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Electrochemistry question

2004 · Shift 0 · Q39
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Electrochemistry question

2004 · Shift 0 · Q39

JEE MainChemistryElectrochemistryMCQ+4 / −1
In a cell that utilises the reaction ZnZnZn(s) + 2H+H^+H+ (aq) →\to→ Zn2+Zn^{2+}Zn2+(aq) + H2H_2H2​(g) addition of H2SO4H_2SO_4H2​SO4​ to cathode compartment, will
  1. A
    lower the E and shift equilibrium to the left
  2. B
    increases the E and shift equilibrium to the left
  3. C
    increase the E and shift equilibrium to the right
  4. D
    Lower the E and shift equilibrium to the right
View written solutionFree

Correct answer: C

  1. Identify the cell reaction

The cell reaction is Zn(s)+2H+(aq)→Zn2+(aq)+H2(g)Zn(s) + 2H^+(aq) \rightarrow Zn^{2+}(aq) + H_2(g)Zn(s)+2H+(aq)→Zn2+(aq)+H2​(g)

  • Oxidation at anode: Zn(s)→Zn2+(aq)+2e−Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-Zn(s)→Zn2+(aq)+2e−
  • Reduction at cathode: 2H+(aq)+2e−→H2(g)2H^+(aq) + 2e^- \rightarrow H_2(g)2H+(aq)+2e−→H2​(g)

So, the cathode compartment contains H+H^+H+ ions.


  1. Write the reaction quotient

For the overall reaction, Q=[Zn2+] PH2[H+]2Q = \frac{[Zn^{2+}]\,P_{H_2}}{[H^+]^2}Q=[H+]2[Zn2+]PH2​​​

since solid ZnZnZn is omitted.


  1. Use the Nernst equation

At a given temperature, E=E∘−0.05912log⁡QE = E^\circ - \frac{0.0591}{2}\log QE=E∘−20.0591​logQ

Substituting QQQ: E=E∘−0.05912log⁡([Zn2+]PH2[H+]2)E = E^\circ - \frac{0.0591}{2}\log\left(\frac{[Zn^{2+}]P_{H_2}}{[H^+]^2}\right)E=E∘−20.0591​log([H+]2[Zn2+]PH2​​​)


  1. Effect of adding H2SO4H_2SO_4H2​SO4​ to cathode compartment

H2SO4H_2SO_4H2​SO4​ is a strong acid, so adding it increases [H+][H^+][H+].

From Q=[Zn2+]PH2[H+]2Q = \frac{[Zn^{2+}]P_{H_2}}{[H^+]^2}Q=[H+]2[Zn2+]PH2​​​

if [H+][H^+][H+] increases, then QQQ decreases.

Now from Nernst equation:

  • If QQQ decreases, then log⁡Q\log QlogQ decreases.
  • Therefore, the term −0.05912log⁡Q-\frac{0.0591}{2}\log Q−20.0591​logQ increases.
  • Hence, cell emf EEE increases.

  1. Effect on equilibrium

The reaction consumes H+H^+H+: Zn(s)+2H+(aq)→Zn2+(aq)+H2(g)Zn(s) + 2H^+(aq) \rightarrow Zn^{2+}(aq) + H_2(g)Zn(s)+2H+(aq)→Zn2+(aq)+H2​(g)

By Le Chatelier’s principle, increasing concentration of a reactant (H+H^+H+) shifts the equilibrium to the right to consume added H+H^+H+.


  1. Check options
  • A: lower EEE, shift left — incorrect
  • B: increase EEE, shift left — incorrect
  • C: increase EEE, shift right — correct
  • D: lower EEE, shift right — incorrect

  1. Final answer

The addition of H2SO4H_2SO_4H2​SO4​ to the cathode compartment will:

  • increase the emf EEE
  • shift equilibrium to the right

So the correct option is C.

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