What are Faraday's Laws of Electrolysis?
Faraday's laws quantify the amount of substance produced or consumed during electrolysis. The first law links mass to charge (Q); the second compares masses of different substances following stoichiometry.
Faraday's first law: mass = (Q × M) / (n × F), where F = 96,485 C/mol of electrons. The mass of substance deposited is proportional to charge passed and inversely proportional to the number of electrons.
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Step-by-step worked examples
How much copper is deposited when 9,648.5 C of charge passes through a CuSO₄ solution? (Cu: M = 64 g/mol, n = 2)
m = (Q × M) / (n × F) m = (9648.5 × 64) / (2 × 96485) m = (617,504) / (192,970) = 3.2 g
Silver is plated when 48,242.5 C passes through AgNO₃. (Ag: M = 108 g/mol, n = 1). Mass of Ag?
m = (Q × M) / (n × F) m = (48242.5 × 108) / (1 × 96485) m = (5,210,190) / (96,485) = 54 g
How many coulombs deposit 1 g of copper? (n = 2, M = 64)
Q = (m × n × F) / M Q = (1 × 2 × 96485) / 64 Q = 192,970 / 64 = 3,015.5 C
Flashcards
Quick quiz
Q1.12,500 C deposits 1.5 g of metal. If n = 2, what is the molar mass?
Q2.What is the Faraday constant?
Q3.If 48,242.5 C deposits 54 g of Ag, what is n?
Q4.Does increasing charge increase or decrease mass deposited?
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Common mistakes
Confusing F (Faraday constant) with the Faraday unit. — Correct: F = 96,485 C/mol; 1 Faraday = charge to deposit 1 mole of singly-charged ions.
Forgetting to use n in the formula. — Correct: n is the number of electrons per ion/atom; Cu²⁺ has n = 2, Ag⁺ has n = 1.
Using molar mass in moles instead of grams. — Correct: M is molar mass in g/mol; multiply by n (in moles of electrons) in the numerator.
Assuming all metals have n = 1. — Correct: n depends on the charge; Cu²⁺ → Cu is n = 2; Al³⁺ → Al is n = 3.
FAQ
What is electrolysis?
The use of electrical energy to drive a non-spontaneous chemical reaction. Oxidation at anode, reduction at cathode.
How do you calculate charge from current and time?
Q = I × t (charge = current in amperes × time in seconds), measured in coulombs.
Can Faraday's law predict gas evolution?
Yes, H₂, Cl₂, and other gases follow the same law. For H₂: 2H⁺ + 2e⁻ → H₂ (n = 2).
What is electroplating?
Electrolysis used to coat one metal with another (e.g., gold-plating). Faraday's law calculates the plate thickness.