Electrolysis of Water

Description

Water containing a dissolved salt is electrolyzed in an apparatus where the volumes of gas produced can be collected and measured. The gaseous products are tested and identified.

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• Electric currents transport electric energy through matter. To have an electric current there must be charge carriers in the matter and there must be a force that moves those carriers. The charge carriers in the electrolytic solution are positive and negative ions.
• Pure water conducts electricity poorly because it contains very few positive and negative ions. Sodium sulfate is added to provide charge carriers in the water.

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1. Dissolve 1 g sodium sulfate in about 150 mL of distilled water. Add food coloring.
2. Open the stopcock on the Hoffman apparatus, and fill the apparatus with the water-salt solution.
3. Connect the platinum electrodes to the DC power supply.
4. Close the stopcocks on the apparatus.
5. Plug in the power supply and slowly turn up voltage until you see bubbles forming at the electrodes.
6. Run for approximately 5-10 minutes or until you have about 20 mL of oxygen gas collected.
7. Collect hydrogen gas in a plastic-tape-wrapped test tube (keep inverted) and test with a lighted splint. You should hear a "pop."
8. Collect oxygen in the other plastic-tape-wrapped test tube and test with glowing splint. The splint should reignite.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 044 Electrolysis of Water

• When a direct electric current is passed through water to which a small amount of sodium sulfate has been added, a chemical reaction takes place, and two different gases are produced.
• Elemental hydrogen is formed at one electrode and elemental oxygen at the other.

Watch the movie and answer the questions.

1. Write the balanced chemical reaction.
2. Write the formula of the product at the negative electrode.
3. Write the formula of the product at the positive electrode.
4. Should the test tube from the anode (positive electrode in elecrolysis) compartment, which has the smaller volume of gas collected, be held with its mouth pointing up or down?
5. Explain.

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Purpose

To measure the relative gas volumes from the electrolysis of water.

To study the gaseous electrolysis products.

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•  Hoffman apparatus with 2 platinum electrodes
• approximately 150 mL distilled water mixed with 1 g sodium sulfate and dyed with food coloring
• 2 medium size test tubes (25 x 150 mm, wrapped in plastic tape)
• 2 wooden splints
• matches
• DC power supply
• 2 wires with alligator clips
• distilled water
• Bunsen burner
• 1-L beaker
• stirring rod

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• Larger test tubes will produce a "pop" or "whoosh" when testing the H₂ gas.
• Be sure to allow enough oxygen gas to form before testing (approximately 20 mL).
• Carbon electrodes also work in the Hoffman apparatus.
• Have student volunteers test the gases.
• Make sure that students observe the gas levels for H₂ and O₂ before any gas is removed. Before testing, ask them to predict which tube contains H₂ and which tube contains O₂.

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Teacher preparation: 20 minutes

Presentation: 25 minutes

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There is an electric shock hazard possible with the power supply. An explosion might cause test tubes filled with gas to shatter.

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• Use caution when handling electrical apparatus. A tingling sensation implies an electrical shock hazard. Use an outlet with a ground fault interrupter as a source.
• Do not allow all of the water to run out of the apparatus while producing hydrogen and oxygen gas.
• Do not displace so much water that the level drops below the cross tube and allows mixing of the gases.
• Wrap the test tubes used for testing gases with plastic tape.

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Dispose of the sodium sulfate solution at the sink. Rinse Hoffman apparatus and platinum electrodes with distilled water. Dispose of burned splints and matches with ordinary solid waste.

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Presentation Question:

• Should the test tube from the anode compartment, which has the smaller volume of gas collected, be held with its mouth pointing up or down?
  • If the gas is more dense than air, the mouth should be held up. The gas in this test tube is oxygen, which is more dense than air, so it should be held mouth-up. Gases are fluids, so the less dense fluids float on the more dense air, and the more dense fluids sink. Therefore, oxygen sinks in air. Hydrogen, however, rises in air and its test tube should be held mouth-down.

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• When a direct electric current is passed through water to which a small amount of sodium sulfate has been added, a chemical reaction takes place, and two different gases are produced.
• Elemental hydrogen is formed at one electrode and elemental oxygen at the other.
• The balanced equation for this reaction is:
  • 2 H₂O(l) --> 2 H₂(g) + O₂(g)
• Note the relationship between the coefficients of the balanced equation and the volumes of hydrogen and oxygen produced during the electrolysis.

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• electrolysis
• oxygen
• hydrogen
• dioxygen
• dihydroge
• anode
• cathode
• oxidation
• reduction
• redox
• burn
• combustion
• explosion

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