(a) What is electrolysis? (b) A current of 2A is passed through a copper voltameter for 5 minutes. If the electrochemical equivalent of copper is 3.27 x 10\...

Answer Details

(a) What is electrolysis?

Electrolysis is the chemical decomposition of an electrolyte (a molten ionic compound or an aqueous solution of one) caused by the passage of a direct electric current through it. The current makes the positive ions (cations) move to the cathode and the negative ions (anions) move to the anode, where they are discharged and new substances are set free.

(b) Mass of copper deposited

This uses Faraday's first law of electrolysis, which states that the mass \(m\) of a substance liberated at an electrode is proportional to the quantity of charge \(Q\) passed, so that \(m = zQ = zIt\), where \(z\) is the electrochemical equivalent.

Given: current \(I = 2\ \text{A}\); time \(t = 5\ \text{min} = 5 \times 60 = 300\ \text{s}\); electrochemical equivalent \(z = 3.27 \times 10^{-7}\ \text{kg C}^{-1}\).

Working:

\[ m = zIt = (3.27 \times 10^{-7}) \times 2 \times 300 \] \[ m = (3.27 \times 10^{-7}) \times 600 \] \[ m = 1.962 \times 10^{-4}\ \text{kg} \]

The mass of copper deposited is \(1.962 \times 10^{-4}\ \text{kg}\), which is about \(0.196\ \text{g}\).

Key point: the time must be converted from minutes to seconds before substituting, because the ampere is one coulomb per second; forgetting to change 5 min into 300 s is the most common error and would give an answer 60 times too small.