medium difficulty9 min

Electrolysis and Faraday's Laws

Electrolysis is the use of electrical energy to drive a non-spontaneous chemical reaction, and Faraday's laws relate the amount of substance deposited or liberated to the quantity of electricity passed.

In one line: Current gives charge, charge gives electrons, electrons give product.

Practice This Concept

Understand it

In electrolysis, oxidation occurs at the anode and reduction occurs at the cathode, but the electrode signs differ from a galvanic cell because an external power source drives the reaction.
The mass deposited is directly proportional to charge passed.
Charge is current multiplied by time.
One faraday, approximately 96500 coulombs, corresponds to one mole of electrons.
The product formed during electrolysis depends on electrode material, ion concentration, and electrode potentials.

Example

During electrolysis of molten NaCl, sodium ions are reduced at the cathode to sodium metal, and chloride ions are oxidised at the anode to chlorine gas.

Rule to remember

Faraday relation: Q = It.

Mass deposited: m = ZIt.

Also, moles of electrons = Q/F, where F = 96500 C mol-1 approximately.

For Mn⁺ + ne- → M, moles of metal deposited = Q/(nF), and mass = molar mass x Q/(nF).

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Calculate mass of copper deposited when 1.93 A current passes through CuSO₄ solution for 500 s.

1Q = It = 1.93 × 500 = 965 C.
2For Cu²⁺ + 2e⁻ → Cu, n = 2.
3Moles of Cu = 965/(2 × 96500) = 0.005 mol.
4Mass = 0.005 × 63.5 = 0.3175 g.
5The small mass is reasonable because the charge is only one-hundredth of a faraday.

Method you can reuse

1Write the electrode half-reaction.
2Count electrons needed per mole of product.
3Convert time to seconds.
4Calculate charge using Q = It.
5Convert charge to moles of electrons using Q/F.
6Use the half-reaction ratio to find product amount and then mass or volume.

Visual tip

A diagram is not compulsory for every question, but a simple electrolytic cell showing battery, anode, cathode, electrolyte, cation movement to cathode, and anion movement to anode helps distinguish it from a galvanic cell.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How much charge passes when a current of 2 A flows for 30 minutes?

Think first, then show answer

Q = It = 2 × 30 × 60 = 3600 C.

Common wrong answer

A common wrong answer is using n = 1 for copper deposition instead of n = 2 for Cu²⁺ + 2e⁻ → Cu.

Don't mix this up

Students often use time in minutes directly instead of converting it to seconds, or forget to divide by the number of electrons required for deposition.

Exam tip

For numerical questions, write the half-reaction first. The number of electrons in that half-reaction decides the mole ratio between electrons and product.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask mass deposited, gas volume liberated, time required, current required, products at electrodes, or comparison of products using electrode potentials.

1
Enough for 1 mark
Electrolysis is the use of electrical energy to drive a non-spontaneous chemical reaction, and Faraday's laws relate the amount of substance deposited or liberated to the quantity of electricity passed.
2
Add for 2 marks
Example: During electrolysis of molten NaCl, sodium ions are reduced at the cathode to sodium metal, and chloride ions are oxidised at the anode to chlorine gas.
3
Add for 3 marks
In electrolysis, oxidation occurs at the anode and reduction occurs at the cathode, but the electrode signs differ from a galvanic cell because an external power source drives the reaction.

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