Electrolysis II – Factors Affecting Preferential Discharge of Ions SS2 Chemistry Lesson Note

FACTORS AFFECTING PREFERENTIAL DISCHARGE OF IONS

The discharge of ions at the electrodes is governed by three conditions, namely

1. The position of ions in the electrochemical series.
2. The concentration of ions.
3. The nature of the electrodes

RELATIVE POSITIONS OF THE IONS IN THE ELECTROCHEMICAL SERIES

Relative position of ions in the electrochemical series. When all other factors are constant a cation (cathode ion) which is lowest in the series (less electropositive) will be discharged in preference to the one higher up (more electropositive). This is because the less electropositive element gains electron(s) more readily from the cathode and so becomes reduced and discharged as a neutral atom while the more electropositive element remains in the solution as positive ions.

NOTE: K+, Na+ and Cat are never discharged at all from aqueous solution. This is because of the large gap between them and H+. However, K+, Na+ and Cat are discharged during the electrolysis of their molten salt.

An anion which is higher up in the series (less electronegative) is preferentially discharged to the one lower down the series (more electronegative). This is because the less electronegative ion loses an electron(s) more readily than the more electronegative ion.

NOTE: F, SO42 and NO3 are never discharged from the aqueous solution because of the large gap between them and OH”.

CONCENTRATION OF IONS

When the concentration of an ion in the electrolyte is increased, the ion tends to increase its chances of being discharged. The influence of concentration, however, is effective only when the two competing ions are closely positioned in the electrochemical series. The effect of concentration becomes less important as the positions of the competing ions become further apart in the series.

NATURE OF ELECTRODES

Inert electrodes (e.g. Platinum and graphite) do not take part in the electrolytic reactions. However, platinum is attacked by liberated chlorine and graphite is attacked by liberated oxygen. Some electrodes have a strong affinity (love) for certain ions and influence the discharge of such ions. For example, in the electrolysis of aqueous NaCl using a mercury cathode, Na+ will be discharged at the cathode to form sodium amalgam, Na/Hg

Na+(aq) + Hg(s) + e- Na/Hg(1)

Also, in the electrolysis of CuSO4 solution using copper anode, neither the SO42 nor the OH will be discharged. Rather, the Cu atoms will lose electrons more readily and go into solution as Cu2+, hence, the copper anode is known as a reactive electrode