Introduction to electrolysis
Francine Taylor-Campbell, Contributor

Huddled under a tree, fourth-form Montego Bay High School students do a last-minute review of their Spanish notes on September 27, 2005. - Claudine Housen/Staff Photographer

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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.

Electrolytes contain ions which are charged atoms, or charged radicles, (groups of atoms) which carry the electric current. Eg Na+, OH-

Strong electrolytes consist totally of ions that are fully ionised, for example all salts, strong acids and alkalis. These have high electrical conductivity.

Weak electrolytes consist mainly of molecules with relatively few ions, for example weak acids and weak bases, such as ethanoic acid and aqueous ammonia. In solution, a large proportion of the molecules remains undissociated.

NH3 (aq) + H2O (l) = NH4+(aq) + OH-(aq)

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

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

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

Identify ions

To identify the ions present in electrolytes (i) Write the formula of the compound e.g. Al2(SO4)3

(ii) Write a balanced equation to separate it into atoms or radicles e.g. Al2(SO4)3 = 2Al + 3SO4

(iii) Put the appropriate charge on the atoms or radicles (numerically equal to the valency) that is Al2(SO4)3 = 2Al3 + + 3SO42-

(iv) Check to see that the total charge on the ions is zero (2x3+) + (3x2-) = 0

In the case of aqueous solutions, relatively very small numbers of H+ and OH- ions are present owing to the extremely slight ionisation of water H2O == OH-(aq) + H+ (aq)

Q1 (a) Write equations for the ionisation of (i) H2SO4 (ii) CuSO4 (iii) FeCl3 (iv) Al2O3 (v) AlPO4 (vi) (NH4)3PO4 (b) Give the formulae of the ions and, or molecules present in (i) CuSO4(aq) (ii) NaCl(aq) iii. NH3(aq) iv. CH3CO2H(aq).

Factors affecting the formation of the products during electrolysis

(i) The degree of electropositiveness and electronegativeness of the ions. At the cathode, if more than one type of positive ion arrives at the cathode, the one which gives up its charge most readily, that is the least electropositive ion is discharged e.g. H+ and Na+, H+ is preferentially discharged, 2H+ + 2e = H2 Thus the ion derived from the element lower in the electrochemical series is discharged. At the anode, the least electronegative ion is discharged, that is the ion which gives up its charge more readily. Thus, in the electrolysis of dilute aqueous sodium chloride OH- ions are discharged in preference to Cl- ions that is 4OH-(aq) = 2H2O(l) + O2(g) + 4e

There are exceptions to this rule:

(ii) Owing to high concentration of an electrolyte, its anion may be discharged in preference to a less electronegative ion, example with concentrated aqueous sodium chloride, Cl- ions are discharged in preference to OH- ions (from water) 2Cl- = Cl2 + 2e (

iii) The type of electrode. In some cases, that is for active electrodes, the anode may dissolve to provide electrons e.g. a copper electrode in the electrolysis of aqueous copper sulphate. Cu(s) = Cu2+(aq) + 2e (left at the anode) since this occurs more readily than the discharge of the anions. Platinum and carbon (graphite) are inert electrodes; they do not form ions during electrolysis.

Francine Taylor-Campbell is an independent contributor.