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Home , Electrolysis of water

NCSU – Dept. of Chemistry – Lecture Demonstrations

Electrolysis of

Description: Hoffman apparatus is used to demonstrate the of water, alternatively, a power source and can show the same demonstration in a Petri dish.

Materials:

Hoffman apparatus (Dabney 125) DC power supply 1 M Na 2SO 4 (alternative) Petri dish (alternative) Bromothymol blue (alternative) Pt wires (alternative)

Procedure:

For large lecture halls, project demonstration using a document camera. For Petri dish alternative, demonstration is best performed on an overhead projector.

1. For Hoffman apparatus, hook up leads to DC power supply (potential of 20 V). Observe bubbles forming (H 2 and O 2 separately). After some time, a difference in the volume of the two will be observed. If testing for pH at each , 0.05% bromothymol blue solution can be added. A yellow color will be observed at the and a blue color will be observed at the . The gases generated in this reaction can be collected by opening the stopcock into an inverted test tube and testing the presence of a flammable using a glowing wood splint.

2. For an alternate demonstration, hook up the Pt wires to the DC power supply set at a potential of 20 V (make sure power supply is off). Hook Pt wires over the lip of the Petri dish on opposite sides. Prepare a solution of 1M Na 2SO 4 and add a few drops of 0.05% bromothymol blue. Neutralize this solution by either the addition of 0.1 M H 2SO 4 or 0.1 M NaOH. Add enough of the Na 2SO 4 solution to the Petri dish to cover the Pt electrodes. Turn on the power supply and observe the color changes at each electrode (similar to those in the Hoffman apparatus).

Discussion:

Water is decomposed into and using . The Hoffman apparatus allows for the separation of these gases as their generation occurs at separate electrodes. According to the two half reactions shown below, H 2 is produced at the cathode while, O 2 is produced at the anode. Over time a difference in the individual volumes of the two gases is observed in a ratio of 2:1 NCSU – Dept. of Chemistry – Lecture Demonstrations Electrochemistry

H2:O 2. This of course is due to the ratio of H to O atoms in H 2O. If bromothymol blue is added in this demonstration a yellow color is observed at the anode due to the formation of H 1+ and a blue color is observed at the cathode due to the formation of OH 1-.

1+ 1- Anode 2 H 2O → O 2 + 4 H + 4 e 1- 1- Cathode 2 H 2O + 2 e → H 2 + 2 OH

Safety: Be sure that power supply is turned off when not in use.

Disposal: Na 2SO 4 solution can be disposed of in an aqueous waste container. Any used electrodes should be rinsed and dried.

References:

Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of Chemistry ; The University of Wisconsin Press: 1992; Vol. 4, p 156-165.

Stauffer, M. T.; Fox, J. P. J. Chem. Educ. 2008 , 85 , 523.

Hendricks, L. J.; Williams, J. T. J. Chem. Educ. 1982 , 59 , 586. http://forums.jce.divched.org:8000/[email protected]@.1ad4b3bd/2

Video : http://www.youtube.com/watch?v=vDyaI0yaiEw (uses sharpened pencils as electrodes) http://www.youtube.com/watch?v=OTEX38bQ-2w