8.4. Electrolysis

Video

Text

The reaction between copper and chlorine

Copper reacts with chlorine gas and forms cupric chloride, CuCl₂.Copper reacts with chlorine gas and forms cupric chloride, CuCl2.

Cu(s) + Cl2(g) → CuCl2(s)

  • The reaction is spontaneous and exothermic.
  • The electrons perform work.
  • What happens if we run the reaction ”backwards”?
  • Electrolysis = splitting (lysis) with the help of electricity.

Electrolysis of cupric chloride, CuCl2

Electrolysis of cupric chloride.Electrolysis of cupric chloride.

Cathode reaction (reduction)

  • Electrons are pushed into the cathode
  • Aqueous copper ions are reduced to solid copper:
    • Cu2+(aq) + 2e → Cu(s)

Anode reaction (oxidation)

  • Electron shortage
  • Chloride ions pulled towards the anode, oxidized to chlorine gas:
    • Cl(aq) → Cl2(g) + 2e

Production of sodium hydroxide

Sodium hydroxide is produced by electrolysis of sodium chloride.Sodium hydroxide is produced by electrolysis of sodium chloride.

Anode reaction

Similar to the electrolysis of cupric chloride:

  • Electron shortage in the anode
  • Chloride ions pulled towards the anode, oxidized to chlorine gas:
    • Cl(aq) → Cl2(g) + 2e

Cathode reaction

Two possibilities:

1. Na+(aq) + e → Na(s)

\(e^0_{\text{Na}} = -2.71 \text{V}\)

2. 2H2O + 2e → H2(g) + 2OH(aq) 

\(e^0_{\text{H}_2\text{O}} = -0.83 \text{V}\)

It can only be the second one, because:

  1. Any sodium atoms that form would immediately react with water:
    • 2Na(s) + 2H2O → 2Na+(aq) + 2OH(aq) + H2(g)
  2. The standard electrode potential for water is greater than the electrod potential for sodium, making the water a stronger oxidizing agent.
    • \(e^0_{\text{H}_2\text{O}} > e^0_{\text{Na}}\) ⇒ Reaction 2 takes place rather than reaction 1.

Total reaction

2NaCl(aq) + 2H2O → Cl2(g) + H2(g) + 2NaOH(aq)

The concentrated sodium hydroxide solution is let out through a pipe.