Electrolysis Trick: Which Metal Gets Deposited First? ⚡️

❓ Question

1 M aqueous solutions of each of $\mathrm{Cu(NO_3)_2},\ \mathrm{AgNO_3},\ \mathrm{Hg_2(NO_3)_2},\ \mathrm{Mg(NO_3)_2}$ are electrolysed using inert electrodes.

Statement (I): With increasing (more negative) cathode potential (i.e. increasing applied voltage), the sequence of deposition of metals on the cathode will be Ag, Hg and Cu.
Statement (II): Magnesium will not be deposited at cathode; instead oxygen gas will be evolved at the cathode.

Decide which statement(s) are correct and explain.

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🖼️ Question Image

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✍️ Short Solution

Key idea: Which species is reduced at the cathode depends on standard (or actual) reduction potentials. The species with the most positive reduction potential is reduced first (at the least negative cathode potential). As the applied cathode potential is driven more negative, species with lower reduction potentials begin to reduce.

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🔹 Standard reduction potentials (approx., vs SHE)

• $\mathrm{Ag^+ + e^- \to Ag}$: $E^\circ \approx +0.80\ \mathrm{V}$
  

• $\mathrm{Hg_2^{2+} + 2e^- \to 2Hg}$: $E^\circ \approx +0.80\ \mathrm{V}$ (very close to Ag)

• $\mathrm{Cu^{2+} + 2e^- \to Cu}$: $E^{\circ }\approx +0.34\mathrm{V}$

• $\mathrm{M}{\mathrm{g}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{M}\mathrm{g: }$$E^\circ \approx -2.37\ \mathrm{V}$
  

Also important: water reduction in neutral/basic aqueous solution:

$$2\,\mathrm{H_2O} + 2e^- \to \mathrm{H_2} + 2\,\mathrm{OH^-},\quad E^\circ \approx -0.83\ \mathrm{V\ (pH\ 7)}$$

(or $\mathrm{2H^+ + 2e^- \to H_2}$ at 0 V under strongly acidic conditions).

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🔹 Interpreting the numbers

• Because Ag⁺ and Hg₂²⁺ have the highest (most positive) reduction potentials, they will be reduced (deposit) first as cathode potential becomes negative. Between Ag and Hg the potentials are very close; small kinetic/overpotential differences determine which appears first practically — Ag⁺ generally deposits easily, so sequence Ag → Hg → Cu is consistent with the potentials given. Thus Statement (I) is essentially correct.

• Magnesium ion has a very negative reduction potential (≈ −2.37 V). Before Mg²⁺ can be reduced to Mg metal, the applied potential required would already be negative enough to reduce water to hydrogen. Therefore in aqueous electrolysis Mg is not deposited; instead hydrogen gas (not oxygen) evolves at the cathode. Oxygen evolution occurs at the anode (oxidation of water). So Statement (II) is incorrect because it names the wrong gas (oxygen) and the wrong electrode process.

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🧮 Image Solution

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✅ Conclusion & Final Answer

• Statement (I): True (Ag, then Hg, then Cu is the correct order of deposition as cathode potential becomes more negative).

• Statement (II): False — Mg²⁺ will not be deposited in aqueous solution; hydrogen gas (H₂) is evolved at the cathode, while oxygen evolves at the anode.