Q.Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous ions under suitable conditions.
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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). …
The key idea is that a substance can oxidise Fe2+ to Fe3+ if its standard reduction potential is greater than E∘(Fe3+/Fe2+)=+0.77 V.
- The half-reaction for oxidation is: Fe2+→Fe3++e− (reverse of the reduction).
- Any oxidising agent with E∘>+0.77 V will spontaneously accept electrons from Fe2+.
- From the standard potential series, three common examples are:
- Cl2: Cl2+2e−→2Cl−, E∘=+1.36 V
- Br2: Br2+2e−→2Br−, E∘=+1.09 V …
The key idea is that a substance can oxidise FeX2+ to FeX3+ if its standard reduction potential is greater than +0.77 V (the E∘ for FeX3+/FeX2+). Three such oxidising agents are FX2, ClX2, and OX3.
Why Standard Electrode Potentials Decide the Reaction
Oxidation is the loss of electrons. For FeX2+ to be oxidised to FeX3+, it must give away an electron. That electron must be accepted by some other species — the oxidising agent. The tendency of a species to accept electrons is measured by its standard reduction potential (E∘). The more positive the E∘, the stronger the oxidising agent.
The half-reaction for ferrous oxidation is:
FeX3++eX−FeX2+E∘=+0.77 V
This value tells us that FeX3+ has a moderate tendency to get reduced back to FeX2+. To push the reaction in the opposite direction — to force FeX2+ to lose an electron — we need an oxidising agent that is even more eager to gain electrons. In other words, the oxidising agent's reduction potential must be greater than +0.77 V.
A species X can oxidise FeX2+ to FeX3+ if:
E∘(X/X−)>+0.77 V
Step-by-step selection
-
Recall the standard reduction potentials of common oxidising agents. From the standard table (at 25∘C, 1 atm, 1 M), we have:
- FX2+2eX−2FX−: E∘=+2.87 V
- OX3+2HX++2eX−OX2+HX2O: E∘=+2.07 V
- ClX2+2eX−2ClX−: E∘=+1.36 V
- MnOX4X−+8HX++5eX−MnX2++4HX2O: E∘=+1.51 V
- CrX2OX7X2−+14HX++6eX−2CrX3++7HX2O: E∘=+1.33 V
- BrX2+2eX−2BrX−: E∘=+1.09 V
- IX2+2eX−2IX−: E∘=+0.54 V
-
Compare each with +0.77 V. Any species with E∘>0.77 V can, in principle, oxidise FeX2+. Those with E∘<0.77 V (like IX2) cannot.
-
Select three distinct substances that are commonly available and clearly satisfy the condition. Good choices are:
- Fluorine gas (FX2): E∘=+2.87 V — the strongest oxidising agent known.
- Chlorine gas (ClX2): E∘=+1.36 V — a classic laboratory oxidiser.
- Ozone (OX3): E∘=+2.07 V — a powerful oxidant used in water treatment.
Other valid options include KMnOX4 (acidified), KX2CrX2OX7 (acidified), or BrX2, but the question asks for three substances, and the above are unambiguous. …
Method: Using Standard Electrode Potentials to Predict Redox Feasibility
Method name: The Ecell∘ Sign Rule (or Spontaneity Rule)
Concept behind the method
A substance can oxidise ferrous ions (Fe2+) if it can accept electrons from Fe2+, converting Fe2+ to Fe3+.
The half-reaction for oxidation of ferrous ions is:
Fe2+→Fe3++e−E∘=+0.77 V
For a reaction to be spontaneous (feasible), the overall cell potential must be positive:
Ecell∘=Ecathode∘−Eanode∘>0
Here, the anode (oxidation) is Fe2+→Fe3++e− with E∘=+0.77 V.
The cathode (reduction) is the substance that will accept electrons — its E∘ must be greater than +0.77 V.
Steps
-
Identify the half-reaction for the substance to be oxidised
- Here: Fe2+→Fe3++e−, E∘=+0.77 V
-
Recall the condition for a substance to act as an oxidising agent
- The oxidising agent (cathode) must have a higher reduction potential than the substance being oxidised (anode).
-
Consult the standard electrode potential table
- Look for half-reactions with E∘>+0.77 V.
-
Select three substances whose reduction potentials are greater than +0.77 V.
Three substances that can oxidise Fe2+
| Substance | Reduction half-reaction | E∘ (V) | …
Here are the common mistakes students make when tackling this exact type of question from Standard Electrode Potentials, along with clear strategies to avoid each.
Mistake 1: Confusing “Oxidise” with “Reduce”
- The error: Students think any substance with a higher reduction potential than Fe2+/Fe will oxidise Fe2+. They often pick metals like Zinc (E∘=−0.76 V) because “it’s more reactive.”
- Why it’s wrong: To oxidise Fe2+ (turn it into Fe3+), the oxidising agent must itself be reduced. This means the oxidising agent must have a higher reduction potential than the Fe3+/Fe2+ couple (+0.77 V). A lower potential means the substance is a reducing agent, not an oxidising agent.
- How to avoid: Always write the half-reaction for the species being oxidised:
Fe2+→Fe3++e−(E∘=+0.77 V)
Then, the oxidising agent must have a more positive E∘ than +0.77 V.
Mistake 2: Forgetting the Correct Half-Cell for Iron
- The error: Using the Fe2+/Fe couple (−0.44 V) instead of the Fe3+/Fe2+ couple (+0.77 V).
- Why it’s wrong: The question asks to oxidise ferrous ions (Fe2+) to ferric ions (Fe3+). The relevant half-reaction is:
Fe3++e−⇌Fe2+E∘=+0.77 V
The Fe2+/Fe couple is for reduction to metallic iron, which is a different process.
- How to avoid: Memorise the key couples for iron:
- Fe3+/Fe2+: +0.77 V (for oxidation of Fe2+)
- Fe2+/Fe: −0.44 V (for reduction to metal)
Mistake 3: Picking Substances with E∘ Exactly Equal to +0.77 V
- The error: Choosing I2/I− (+0.54 V) or Cu2+/Cu (+0.34 V) because they are “close” to +0.77 V.
- Why it’s wrong: For a spontaneous reaction, the oxidising agent must have E∘>+0.77 V. If E∘ is lower, the reaction is non-spontaneous under standard conditions. The reaction will not proceed.
- How to avoid: Use the spontaneity rule:
ΔG∘=−nFEcell∘
For a spontaneous reaction, Ecell∘>0. Here:
Ecell∘=Eoxidising agent∘−0.77 V
So you need Eoxidising agent∘>0.77 V.
Mistake 4: Not Checking the Table Carefully
- The error: Listing substances like Cl2 (+1.36 V) but forgetting that Cl2 is a gas and may not be “suitable” under all conditions (e.g., in solution, it can also oxidise water).
- Why it’s wrong: The question says “under suitable conditions.” While Cl2 works in principle, students often miss that MnO4− (+1.51 V) or Cr2O72− (+1.33 V) are more common lab oxidising agents for Fe2+.
- How to avoid: List three distinct substances from the table with E∘>+0.77 V. Good choices:
- Cl2 (+1.36 V)
- MnO4− in acid (+1.51 V)
- Cr2O72− in acid (+1.33 V) …
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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