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Electrochemistry question

2019 · 10 Jan · Shift 2 · Q11
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Electrochemistry question

2019 · 10 Jan · Shift 2 · Q11

JEE MainChemistryElectrochemistryMCQ+4 / −1
In the cell Pt ∣(s)∣\left| {\left( s \right)} \right|∣(s)∣ H2H_2H2​(g, 1 bar) ∣HCl(aq)∣\left| {HCl\left( {aq} \right)} \right|∣HCl(aq)∣ AgCl ∣(s)∣\left| {\left( s \right)} \right|∣(s)∣ Ag(s)|Pt(s) the cell potential is 0.92 V when a 10–6 molal HClHClHCl solution is used. The standard electrode potential of (AgCl/ AgCl−AgCl^-AgCl− ) electrode is : {\left\{ {} \right.{ Given, 2.303RTF=0.06V{{2.303RT} \over F} = 0.06VF2.303RT​=0.06V at 298}\left. {298} \right\}298}
  1. A
    0.94 V
  2. B
    0.40 V
  3. C
    0.76 V
  4. D
    0.20 V
View written solutionFree

Correct answer: D

  1. Identify the half-cells

The cell is:

Pt∣H2(g,1 bar)∣HCl(aq)∣AgCl(s)∣Ag(s)∣Pt\text{Pt} | H_2(g,1\,\text{bar}) | HCl(aq) | AgCl(s) | Ag(s) | \text{Pt}Pt∣H2​(g,1bar)∣HCl(aq)∣AgCl(s)∣Ag(s)∣Pt

This consists of:

  • Left electrode: hydrogen electrode
  • Right electrode: silver-silver chloride electrode

So,

Ecell=Eright−EleftE_{\text{cell}} = E_{\text{right}} - E_{\text{left}}Ecell​=Eright​−Eleft​

Given:

Ecell=0.92 VE_{\text{cell}} = 0.92\,\text{V}Ecell​=0.92V

and concentration of HCl is 10−610^{-6}10−6 molal, so for this dilute solution we take

[H+]=[Cl−]=10−6[H^+] = [Cl^-] = 10^{-6}[H+]=[Cl−]=10−6

  1. Potential of hydrogen electrode

For the hydrogen electrode reaction

2H++2e−→H2(g)2H^+ + 2e^- \rightarrow H_2(g)2H++2e−→H2​(g)

Nernst equation:

EH+/H2=E∘−0.062log⁡PH2[H+]2E_{H^+/H_2} = E^\circ - \frac{0.06}{2} \log \frac{P_{H_2}}{[H^+]^2}EH+/H2​​=E∘−20.06​log[H+]2PH2​​​

Since E∘=0E^\circ=0E∘=0 and PH2=1P_{H_2}=1PH2​​=1 bar,

EH+/H2=−0.062log⁡1(10−6)2E_{H^+/H_2} = -\frac{0.06}{2} \log \frac{1}{(10^{-6})^2}EH+/H2​​=−20.06​log(10−6)21​

=−0.03log⁡(1012)= -0.03 \log (10^{12})=−0.03log(1012)

=−0.03×12=−0.36 V= -0.03 \times 12 = -0.36\,\text{V}=−0.03×12=−0.36V

So,

Eleft=−0.36 VE_{\text{left}} = -0.36\,\text{V}Eleft​=−0.36V

  1. Potential of AgCl/Ag electrode

The electrode reaction is

AgCl(s)+e−→Ag(s)+Cl−AgCl(s) + e^- \rightarrow Ag(s) + Cl^-AgCl(s)+e−→Ag(s)+Cl−

For this electrode,

E=E∘−0.06log⁡[Cl−]E = E^\circ - 0.06 \log [Cl^-]E=E∘−0.06log[Cl−]

Since [Cl−]=10−6[Cl^-] = 10^{-6}[Cl−]=10−6,

Eright=E∘−0.06log⁡(10−6)E_{\text{right}} = E^\circ - 0.06 \log(10^{-6})Eright​=E∘−0.06log(10−6)

=E∘−0.06(−6)= E^\circ - 0.06(-6)=E∘−0.06(−6)

=E∘+0.36= E^\circ + 0.36=E∘+0.36

  1. Use cell potential

Now,

Ecell=Eright−EleftE_{\text{cell}} = E_{\text{right}} - E_{\text{left}}Ecell​=Eright​−Eleft​

0.92=(E∘+0.36)−(−0.36)0.92 = (E^\circ + 0.36) - (-0.36)0.92=(E∘+0.36)−(−0.36)

0.92=E∘+0.720.92 = E^\circ + 0.720.92=E∘+0.72

E∘=0.92−0.72=0.20 VE^\circ = 0.92 - 0.72 = 0.20\,\text{V}E∘=0.92−0.72=0.20V

  1. Final answer

Therefore, the standard electrode potential of the (AgCl/Ag,Cl−)(AgCl/Ag,Cl^-)(AgCl/Ag,Cl−) electrode is

0.20 V\boxed{0.20\,\text{V}}0.20V​

So the correct option is D.

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