Electrolysis

■ Electrolysis is the decomposition of an electrolyte by an electric current. This is carried out in an electrolytic cell.

■ An electrolyte is a molten compound or the solution of a compound, which conducts an electric current and is decomposed by it.

■ The ions are only free to move when molten or in solution. ■ Electrolytes contain ions which are charged atoms, or charged radicles (groups of atoms) which carry the electric current. E.g., Na+, OH

■ Strong electrolytes consist totally of ions, i.e., are fully ionized, e.g., all salts, strong acids and alkalis. These have high electrical conductivity.

■ Weak electrolytes consist mainly of molecules with relatively few ions, e.g., weak acids and weak bases such as ethanoic acid and aqueous ammonia. In solution, a large proportion of the molecules remain undissociated. NH3 (aq) + H2O (l) NH +( aq) +

4 OH-(aq).

■ The electrodes in the electrolytic cell are the anode, which is the positive electrode, and the cathode, which is the negative electrode. During electrolysis, the anode gains electrons and the cathode loses electrons. Electrons flow from the anode to the cathode in an electrolytic cell.

■ Electrodes which react and take part in electrolysis are called active electrodes (e.g., copper). Those which do not take part in the reaction are called inert electrodes (e.g., graphite, platinum).

■ Anions are negative ions which travel towards the anode during electrolysis, e.g., OH- and Cl- where they may give up electrons (2Cl-(aq) Cl2(g) + 2e).

■ Cations are positive ions which travel towards the cathode during electrolysis, e.g., H+, Na+, Cu2 +, where they may gain electrons (Cu + 2e Cu).

2

■ 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 (oxidized) and forms atoms.

■ When electrolysing aqueous solutions, the ions of the substance being electrolysed and water are present. The discharge of ions at the electrodes will, therefore, be dependent on position of the ion in the electrochemical series, the concentration of the solution, and the type of electrode being used.

■ At the cathode, if more than one type of positive ion arrives at the cathode, the one which gives up its charge most readily, i.e., the least electropositive ion, is discharged, e.g., H+ and Na+, H+ is preferentially discharged 2H+ (aq)+ 2e H2 (g). Thus, the ion derived from the element lower in the electrochemical series is discharged. At the anode, the least electronegative ion is discharged, i.e., the ion which gives up its charge more readily. Thus, in the electrolysis of dilute aqueous sodium chloride, OHions are discharged in preference to Cl- ions

■ Due to the high concentration of an electrolyte, its anion may be discharged in preference to a less electronegative ion. For example, with concentrated aqueous sodium chloride, Cl- ions are discharged in preference to OH- ions (from water). Let us attempt a few questions.

1. Electrolysis involves the chemical decomposition of a compound, either when molten or in aqueous solution, by the passage of an electric current.

(a) Explain why aqueous calcium nitrate can be electrolysed, but liquid pentane cannot.

(b) State the products of the electrolysis of molten sodium chloride.

(c) State the products of the electrolysis of concentrated aqueous sodium chloride.

(d) Describe the essential details of the manufacture of aluminium by electrolysis.

ANSWERS

(a) Electrolysis requires the presence of positive and negative ions to conduct an electric current. This means that the substance should be ionic or have the possibility to generate ions. Aqueous calcium nitrate is an ionic compound which forms the ions Ca2+, H+, OH-, NO3- in solution. Pentane is a covalent compound which consists of molecules and not ions. Hence, pentane cannot be electrolysed.

(b) In a molten substance such as molten sodium chloride (NaCl (l)), only 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 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) Cl2(g) + 2e

(c) In concentrated aqueous sodium chloride, the ions Na+, H+, OH- and Cl- are present. Normally, ions in aqueous solution are discharged in preference based on their position in the electrochemical series. So, ions lower in the series are discharged first. In this case, since a concentrated solution of NaCl (aq) is being used, higher concentration of Cl- ions means that these ions will be discharged in preference to OH- ions. Products at the cathode, H+ ions, will be discharged as H2 (g). Products at the anode, Cl- ions, will be discharged as Cl2 (g).

(d) In the extraction of aluminium, the ore (bauxite) is purified and dissolved in molten cryolite (Na AlF ) from which it produces

3 6 the ions Al3 and O2 -. The aluminium oxide/ cryolite solution is

+ then electrolysed in a cell using graphite (carbon) electrodes. Aluminium is discharged at the cathode and collected at the bottom of the cell.

Al3 (l) + 3e Al (l)

+

The oxide ions (-ve) move towards the anode, where they lose electrons to form oxygen, which is collected. 2O2-(l)

O2 (g) + 4e.

Francine Taylor-Campbell is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com