Lecture 2 – Preferential Discharge

Electrolysis of Aqueous electrolytes:

In the electrolysis of Lead (II) Bromide, only two ions were present. One positive and the other negative. On the passage of current via the solution, these ions moved to their respective electrodes and got discharged into their atomic form.

Consider the case of the electrolysis of the salt solution of Sodium Chloride. The aqueous solution here contains 4 ions, 2 positive and 2 negative.

In presence of a potential difference across the electrodes terminals, the positively charged anode will attract both the negative ions of Cl^- and OH^-. However, only one of them will be discharged at the anode to its atomic form. In this particular case, hydroxide ions being easier of the 2 ions, to discharge will be given off at the anode.

Oxygen gas will be discharged at the anode due to Preferential discharge.

The negatively charged cathode will attract both the positive ions of Na⁺ and H⁺. Sodium is more reactive than hydrogen, therefore it is easier for hydrogen to gain electrons from the cathode than sodium. Hydrogen will gain electrons at the cathode to form hydrogen gas.

Thus, the overall reaction of the electrolysis of aqueous sodium chloride becomes:

Table 1: Preferential Discharge

Important Note:

It is important to note that in the electrolysis of all salt solutions EXCEPT Copper salts – HYDROGEN gas will be given off at the cathode. This is because Hydrogen is easier to discharge than more reactive metals. However, in the case of Copper salts, copper ions are easier to discharge than hydrogen, and are obtained at the cathode.

Preferential discharge & Concentration of Electrolyte:

Concentration of electrolytes affects the results of electrolysis.

Example:

Electrolysis of dilute HCl:

Oxygen gas at the anode

Hydrogen gas at the cathode

Electrolysis of concentrated HCl:

Chlorine gas at the anode

Hydrogen gas at the cathode