Electrolysis of Water 1

A simple demonstration of water electrolysis that shows changes in pH in solution at the electrodes.

Equipment

  • 2 pencils
  • 9 V battery
  • wires with alligator clips
  • Petri dish

Reagents

  • water
  • universal indicator

Presentation

  1. Add a few mm of water with universal indicator to the Petri dish
  2. Position the pencils so that the graphite points are submerged.  A wire clamp is used to hold them in place.
  3. After discussing the chemistry that accompanies the electrolysis reactions at the anode and cathode (see below), the 9V battery is connected to the two pencils. 

Details

The electrolysis reactions at the two electrodes are:

cathode (reduction):  2 H2O (l) + 2 e- → H2 (g) + 2 OH- (aq)  (balanced with base)

anode (oxidation):  2 H2O (l) → O2 (g) + 4 H+ (aq) + 4 e-  (balanced with acid)

Adding the half equations gives the overall water splitting reaction:

2 H2O (l) → 2 H2 (g) + O2 (g)

Looking at the half reactions that are occuring in different locations of the Petri dish, it is clear that at the cathode one should see an increase in [OH-], while at the anode there is an increase in [H+] (or more correctly, an increase in [H3O+].  This is clearly visualized in this demo by the change in pH shown by the pH indicator.  From the pH changes in the dish, one can quickly decide where the hydrogen bubbles and oxygen bubbles are being generated.  To show that this is due to the electrolysis reactions, the demonstrator can switch the leads on the battery and the color changes will swap electrodes as well.  You can also show swirl the solution in the dish so that the high and low [H3O+] regions mix and the color is returned to a uniform green.  This is a semi-quantitative way of confirming that the overall reaction is stoichiometric; the reduction at one electrode is balanced with oxidation at the other.

 

Safety

explosion

Special Safety Notes

  • the electrodes are generating hydrogen and oxygen gases, keep demo away from open flames.
elec1
The demo set up before connecting the 9V battery.
batt
After connecting the 9V battery.