18
jun
10

Electrolisis del agua

ELECTROLYSIS OF WATER WITHOUT A HOFFMAN APPARATUS 


 PURPOSE 

These two experiments or demonstrations allow a detailed study of the electrolysis of water without the use of the traditional Hoffman apparatus.

DESCRIPTION

A careful study of the electrolysis of water can be used to introduce students in a first-year college-prep course to oxidation-reduction reactions or to illustrate the use of an external source of energy to drive a non-spontaneous chemical reaction. In Part A, a power source is used to electrolyze water and the gases produced are identified. In Part B, a 9-V battery is used to electrolyze water and the ions produced at each electrode are identified.

TIME REQUIRED

1-2 class periods.

 MATERIALS 

Part A

Chemicals

1.0 M MgSO4 solution (dissolve 246 g MgSO4.7H2Oin distilled or deionized water to make 1.0 liter of solution)*

Equipment

low voltage DC power supply, battery charger, or fresh 6-V lantern battery

600-mL beaker

18 x 150 mm test tubes

ring stand

utility clamps

insulated stainless steel electrodes

wood splints

Part B

Chemicals

universal indicator*

1.0 M MgSO4 solution (see Part 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)

Equipment

10-mm ID glass tubing

clamp clothes pins

mechanical pencil leads

9-V battery with snap top and micro alligator clips

*See Modifications/Substitutions

 HAZARDS 

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 Part B with care; both cause skin irritation. Goggles must be worn throughout the experiments.

MODIFICATIONS/SUBSTITUTIONS

  1. 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 part. Do not substitute sulfuric acid solution for the electrolyte in Part A. Since Part 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 Part A and avoid confusion.
  2. A large, wide mouth jar could be substituted for the beaker in Part A.
  3. 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.)
  4. 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.
  5. NaOH is available at grocery stores as lye.
  6. Red cabbage juice indicator may be substituted for the universal indicator in Part B. Make red cabbage indicator by pouring boiling water over chopped red cabbage; let stand until cool; decant.
  7. Although the same color changes can be observed in Part B with nichrome wire electrodes, oxygen bubbles are not observed at the anode. To avoid confusion, it is suggested that carbon electrodes be used.
  8. Micro alligator clips are available from Radio Shack stores.

PROCEDURE

Part A

  1. Add 500 mL 1.0 M MgSO4 solution to a 600-mL beaker. Set up electrodes as illustrated in Fig. 1. Fill two test tubes with MgSO4 solution and invert into the beaker without allowing any air to enter the test tubes.
  2. Attach leads from the power supply to the electrodes and collect the gases produced until one tube is nearly full. Have students observe relative volumes of the two gases produced.
  3. 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 lessor volume and a glowing splint to test the gas of greater volume.
  4. 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.

Part B

  1. Add 1-3 drops of universal or red cabbage indicator to small samples of 0.10 M HCl, pH 7 buffer, and 0.10 M NaOH. Observe the color of the indicator in acidic, neutral, and basic solutions.
  2. Bend 40 cm of the glass tubing as illustrated in Fig. 2. Support with two clothes pins as shown.
  3. Add universal indicator or red cabbage indicator to about 40 mL of 1.0 M MgSO4 solution. Mix thoroughly and fill the glass tube with the solution.
  4. Attach the snap top to the 9-V battery and attach the micro alligator clips to the wire leads. Attach the alligator clips to the carbon electrodes (pencil leads) and insert one electrode into each arm of the glass tubing.
  5. Allow the electrolysis to proceed until a significant change is observable at each electrode.
  6. 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.
  7. 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.
  8. Return the solution to the cell and electrolyze again. When a significant change is evident, reverse the electrodes and observe the changes that occur.
  9. Following both activities, students should:
    1. cite evidence for reaction in the cell in Part A,
    2. identify gas produced at each electrode in Part A,
    3. cite evidence for reaction in the cell in Part B,
    4. identify the substance causing the color change at each electrode in Part B,
    5. write the balanced equation for the reaction occurring at each electrode, assuming that water is electrolyzed,
    6. write the net equation for the reaction occurring in the cell,
    7. trace the flow of electrons in the system,
    8. identify the purpose: of the battery, and
    9. explain the purpose: of the magnesium sulfate in the solution.

 DISPOSAL 

Magnesium sulfate solutions may be flushed down the drain with plenty of water.

 DISCUSSION 

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.

oxidation./anode:

2H2O-“>O2 + 4H+ + 4e- E° = -1.23 v

reduction/cathode:

4H2O + 4e”> 2H2 + 4OH- E° = -0.83 v


net reaction:

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 Part A. In Part 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.

TIPS

  1. It is suggested that Part A be conducted as a demonstration and that, while it is running, students conduct Part 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.
  2. 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.

 REFERENCES 

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 Part 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.

Para que tengas una mejor perspectiva del experimento checa la siguiente pagina web:

http://www.rena.edu.ve/primeraetapa/experimentos/swf/obtiene_hidrogeno_agua.swf

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