Q.What will happen during the electrolysis of aqueous solution of CuSO4 in the presence of Cu electrodes? (Two or more than two options may be correct.)
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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 …
The key idea is Faraday's laws of electrolysis applied to an electrolytic cell with active (copper) electrodes. Unlike inert electrodes, the copper anode itself can participate in the reaction.
-
Cathode reaction: Cu2+ ions in solution gain electrons and deposit as copper metal:
Cu2++2e−→Cu(s) — so copper deposits at the cathode.
-
Anode reaction: Since the anode is made of copper, it can oxidise (dissolve) more easily than any anion in solution (like SO42− or OH−). The reaction is: …
In the electrolysis of aqueous CuSO4 with copper electrodes, the anode itself is oxidised (copper dissolves) and Cu2+ ions from the solution are reduced at the cathode (copper deposits). No gas evolution occurs. The correct options are (i) and (ii).
This is a classic case of electrolysis with active electrodes. The key idea is that when the electrodes are made of the same metal as the cation in the electrolyte, the electrode reactions change completely — the anode is no longer inert, so it participates in the reaction.
Let’s break it down.
1. What are the species present in the solution?
Aqueous CuSO4 dissociates into Cu2+ and SO42− ions. Water itself is also present, providing H+ and OH− ions (though in very low concentration).
So at the electrodes, we have competing possibilities:
- At cathode (reduction): Cu2+ ions can be reduced to Cu metal, or H+ from water can be reduced to H2 gas.
- At anode (oxidation): SO42− ions can be oxidised (very difficult), OH− from water can be oxidised to O2 gas, or the copper metal of the anode itself can be oxidised to Cu2+.
The standard reduction potentials tell us which reaction is favoured.
2. Why copper deposits at the cathode — not hydrogen
The reduction potentials (at 298 K, 1 M, vs SHE) are:
Cu2++2e−→Cu(s)E∘=+0.34 V
2H++2e−→H2(g)E∘=0.00 V
A more positive E∘ means a greater tendency to be reduced. Cu2+ reduction is much more favourable than H+ reduction. So at the cathode, copper ions are reduced to copper metal, which deposits on the cathode.
A common mistake is to think that because water is present, hydrogen gas must evolve. But unless the Cu2+ concentration is extremely low, copper deposition is strongly favoured. In standard conditions, copper plates out first.
So option (i) Copper will deposit at cathode is correct.
3. What happens at the anode — the crucial difference
If the anode were inert (like platinum or graphite), the only possible oxidation would be of OH− to O2:
4OH−→O2+2H2O+4e−E∘=+0.40 V
But here the anode is copper metal. Copper can itself be oxidised:
Cu(s)→Cu2++2e−E∘=−0.34 V
The oxidation potential (the reverse of the reduction potential) is +0.34 V for copper dissolution, compared to +0.40 V for oxygen evolution. A lower oxidation potential means the reaction is easier. So copper metal from the anode dissolves into the solution as Cu2+ ions, rather than oxygen being produced. …
Method: Electrode Potential Comparison for Electrolysis (Active Electrodes)
This method uses standard oxidation/reduction potentials, but with an added check: whether the electrode material itself can react (an "active" electrode) instead of only the ions in solution.
Steps:
-
List the species present.
Aqueous CuSO4 with copper electrodes: Cu2+, SO42−, water — and, crucially, the copper metal of the electrodes themselves.
-
At the cathode, compare reduction potentials:
- Cu2++2e−→Cu(s); E∘=+0.34 V
- 2H++2e−→H2(g); E∘=0.00 V
Cu2+ reduction is more favourable, so copper deposits at the cathode.
-
At the anode, first check whether the electrode itself can be oxidised.
Because this anode is copper metal (an "active" electrode, not inert Pt/graphite), compare:
- Cu(s)→Cu2++2e−; Eox∘=+0.34 V
- 4OH−→O2+2H2O+4e−; Eox∘≈+0.40 V …
Here are the most common mistakes students make on this exact type of "active electrode electrolysis" question, along with how to avoid each.
1. Assuming Oxygen Is Released at the Anode, As With Inert Electrodes
The Mistake:
Students remember that with Pt/graphite electrodes, water is oxidised to O2 at the anode, and wrongly apply the same conclusion here.
How to Avoid:
Always check the electrode material first. If the anode is made of the same metal as the cation in solution (copper anode in CuSO4), the metal itself is oxidised in preference to water — no gas is evolved at all.
2. Forgetting to Compare the Electrode's Own Oxidation Potential
The Mistake:
Students only compare SO42− and H2O as candidates for anode oxidation, never considering that the copper electrode itself can be oxidised.
How to Avoid:
Write out Cu(s)→Cu2++2e− (Eox∘=+0.34 V) alongside 4OH−→O2+2H2O+4e− (Eox∘≈+0.40 V). The lower oxidation potential (copper) is favoured, so the anode dissolves instead of evolving oxygen.
3. Thinking Copper Can Deposit at the Anode
The Mistake:
Students pick "copper deposits at anode" because copper is visibly involved at both electrodes.
How to Avoid:
Deposition is always a reduction process, and the anode is always where oxidation occurs. Copper metal is produced by oxidation (dissolves) at the anode, and consumed by reduction (deposits) only at the cathode — never the reverse.
4. Missing the Real-World Link to Electrorefining
The Mistake:
Students don't connect this setup to a named process, so they can't sanity-check their answer.
How to Avoid: …
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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