CSEC Chemistry: Preferential Discharge of Ions and the Electrochemical Series

In this post we will cover:

• predicting chemical reactions making use of electrochemical series (SA 9.7)

If a molten electrolyte is electrolysed, there is no competition for discharge at the electrodes since there is only one type of cation and one type of anion. However, when an aqueous electrolyte (i.e. dissolved in water) is electrolysed, hydrogen ions and hydroxide ions (from the partial ionization of water) are also present in addition to the ions from the electrolyte.

The partial ionization of water is as follows:

H₂O(l) ⇋ H⁺(aq) + OH⁻(aq)

When the current is passed through an aqueous solution, multiple types of ion move towards each electrode. One type of ion is discharged in preference to the other.

For example, when a current is passed through an aqueous solution of sodium sulphate, both hydrogen and sodium ions (cations) move towards the cathode, and both hydroxide and sulphate ions (anions) move towards the anode.

The ion which is preferentially discharged at either electrode is determined based on its position on the electrochemical series.

This is the basic principle behind using the electrochemical series in predicting chemical reactions:

When more than one cation is present at the cathode, the one which is preferentially discharged is the one lower in the electrochemical series (also called the reactivity series).

When more than one anion is present at the anode, the one discharged preferentially is the one lower in the electrochemical series.