Aqueous sodium chloride

Now electrolyse sodium chloride dissolved in water. The water adds ions of its own, and that changes the product at the cathode.

Activity 12.2.2

You will need: sodium chloride solution, carbon rods, conducting wires, a 9 V battery

Connect the two carbon rods to the battery terminals with wires, then dip both electrodes in the aqueous sodium chloride solution (figure 12.2.4). Observe and report.

The observation

Gas bubbles come off both electrodes.

The solution mainly contains Na⁺ and Cl^- ions. Besides these, a very small amount of H⁺ and OH^- ions forms from the slight dissociation of water molecules (figure 12.2.5).

Water's own ions

Water is a molecule with covalent bonds, but it has been found that in pure water a small fraction of the molecules dissociate into H⁺ and OH^- ions. At 25 °C the concentration of each in pure water is 1.0 × 10^-7 mol dm^-3.

At the cathode (negative)

Na⁺ and H⁺ ions move to the negative terminal. Hydrogen is below sodium in the activity series, so it is the H⁺ ions that are reduced — hydrogen gas bubbles off.

At the anode (positive)

Cl^- and OH^- ions are attracted; here the Cl^- ions are the more likely to be oxidised, so chlorine gas bubbles off.

2H⁺ (aq) + 2e → H₂ (g)

2Cl^- (aq) → Cl₂ (g) + 2e

2H⁺ (aq) + 2Cl^- (aq) → H₂ (g) + Cl₂ (g)

(1) cathode; (2) anode; (1) + (2) overall

What is left behind

At the start the solution held Na⁺, H⁺, Cl^- and OH^- ions. The H⁺ and Cl^- leave as the gases H₂ and Cl₂, so Na⁺ and OH^- ions stay behind. So this reaction can be used in industry to make sodium hydroxide (NaOH).

Salt water gives hydrogen, chlorine and NaOH

Molten salt gives sodium at the cathode; salt solution gives hydrogen instead, because H⁺ is reduced before Na⁺. Sodium hydroxide is left in solution.