Electrolysis of brine and copper(II) sulfate

Last time

Before we start: last time, in Writing and balancing equations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Salt water that makes three things

Seawater is a cheap, endless supply of salt. Run electricity through concentrated salt solution, called brine, and you get chlorine, hydrogen and sodium hydroxide. All three are valuable, and the process is one of the biggest in the chemical industry.

Concentrated sodium chloride solution

Brine contains four ions: Na⁺ and Cl⁻ from the salt, and H⁺ and OH⁻ from the water. At the cathode, sodium is more reactive than hydrogen, so hydrogen gas forms. At the anode, the solution is concentrated, so chloride ions lose electrons and chlorine gas forms. The sodium ions and the hydroxide ions are left, so the solution becomes sodium hydroxide.

The overall equation

Two NaCl and two H₂O give two NaOH, plus H₂, plus Cl₂. Check the atoms: two Na, two Cl, four H, two O on each side. It balances. Notice that the sodium ions never change. They are spectators, and stay in the solution.

Testing the products

Chlorine bleaches damp blue litmus paper. It turns white. Hydrogen gives a squeaky pop with a lit splint. Sodium hydroxide is alkaline. Universal indicator turns purple, and litmus turns blue.

Uses of the three products

Chlorine is used to kill bacteria in water, to make bleach, and to make plastics such as PVC. Hydrogen is used as a fuel, and to make ammonia in the Haber process. Sodium hydroxide is used to make soap, paper and bleach.

Copper sulfate with inert electrodes

Aqueous copper sulfate has Cu²⁺, SO₄²⁻, H⁺ and OH⁻. Copper ions go to the cathode, and hydroxide ions to the anode. Copper forms at the cathode, and oxygen at the anode. The blue colour fades, as the copper ions are used up.