Electrolysis of Water Teaching Notes OVERVIEW The electrolysis of water is a useful demonstration of the set up and operation of an electrolytic cell. Students should understand the flow of electrons to and from the power source, be able to identify the half-cells and half-reactions as anode and cathode, and relate these two terms to the processes of reduction and oxidation. Two options are provided for the lab. One involves using a Brownlee electrolysis apparatus, a relatively inexpensive piece of lab equipment. It is not difficult, however, to assemble your own setup. This option allows students to collect the hydrogen and oxygen gases produced by each half-reaction. Tests may be performed to indicate which gas is produced at each half-cell. Phenolphthalein may also be used to indicate the production of hydroxide ions at the cathode, causing the solution to become basic. A second option, which requires less equipment and preparation is also provided. Students should notice the production of gases at the two electrodes and will be able to note the change in pH at the cathode, as noted above.
The electrolysis of water Electroplating An electrochemical cell
To use electrolysis to separate water into hydrogen and oxygen gas.
Electrochemistry: Electrolysis of Water Teaching Notes
Practice will likely be needed to invert the test tubes without admitting air into the test tubes.
Practice will also be needed to test for the presence of hydrogen and oxygen gases in the test tubes.
RESULTS Option 1: Brownlee Electrolysis Apparatus 1.
An option to having all students perform the electrolysis lab and other experiments in this unit is to divide the class into groups. Each group would them prepare one experiment as a demonstration for the rest of the class. Posters could be made to help with the presentation. Suggested lab demonstrations: § § §
Small amounts of explosive hydrogen gas will be generated. Safety goggles should be worn when testing for the presence of hydrogen gas.
Describe what you observe occurring at the two electrodes. Make special note of the relative amounts of gas that form in each of the two test tubes – are they equal amounts? If not, indicate which test tube is attached to which post of the power source or battery. Students should observe gas bubbles forming at each electrode. The rate of gas production may be more evident at the cathode, which is connected to the negative post of the battery. This is the site of hydrogen gas production, and twice as much hydrogen gas will form as oxygen gas.
Describe any colour changes that occurred after the addition of phenolphthalein. A pink or red colour should appear near the cathode, the half-cell connected to the negative post of the battery or power source.
Option 2 1.
negative post of the battery). In this half-cell hydroxide ions are being produced
Record your observations for the two beakers. Carefully observe the ends of the wire. Watch for any colour changes that occur.
4H2 O + 4 e - → 2 H2 + 4 OH-
which will result in the solution becoming slightly basic. This will cause phenolphthalein to turn pink.
Gas bubbles should be noted forming on the ends of both wires. After adding phenolphthalein a red or pink colour should appear near the end of the wire attached to the negative post of the battery.
CONCLUSIONS AND QUESTIONS
Option 1: Brownlee Electrolysis Apparatus 1.
Why does more gas form in one test tube than in the other? Explain in terms of the half-reactions that occur in each test tube, identifying each test tube by which post they are attached to at the power source. Also identify each test tube half-reaction as either the anode or the cathode.
What gas was formed in Beaker A? Write the halfreaction that occurred in this beaker, identify it as either oxidation or reduction, and label it as the anode or cathode. The half-reaction in Beaker A: Beaker A, cathode, reduction: 4H2O + 4 e- → 2 H 2 + 4 OH-
Finally explain the flow of electrons through the system.
What gas was formed in Beaker B? Write the halfreaction that occurred in this beaker, identify it as either oxidation or reduction, and label it as the anode or cathode.
The balanced equation Beaker B, anode, oxidation: 2 H2 O(l) → 2 H2 (g) + O2 (g) tells us that for every mole of oxygen gas that is produced two moles of hydrogen gas will form. Thus we should expect to see twice the volume of hydrogen gas produced relative to oxygen.
2H 2O → O 2 + 4 H + + 4 e-
The negative post of the battery, or power source, supplies the electrons required for the reduction reaction occurring in Beaker A. The electrons produced in Beaker B during the oxidation of water return to the power source via the positive post.
Hydrogen gas will be produced at the cathode by reduction: 4H2O + 4 e - → 2 H2 + 4 OHThe electrons required for this reduction will be supplied by the negative post of the battery or power source. Oxygen will form at the anode by oxidation: 2H2 O → O2 + 4 H+ + 4 e 2.
Explain the flow of electrons through the system.
What was the purpose of adding phenolphthalein to the solution in Beaker A? Phenolphthalein is an acid-base indicator that turns pink under basic conditions. In Jar A the production of hydroxide ions will cause the solution to become basic. Students should observe that the pink colour first appears around the wire.
Explain the colour changes that occurred after the addition of phenolphthalein. A red colour may begin to appear around the test tube acting as the cathode (and attached to the
Electrochemistry: Electrolysis of Water Teaching Notes