Electrolysis
Francine Taylor-Campbell, Contributor

Oberlin High School, 2005 winner of the school choir competition, 'All Together Sing', performing on October 19, 2006. - Norman Grindley/Deputy Chief Photographer

In the electrolysis of molten substances only two kinds of ions are present and both are discharged. The cation is discharged at the cathode where it gains electrons (reduced) and forms atoms. The molten metal generally coats the cathode. The anion is discharged at the anode by losing electrons (oxidised) and forms atoms.

When electrolysing aqueous solutions, the ions of the substance being electrolysed and water is present. The discharge of ions at the electrodes will, therefore, be dependent on the position of the ion in the electrochemical series, concentration, and the type of electrode being used. (Please review last week's lesson to understand this concept.)

Exercise

1. Write electrode reactions for the electrolysis of molten sodium chloride.

In a molten substance such as NaCl (l), two ions are present, a positive ion (cation) Na+ and a negative ion (anion) Cl-.

At the cathode: Na + migrates towards the cathode (negative electrode) and gains electrons to form Na, which is deposited on to the electrode.

Reaction: Na+(l) + e =Na (s)

At the anode: Cl-ions migrate towards the anode (positive electrode) and lose electrons to form chlorine gas.

Reaction: 2Cl-(l) = Cl 2 (g) + 2e

Electrolysis of dilute sulphuric acid

Ions present are H+_and OH- from water and H+ and SO42- from sulphuric acid.

At the cathode: H+ ions migrate towards the cathode and are discharged as hydrogen gas. 2H+(aq) + 2e =H2(g)

At the anode: OH- and SO42- migrate towards the anode, but the OH-ion is discharged in preference to the SO42- ion. 4OH- (aq) = 2H2O (l) + O2(g) + 4e

NOTE

Since four mol of electrons are produced at the anode, then these four mol of electrons must be used up at the cathode. So in this reaction, for every one mol of oxygen gas produced, two mol of hydrogen gas are formed.

Electrolysis of aqueous sodium chloride using Pt/C electrode

Ions present: Na+, H+, OH- and Cl-

At the cathode: Na+ and H+ migrate towards the cathode but H+ is discharged in preference to Na+ (lower in electrochemical series).

2H+(aq) + 2e = H2(g)

At the anode: OH - and Cl - ions migrate towards the anode. If the sodium chloride solution is diluted, then OH- ions are discharged in preference to Cl-.

4OH-(aq) = 2H2O (l) + O2(g) + 4e

If the solution is concentrated, then Cl- ions are discharged preferentially and in this case a carbon electrode is used instead of platinum, as they are resistant to attack by chlorine.

2Cl-(aq) = Cl2(g) + 2e

Electrolysis of aqueous copper (II) sulphate solution

Ions present: H+, Cu2+, OH- and SO42-

At the cathode: Cu2+ and H+ ions migrate towards the cathode where the Cu2+ ions are discharged in preference to H+ ions as it is lower in the series.

Cu2+ (aq) + 2e = Cu (s).

This copper is deposited on the cathode,which gets thicker. The blue copper sulphate solution gradually fades.

At the anode: Two reactions can occur depending on the type of electrode used. Using Pt/C electrodes, OH- ions are discharged in preference to SO42-. 4OH-(aq) = 2H2O(l) + O2(g) + 4e. The solution becomes acidic as Cu2+ and OH- ions are removed leaving H+ and SO42-ions.

Using copper electrode as the anode results in the copper from the electrode being converted to copper ions (a process which requires less energy) and the anode dissolves and gets thinner. Cu(s) = Cu2+ (aq) + 2e. The concentration of the solution is unchanged (the blue colour remains the same).

Francine Taylor-Campbell is an independent contributor.