Introduction: (Initial Observation)
A careful study of the electrolysis of water can be used to introduce students to oxidation-reduction reactions or to illustrate the use of an external source of energy to drive a non-spontaneous chemical reaction. In experiment A, a power source is used to electrolyze water and the gases produced are identified. In experiment B, a 9-V battery is used to electrolyze water and the ions produced at each electrode are identified.
At the positive electrode (anode), water is oxidized to form oxygen gas and hydrogen ion. At the negative electrode (cathode), water is reduced to form hydrogen gas and hydroxide ion.
2H2O- ==> O2 + 4H+ + 4e- E° = -1.23 v
4H2O + 4e ==> 2H2 + 4OH- E° = -0.83 v
2H2O- ==> 2H2 + O2 E° = -2.06 v
The relative volumes of the gases produced and the reactions of the gases to the glowing and burning splints can be correlated with the production of oxygen at the anode and hydrogen at the cathode in experiment A. In experiment B, the colors produced at each electrode can be used to identify the production of hydrogen ion at the anode and hydroxide ion at the cathode.
If these activities are used to introduce students to oxidation-reduction reactions, the terms oxidation, reduction, anode, and cathode can be associated with the processes observed and then generalized to similar processes. If, on the other hand, these activities are used to illustrate electrolysis after students have been introduced to oxidation-reduction, these terms can be reviewed as part of the discussion of the activities.
These two experiments or demonstrations allow a detailed study of the electrolysis of water without the use of the traditional Hoffman apparatus. For this project often students decide to study the effect of voltage on the rate of electrolysis.
What is the effect of voltage on the rate of electrolysis.
This is a very good and practical application for someone who needs to produce hydrogen and oxygen gases by electrolysis of water.
You may also be able to expand your experiments in order to find answer to any of the following questions:
- What gas will be produced in an anode?
- What gas will be produced in a cathode?
- What is the ratio of produced gases?
- How do ionic compounds help the electrolysis of water?
- How does electrolysis of water affect the pH of water in or around electrodes?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
For the effect of voltage on the rate of electrolysis you may define the following variables:
Independent variable (also known as manipulated variable) is the voltage used for electrolysis.
Dependent variable (also known as responding variable) is the the rate of electrolysis. The amount of water electrolyzed within an hour or the amount of oxygen gas produced within an hour will be reported as the rate of electrolysis.
Constants are the solution (amount and type), electrodes (type and size).
Controlled variable is the temperature (ambient temperature and electrolyte temperature).
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. This is a sample hypothesis:
The rate of electrolysis and production of hydrogen and oxygen will increase by any increase in the voltage.
This hypothesis is being tested in experiment C.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Following are the procedures for 3 different experiments.
Identify gases produced by water electrolysis
- Add 500 mL 1.0 M MgSO4 solution to a 600-mL beaker. Set up electrodes as illustrated in diagram bellow. Fill two test tubes with MgSO4 solution and invert into the beaker without allowing any air to enter the test tubes.
- Carefully slide electrodes under the test tubes. Electrodes can be stainless steel or aluminum wire. At least one inch of the electrode wire must enter the test tube. The part of the electrode that is out of the test tube but in the electrolyte, must be insulated using electric tape or nail polish.
- Attach leads from the power supply to the electrodes and collect the gases produced until one tube is nearly full. Observe relative volumes of the two gases produced.
- Remove each test tube of gas from the beaker by placing a thumb over the mouth of the tube. Use a lighted splint to test the gas of lesser volume and a glowing splint to test the gas of greater volume.
- Note the behavior and relative volume of the gas produced at the electrode connected to the positive terminal of the power source and at the electrode connected to the negative terminal.
Following this activity, student should:
a. cite evidence for reaction in the cell
b. identify gas produced at each electrode
c. write the balanced equation for the reaction occurring at each electrode, assuming that water is electrolyzed,
d. trace the flow of electrons in the system,
e. identify the purpose: of the battery, and
f. explain the purpose: of the magnesium sulfate in the solution.
Identify ions produced during the water electrolysis
- Use a universal pH indicator paper and test small samples of acid (such as 0.10 M HCl), pH 7 buffer (or distilled water), and alkaline solution (such as 0.10 M NaOH). Observe the color of the indicator in acidic, neutral, and basic solutions. (This step is optional)
- Get a U shape glass tubing and secure it to stand vertically (Possibly using a clamp or tape).
- Drop 3 to 5 pieces of universal indicator paper in about 40 mL of 1.0 M MgSO4 solution. Let it soak so that the indicator chemicals are dispersed in the solution. Mix thoroughly, remove the papers and fill the glass tube with the solution.
- Attach 1 carbon electrode to one alligator clip of a red connection wires and connect the other end of the wire to the + pole of a 9 V battery.
- Attach another carbon electrode to one alligator clip of a black connection wires and connect the other end of the wire to the – pole of a 9 V battery.
- Insert one electrode into each arm of the glass tubing.
- Allow the electrolysis to proceed until a significant change is observable at each electrode.
- Note the color change produced at the electrode attached to the positive terminal of the battery and at the electrode attached to the negative terminal of the battery.
- Carefully remove the electrodes and empty all of the solution from the cell into a clean, dry beaker. Rinse the cell with some of the solution and return to the beaker. Observe the color of the solution when thoroughly mixed.
- Return the solution to the cell and electrolyze again. When a significant change is evident, reverse the electrodes and observe the changes that occur.
- Following this activity, student should:
a. cite evidence for reaction in the cell
b. identify the substance causing the color change at each electrode
c. write the balanced equation for the reaction occurring at each electrode, assuming that water is electrolyzed,
d. write the net equation for the reaction occurring in the cell,
e. trace the flow of electrons in the system,
f. identify the purpose: of the battery, and
g. explain the purpose: of the magnesium sulfate in the solution.
Magnesium sulfate solutions may be flushed down the drain with plenty of water.
Although power sources used are relatively weak, electrodes should not be handled while cells are operating. Care should be exercised when testing the hydrogen gas with a burning splint; the test tube should not be cracked or chipped. Handle the acid and base in experiment B with care; both cause skin irritation. Goggles must be worn throughout the experiments.
What is the effect of voltage on the rate of electrolysis
- Add 500 mL 1.0 M MgSO4 solution to a 600-mL beaker. Set up electrodes as illustrated in diagram bellow.
- Prepare two test tubes with known volume. If you don’t know the volume of the tubes, fill them up with water and then transfer the water to a measuring cylinder to see the volume. You may also use measuring cylinders instead of test tubes.
- Fill the test tubes with MgSO4 solution and invert into the beaker without allowing any air to enter the test tubes.
- Use a 3-volt battery or a 3-volt power supply as the power source. Attach leads from the power supply to the electrodes and set the timer at the same time. Collect the gases produced until one tube is full. Stop the timer and record the number of seconds it took for the one test tube to fill up.
- Repeat the step 2 with 6-volt and with 9-volt batteries or power supplies and record your results in a data table like this:
Voltage Time in seconds
Materials and Equipment:
For experiment A
1.0 M MgSO4 solution (dissolve 246 g MgSO4.7H2Oin distilled or deionized water to make 1.0 liter of solution)*
low voltage DC power supply, battery charger, or fresh 6-V lantern battery
18 x 150 mm test tubes
insulated stainless steel electrodes
For experiment B
1.0 M MgSO4 solution (see experiment A)
0.10 M HCl solution (8.3 mL conc. HCl per liter)*
0.10 M NaOH (4.0 g NaOH per liter)*
pH 7 buffer (dissolve 6.8 g K2HPO4 in a small amount of distilled water, add 296 mL 0.10 M NaOH solution and dilute to 1.0 liter)
10-mm ID U shape glass tubing (SchoolOrders.com Part# 5554)
clamp clothes pins
mechanical pencil leads
9-V battery with snap top and micro alligator clips
pH indicator Paper (MiniScience.com product code PHBOOK114)
- MgSO4·7H2O is available as Epsom salts at drugstores. 1.0 M NaNO3 solution (85 g NaNO3 dissolved in enough distilled or deionized water to make 1.0 liter) can be used in place of 1.0 M MgSO4 solution in either experiment. Do not substitute sulfuric acid solution for the electrolyte in experiment A. Since experiment B requires that a neutral salt be used in order to observe the color changes due to the formation of hydrogen and hydroxide ions, it is better to use the same salt in experiment A and avoid confusion.
- A large, wide mouth jar could be substituted for the beaker in experiment A.
- Stainless steel electrodes are available from Fisher Scientific (S52017 Electrolysis Kit) or two large paper clips partially wrapped with electrical tape may be used. (See Fig. 1.)
- HCl solution is available from hardware stores as muriatic acid, 28% HCl; substitute 3.5 mL muriatic acid for 8.3 mL conc. HCl solution.
- NaOH is available at grocery stores as lye.
- Red cabbage juice indicator may be substituted for the universal indicator in experiment B. Make red cabbage indicator by pouring boiling water over chopped red cabbage; let stand until cool; decant.
- Although the same color changes can be observed in experiment B with nichrome wire electrodes, oxygen bubbles are not observed at the anode. To avoid confusion, it is suggested that carbon electrodes be used.
- Micro alligator clips are available from MiniScience.com (product Code 9119).
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
If you do any calculations, write your calculations in this section of your report.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
- It is suggested that experiment A be conducted as a demonstration and that, while it is running, students conduct experiment B as an experiment. Both could be done in one period if the tubes and battery snap tops with micro alligator clips attached are prepared in advance.
- The color changes observed with the universal indicator are more pronounced than those with red cabbage indicator. With either it may be necessary to adjust the pH of the magnesium sulfate solution to 7 with buffer solution because of dissolved carbon dioxide in the distilled water. Universal indicator is green at pH 7; red cabbage indicator is purple at pH 7.
List your reference/ bibliography in this section of your report.
Hendricks, L.J. and Williams, J.T., J. Chem. Ed., 59, 586 (1982).
– This article describes the construction of an electrolysis cell that is more complex than the one used in experiment B.
Skinner, B.F., J. Chem. Ed., 58, 1017 (1981).
– This article describes the use of universal and red cabbage juice indicators to demonstrate the electrolysis of water in a crystallizing dish.
Talesnick, I. “Demonstration 160: Electrolysis–Sodium Sulfate,” Queen’s Demonstration Workshop. Queen’s U., Kingston, ON, 1980.
– This demonstration uses a Hoffman apparatus to illustrate the electrolysis of water containing sodium sulfate. With this apparatus, one can illustrate the entire anode and cathode processes at once.
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