Question

During electrolysis of acidulated water, which gas is collected at cathode and which gas is collected at anode?

(A) Hydrogen, Oxygen
(B) Oxygen, Hydrogen
(C) Hydronium, Hydroxyl
(D) none of the above

Answer 1

Hint: First determine the cations and anions present in the solution. Then determine which ions are selectively discharged at respective electrodes.

Complete step by step answer:
Acidulated water represents the water to which an acid is added. Assume that some sulphuric acid is added to acidulated water. Write the balanced chemical equations for the ionization of acidulated water.
\[{{\text{H}}_2}{\text{O }} \rightleftharpoons {\text{ }}{{\text{H}}^ + }{\text{ + O}}{{\text{H}}^ - } \\
{{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{ }} \rightleftharpoons {\text{ 2}}{{\text{H}}^ + }{\text{ + SO}}_4^{2 - } \\\]
Water dissociates to a very small extent as water is a weak electrolyte. The dissociation of water gives hydronium ions (protons) and hydroxide ions. Sulphuric acid is a strong acid. It completely dissociates in aqueous solution to provide protons and sulphate ions.
Thus, the solution contains protons, hydroxide ions and sulphate ions.
Under the influence of an electric field, cations move towards cathode and anions move towards anode.
At cathode, the proton accepts an electron and is reduced to a hydrogen atom. Two hydrogen atoms combine to form hydrogen molecules.
\[{{\text{H}}^ + }{\text{ + }}{{\text{e}}^ - }{\text{ }} \to {\text{ H}} \\
{\text{H + H }} \to {\text{ }}{{\text{H}}_2} \\\]
At anode, hydroxide ion is discharged in preference to sulphate ion
\[{\text{O}}{{\text{H}}^ - }{\text{ }} \to {\text{ OH + }}{{\text{e}}^ - } \\
{\text{4OH }} \to {\text{ 2 }}{{\text{H}}_2}{\text{O + }}{{\text{O}}_2} \\\]
Thus, during electrolysis of acidulated water, gas is collected at cathode is hydrogen and gas collected at anode is oxygen.

Hence, the correct option is (A) Hydrogen, Oxygen.

Note: During electrolysis, electrical energy is passed through the electrolyte present in the electrolysis cell. Electrical energy then drives a non-spontaneous chemical reaction. Thus, during electrolysis, electrical energy is converted to chemical energy.