Electrolysis of Water Teaching Notes
The electrolysis of water is a useful demonstration Small amounts of explosive hydrogen gas will
of the set up and operation of an electrolytic cell. be generated. Safety goggles should be worn
Students should understand the flow of electrons to when testing for the presence of hydrogen gas.
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. PROCEDURE
Two options are provided for the lab. One involves Practice will likely be needed to invert the test
using a Brownlee electrolysis apparatus, a relatively tubes without admitting air into the test tubes.
inexpensive piece of lab equipment. It is not
difficult, however, to assemble your own setup. This Practice will also be needed to test for the
option allows students to collect the hydrogen and presence of hydrogen and oxygen gases in the
oxygen gases produced by each half-reaction. Tests test tubes.
may be performed to indicate which gas is produced
at each half-cell. Phenolphthalein may also be used RESULTS
to indicate the production of hydroxide ions at the
cathode, causing the solution to become basic.
Option 1: Brownlee Electrolysis Apparatus
A second option, which requires less equipment and
preparation is also provided. Students should notice 1. Describe what you observe occurring at the two
the production of gases at the two electrodes and electrodes. Make special note of the relative
will be able to note the change in pH at the cathode, amounts of gas that form in each of the two test
as noted above. tubes – are they equal amounts? If not, indicate
which test tube is attached to which post of the
An option to having all students perform the power source or battery.
electrolysis lab and other experiments in this unit is
to divide the class into groups. Each group would Students should observe gas bubbles forming at
them prepare one experiment as a demonstration for each electrode. The rate of gas production may be
the rest of the class. Posters could be made to help more evident at the cathode, which is connected to
with the presentation. Suggested lab the negative post of the battery. This is the site of
demonstrations: hydrogen gas production, and twice as much
hydrogen gas will form as oxygen gas.
The electrolysis of water
Electroplating 2. Describe any colour changes that occurred after the
An electrochemical cell addition of phenolphthalein.
PURPOSE A pink or red colour should appear near the
cathode, the half-cell connected to the negative post
of the battery or power source.
To use electrolysis to separate water into
hydrogen and oxygen gas.
Electrochemistry: Electrolysis of Water Teaching Notes 1
Option 2 negative post of the battery). In this half-cell
hydroxide ions are being produced
1. Record your observations for the two beakers.
Carefully observe the ends of the wire. Watch for 4H2O + 4 e- 2 H2 + 4 OH-
any colour changes that occur.
Gas bubbles should be noted forming on the ends of which will result in the solution becoming slightly
both wires. After adding phenolphthalein a red or basic. This will cause phenolphthalein to turn pink.
pink colour should appear near the end of the wire
attached to the negative post of the battery.
CONCLUSIONS AND QUESTIONS 1. What gas was formed in Beaker A? Write the half-
reaction that occurred in this beaker, identify it as
Option 1: Brownlee Electrolysis Apparatus either oxidation or reduction, and label it as the
anode or cathode.
1. Why does more gas form in one test tube than in the
other? Explain in terms of the half-reactions that The half-reaction in Beaker A:
occur in each test tube, identifying each test tube by
which post they are attached to at the power source. Beaker A, cathode, reduction:
Also identify each test tube half-reaction as either 4H2O + 4 e- 2 H2 + 4 OH-
the anode or the cathode.
2. What gas was formed in Beaker B? Write the half-
Finally explain the flow of electrons through the reaction that occurred in this beaker, identify it as
system. either oxidation or reduction, and label it as the
anode or cathode.
The balanced equation
Beaker B, anode, oxidation:
2 H2O(l) 2 H2 (g) + O2 (g)
2H2O O2 + 4 H+ + 4 e-
tells us that for every mole of oxygen gas that is
produced two moles of hydrogen gas will form. Thus 3. Explain the flow of electrons through the system.
we should expect to see twice the volume of
hydrogen gas produced relative to oxygen. The negative post of the battery, or power source,
supplies the electrons required for the reduction
Hydrogen gas will be produced at the cathode by reaction occurring in Beaker A. The electrons
reduction: produced in Beaker B during the oxidation of water
4H2O + 4 e- 2 H2 + 4 OH- return to the power source via the positive post.
The electrons required for this reduction will be 4. What was the purpose of adding phenolphthalein to
supplied by the negative post of the battery or power the solution in Beaker A?
Phenolphthalein is an acid-base indicator that turns
Oxygen will form at the anode by oxidation: 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
2H2O O2 + 4 H+ + 4 e-
first appears around the wire.
2. 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 2