☰ Contents · Physics

Electrolysis and Faraday’s laws

Lessons 42–43 · 2 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, J.E. Usarov, K.T. Suyarov, M.K. Yuldasheva. Physics, textbook for Grade 8 of general secondary schools, revised and enlarged 3rd edition. “O‘qituvchi” Publishing and Printing Creative House, Tashkent, 2019
42

Electrolysis. Faraday’s first law

Textbook: pp. 120–122
GoalKnow electrolysis and Faraday’s first law m = kq = kIt, and use the electrochemical equivalent k.
New words
electrolysis · elektrolizelectrochemical equivalent · elektrokimyoviy ekvivalentFaraday’s law · Faradey qonunimass of deposited substance · ajralgan modda massasi
Explanation

When current passes through an electrolyte, substance is released at the electrodes — this is electrolysis. The mass m of substance released at an electrode is directly proportional to the charge q that has passed through the electrolyte (Faraday’s first law): m = k·q = k·I·t. Here k is the electrochemical equivalent of the substance; it equals the mass released when 1 C passes and depends on the substance. k is usually given in mg/C: 1.118 for silver, 0.329 for copper, 0.304 for nickel, 0.367 for chlorine and 0.094 for aluminium. If the current or the time grows, the mass released grows equally. Solutions such as copper sulfate are poisonous: only the teacher shows this experiment; never do it yourself at home and never put a solution into your mouth.

Worked examples
Cu: k = 0.329 mg/C, I = 2 A, t = 1000 s. q = 2000 C; m = 0.329 · 2000 = 658 mg.
Al (k = 0.094 mg/C): to release 470 mg we need q = m : k = 470 : 0.094 = 5000 C; at I = 5 A, t = 5000 : 5 = 1000 s.
Class activity

Make a table in your notebook: for k = 0.329 mg/C (copper) and different q (1000, 2000, 3000 C) calculate m. See that the mass is directly proportional to the charge. Only the teacher does the experiment.

Practice
1
State Faraday’s first law and write its formula.
2
For copper k = 0.329 mg/C. How many mg of copper is released when 3000 C passes?
3
Nickel (k = 0.304 mg/C) is deposited by a current of 4 A in 500 s. The mass in mg?
4
Why must the electrolysis experiment not be done at home?