The acid in electrolysis of water is dilute

Analysis

The question asks for the specific acid used to facilitate the electrolysis of water. Pure water is a poor conductor of electricity because it has very few ions. To make it conduct electricity effectively, we add a small amount of an electrolyte (an acid, base, or salt).

Concept

In the electrolysis of water, we aim to decompose water ($H_2O$) into hydrogen gas and oxygen gas. The acid added must be non-volatile and must not interfere with the discharge of the hydrogen and hydroxide ions at the electrodes.

• HNO₃ (Nitric acid): This is an oxidizing acid. It can interfere with the products formed at the electrodes.
• CH₃COOH (Ethanoic acid): This is a weak acid and does not provide enough ions to make the water a good conductor.
• H₂SO₄ (Tetraoxosulphate(VI) acid): This is a strong, non-volatile acid. It provides a high concentration of $H^+$ and $SO_4^{2-}$ ions, which increases the conductivity of the water without being consumed in the reaction or producing unwanted side products at the electrodes.
• HCl (Hydrochloric acid): This is a volatile acid. During electrolysis, chloride ions ($Cl^-$) would be preferentially discharged at the anode instead of hydroxide ions, leading to the production of chlorine gas instead of oxygen gas.

Solution

1. Requirement: We need an electrolyte that increases conductivity without being discharged itself.
2. Process: Dilute tetraoxosulphate(VI) acid ($H_2SO_4$) is preferred because the sulphate ion ($SO_4^{2-}$) is much harder to discharge than the hydroxide ion ($OH^-$) from water.
3. Result: Consequently, $OH^-$ ions are discharged at the anode to produce oxygen, and $H^+$ ions are discharged at the cathode to produce hydrogen. This allows the electrolysis of water to proceed efficiently.

Tip

When choosing an electrolyte for electrolysis, always remember the Electrochemical Series. You want an electrolyte whose ions are "less easily discharged" than the ions of the solvent (water), so that the water itself is the substance being decomposed.