Colorful Electrolysis -- Decomposition of Water

Colorful Electrolysis -- Decomposition of Water

Credit: Yan Luo

 
Author: Yan Luo
Description:
This colorful water electrolysis demonstration is a decomposition reaction of water molecules to produce hydrogen and oxygen gases. The mole ratio of hydrogen and oxygen atoms in water molecules is shown not to be 1:1. H+ is a temporary direct product of the reaction before they soon form nonpolar covalent bonds with each other as H2 gas. The temporary H+ is acidic in solution which changes the color of the bromothymol blue solution to yellow.Also because of the temporaryness, only the solution at the cathode instead of the entire dish changes color.
Usage:
Demonstration for decomposition reactions. It can also be used to demonstrate electrolysis reactions and the use of indicators.
 
Futher Images
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Credits, from left: Yan Luo, Yan Luo, Yan Luo, Yan Luo
 
Safety:
Regular laboratory guidelines (eg. gloves, goggles, etc) should be followed.
 
Equipment:
A petri dish Two wires with alligator clips A petri dish A stir rod A 9V battery Two mechanical pencil leads
 
Materials:
Bromothymol blue solution (with an eye dropper) A little (about 1 gram) table salt (or any highly soluable, neutral salt) Water (enough to fill 3/4 of the petri dish)
 
Procedure:
Preperation: Petri Dish Fill 3/4 of the petri dish with water. Put a little (about 1 gram) table salt (or any other highly soluable, neutral salt) into the petri dish. Dissolve the salt completely with water using a stir rod [to introduce ions into the solution to allow for a complete circuit]. Use an eye dropper to add 2-3 drops (amount may vary-depends on the color) of bromothymol blue solution into the petri dish until the blue color can easily be observed. Electric Circuit: Connect both wires each to a terminal of the battery with no other connection between the two wires [to prevent a short circuit]. Connect the other side of a wire to a mechanical pencil lead. Follow the same procedure to connect the other wire to another mechanical pencil lead. Make sure the two mechanical pencil leads are not directly connected [to prevent a short circuit]. Demonstration: Place both of the pencil leads into the petri dish without letting the pencil leads touching each other. What to observe: Does the solution change color? If so, what are the initial and final colors? Is there a specific location inside the petri dish around which the color changes? Does solution in the entire dish change color? What are some differences between the two ends in which a mechanical pencil lead is immersed in solution (in terms of color, amount of bubbles, etc)? What happens after the electric circuit setup is removed and the solution is allowed to sit for 5 minutes?
 
Notes:
Bromothymol blue turns yellow in acdic solutions. When oxygen is produced as the gas product in the anode of the electrolysis reaction, the oxidation half reaction is shown as below: $text{2H}_2text{O}_{(l)} to text{O}_2_{(g)} + text{4H}^{+}_{(aq)} + text{4e}^{-}_{(aq)}$ The formation of H+ causes the color of bromothymol blue change to yellow. There is more gas produced on the cathode side than on the anode side, indicating that there are more moles of hydrogen atoms than oxygen atoms in water molecules (the mole ratio of hydrogen to oxygen in water molecules is not 1:1, but indeed 2:1). The formation of H+ is temporary, and after the electric circuit setup is removed and the solution is allowed to sit for 5 minutes, the color of the solution changes back to its original color [blue]. ______________________________________________________________________________________________________ Related videos can be view here: Animation Click for video Acidity Click for video Standard setup with gas testing Click for video Distilled water Click for video
 
Disposal:
The liquid waste can be disposed directly down the sink. The battery should be recycled or reused.
 
Difficulty:No specific experience required
Preparation Time:2 minutes Demonstration Time: 1 minutes
Availability of Materials:Local grocery store
Cost of materials:$2
Last Updated:Tue 09 Aug 2011 12:44:46 EDT Viewed:132746 times viewed
Source:ScienceFix.com, Darren Fix, August 11, 2009, ScienceFix.com