☰ Contents · Physics

Electrolysis problems and uses

Lessons 44–45 · 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
44

Problem solving

Textbook: pp. 125–126
GoalSolve electrolysis problems: find mass, charge, current and time, and calculate layer thickness.
New words
m = kIt · m = kItlayer thickness · qatlam qalinligidensity · zichlikchecking units · birliklarni tekshirish
Explanation

In electrolysis problems the basic formula is m = k·I·t; from it t = m/(kI), I = m/(kt) and q = m/k follow. k is usually given in mg/C, so take the mass in mg (1 g = 1000 mg) and the time in seconds (1 min = 60 s, 1 h = 3600 s). In layer-thickness problems the released mass is linked by m = ρ·V = ρ·S·h: ρ is the density, S the area, h the thickness; keep the units consistent (for example g/cm³, cm², cm). It helps to check the result by comparing two ways of solving. The experiments in the problems are for calculation only: electrolysis involves poisonous solutions and current, so it is not done without a teacher.

Worked examples
The area to be coated is S = 110 cm², layer h = 0.01 mm = 0.001 cm, ρ ≈ 9 g/cm³: m = 9 · 110 · 0.001 = 0.99 g = 990 mg. With k = 0.33 mg/C and I = 3 A: t = m : (kI) = 990 : (0.33 · 3) = 1000 s.
Silver (k = 1.118 mg/C): 1118 mg was released in 500 s. The current I = m : (k·t) = 1118 : (1.118 · 500) = 2 A.
Class activity

Make your own problem: choose a metal and its k, give a current and a time, and ask for the mass released. Solve it in your notebook and swap with a classmate. Paper only.

Practice
1
How is time found from m = kIt?
2
How many seconds does it take to deposit 658 mg of copper (k = 0.329 mg/C) with 2 A?
3
What charge in C is needed to release 367 mg of chlorine (k = 0.367 mg/C)?
4
Why must grams be converted to mg when computing with k in mg/C?