In aqueous solution, a salt or an ionic compound ionizes to positive and negative ions. If the solvent is water, it will also ionize to give H and OH ions. The ions present are therefore(Na, Cl, H and OH). The order of selective discharge either at the anode or cathode depends on the following factors.

• Positive of the ions in the electrochemical series
• Concentration of the ions
• Nature of electrode

Position of ions on the electrochemical series

The electrochemical series is an arrangement of ions according to their reactivity. Ions that are lower down the series will be discharged in preference to those above it on the series. The table below show the relative position of ions on the electrochemical series.

Concentration of the ions

Ions concentration infleunces the ion to be discharged during electrolysis. The more concentrated an ion is in the electrolyte, more likely it is going to be discharged during electrolysis. This true when the ions are very close to each other on the electrochemical series. However if they  are far apart on the electrochemical series, the one lower down the series is discharged in preference.

Nature of Electrode

The ion discharged during electrolysis also depends on the nature of the electrode used. Some electrodes will preferentially discharge a particular ion while others will prefer another ion. For example in the electrolysis of aqueous sodium chloride, the ion present are Cl^-,Na⁺, OH^-. Both Na⁺ and H⁺ ions migrate to the cathode while Cl^- and OH^- migrate to the anode.

Examples of electrolysis

1. Electrolysis of molten substances e.g lead(II) bromide.The apparatus below can be used for this purpose.

Solid lead(II) bromide is placed in the crucible and heated. It is observed that the bulb does not light while the lead(II) bromide I still in the  form , showing that no electric current is passing through the solid lead(II) bromide. As soon as the lead(II) bromide melts, the bulb lights, showing that ions are present in the molten state. If the set up is allowed to run for some time, bromine could be seen escapingat the anode as a reddish brown vapour while lead is deposited at the cathode, this is because during electrolysis, the Pb²⁺ ions migrate to the cathode where they acquire or gain electrons to become lead metal.

Also the bromine ion migrate to the anode where they lose electrons to the electrode to become bromine gas.

Ions present lead and bromine

The cathode is ade up of carbon and the anode is made up of the carbon.

The reaction at the cathode  and anode respectively

However in aqueous solution, there are two types of positive and two types of negative ions. Two types of ions are from the solute and two from water( solvent).

1. Electrolysis of sodium chloride using electrodes

If all other conditions that is concentration and nature of elcetrodes are constant, then ions lower down the electrochemical series will be discharged in preference to those higher up the series. In the electrolysis of dilute sodium chloride using carbon electrodes, ions present are Na⁺,Cl^-,H⁺ and OH^-. During electrolysis, Na⁺ and H⁺ ions move to the cathode while OH^- and Cl^- ions move to the anode.

At the cathode, H⁺ being lower than Na⁺ on the electrochemical series gains an electron and is discharged in preference to sodium ion

Two atoms of hydrogen associate to form a molecule of hydrogen gas which is then discharged.

At the anode, hydroxide ions being lower in the series than chloride ion loses an electron and is discharged in preference to the chloride ion.

Summary

Oxygen is thus liberated at the anode. In the electrolysis o dilute sodium chloride therefore, hydrogen is released at the cathode while oxygen is released at the anode. The solution becomes more and more concentrated with sodium ions and chloride ions as electrolysis continues.