Unit 4

Electricity and Magnetism

Class 12 Physics

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Chemical Effect of Current

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Contents / Quick Navigation

Introduction
Electrolysis Terms
Faraday’s First Law
Faraday’s Second Law
Faraday Constant
Experimental Verification
Worked Numericals
Exam Questions

Introduction

When a direct current passes through a suitable electrolyte, matter is transported by ions and chemical changes occur at the electrodes. This is the chemical effect of electric current. The quantitative relationship between electric charge and the amount of substance liberated or deposited is described by Faraday’s laws of electrolysis.

Diagram 1: Basic electrolytic cell

Positive ions move to the cathode; negative ions move to the anode.

Electrolysis: The process in which an electrolyte undergoes chemical decomposition or electrode reactions due to the passage of electric current.

Important Terms

Term	Meaning
Electrolyte	A substance whose molten state or solution conducts through mobile ions and participates in electrode reactions.
Electrode	A conductor through which current enters or leaves the electrolyte.
Anode	Positive electrode in an electrolytic cell; oxidation occurs here.
Cathode	Negative electrode in an electrolytic cell; reduction occurs here.
Cation	Positive ion; moves toward the cathode.
Anion	Negative ion; moves toward the anode.
Electrochemical equivalent	Mass deposited/liberated per unit charge; symbol Z; SI unit kg C⁻¹.
Chemical equivalent	Molar/atomic mass divided by valency (equivalent mass).

Remember: In an electrolytic cell, the anode is connected to the positive terminal and the cathode to the negative terminal of the external D.C. supply.

Faraday’s First Law of Electrolysis

Law: The mass of a substance liberated or deposited at an electrode is directly proportional to the total charge passed through the electrolyte, provided other relevant conditions are fixed.

m ∝ Q m = ZQ = ZIt where m = deposited mass (kg), Z = electrochemical equivalent (kg C⁻¹), I = current (A), t = time (s), Q = It (C).

Diagram 2: First-law verification with a voltameter

Measure deposited mass for different values of current and/or time while keeping the electrolyte system unchanged.

Diagram 3: First-law graph

A straight line through the origin verifies m ∝ Q; its slope gives Z.

Derivation of the working relation

For steady current I flowing for time t, Q = It. Faraday’s first law gives m = ZQ. Therefore:

m = ZIt

Dimensional/unit check: ZIt = (kg C⁻¹)(C s⁻¹)(s) = kg.

Faraday’s Second Law of Electrolysis

Law: When the same quantity of electricity passes through different electrolytes, the masses of substances liberated or deposited are proportional to their chemical equivalent masses.

m ∝ E m₁/m₂ = E₁/E₂ where E is chemical equivalent (equivalent mass).

Diagram 4: Series voltameters for the second law

Series connection ensures the same charge passes through every electrolyte.

Combining the first and second laws gives:

m = (E/F)Q = EIt/F

Thus the electrochemical equivalent is:

Z = E/F

Faraday Constant

Faraday constant (F): The magnitude of electric charge carried by one mole of elementary charges (approximately one mole of electrons). F ≈ 9.6485 × 10⁴ C mol⁻¹

For school numericals, F is often taken as 9.65 × 10⁴ C mol⁻¹.

m = (M/nF)Q where M = molar mass (kg mol⁻¹ or g mol⁻¹ consistently), n = number of electrons involved per ion/formula unit in the electrode reaction.

Link with elementary charge: Since one mole contains N_A particles, F = N_Ae so measurement of F historically provides a route to the elementary charge when N_A is known.

Experimental Verification of Faraday’s Laws

Verification of the first law

Clean, dry and weigh the cathode of a suitable voltameter (for example, a copper voltameter).
Pass a steady current I for a measured time t.
Remove, wash, dry and reweigh the cathode. The increase in mass is m.
Repeat for different I or t while keeping the electrolyte/electrode system the same.
Plot m against Q = It. A straight line through the origin verifies m ∝ Q.

Verification of the second law

Connect different voltameters in series so that the same current flows through each for the same time.
Measure masses m₁, m₂, … deposited/liberated.
Compare m₁/m₂ with the ratio of the corresponding chemical equivalents E₁/E₂.
Agreement within experimental uncertainty verifies the second law.

Exam important: “Same charge” is essential in the second law. Series connection is used because the same current passes through each cell for the same time.

Worked Numerical Examples

Example 1: Using electrochemical equivalent. A current of 2.0 A passes for 20 min through an electrolyte with Z = 3.3×10⁻⁷ kg C⁻¹. Find the deposited mass.

t = 1200 s, Q = It = 2400 C.

m = ZIt = 3.3×10⁻⁷ × 2400 = 7.92×10⁻⁴ kg = 0.792 g

Example 2: Copper deposition. Find the mass of Cu deposited by 3.0 A in 30 min. Take M(Cu)=63.5 g mol⁻¹, n=2 and F=96500 C mol⁻¹.

Q = 3×1800 = 5400 C.

m = MQ/(nF) = 63.5×5400/(2×96500) ≈ 1.78 g

Example 3: Second law. The same charge deposits 1.08 g of Ag (equivalent mass 107.9 g/eq). What mass of Cu is deposited if Cu equivalent mass is 31.75 g/eq? m_Cu/1.08 = 31.75/107.9 m_Cu ≈ 0.318 g

Applications and Scientific Context

The syllabus focus is quantitative electrolysis, but the same principles explain electroplating, electrolytic refining and controlled electrodeposition. These applications should be treated as context, not as a replacement for learning Faraday’s laws.

Diagram 5: Electroplating principle

The article to be coated acts as the cathode; metal ions are supplied through the electrolyte.

Common exam mistakes:

Using minutes directly in m = ZIt instead of converting to seconds.
Mixing grams with kilograms without matching the unit used for Z or M.
Using atomic mass instead of equivalent mass in the second law.
Forgetting ion valency/electron number n in m = MQ/(nF).
Reversing anode and cathode signs for an electrolytic cell.

Important Exam Questions

Short-answer

Define electrolysis, electrolyte, anode and cathode.
Define electrochemical equivalent and state its SI unit.
State Faraday’s first law of electrolysis.
State Faraday’s second law of electrolysis.
What is Faraday constant? Write its approximate value and unit.

Long-answer / derivation

State and explain both Faraday laws and obtain m = EIt/F.
Describe an experiment to verify Faraday’s first law.
Describe an experiment to verify Faraday’s second law using series voltameters.
Show that Z = E/F and explain each symbol.

Numerical questions

A current of 1.5 A flows for 40 min. If Z = 1.12×10⁻⁶ kg C⁻¹, calculate the deposited mass.
Calculate the charge required to deposit 1.0 g of Ag using M = 107.9 g mol⁻¹, n=1.
The same charge passes through AgNO₃ and CuSO₄ cells. Compare the deposited masses using equivalent masses.

Diagram questions

Draw and label a basic electrolytic cell.
Draw a copper voltameter setup for first-law verification.
Draw series voltameters for second-law verification.
Sketch m versus Q and identify its slope.

One-Minute Revision

Electrolysis is chemical change produced by current through an electrolyte.
Cations move to cathode; anions move to anode.
First law: m ∝ Q, so m = ZIt.
Z is electrochemical equivalent; SI unit kg C⁻¹.
Second law: for equal charge, m ∝ chemical equivalent E.
m₁/m₂ = E₁/E₂.
Combined law: m = EIt/F.
Z = E/F.
F ≈ 9.65×10⁴ C mol⁻¹.
Also F = N_Ae.
Convert time to seconds and keep mass units consistent.

Diagram Practice

Electrolytic cell
Copper voltameter
m–Q graph
Series voltameters
Electroplating setup

Source handling: The original Nepal eNotes PDF is embedded above using the verified Google Drive file. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.

Reference scope used: NEB/CDC Class 12 Physics syllabus coverage for Chemical Effect of Current: Faraday’s laws of electrolysis, Faraday constant and verification of the laws.