Download Electrolysis of Water

NCSU – Dept. of Chemistry – Lecture Demonstrations
Electrochemistry
Electrolysis of Water
Description: Hoffman apparatus is used to demonstrate the electrolysis of
water, alternatively, a power source and electrodes can show the same
demonstration in a Petri dish.
Materials:
Hoffman apparatus (Dabney 125)
1 M Na2SO4 (alternative)
Bromothymol blue (alternative)
DC power supply
Petri dish (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 (H2 and O2 separately). After some time,
a difference in the volume of the two gases will be observed. If testing for
pH at each electrode, 0.05% bromothymol blue solution can be added. A
yellow color will be observed at the anode and a blue color will be
observed at the cathode. 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 gas 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 Na2SO4 and add a few drops of 0.05% bromothymol blue. Neutralize
this solution by either the addition of 0.1 M H2SO4 or 0.1 M NaOH. Add
enough of the Na2SO4 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 hydrogen and oxygen using electricity. 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, H2 is
produced at the cathode while, O2 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:O2. This of course is due to the ratio of H to O atoms in H2O. If bromothymol
blue is added in this demonstration a yellow color is observed at the anode due
to the formation of H1+ and a blue color is observed at the cathode due to the
formation of OH1-.
Anode
Cathode
2 H2O → O2 + 4 H1+ + 4 e12 H2O + 2 e1- → H2 + 2 OH1-
Safety: Be sure that power supply is turned off when not in use.
Disposal: Na2SO4 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