Q.In the electrolysis of aqueous sodium chloride solution which of the half cell reaction will occur at anode?
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Faraday's Laws of Electrolysis
Imagine you're trying to plate a copper spoon with silver. You drop the spoon into a solution containing silver ions, connect it to a battery, and wait. How much silver actually deposits? Does it depend on how long you wait? On how strong the battery is? On what metal you're using?
Faraday's laws answer exactly these questions. They connect the invisible world of electrons flowing through a wire to the visible world of atoms depositing on a surface.
The Intuition First
Think of electrolysis as a counting problem. Each silver ion (Ag+) needs exactly one electron to become a neutral silver atom (Ag). So if you push a certain number of electrons through the circuit, you should get exactly that many silver atoms deposited.
The first law says: more charge → more mass deposited. Double the charge, double the mass. It's a direct proportionality.
The second law says: different elements need different amounts of charge per atom. A copper ion (Cu2+) needs two electrons to become neutral copper, so for the same amount of charge, you get half as many copper atoms as silver atoms.
The Precise Statements
First Law: The mass of a substance liberated at an electrode is directly proportional to the quantity of electric charge passed through the electrolyte.
m∝Qorm=ZQ
where Z is the electrochemical equivalent of the substance.
Second Law: When the same quantity of charge is passed through different electrolytes, the masses of substances liberated are proportional to their chemical equivalents (equivalent weights).
E1m1=E2m2
Here E is the equivalent weight: atomic mass divided by the number of electrons transferred per ion (n). For silver (Ag+, n=1), E=107.87 g. For copper (Cu2+, n=2), E=63.55/2=31.77 g.
The Combined Law
These two laws merge into one powerful equation:
m=FQ×E
where F is Faraday's constant — the charge carried by one mole of electrons: F=96485 coulombs per mole.
Since Q=I×t (current × time), you can write:
m=FI×t×E
This is the working formula for every electrolysis calculation in your exams.
To avoid confusion: equivalent weight E is always atomic mass divided by n (the number of electrons gained or lost per ion). For Al3+, n=3; for O2 gas (from water), each oxygen atom loses 2 electrons, but the molecule has 2 atoms, so n=4 per O2 molecule.
A Worked Example
Problem: How much copper deposits when a current of 2.0 A flows through a copper sulfate solution for 30 minutes? (Atomic mass of Cu = 63.5 g/mol, n=2)
Step 1: Find the equivalent weight.
E=263.5=31.75 g/mol
Step 2: Find total charge.
Q=I×t=2.0×(30×60)=3600 C
Step 3: Apply the combined law.
m=FQ×E=964853600×31.75=1.185 g
So about 1.2 grams of copper deposits. …
Why this formula?
Faraday's Laws of Electrolysis: Why the Formulas Hold
Faraday's Laws of Electrolysis describe the quantitative relationship between the amount of electricity passed through an electrolyte and the mass of substance liberated at the electrodes. Let's build the reasoning step-by-step.
1. The Core Idea: Charge Carries Matter
Electrolysis works because ions (charged particles) move toward electrodes and undergo redox reactions.
- At the cathode (negative electrode), cations gain electrons (reduction).
- At the anode (positive electrode), anions lose electrons (oxidation).
The key insight: Each ion that reacts carries a fixed amount of charge.
- For a monovalent ion (e.g., Na+), charge = 1.602×10−19C (the elementary charge e).
- For a divalent ion (e.g., Cu2+), charge = 2e.
Thus, the total charge passed (Q) is directly proportional to the number of ions that have reacted (N):
Q=N⋅ze
where:
- z = valency (number of electrons transferred per ion)
- e = elementary charge (1.602×10−19C)
2. From Number of Ions to Mass
The number of ions N is related to the mass (m) of substance liberated via Avogadro's number (NA) and molar mass (M):
N=Mm⋅NA
Substitute into Q=Nze:
Q=(Mm⋅NA)⋅ze
3. Introducing Faraday's Constant
The product NAe appears repeatedly — it's called Faraday's constant (F):
F=NAe≈96485C mol−1
So:
Q=Mm⋅zF
Rearrange for mass:
m=zFQM
This is the unified formula for both of Faraday's laws.
4. Why Two "Laws"? — They Are the Same Idea
Faraday originally stated two laws, but they are logical consequences of the same charge–mass relationship:
First Law (Direct Proportionality)
Mass liberated is directly proportional to the charge passed.
From m=zFQM, if M, z, and F are constant, then:
m∝Q
Why? Because each ion needs a fixed charge to react — more charge means more ions, hence more mass.
Second Law (Electrochemical Equivalent)
For the same charge, masses liberated are proportional to equivalent weights.
Equivalent weight E=zM (mass per mole of electrons transferred).
From m=FQ⋅zM=FQ⋅E, if Q is fixed:
m∝E
Why? For the same charge, the number of electrons transferred is fixed. A substance with a smaller z (fewer electrons per ion) will liberate more moles of substance, hence more mass per mole.
5. Practical Formula for Exams …
Concept: Electrolysis of aqueous NaCl — at the anode, oxidation occurs. Standard electrode potentials alone favour water oxidation, but in practice, overpotential reverses this.
Reasoning:
- At the anode, we compare the oxidation of water (E∘=1.23 V) and oxidation of chloride ions (E∘=1.36 V).
- Purely by standard potentials, water (lower potential) should oxidise first. …
In the electrolysis of aqueous NaCl, the anode is where oxidation occurs. The two competing oxidations are water (to O2, standard potential +1.23 V) and chloride ions (to Cl2, standard potential +1.36 V). Purely on standard thermodynamic potentials, water should oxidise first — but in practice, the high overpotential for oxygen evolution on real electrodes makes chloride oxidation the reaction that actually occurs. Option (iv) is correct.
-
Identify what happens at the anode.
The anode is the electrode where oxidation takes place — loss of electrons. So we need to look at the half‑reactions that are written as oxidations (electrons on the right). Options (i) and (iii) are reductions (electrons on the left), so they cannot occur at the anode. That leaves (ii) and (iv).
-
Understand the competition.
In aqueous NaCl, the solution contains Na+, Cl-, H+, OH-, and water molecules. At the anode, two species can be oxidised:
- Water: 2H2O(l)→O2(g)+4H+(aq)+4e−
- Chloride ions: Cl−(aq)→21Cl2(g)+e−
-
Compare the standard potentials.
- Water oxidation: E∘=+1.23 V
- Chloride oxidation: E∘=+1.36 V
Purely thermodynamically (standard conditions, no kinetic barriers), the reaction needing the lower potential is favoured — so water oxidation looks like it should win.
-
But real electrodes have overpotential — the deciding factor. …
Method: Standard Electrode Potential Comparison + Overpotential for Anode Reactions
In electrolysis, the anode is where oxidation occurs (loss of electrons). Comparing standard oxidation potentials alone is not sufficient — the reaction that actually occurs also depends on overpotential (a kinetic factor).
Steps
-
Identify the species present at the anode
In aqueous NaCl:
- Cl− (from NaCl)
- H2O (solvent)
-
Write possible oxidation half-reactions
- (ii) 2H2O(l)→O2(g)+4H+(aq)+4e−; E∘=+1.23 V
- (iv) Cl−(aq)→21Cl2(g)+e−; E∘=+1.36 V
(Options A and C are reduction reactions — they occur at the cathode, not anode.)
-
Compare standard oxidation potentials
Water oxidation (+1.23 V) has the lower standard potential, so purely thermodynamically it should be favoured.
-
Apply the overpotential correction …
Common Mistakes Students Make on This Question
Mistake 1: Confusing Anode and Cathode Reactions
- The error: Students often pick reduction reactions (like A or C) for the anode, forgetting that oxidation always occurs at the anode.
- How to avoid: Memorise the mnemonic "An Ox – Red Cat" (Anode = Oxidation, Cathode = Reduction). Before answering, ask yourself: "Is this a loss of electrons?" Only reactions where electrons appear on the right side (products) are oxidation.
Mistake 2: Not Checking Which Species Actually Gets Oxidised
- The error: Students see option D (Cl−→Cl2) and assume it's correct without comparing the overpotential and concentration effects in aqueous NaCl.
- How to avoid: In aqueous solutions, water itself can oxidise (option B, E∘=1.23 V). Even though Cl− oxidation has E∘=1.36 V, the overpotential for oxygen evolution is high (~0.4–0.6 V) on common electrodes like platinum. This means the actual voltage needed for water oxidation is higher than 1.23 V, making chloride oxidation kinetically favoured in concentrated NaCl. Always consider:
- Standard potentials
- Overpotential (kinetic barrier)
- Concentration (Le Chatelier's principle)
Mistake 3: Misreading the Sign Convention for E∘
- The error: Students see Ecell∘ labelled on each half-reaction and treat them as if they are reduction potentials. But option B and D are written as oxidation half-reactions — the given E∘ values are actually the oxidation potentials (reverse of standard reduction potentials).
- How to avoid: Always convert to a standard reduction potential table in your mind. For oxidation reactions, the true tendency to occur is opposite to the sign shown. For example:
- Cl−→21Cl2+e− has Eox∘=1.36 V → reduction potential for Cl2+e−→Cl− is +1.36 V.
- 2H2O→O2+4H++4e− has Eox∘=1.23 V → reduction potential for O2+4H++4e−→2H2O is +1.23 V.
- Higher reduction potential = easier to reduce = harder to oxidise. So water (Ered∘=1.23 V) is harder to oxidise than chloride (Ered∘=1.36 V) based on thermodynamics alone — but overpotential reverses this.
Mistake 4: Ignoring the "Aqueous" Condition
- The error: Students pick option A (Na+→Na) because they think of molten NaCl electrolysis. In aqueous solution, water is present and Na+ reduction (E∘=−2.71 V) is not possible because water reduction to H2 (E∘=−0.83 V at pH 7) occurs first.
- How to avoid: For aqueous solutions, always check if water can react more easily than the ion. Use the electrochemical series and remember:
- Cations of highly active metals (Group 1, 2) are not reduced in water — H2O gets reduced instead.
- Anions like Cl− may or may not be oxidised depending on concentration and electrode material. …
Showing the 12 most recent of 22 on this concept.
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.Molten Al2O3 was electrolyzed between carbon electrodes. The mass (in g) of aluminium produced at cathode when 965 amperes current is passed through it for 1000 seconds is (F=96500 C mol−1) (A) 30 (B) 90 (C) 60 (D) 45
›Reveal solutionSolution
This tests Faraday's laws of electrolysis applied to Al2O3 electrolysis; the answer is 90 g.
Concept and Intuition
In electrolytic reduction of molten Al2O3, Al3+ ions gain 3 electrons at the cathode to form Al metal. The amount of substance deposited depends on the total charge passed and the number of electrons needed per mole of product (Faraday's second law).
Step-by-Step Solution
- Charge passed: Q=I×t=965A×1000s=9.65×105C.
- Moles of electrons transferred: ne=Q/F=965009.65×105=10mol.
- Cathode reaction: Al3++3e−→Al, so 3 mol electrons deposit 1 mol Al.
- Moles of Al deposited =10/3mol. …
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.The number of Faradays involved in the conversion of 0.25 mol of Al3+ to Al is x and number of Faradays involved in the conversion of 1100 mL of 0.5 M Cu2+ to Cu is y. The values of x and y respectively are (A) 0.75, 1.1 (B) 0.25, 2.2 (C) 0.50, 3.3 (D) 1.00, 2.2
›Reveal solutionSolution
Faradays needed = (moles of ion) × (charge/electrons needed per ion); working both cases gives x=0.75, y=1.1.
Concept and Intuition
One Faraday (1 F) supplies one mole of electrons. To deposit/reduce a metal ion Mn+ completely, you need n Faradays per mole of that ion, since Mn++ne−→M.
Step-by-Step Solution
- Al3++3e−→Al: for 0.25 mol Al³⁺, Faradays required x=3×0.25=0.75 F.
- Moles of Cu²⁺ =10001100 L×0.5 mol/L=0.55 mol. …
- AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQQ.The number of moles of H2 gas liberated at cathode, when 10 milli-ampere current is passed through dilute aqueous solution of NaCl for about 19.3×104 seconds is (F=96500 C mol−1) (A) 0.50 (B) 0.02 (C) 0.01 (D) 0.15
›Reveal solutionSolution
Faraday's law gives moles of electrons passed; since dilute aqueous NaCl electrolysis reduces water (not Na+) at the cathode, 2 electrons give 1 mole of H2, so n(H2)=0.01 mol.
Concept and Intuition
In dilute aqueous NaCl, Na+ is not reduced at the cathode (it is a much harder ion to reduce than water); instead water itself is reduced, liberating H2 gas and OH−. Faraday's laws connect the charge passed to the moles of product via the number of electrons in the half-reaction.
Step-by-Step Solution
- Charge passed: Q=I×t=(10×10−3A)×(19.3×104s)=1930 C.
- Moles of electrons: ne=Q/F=1930/96500=0.02 mol.
- Cathode half-reaction in dilute aqueous NaCl: 2H2O+2e−→H2+2OH− — 2 mol electrons give 1 mol H2. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.When 3 amp current was passed through an aqueous solution of salt of a metal M (atomic weight 106.4 u) for 1 hour, 2.977 g of Mn+ was deposited at cathode. The value of n is (1 F = 96500 C mol−1) (A) 2 (B) 1 (C) 3 (D) 4
›Reveal solutionSolution
Applying Faraday's first law of electrolysis to the given charge, mass deposited, and atomic weight gives n=4.
Concept and Intuition
Faraday's law relates the mass of metal deposited at the cathode to the charge passed:
m=nFQM
where Q=It is total charge, M is the atomic weight of the metal, n is the number of electrons needed per metal ion (i.e. its charge Mn+), and F is Faraday's constant. Rearranging for n lets you deduce the ionic charge state from an experimental deposition measurement.
Step-by-Step Solution
- Total charge passed: Q=It=3A×3600s=10800C.
- Rearrange Faraday's law: n=mFQM. …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.At 27 °C, the pH of 1 L of aqueous copper sulphate is 5.5. This solution was electrolyzed using two Pt electrodes for some time. What is the pH of remaining copper sulphate solution? (A) 5.5 (B) More than 5.5 but less than 7.0 (C) 7.5 (D) Less than 5.5 but more than zero
›Reveal solutionSolution
Electrolysis with inert Pt electrodes deposits Cu at the cathode and liberates O2 + H+ at the anode, so the solution becomes progressively more acidic — pH drops below 5.5 but stays above zero.
Concept and Intuition
With Pt (inert) electrodes, Cu2+ is preferentially reduced at the cathode (its reduction potential is far more favourable than reducing water/H⁺ to H2), while at the anode water is oxidized to oxygen (sulfate is essentially inert to oxidation under these conditions). This liberates H+ ions at the anode without any corresponding consumption of H+ at the cathode, so the net effect is generation of acid.
Step-by-Step Solution
- Cathode: Cu2++2e−→Cu(s) — copper deposits, no H+ consumed.
- Anode: 2H2O→O2+4H++4e− — H+ ions are released into solution.
- Overall: 2CuSO4+2H2Oelectrolysis2Cu+O2+2H2SO4.
- As electrolysis proceeds, [H+] increases, so pH decreases below the starting 5.5. …
- AP EAPCET 2024Set eng-2024-05-19-AN1 markMCQQ.2.644 g of metal (M) was deposited when 8040 coulombs of electricity was passed through molten MF2 salt. What is the atomic mass of M? (F=96500 Cmol−1) (A) 63.47 u (B) 65.54 u (C) 31.74 u (D) 61.48 u
›Reveal solutionSolution
This tests Faraday's laws of electrolysis applied to a molten metal fluoride salt to find the atomic mass of the deposited metal. The answer is (A) 63.47 u.
Concept and Intuition
In electrolysis, the amount of substance deposited at an electrode is directly related to the total charge passed via Faraday's laws. For a metal M in the salt MF2, the metal exists as the M2+ ion, which requires exactly 2 moles of electrons to be reduced to 1 mole of neutral metal atoms.
Step-by-Step Solution
- Total charge passed: Q=8040 C.
- Moles of electrons passed: ne=FQ=965008040=0.08332 mol.
- Since M2++2e−→M, moles of metal deposited: nM=2ne=20.08332=0.04166 mol. …
- AP EAPCET 2024Set eng-2024-05-19-AN1 markMCQQ.The anode and cathode used in electrolytic refining of copper respectively are (A) Pure copper, impure copper (B) Impure copper, pure copper (C) Pure copper, pure zinc (D) Impure copper, pure zinc
›Reveal solutionSolution
Electrolytic refining of copper: the impure metal is oxidised (dissolved) at the anode and pure metal is deposited at the cathode.
Concept and Intuition
In electrorefining, the metal to be purified is made the anode so that it oxidises and goes into solution as Cu2+, while a strip of the pure metal is made the cathode where Cu2+ is reduced and deposited as pure copper. The electrolyte is acidified CuSO4 solution, and the less noble impurities stay in solution or fall as "anode mud" while more noble impurities collect as anode mud too (e.g. Ag, Au).
Step-by-Step Solution
- Anode reaction (oxidation): Cu(s, impure)→Cu2++2e− — this must be the impure block since it is consumed/dissolved. …
- AP EAPCET 2024Set eng-2024-05-20-FN1 markMCQQ.Aqueous CuSO4 solution was electrolysed by passing 2 amp of current for 10 min. What is the weight (in g) of copper deposited at cathode? (Cu = 63 u; F = 96500 C mol−1) (A) 0.195 (B) 0.39 (C) 0.78 (D) 1.56
›Reveal solutionSolution
Faraday's law with n=2 electrons per Cu deposited gives ≈0.39 g of copper — answer (B).
Concept and Intuition
Faraday's first law: the mass deposited at an electrode is proportional to the total charge passed, via m=FQ×nM, where n is the number of electrons needed to deposit one atom/ion (here Cu2++2e−→Cu, so n=2).
Step-by-Step Solution
- Charge passed: Q=I×t=2 A×(10×60) s=2×600=1200 C.
- Moles of electrons: ne=Q/F=1200/96500=0.012435 mol. …
- AP EAPCET 2023Set eng-2023-05-15-AN1 markMCQQ.The number of Faradays required to completely deposit magnesium from 1 L of 0.1 M MgCl2 aq. solution is (A) 0.2 (B) 2 (C) 0.1 (D) 0.4
›Reveal solutionSolution
Depositing all the Mg2+ from 0.1 mol of MgCl2 needs 2 electrons per ion, so 0.1×2=0.2 Faradays are required.
Concept and Intuition
Electrodeposition amount is governed by Faraday's laws of electrolysis: the moles of electrons (Faradays) needed equal the moles of the species times the number of electrons transferred in reducing (or oxidizing) that species. For a metal ion of charge n+, depositing it as the neutral metal requires n electrons per ion.
Step-by-Step Solution
- Moles of MgCl2 in 1 L of 0.1 M solution: n=M×V=0.1 mol/L×1 L=0.1 mol, giving 0.1 mol of Mg2+ ions (1:1 stoichiometry, MgCl2→Mg2++2Cl−).
- Write the cathodic deposition reaction: Mg2++2e−→Mg — each Mg2+ ion needs 2 electrons. …
- AP EAPCET 2023Set eng-2023-05-16-FN1 markMCQQ.The time required (in hours) to reduce 3 mol of Fe3+ ions to Fe2+ ions with 2.0 amperes of current is (1 F=96500 C mol−1) (A) 30.2 (B) 40.2 (C) 10.2 (D) 15.2
›Reveal solutionSolution
Reducing Fe3+ to Fe2+ is a one-electron process; use Q=nF and t=Q/I, converting seconds to hours.
Concept and Intuition
Faraday's law connects moles of electrons transferred to charge passed: Q=ne×F, where ne is moles of electrons and F=96500 C/mol. The reduction half-reaction Fe3++e−→Fe2+ shows exactly 1 electron per Fe ion reduced.
Step-by-Step Solution
- Moles of electrons needed for 3 mol Fe3+ = 3 mol (1:1 ratio).
- Charge required: Q=3×96500=289500 C.
- Time: t=Q/I=289500/2.0=144750 s. …
- AP EAPCET 2022Set eng-2022-07-04-AN1 markMCQQ.38.6 amperes of current is passed for 100 seconds through an aqueous CuSO4 solution using platinum electrodes. The mass of copper consumed from the solution and volume of gas liberated at STP are respectively (molar mass of Cu = 63.54 g mol−1). (A) 6.37 g, 0.448 L (B) 0.63g, 0.224 L (C) 1.27g, 0.224 L (D) 4g, 0.448 L
›Reveal solutionSolution
This tests Faraday's laws of electrolysis for a two-electrode cell (Cu deposited at cathode, O2 evolved at the inert Pt anode). Answer: 1.27 g Cu, 0.224 L O2.
Concept and Intuition
When current flows through an electrolyte, the amount of substance deposited/liberated at each electrode is proportional to the charge passed, via ne−=Q/F. With Pt (inert) electrodes in aqueous CuSO4: at the cathode Cu2+ ions are reduced to Cu metal (2 electrons per Cu atom); since Pt itself does not dissolve at the anode, the anode reaction is oxidation of water to O2 (4 electrons per O2 molecule), because SO42− is not easily oxidised compared to water.
Step-by-Step Solution
- Charge passed: Q=It=38.6 A×100 s=3860 C.
- Moles of electrons transferred: ne−=Q/F=3860/96500=0.04 mol.
- Cathode: Cu2++2e−→Cu, so moles of Cu =0.04/2=0.02 mol.
- Mass of Cu =0.02 mol×63.54 gmol−1=1.2708 g≈1.27 g. …
- AP EAPCET 2022Set eng-2022-07-05-FN1 markMCQQ.96.5 amperes current is passed through the molten AlCl3 for 100 seconds. The mass of aluminum deposited at the cathode is (Atomic weight of Al = 27 u) (A) 0.90 g (B) 0.45 g (C) 1.35 g (D) 1.8 g
›Reveal solutionSolution
This is a straightforward Faraday's-law electrolysis calculation for aluminium deposition from molten AlCl3; the mass deposited is 0.90 g.
Concept and Intuition
Electrolysis obeys Faraday's laws: the amount of substance deposited at an electrode is proportional to the quantity of electricity (charge) passed, and the proportionality depends on the valency (charge) of the ion being discharged. One mole of electrons (1 Faraday =96500 C) discharges 1/n mole of an ion of charge n+. For Al3+, n=3, so three moles of electrons are needed to deposit one mole of Al metal.
Step-by-Step Solution
- Charge passed: Q=I×t=96.5 A×100 s=9650 C.
- Moles of electrons passed: ne=Q/F=9650/96500=0.1 mol. …
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