Q.Suggest a list of metals that are extracted electrolytically.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Standard Electrode Potentials
Standard Electrode Potentials: A Number for "How Badly It Wants Electrons"
Dip a zinc rod into a zinc-salt solution and a tiny tug-of-war begins at the surface:
metal atoms tend to dissolve as ions (leaving electrons behind on the rod) while ions
from the solution tend to deposit as metal (consuming electrons). The rod ends up with
a characteristic electrical potential relative to the solution — the electrode potential. It is a direct measure of the tendency of that redox couple to gain or
lose electrons.
The Core Idea
Different couples pull electrons with very different strengths. Copper's ion grabs
them readily; zinc's barely wants them. Put a number on each couple and you can
predict, before mixing anything, who will oxidise whom.
Two conventions make the numbers comparable:
- Standard conditions. Every species at unit concentration (1 M), any gas at 1 atm, temperature 298 K. The potential measured then is the standard electrode potential, written E⊖.
- A common zero. Potentials can only be measured as differences, so one electrode is defined as the reference: the standard hydrogen electrode (SHE), 2H++2e−→H2, is fixed at exactly 0.00 V. Every E⊖ is the voltage of a couple measured against it.
By convention the values are tabulated for the reduction direction:
Oxidised form+ne−→Reduced formE⊖ (in volts, at 298 K)
Reading the Table
The standard-potential table (Table 7.1 in the Class 11 chapter) runs from
F2/F− at +2.87 V down to Li+/Li at −3.05 V.
Two rules unlock it:
- More positive E⊖ → stronger oxidising agent (the oxidised form is hungrier for electrons). F₂ tops the table; that is why fluorine oxidises almost everything.
- More negative E⊖ → stronger reducing agent (the reduced form gives electrons up most easily). Li, K, Ca, Na at the bottom are the great electron donors. A negative E⊖ means the couple is a stronger reducing agent than the H⁺/H₂ couple; a positive one, weaker.
Predicting Whether a Reaction Goes
For any proposed redox reaction, the species being reduced acts as the cathode couple
and the species being oxidised as the anode couple:
Ecell⊖=Ecathode⊖−Eanode⊖
A positive Ecell⊖ means the reaction is feasible
(spontaneous) under standard conditions; a negative one means the reverse reaction
is the spontaneous direction.
Worked feel: can Fe³⁺ oxidise iodide? E⊖(Fe3+/Fe2+)=+0.77 V is above E⊖(I2/I−)=+0.54 V, so
Ecell⊖=+0.23 V — yes. Can silver metal reduce Fe³⁺?
0.77−0.80=−0.03 V — no.
This is also the logic of the activity series: a metal displaces, from solution, …
Why this formula?
Galvanic Corrosion: Why the Key Formulas Hold
Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte. The key formula that governs this is the mixed potential theory, which leads to the galvanic current and corrosion rate expressions.
Let's build the reasoning step-by-step.
1. The Core Idea: Two Electrodes, One Circuit
When metals M₁ (more active, e.g., zinc) and M₂ (more noble, e.g., copper) are connected:
- M₁ acts as the anode — it oxidizes (corrodes):
M1→M1n++ne−
- M₂ acts as the cathode — it reduces something (e.g., oxygen or H⁺):
O2+2H2O+4e−→4OH−(in neutral/alkaline)
or
2H++2e−→H2(in acidic)
The two metals are electrically connected (via a wire or direct contact), and the electrolyte completes the circuit. Electrons flow from M₁ to M₂.
2. The Mixed Potential: Why It Exists
Each metal, when alone in the electrolyte, has its own open-circuit potential (OCP) — the equilibrium potential for its half-reaction. For M₁, it's Ecorr,1; for M₂, it's Ecorr,2.
When connected, the system cannot stay at two different potentials. The entire metal couple must reach a single potential — the mixed potential Emix.
- Emix lies between Ecorr,1 and Ecorr,2.
- At Emix, the total anodic current from M₁ equals the total cathodic current from M₂ (charge conservation):
Ianode=Icathode
This is the fundamental equation of galvanic corrosion.
3. Deriving the Galvanic Current
Assume each electrode follows Butler-Volmer kinetics (for activation-controlled reactions). For the anode (M₁), the anodic current density ia at potential E is:
ia=i0,1exp(RTαaF(E−E0,1))
For the cathode (M₂), the cathodic current density ic is:
ic=i0,2exp(−RTαcF(E−E0,2))
Where:
- i0,1,i0,2 = exchange current densities
- αa,αc = transfer coefficients (typically ~0.5)
- F = Faraday constant
- R = gas constant
- T = temperature
- E0,1,E0,2 = standard reduction potentials
At the mixed potential Emix:
Igalvanic=A1⋅ia(Emix)=A2⋅ic(Emix)
Where A1 and A2 are the surface areas of the anode and cathode.
Why this holds: The net current from the anode must exactly balance the net current consumed at the cathode — otherwise, charge would accumulate, which is impossible in a steady-state circuit.
4. The Corrosion Rate Formula
The corrosion rate (mass loss per time) of the anode is given by Faraday's law:
Corrosion rate=n⋅F⋅ρIgalvanic⋅M
Where:
- M = molar mass of the anode metal
- n = number of electrons transferred per atom
- ρ = density of the metal
- F = Faraday constant (96,485 C/mol)
Why this holds: Each mole of metal oxidized releases n moles of electrons. The total charge passed Q=Igalvanic⋅t corresponds to moles of metal lost:
moles lost=nFQ=nFIgalvanic⋅t
Multiply by M/ρ to get volume or thickness loss.
5. The Area Effect: Why It Matters
From the mixed potential equation:
A1⋅ia(Emix)=A2⋅ic(Emix)
If the cathode area A2 is large relative to the anode area A1, then ia(Emix) must be large to balance the current. This means:
- Small anode + large cathode → severe galvanic corrosion (high current density on the anode). …
Concept: Standard Electrode Potentials – metals with highly negative reduction potentials (E∘≪0) cannot be reduced by chemical reducing agents; they require electrolysis.
Reasoning:
- Electrolytic extraction is used for metals that are stronger reducing agents than carbon (i.e., above aluminium in the reactivity series).
- These metals have E∘ values more negative than about −1.5 V, making chemical reduction impossible. …
Metals that are extracted electrolytically are those too reactive to be reduced by carbon or other common chemical reducing agents — typically metals high in the electrochemical series (like Na, Mg, Al, Ca, K). The key idea: electrolysis supplies the large negative reduction potential needed to force their ions to gain electrons. The final list includes sodium, magnesium, aluminium, calcium, potassium, and a few others.
Why Electrolysis? The Concept of Standard Electrode Potentials
The method used to extract a metal from its ore depends on how tightly the metal holds onto its electrons — or, more precisely, how easily its ions can be reduced. This is quantified by the standard reduction potential (E∘).
Metals with very negative E∘ values (like ENa+/Na∘=−2.71 V) are strong reducing agents themselves — their ions are extremely reluctant to accept electrons. Carbon, the common reducing agent in pyrometallurgy, simply cannot supply enough reducing power. For example, carbon can reduce zinc oxide (EZn2+/Zn∘=−0.76 V) but fails for magnesium oxide (EMg2+/Mg∘=−2.37 V).
Electrolysis solves this by using an external voltage to force the reduction at the cathode. The applied voltage must exceed the decomposition potential of the molten or dissolved compound. This is why the most reactive metals — those at the top of the reactivity series — are extracted electrolytically.
For a metal ion Mn+, the minimum voltage required is Ecell=Eanode∘−Ecathode∘ (plus overpotential). The more negative Ecathode∘, the larger the voltage needed.
Step-by-Step Reasoning
-
Identify the reactivity criterion.
Metals with E∘<−1.0 V (roughly) are almost always extracted by electrolysis. This includes groups 1 and 2, aluminium, and a few others. Carbon reduction works for metals like Zn, Fe, Pb, Cu (moderately negative to positive E∘).
-
List the common electrolytically extracted metals from the NCERT/standard syllabus.
- Sodium (Na): Extracted by electrolysis of molten NaCl (Downs process). E∘=−2.71 V.
- Magnesium (Mg): Electrolysis of molten MgCl2 (from seawater or dolomite). E∘=−2.37 V.
- Aluminium (Al): Hall–Héroult process — electrolysis of Al2O3 dissolved in molten cryolite (Na3AlF6). E∘=−1.66 V.
- Calcium (Ca): Electrolysis of molten CaCl2. E∘=−2.87 V.
- Potassium (K): Electrolysis of molten KCl (often with CaCl2 to lower melting point). E∘=−2.93 V.
- Lithium (Li): Electrolysis of molten LiCl/KCl eutectic. E∘=−3.04 V.
-
Consider borderline cases.
- Beryllium (Be): E∘=−1.85 V — extracted electrolytically from molten BeF2 or BeCl2.
- Strontium (Sr) and Barium (Ba): E∘≈−2.9 V — also electrolytic.
- Zinc (Zn): E∘=−0.76 V — usually extracted by carbon reduction (roasting then reduction), not electrolysis. However, electrorefining of zinc uses electrolysis, but that's purification, not extraction. The question asks for extraction, so Zn is excluded.
-
Watch out for aluminium — a classic exam point. …
Method: Electrochemical Series Analysis for Electrolytic Extraction
Why This Method Works
Metals are extracted electrolytically when they are too reactive to be reduced by carbon or other chemical reducing agents. The key is their standard electrode potential (E⊖): more negative values mean stronger reducing power and greater difficulty in chemical reduction.
Steps
-
Recall the electrochemical series — metals with E⊖<−0.83 V (more negative than aluminium) are typically extracted by electrolysis of their molten salts.
-
Identify the threshold: Carbon reduction works for metals with E⊖>−0.76 V (e.g., zinc, iron). Below this, electrolysis is required.
-
List metals with highly negative E⊖:
- Lithium (−3.04 V)
- Potassium (−2.93 V)
- Calcium (−2.87 V)
- Sodium (−2.71 V)
- Magnesium (−2.37 V)
- Aluminium (−1.66 V) …
Common Mistakes: Standard Electrode Potentials & Electrolytic Extraction
Mistake 1: Confusing Electrolytic with Electrochemical
The error: Students think any metal with a negative E∘ value is extracted electrolytically. This is wrong — many negative E∘ metals (like Zn, Fe) are extracted by other methods.
Why it happens: You memorise "negative E∘ = electrolysis" without understanding why.
How to avoid: Remember the reactivity-electrolysis rule:
- Only metals more reactive than carbon (roughly, above Al in the reactivity series) must be extracted electrolytically
- These have E∘ values more negative than about −1.6 V (aluminium's value), so carbon cannot reduce their oxides
Mistake 2: Including Transition Metals Like Iron or Zinc
The error: Listing Fe (E∘=−0.44 V), Zn (E∘=−0.76 V), or Ni (E∘=−0.25 V) as electrolytically extracted.
Why it happens: You see a negative E∘ and assume electrolysis is needed.
How to avoid: Check if carbon can reduce the metal oxide:
| Metal | E∘ (V) | Extraction method |
|---|---|---|
| Fe | −0.44 | Carbon reduction (blast furnace) |
| Zn | −0.76 | Carbon reduction (roasting then reduction) |
| Al | −1.66 | Electrolytic (carbon can't reduce) |
| Na | −2.71 | Electrolytic |
Key insight: Carbon can reduce oxides of metals below Al in the reactivity series.
Mistake 3: Forgetting the Hall-Héroult Process Details
The error: Saying "aluminium is extracted by electrolysis of molten Al₂O₃" but forgetting:
- Al₂O₃ is mixed with cryolite (Na3AlF6) to lower the melting point
- The electrodes are carbon (graphite)
How to avoid: For Al extraction, always mention:
- Molten Al2O3 (not aqueous — Al³⁺ would reduce water instead)
- Cryolite as flux (lowers melting point from 2072 ∘C to ∼950 ∘C)
- Carbon anodes (get consumed: C+O2−→CO2)
Mistake 4: Thinking Mg, Ca, or K Are Obtained by Chemical Reduction
The error: Claiming that reactive metals like magnesium, calcium, or potassium are extracted by carbon (or other chemical) reduction — or leaving them off the electrolytic list entirely.
Why it's wrong: Mg, Ca, and K are far too electropositive (very negative E∘: −2.37 V, −2.87 V, −2.93 V) to be reduced by carbon or any ordinary chemical reducing agent. In practice they are obtained electrolytically from their molten salts — Mg from molten MgCl2, Ca from molten CaCl2, and K from molten KCl.
The correct list: The standard metals extracted electrolytically are:
- Aluminium (Hall-Héroult process)
- Sodium (Downs cell: molten NaCl)
- Potassium (electrolysis of molten KCl)
- Magnesium (electrolysis of molten MgCl₂) …
Showing the 12 most recent of 28 on this concept.
- AP EAPCET 2026Set eng-2026-05-13-FN1 markMCQQ.X,Y and Z represent three electrodes Al3+/Al, Cu2+/Cu and Ag+/Ag with E° values −1.66, 0.34 and 0.80 V respectively. The correct order of oxidising power of these three electrodes is (A) X>Y>Z (B) Z>Y>X (C) X=Y=Z (D) Y>Z>X
›Reveal solutionSolution
This tests reading standard reduction potentials as a measure of oxidising power; higher (more positive) E° means stronger oxidising power, giving Z>Y>X.
Concept and Intuition
The standard reduction potential E° of an electrode couple Mn+/M measures how readily Mn+ is reduced to M. A more positive E° means the ion has a greater tendency to be reduced — i.e., it is a stronger oxidising agent (it more readily takes electrons from something else, getting reduced itself). So ranking electrodes by oxidising power is the same as ranking them by E° value, from most positive (strongest oxidiser) to most negative (weakest oxidiser / strongest reducing agent in its reduced form).
Step-by-Step Solution
- Identify the labels: X=Al3+/Al (E°=−1.66 V), Y=Cu2+/Cu (E°=0.34 V), Z=Ag+/Ag (E°=0.80 V).
- Rank by E° value (most positive = strongest oxidising power): Z (0.80)>Y (0.34)>X (−1.66). …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.ΔG∘ (in kJ mol−1) for the cell reaction given below is 2Al(s)+3Cu2+(aq)→2Al3+(aq)+3Cu(s) (Given: EAl3+∣Al∘=−1.66 V ; ECu2+∣Cu∘=+0.34 V ; F = 96500 C mol−1) (A) -1158 (B) -579 (C) -386 (D) -772
›Reveal solutionSolution
Standard electrochemistry problem: find Ecell∘ from the two standard reduction potentials, count the electrons transferred in the balanced equation, then apply ΔG∘=−nFE∘. Result: −1158 kJmol−1.
Concept and Intuition
The standard Gibbs free energy change of a cell reaction is related to its standard cell potential by
ΔG∘=−nFEcell∘
where n is the number of moles of electrons transferred in the balanced overall reaction, and F is the Faraday constant. Here, Al is oxidised (its half-reaction is reversed relative to the reduction potential given, so it becomes the anode), and Cu2+ is reduced (cathode). The cell potential is always cathode potential minus anode potential (both taken as standard reduction potentials, without flipping signs manually):
Ecell∘=Ecathode∘−Eanode∘
Step-by-Step Solution
- Identify cathode (reduction): Cu2++2e−→Cu, E∘=+0.34 V.
- Identify anode (oxidation, but use its reduction potential in the formula): Al3++3e−→Al, E∘=−1.66 V.
- Ecell∘=0.34−(−1.66)=2.00 V.
- Balance the overall reaction: 2Al→2Al3++6e− and 3Cu2++6e−→3Cu, so total electrons transferred n=6. …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.Which of the following reactions is non-spontaneous? (A) 2F2+2H2O→4HF+O2 (B) Cl2+H2O→HCl+HOCl (C) Br2+H2O→HBr+HOBr (D) 2I2+2H2O→4HI+O2
›Reveal solutionSolution
Comparing the standard reduction potentials of the halogens with that of O2/H2O shows only F2 (and, via ordinary hydrolysis, Cl2/Br2) can act spontaneously on water; iodine cannot oxidise water to O2. The answer is (D).
Concept and Intuition
Whether a halogen reacts with water depends on its oxidising power (standard reduction potential) relative to the species it must oxidise. Fluorine has such an exceptionally high reduction potential that it can directly oxidise water all the way to O2 gas, releasing HF — a highly spontaneous, even violent reaction. Chlorine and bromine are weaker oxidants and instead undergo a milder disproportionation-type hydrolysis, forming the hydrohalic acid and the hypohalous acid (HOX), which is also spontaneous (though the equilibrium lies less and less to the product side going down the group). Iodine is the weakest oxidant of these halogens; its reduction potential is too low to drive the oxidation of water to O2, so a reaction analogous to fluorine's (2I2+2H2O→4HI+O2) does not occur spontaneously.
Step-by-Step Solution
- (A) 2F2+2H2O→4HF+O2: fluorine's reduction potential (E°≈2.87 V) vastly exceeds that of O2/H2O (E°≈1.23 V), so this reaction is strongly spontaneous — not the answer.
- (B) Cl2+H2O→HCl+HOCl: this simple hydrolysis (disproportionation) reaction is spontaneous and is the actual observed reaction of chlorine with water — not the answer. …
- AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQQ.Identify the sets in which both the metals react with water? I. Be, Mg II. Li, Mg III. Na, K IV. K, Ca Correct answer is (A) II, IV only (B) II, III, IV only (C) III, IV only (D) I, II only
›Reveal solutionSolution
Only Be fails to react with water; sets II, III and IV have both metals reacting → (B).
Concept and Intuition
Among the light s-block metals, reactivity with water rises down and across the alkali/alkaline-earth series. Beryllium is anomalous — it does not react with water even as steam (protective oxide, high hydration/ionisation energy). Magnesium reacts slowly with hot water/steam; alkali metals (Li, Na, K) react with water, and Ca reacts readily with cold water.
Step-by-Step Solution
- Set I (Be, Mg): Be does not react with water → invalid.
- Set II (Li, Mg): Li reacts with water, Mg reacts with hot water/steam → valid.
- Set III (Na, K): both react vigorously with water → valid.
- Set IV (K, Ca): both react with water → valid.
- Valid sets = II, III, IV → option (B).
Common Mistakes …
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.For which of the following the E⊖(M3+/M2+) is negative? (A) Mn (B) Co (C) Fe (D) Cr
›Reveal solutionSolution
Cr3+/Cr2+ has a negative standard reduction potential because Cr2+ is unstable/strongly reducing (readily loses an electron to reach the stable d3 Cr3+), unlike Mn, Fe, Co which favour +2. The answer is (D) Cr.
Concept and Intuition
The sign and magnitude of E⊖(M3+/M2+) tells us the relative thermodynamic stability of the +2 vs +3 oxidation state for a transition metal:
- A large positive E⊖ means M3+ is a strong oxidising agent — i.e., M2+ is the thermodynamically favoured/stable state (electron gain is favourable).
- A negative E⊖ means the reverse: M2+ readily loses an electron to become M3+ — i.e., M3+ is the stable state and M2+ is a strong reducing agent.
Extra stability of a particular dn configuration (half-filled d5, or in this case the stability trend of d3 for Cr3+) strongly influences these potentials. Known standard values (approx.): Mn3+/Mn2+=+1.57 V, Co3+/Co2+=+1.82 V, Fe3+/Fe2+=+0.77 V, Cr3+/Cr2+=−0.41 V.
Step-by-Step Solution
- Recall/estimate the sign of E⊖(M3+/M2+) for each metal in the options.
- Mn: Mn2+ (d5, half-filled, extra stable) is strongly favoured over Mn3+ (d4) — so E⊖ is a large positive value (+1.57 V). Not the answer.
- Co: Co2+ (d7) is far more stable than Co3+ (d6, in aqueous simple ions) — E⊖ is strongly positive (+1.82 V). Not the answer. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.If EFe2+/Fe∘=−0.441 V and EFe3+/Fe2+∘=0.771 V, the standard emf of the cell reaction Fe(s)+2Fe3+(aq)⟶3Fe2+(aq) is (A) −1.212 V (B) +1.212 V (C) −2.424 V (D) +2.424 V
›Reveal solutionSolution
Combining the Fe2+/Fe and Fe3+/Fe2+ half-cells for the disproportionation-type reaction Fe + 2Fe3+ → 3Fe2+ gives a standard cell potential of +1.212 V.
Concept and Intuition
Standard cell potential is computed as Ecell∘=Ecathode(reduction)∘−Eanode(reduction)∘, regardless of how many electrons each half-reaction involves — E∘ is an intensive quantity and is NOT multiplied when a half-reaction is scaled to balance electrons.
Step-by-Step Solution
- Identify the two half reactions inside the overall reaction Fe(s) + 2Fe3+(aq) → 3Fe2+(aq):
- Oxidation (anode): Fe→Fe2++2e−, using EFe2+/Fe∘=−0.441 V (as a reduction potential).
- Reduction (cathode): Fe3++e−→Fe2+ (doubled to 2Fe3++2e−→2Fe2+ for electron balance), using EFe3+/Fe2+∘=0.771 V — this value does NOT change when the equation is doubled.
- Apply Ecell∘=Ecathode∘−Eanode∘=0.771−(−0.441)=1.212 V. …
- Identify the two half reactions inside the overall reaction Fe(s) + 2Fe3+(aq) → 3Fe2+(aq):
- AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQQ.Which one of the following reactions is not feasible ? (g = gas, l = liquid, s = solid, aq = aqueous) (A) Cl2(g)+2KBr(aq)⟶2KCl(g)+Br2(l) (B) Cl2(g)+2KI(aq)⟶2KCl(aq)+I2(s) (C) Br2(l)+2KI(aq)⟶2KBr(aq)+I2(s) (D) I2(s)+2KBr(aq)⟶2KI(aq)+Br2(l)
›Reveal solutionSolution
This tests the halogen displacement (reactivity) series; the infeasible reaction is I₂ + 2KBr → 2KI + Br₂, option (D).
Concept and Intuition
Among halogens, oxidizing power (and hence the ability to displace a halide from its salt) decreases down the group: F2>Cl2>Br2>I2. A more powerful oxidizing halogen can displace a less powerful one from its halide salt, but the reverse cannot happen spontaneously.
Step-by-Step Solution
- (A) Cl2+2KBr→2KCl+Br2: Cl₂ is a stronger oxidizer than Br₂, so it displaces bromide — feasible.
- (B) Cl2+2KI→2KCl+I2: Cl₂ is stronger than I₂, displaces iodide — feasible.
- (C) Br2+2KI→2KBr+I2: Br₂ is stronger than I₂, displaces iodide — feasible. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.The EM+(aq)∣M(s)⊖ is highest with negative sign for the alkali metal 'x' and lowest with negative sign for the alkali metal 'y'. In flame test, the characteristic colours of x and y are respectively (A) Blue, Yellow (B) Yellow, Violet (C) Yellow, Crimson red (D) Crimson red, Yellow
›Reveal solutionSolution
Li has the most negative EM+/M⊖ among alkali metals (anomalously, due to huge hydration enthalpy) and Na has the least negative. Their flame colours are crimson red (Li) and yellow (Na) respectively.
Concept and Intuition
Standard reduction potentials of alkali metals in water do not follow the simple ionisation-energy trend because hydration enthalpy also matters heavily. Lithium's very small ionic size gives it an unusually large hydration enthalpy, which overcompensates for its lower ionisation energy and makes ELi+/Li⊖ the most negative among the alkali metals — Li is thus the strongest reducing agent in aqueous solution, contrary to what its higher ionisation energy alone might suggest. Sodium, further down this thermodynamic cycle, ends up with the least negative potential among the common alkali metals.
Step-by-Step Solution
- Typical standard reduction potentials (aqueous, 298 K): Li+/Li≈−3.05 V, Na+/Na≈−2.71 V, K+/K≈−2.93 V, Rb+/Rb≈−2.93 V, Cs+/Cs≈−2.92 V.
- "Highest with negative sign" = most negative value = Li (x=Li).
- "Lowest with negative sign" = least negative (closest to zero) = Na (y=Na). …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.Match the following List-I (Transition metal, M): A) Ni B) Mn C) Fe D) Cr List-II (EM2+/M⊖): I) −1.18 II) −0.91 III) −0.25 IV) −0.44 The correct answer is (A) A-III, B-II, C-IV, D-I (B) A-III, B-IV, C-I, D-II (C) A-III, B-I, C-IV, D-II (D) A-I, B-IV, C-II, D-III
›Reveal solutionSolution
This tests recall of standard reduction potentials EM2+/M⊖ for first-row transition metals. The answer is A-III, B-I, C-IV, D-II.
Concept and Intuition
The standard electrode potentials of M2+/M couples for 3d transition metals are largely governed by a combination of enthalpy of atomisation, ionisation enthalpy, and hydration enthalpy, and do not follow a simple monotonic trend across the series. These values are typically memorised from the standard NCERT table.
Step-by-Step Solution
- Recall standard EM2+/M⊖ values (in volts): Cr2+/Cr=−0.91, Mn2+/Mn=−1.18, Fe2+/Fe=−0.44, Ni2+/Ni=−0.25. …
- AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQQ.The cell reaction of a cell is given below 2Cu+→Cu+Cu2+ What is Ecell0 (in V)? (Given: ECu2+/Cu+0=x V; ECu+/Cu0=y V) (A) x−y (B) y−x (C) x+y (D) −x−y
›Reveal solutionSolution
The key is to treat the given cell reaction as the sum of two half‑reactions, each with its own standard potential, and then combine them correctly — the result is Ecell0=y−x, so the correct option is (B).
Why this approach works
Standard electrode potentials are intensive properties: they do not depend on how many electrons are transferred. When we combine half‑reactions to get a full cell reaction, we never multiply the potentials by coefficients — we simply add them (with the appropriate sign for the direction we use). The trick here is that the reaction 2Cu+→Cu+Cu2+ is a disproportionation: one Cu+ is reduced to Cu and the other is oxidised to Cu2+. So we need to identify which half‑reaction runs as reduction and which as oxidation, then combine their potentials.
Step‑by‑step reasoning
- Identify the two half‑reactions hidden in the overall reaction The overall reaction is:
2Cu+→Cu+Cu2+
This can be split into:
- Reduction half: Cu++e−→Cu Its standard potential is given as ECu+/Cu0=y V.
- Oxidation half: Cu+→Cu2++e− This is the reverse of Cu2++e−→Cu+, whose potential is x V. For the reverse reaction, the potential changes sign: Eox0=−x V.
- Combine the half‑reaction potentials The standard cell potential is the sum of the reduction potential of the cathode and the oxidation potential of the anode:
Ecell0=Ered0+Eox0
Here:
- Cathode (reduction): Cu++e−→Cu, Ered0=y
- Anode (oxidation): Cu+→Cu2++e−, Eox0=−x
Therefore:
Ecell0=y+(−x)=y−x
- Check the sign convention …
- AP EAPCET 2024Set eng-2024-05-20-AN1 markMCQQ.The standard reduction potentials of 2H+/H2, Cu2+/Cu, Zn2+/Zn and NO3−,H+/NO are 0.0, +0.34, -0.76 and 0.97 V respectively. Observe the following reactions I. Zn+HCl→ II. Cu+HCl→ III. Cu+HNO3→ Which reactions does not liberate H2(g)? (A) II, III only (B) I, II only (C) I, III only (D) I, II, III
›Reveal solutionSolution
H2 is liberated only when the metal's reduction potential is below 0 V (more easily oxidized than H2); Zn (−0.76 V) liberates H2 with HCl, but Cu (+0.34 V) cannot liberate H2 with either HCl or HNO3.
Concept and Intuition
A metal displaces H2 from a dilute acid only if it is a stronger reducing agent than hydrogen, i.e., its standard reduction potential is more negative than that of 2H⁺/H2 (0.0 V). Cu, with a positive reduction potential, cannot reduce H⁺ to H2 under any of these acids — with HNO3 specifically, the acid itself acts as an oxidizer (reduced to NO) rather than being a source of H2.
Step-by-Step Solution
- Zn²⁺/Zn = −0.76 V is more negative than 2H⁺/H2 = 0.0 V, so Zn CAN reduce H⁺ to H2 — reaction I liberates H2.
- Cu²⁺/Cu = +0.34 V is more positive than 0.0 V, so Cu CANNOT reduce H⁺ to H2 with HCl — reaction II does NOT liberate H2.
- With HNO3, the oxidizing species is NO3⁻/H⁺ → NO (E° = 0.97 V), which is even more strongly oxidizing than H⁺; Cu reacts with HNO3 by reducing NO3⁻ to NO (or NO2), not by liberating H2 — reaction III does NOT liberate H2. …
- AP EAPCET 2024Set eng-2024-05-21-AN1 markMCQQ.Consider the following standard electrode potentials (E0 in volts) in aqueous solution:
Element M3+/M M+/M ; Al −1.66 +0.55 ; Tl +1.26 −0.34. Based on this data, which of the following statements is correct? (A) Tl3+ is more stable than Al3+ (B) Tl+ is more stable than Al3+ (C) Al+ is more stable than Al3+ (D) Tl+ is more stable than Al+ ›Reveal solutionSolution
Combining the two given electrode potentials for each metal (via ΔG=−nFE0) shows Al's +1 state is unstable (disproportionates to Al3+) while Tl's +1 state is the stable one — the classic inert-pair-effect result, so Tl+ is more stable than Al+.
Concept and Intuition
Whether an intermediate oxidation state (M+) is stable or disproportionates depends on the relative reducing/oxidizing strength of the two half-reactions that flank it. We can combine E0(M3+/M) and E0(M+/M) using free energies (which are additive, unlike potentials) to get E0(M3+/M+), and that tells us directly whether M+ wants to disproportionate into M and M3+.
Step-by-Step Solution
- For Al: E0(Al3+/Al)=−1.66 V (n=3), E0(Al+/Al)=+0.55 V (n=1).
- ΔG0(Al3+→Al)=−3F(−1.66)=+4.98F; ΔG0(Al+→Al)=−1F(0.55)=−0.55F.
- ΔG0(Al3+→Al+)=ΔG0(Al3+→Al)−ΔG0(Al+→Al)=4.98F−(−0.55F)=5.53F (for a 2-electron step), so E0(Al3+/Al+)=−5.53F/2F≈−2.77 V — very negative, meaning Al3+ strongly resists being reduced to Al+; equivalently, Al+ is a strong enough reducing agent to be oxidized to Al3+ spontaneously (disproportionates, i.e. Al+ is unstable).
- For Tl: E0(Tl3+/Tl)=+1.26 V, E0(Tl+/Tl)=−0.34 V.
- ΔG0(Tl3+→Tl)=−3F(1.26)=−3.78F; ΔG0(Tl+→Tl)=−1F(−0.34)=+0.34F.
- ΔG0(Tl3+→Tl+)=−3.78F−0.34F=−4.12F (2-electron step), so E0(Tl3+/Tl+)=+4.12F/2F≈+2.06 V — strongly positive, meaning Tl3+ is readily reduced to Tl+: Tl+ is the thermodynamically favoured, stable state and does not disproportionate. …
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