Electrolysis of Water

Summary:  Sodium sulfate solution is electrolyzed to make hyrogen and oxygen gas.  A ignited  wood stick shown to glow more brightly in the presence of concentrated oxygen gas (collected in a test tube).  Hydrogen gas collected in a test tube is ignited using a bunsen burner. 

Hazards: 

Chemicals and Solutions: 

1.0 M Sodium sulfate solution

Materials: 

Electrolysis Apparatus

D.C. Power Supply

Dilute (0.5M or so) Na2SO4 solution. Phenolphthalein may be added to show formation of hydroxide ion at the cathode.

Splint

Matches

Testubes

Procedure: 

Open stopcocks, making sure that the sodium sulfate solution filles each compartment to the tip. Close stopcocks. Attach apparatus to D.C. power supply.

Do a check to make sure the demo works.

As soon as class starts, turn on the power supply. Operate at about 10-12V.

Hydrogen may be collect at the cathode in a small test tube and ignited. Oxygen may be collected at the anode.  Add a glowing splint to show that oxygen supports combustion.

Rinse apparatus immediately after use to avoid corrosion of the electrode.

Hint:  Too much voltage will blow the Variac fuse, best to keep demo under ….

Sodium sulfate solution will eventually mould.

Discussion: 

The overall reaction:

2H2O(l) ----> 2 H2(g)+ O2(g)

Reduction at the cathode:

Oxidation at the anode:

By multiplying the equation for the reaction at the cathode by two and then combining it to the equation for the reaction at the anode you get:

This equation can be simplified, because the H+ and the OH- combine to give water.  You can then subtract 4H2O from each side to get:

To schedule a demonstration, please send an e-mail to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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