Electro Chemistry

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Questions:

Multiple Choice Question

24 x 1 = 24

Cell equation: A + 2B^- \(\rightarrow\)A²⁺+ 2B; A²⁺ + 2e^- \(\rightarrow\)A E^o = +0.34V and log₁₀ K = 15.6 at 300K for cell reactions find E^o for B⁺ + e⁻ \(\rightarrow\) B

(a)

0.80

(b)

1.26

(c)

-0.54

(d)

-10.94

A gas X at 1 atm is bubbled through a solution containing a mixture of 1MY^-and 1MZ^- at 25^oC. If the reduction potential of Z>Y>X, then_____.

(a)

Y will oxidize X and not Z

(b)

Y will oxidize Z and not X

(c)

Y will oxidize both X and Z

(d)

Y will reduce both X and Z

A certain current liberated 0.504gm of hydrogen in 2 hours. How many grams of copper can be liberated by the same current flowing for the same time in a copper sulphate solution ______.

(a)

31.75

(b)

15.8

(c)

7.5

(d)

63.5

For the cell reaction
2Fe³⁺(aq) + 2l⁻(aq) \(\rightarrow\)2Fe²⁺ (aq) + 1₂ (aq)
E^o_cell= 0.24V = at 298K. The standard Gibbs energy (Δ, G^o) of the cell reactions is:

(a)

-46.32 KJ mol⁻¹

(b)

-23.16 KJ mol^-1

(c)

46.32 KJ mol⁻¹

(d)

23.16 KJ mol^-1

Consider the change in oxidation state of Bromine corresponding to different emf values as shown in the diagram below:
\({ BrO }_{ 4 }^{ - }\overset { 1.82V }{ \longrightarrow } { BrO }_{ 3 }^{ - }\overset { 1.5V }{ \longrightarrow } HBrO\overset { 1.595v }{ \longrightarrow } { Br }_{ 2 }\overset { 1.0652V }{ \longrightarrow } Br^{ - }\)
Then the species undergoing disproportional is

(a)

Br₂

(b)

BrO₄^-

(c)

BrO^-₃

(d)

HBrO

In the electrochemical cell: Zn|ZnSO₄ _(0.01M)|| CuSO₄ _(1.0M)|Cu, the emf of this Daniel cell is E₁. When the concentration of ZnSO₄ is changed to 1.0M and that CuSO₄ changed to 0.01M, the emf changes to E₂. From the above, which one is the relationship between E₁ and E₂?

(a)

E₁ < E₂

(b)

E₁ > E₂

(c)

E₂ ≥ E₁

(d)

E₁= E₂

Conductivity of a saturated solution of a sparingly soluble salt AB (1:1 electrolyte) at 298K is 1.85 ×10⁻⁵ S m⁻¹. Solubility product of the salt AB at 298K (Λ^o_m)_AB = 14 ×10⁻³S m² mol⁻¹.

(a)

5.7 x 10^-12

(b)

1.32 x 10⁻¹²

(c)

7.5 x 10⁻¹²

(d)

1.74 x 10^-12

A conductivity cell has been calibrated with a 0.01M, 1:1 electrolytic solution (specific conductance (k = 1.25 x 10^-3cm^-1) in the cell and the measured resistance was 800 Ω at 25^oC. The cell constant is ______.

(a)

10⁻¹ cm⁻¹

(b)

10¹ cm⁻¹

(c)

1 cm⁻¹

(d)

5.7 x 10^-12

The equivalent conductance of M/36 solution of a weak monobasic acid is 6 mho cm² equivalent^-1 and at infinite dilution is 400 mho cm² equivalent^-1. The dissociation constant of this acid is ______.

(a)

1.25 x 10⁻⁶

(b)

6.25 x 10^-6

(c)

1.25 x 10⁻⁴

(d)

6.25 x 10 ^-5

10.

In H₂-O₂fuel cell the reaction occur at cathode is _______.

(a)

O_2(g) + 2H₂O_(l) + 4e^- ⟶ 4OH⁻_(aq)

(b)

H⁺(aq) + OH⁻ (aq) ⟶ H₂O (l)

(c)

2H₂ (g) + O₂ (g) ⟶ 2H₂O (g)

(d)

H⁺ + e^- ⟶ 1/2 H₂

11.

Zinc can be coated on iron to produce galvanized iron but the reverse is not possible. It is because _______.

(a)

Zinc is lighter than iron

(b)

Zinc has lower melting point than iron

(c)

Zinc has lower negative electrode potential than iron

(d)

Zinc has higher negative electrode potential than iron

12.

Among the following cells
I) Leclanche cell
II) Nickel – Cadmium cell
III) Lead storage battery
IV) Mercury cell
Primary cells are ____.

(a)

I and IV

(b)

I and III

(c)

III and IV

(d)

II and III

13.

While charging lead storage battery _______.

(a)

PbSO₄ on cathode is reduced to Pb

(b)

PbSO₄ on anode is oxidised to PbO₂

(c)

PbSO₄ on anode is reduced to Pb

(d)

PbSO₄ on cathode is oxidised to Pb

14.

Which of the following electrolytic solution has the least specific conductance?

(a)

(b)

0.002N

(c)

0.02N

(d)

0.2N

15.

The number of electrons delivered at the cathode during electrolysis by a current of 1A in 60 seconds is ____. (charge of electron = 1.6 ×10⁻¹⁹C )

(a)

6.22 ×10²³

(b)

6.022 ×10²⁰

(c)

3.75 ×10²⁰

(d)

7.48 ×10²³

16.

During electrolysis of molten sodium chloride, the time required to produce 0.1mole of chlorine gas using a current of 3A is _____.

(a)

55 minutes

(b)

107.2 minutes

(c)

220 minutes

(d)

330 minutes

17.

A current strength of 3.86 A was passed through molten Calcium oxide for 41 minutes and 40 seconds. The mass of Calcium in grams deposited at the cathode is_______. (atomic mass of Ca is 40g / mol and 1F = 96500C).

(a)

(b)

(c)

(d)

18.

How many faradays of electricity are required for the following reaction to occur MnO₄^-→ Mn²⁺

(a)

(b)

(c)

(d)

19.

Faraday constant is defined as_______.

(a)

charge carried by 1 electron

(b)

charge carried by one mole of electrons

(c)

charge required to deposit one mole of substance

(d)

charge carried by 6.22 ×10¹⁰ electrons

20.

Electrolyte	KCl	KNO₃	HCl	NaOAC	NaCl
Λ_- (Scm² mol^-1)	149.9	145.0	426.2	91.0	126.5

Calculate Λ^o_HoAC using appropriate molar conductances of the electrolytes listed above at infinite dilution in water at 25^oC_______.

(a)

517.2

(b)

552.7

(c)

390.7

(d)

217.5

21.

The molar conductivity of a 0.5 mol dm^-3 solution of AgNO₃ with electrolytic conductivity of 5.76 ×10⁻³ S cm⁻¹at 298 K is______.

(a)

2.88 S cm²mol^-1

(b)

11.52 S cm²mol^-1

(c)

0.086 S cm²mol^-1

(d)

28.8 S cm²mol^-1

22.

The button cell used is watches function as follows
Zn (s) + Ag₂O (s) + H₂O (l) ⇌ 2Ag (s) + Zn²⁺ (aq) + 2OH^-(aq) the half cell potentials are Ag₂O (s) + H₂O (l) + 2e^- → 2Ag (s) + 2OH^- (aq) E^o= 34V and Zn (s) → Zn²⁺ (aq) + 2e⁻ E⁰ = 0.76V . The cell potential will be_______.

(a)

0.84V

(b)

1.34V

(c)

1.10V

(d)

0.42V

23.

Consider the following half cell reactions.
Mn²⁺ + 2e^- ➝ Mn E^o = -1.18V
Mn²⁺ ➝ Mn³⁺ + e^- E^o = -1.51V
The E^o for the reaction 3Mn²⁺➝ Mn + 2Mn³⁺, and the possibility of the forward reaction are respectively.

(a)

2.69V and spontaneous

(b)

-2.69 and non spontaneous

(c)

0.33V and Spontaneous

(d)

4.18V and non spontaneous

24.

The number of electrons that have a total charge of 9650 coulombs is_______.

(a)

6.22 ×10²³

(b)

6.022 ×10²⁴

(c)

6.022 ×10²²

(d)

6.022 ×10⁻³⁴

Assertion and reason

1 x 2 = 2

25.

Assertion: pure iron when heated in dry air is converted with a layer of rust.
Reason: Rust has the composition Fe₃ O₄
Codes:
a) if both assertion and reason are true and reason is the correct explanation of assertion
b) if both assertion and reason are true but reason is not the correct explanation of assertion
c) assertion is true but reason is false
d) both assertion and reason are false

(a)

if both assertion and reason are true and reason is the correct explanation of assertion

(b)

if both assertion and reason are true but reason is not the correct explanation of assertion

(c)

assertion is true but reason is false

(d)

both assertion and reason are false

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Answers Key:

Multiple Choice Question

24 x 1 = 24

(a)
0.80

Z is tie strongest oxidising agent (High SRP)
X is the strongest reducing agent (Low SRP)

\(\frac{m_{1}}{m_{2}} = \frac{E_{1}}{E_{2}} \)
\(\frac{0.504}{m_{2}} = \frac{1.008}{31.77} \)
m₂ = 15.885 g

ΔG^o = -nFE^o_cell
= -2 x 96500 x 0.24
= - 46320 J mol^-1
= -46.32 KJ mol⁻¹

(E_cell)_A = -1.82 + 1.5 = -0.32V
(E_cell)_B = -1.5 + 1.595 = + 0.095V
(E_cell)_C = -1.595 + 1.0652 = -0.529V
The species undergoing disproportionation is HBrO

E_cell= E^o_cell- \(\frac{0.0591}{2} \log [\frac{Zn^{2+}}{Cu^{2+}}]\)
E_l= E^o_cell- \(\frac{0.0591}{2} \log [\frac{10^{-2}}{1}]\)
E_l= E^o_cell+ 0.0591 ...(1)
E₂= E^o_cell- \(\frac{0.0591}{2} \log [\frac{1}{10^{-2}}]\)
E₂= E^o_cell- 0.0591 ...(2)
E₁ > E₂
Zn_(s)| ⟶ Zn²⁺_(aq) + 2e^-
Cu²⁺_(aq) + 2e^-⟶ Cu_(s)
Zn_(s) + Cu²⁺_(aq) ⟶ Zn²⁺_(aq) + Cu_(s)

For 1:1 electrolyte K_sp = S²
\(=[\frac{ K \times 10^{-3}}{Λ^o}]^2\)
\(=[\frac{1.85 \times 10^{-5} \times 10^{-3}}{14 \times 10^{-3}}]^2\)
= (0.1321 x 10^-3)²
= 0.01745 x 1010^-10
= 1.74 x 10^-12

R = ρ.l/A
Cell constant = R/ρ
\(= k.R (l/ \rho = K)\)
= 1.25 x 10^-3 Ω^-1 cm^-1 x 800Ω
= 1 cm^-1

α = Λ/Λ^o
= 6/400
K_a = α²C
\(= \frac{6}{400} \times \frac{6}{400} \times \frac{1}{36}\)
= 6.25 x 10^-6

10.

(a)
O_2(g) + 2H₂O_(l) + 4e^- ⟶ 4OH⁻_(aq)

11.

E^o_Zn²⁺[Zn]= 0.76V and E^o_Fe²⁺[Fe]= -0.44V
Zinc has higher negative electrode potential than iron, iron cannot be coated on zinc

12.

(a)
I and IV

13.

Charging anode:
PbSO_4(s) + 2e^- ➝ Pb_(s) + SO₄^2-_(aq)
Cathode:
PbSO_4(s) + 2H₂O_(I) ➝ PbO_2(s) + SO₄^2-_(aq) + 2e^-

14.

In general, specific conductance of an electrolyte decreases with dilution.So,0.002N solution has least specific conductance.

15.

Q = It
= 1A x 60s = 60 C
96500 C charge = 6.023 x 10²³ electrons
60 C charge \(= \frac{6.023 \times 10^{23}}{96500}\times 60\)
= 3.744 x 10²⁰ electrons

16.

mass of 1 mole of CI₂ gas = 71
∴ mass of 0.1 mole of Cl₂ gas = 7.1 g mol^-1
m = Zlt
t = m/ZI
\(= \frac{7.1}{\frac{71}{2 \times 96500} \times 3}\) (2Cl^- ➝ Cl₂+ 2e^-)
\(= \frac{2 \times 96500 \times 7.1}{71 \times 3}\)
= 6433.33s = 107.2 min

17.

mg = ZIt
\(= \frac{40 \times 3.86 \times 2500}{2 \times 96500} = 2g\)
(∵ t = 41 min 40 sec = 2500 seconds & \(Z = \frac{m}{n \times 96500} = \frac{40}{x \times 96500})\)

18.

7MnO₄^- + 5e^- → Mn²⁺ + 4H₂O
5 moles of electrons i.e., 5F charge is required.

19.

IF = 96500 C = charge of one mole of e^- charge of 6.022 x 10^-23 electron

20.

(Λ^o_HoAC) = [(Λ^o)_HCl + Λ^o)_NaOAC ] - (Λ^o)_NaCl
= (426.2 + 91) - (126.5)
= 390.7

21.

Λ = k/W x 10⁻³ mol⁻¹ dm³
\(=\frac{ 5.76 \times 10^{−3} S cm^{−1} \times 10^{−3}}{0.5} = mol^{-1} dm^{3}\)
\(=\frac{ 5.76 \times 10^{−3} S cm^{−1} \times 10^{−3}}{0.5}\) S cm^-1 mol⁻¹ dm³
= 11.52 S cm²mol^-1

22.

Anodic Oxidation: (Reverse the given reaction)
(E^o_ox) = 0.76 V Cathodic reduction
E^o_cell= (E^o_ox)+ (E^o_red)
= 0.76 + 0.34 = 1.1 V

23.

Mn²⁺ + 2e^- ➝ Mn E^o = -1.18V
Mn²⁺ ➝ Mn³⁺ + e^- E^o = -1.51V
3Mn²⁺➝ Mn³⁺ + 2Mn³⁺E^o_cell= ?
E^o_cell= (E^o_ox)+ (E^o_red)
= 1.51 - 1.18 and non spontaneous
= -2.69V
Since E°is ve ΔG is +ve and the given forward cell reaction is non- spontaneous