}}^{*} = 0.77 \text{ V})</math>

(3/5, Delhi 2015C) III

Calculate the emf of following cell at 298 K:

<math>Mg_{(s)} | Mg^{2+}(0.1 M) | Cu^{2+}(0.01 M) | Cu_{(s)}</math>

[Given: <math>E_{cell}^o = +2.71 \text{ V}, 1 \text{ F} = 96500 \text{ C mol}^{-1}</math>]

(3/5, Delhi 2014)

Estimate the minimum potential difference needed

to reduce Al₂O₃ at 500°C. The Gibbs energy change

for the decomposition reaction,

<math>\frac{2}{3}</math>Al₂O₃ <math>\rightarrow \frac{4}{3}</math>Al+O₂ is 960 kJ. (F = 96500 C mol^-1) (3/5, Delhi 2014C)

(5 marks)

<math>E^{\circ}_{cell}</math> for the given redox reaction is 2.71 V.

<math>Mg_{(s)}+Cu^{2+}(0.01M)\rightarrow Mg^{2+}(0.001M)+Cu_{(s)}</math>

Calculate E_cell for the reaction. Write the direction of

flow of current when an external opposite potential

applied is less than 2.71 V and

(ii) greater than 2.71 V (Delhi 2019)

Calculate e.m.f and <math>\Delta G</math> for the following cell

<math>Mg_{(s)} | Mg^{2+} (0.001 M) | Cu^{2+} (0.0001 M) | Cu_{(s)}</math>

<math>E_{(Mg^{2*}/Mg)}^{*} = -2.37 \text{ V,} E_{(Cu^{2*}/Cu)}^{*} = +0.34 \text{ V}</math>

(NCERT, AI 2015)

2.4 Conductance of Electrolytic Solutions

MCQ

The unit of molar conductivity is

1. S cm^-2 mol^-1<br>(c) S^-1 cm² mol^-1 (d) Scm2 mol (b) S cm2 mol-1 (2023)

Assertion (A): Conductivity of an electrolyte

increases with decrease in concentration.

Reason (R): Number of ions per unit volume

decreases on dilution.