21.4    Quantitative Electrolysis

E° value in electrolysis

• Recall that the selective discharge of ions during electrolysis is affected by:
  1. the position of ions in the electrochemical series
  2. the concentration of ions

• Cations lower in the electrochemical series will be selectively discharged(reduced). This is because they have a more positive E° value and hence it favours reduction more

• Cations with higher concentration will be selectively discharged(reduced). This is because the value of E° becomes more positive as the concentration of ions increases. A more positive E° favours reduction

Faraday’s laws of electrolysis

• The quantity of charge,Q passed during electrolysis is given by this equation. Q is measured in Coulombs, C

• Faraday’s first law of electrolysis states that the mass of a substance liberated during electrolysis is directly proportional to the quantity of charge passed through during electrolysis

• Faraday’s second law of electrolysis states that the number of Faradays required to discharge one mole of ion at an electrode equals to the number of charges on the ion

• Faraday, F is the amount of charge carried by one mole of electrons. 1 electron → 60 x 10⁻¹⁹ C
  ∴ 6.02 x 10²³ electrons → 96500 C
  Therefore, 1 F = 96500 C

• Calculation examples:

Determining the value of  Avogadro’s constant, L by means of    electrolysis

• The Avogadro’s constant can be determined using electrolysis. This is done by calculating the charge associated with one mole of electrons. Then, Avogadro’s constant can be calculated using:

• The charge on one electron is 60 x 10⁻¹⁹ C, this is done using physics calculation.

• The charge on one mole of electrons is then found from a simple electrolysis experiment.

• The procedure is
  1. Weigh the pure copper anode and pure copper cathode separately
  2. Arrange the apparatus as shown. The variable resistor is used to keep the current constant
  3. Pass a constant electric current for a measured time interval
  4. Remove the cathode and anode and wash and dry them with distilled water and then with propanone
  5. Reweigh the copper cathode and anode

• The cathode increases in mass because copper is deposited, the anode decreases in mass because the copper goes into the solution as The decrease in mass of anode is preferably measured, this is because copper does not always ‘stick’ to the cathode very well, resulting in inaccurate gain in mass.

• Suppose a sample set of experiment data is as shown below:
  -Mass of anode at the beginning = 56.53 g
  -Mass of anode at the end = 56.40 g
  -Mass of copper removed from anode = 0.13 g
  -Quantity of charge passed = 408 C
• 1. Cu²⁺(aq) + 2e⁻ → Cu(s)
  2. 0.13 g → 408 C
     ∴   63.5 g → 63.5 x 408 ÷ 0.14 = 185057.143 C, for two moles of electrons
  3. Therefore, charge on one mole of electrons = 185057.143 ÷ 2 = 99600 C
  4. Therefore, L = 99600 ÷ 60 x 10⁻¹⁹ C = 6.2 x 10²³
  5. This is a good agreement with the accurate value, 6.02 x 10²³.