Acidified water

Finally, electrolyse acidified water with carbon electrodes.

Activity 12.2.4

You will need: distilled water with a little dilute sulphuric acid added, carbon rods, a 9 V battery, connecting wires, a plastic cup

Make holes in the base of the plastic cup and fix the carbon rods in them as in the figure. Seal around the rods with ethyl tape or PVC so no water leaks out (silicone sealer can also be used).
Pour the acidified water into the cup.
Fill two test tubes completely with water, turn them upside down, and place one over each carbon rod.
Connect the 9 V supply as in figure 12.2.8 and record what you see.

The observation

Gas collects in both test tubes, and the volume of gas from the cathode is larger than the volume from the anode.

The acidified water contains H⁺ and SO₄^2- from the ionisation of the dilute sulphuric acid, and H⁺ and OH^- from the dissociation of water. At the cathode, H⁺ — the only positive ion — gains electrons and hydrogen is given off. At the anode, of SO₄^2- and OH^-, the more likely to be oxidised is OH^-, so oxygen is given off.

2H⁺ (aq) + 2e → H₂ (g)

4OH^- (aq) → O₂ (g) + 2H₂O (l) + 4e

2H₂O (l) → 2H₂ (g) + O₂ (g)

(1) cathode; (2) anode; the overall change

Why twice as much hydrogen

The overall equation says the answer: two molecules of hydrogen form for every molecule of oxygen. Equal numbers of molecules of gases take equal volumes, so the hydrogen tube fills about twice as fast as the oxygen tube. You can check it from the electrons too: 4 electrons make one O₂ at the anode, and the same 4 electrons make two H₂ at the cathode. The acid is only there so the water conducts — it is the water that is split.

Acidified water splits into hydrogen and oxygen

Hydrogen at the cathode, oxygen at the anode, in a 2 : 1 ratio by volume.