Demonstrate Water Electrolysis in a Simple Way

Demonstrate Water Electrolysis in a Simple Way

Credit: Yan Luo

 
Author: Yan Luo
Description:

Many middle schools and high schools lack the equipments to perform the water electrolysis demonstration and collect gaseous products for further testing. This particular demonstration adapted from scienceFix.com uses grocery-store-purchasable equipment and materials to perform the water electrolysis experiment with a simple setup.

Usage:
  • Demonstrate water electrolysis, decomposition reaction, and stoichiometry
  • Demonstrate testing methods of H2 and O2
 
Futher Images
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Credits, from left: Yan Luo, Yan Luo, Yan Luo, Yan Luo
 
Safety:

Regular laboratory safety procedure should be followed (eg. gloves and goggles).

 
Equipment:
  • 1 9V battery
  • 2 test-tubes
 
Materials:
  • Water (enough to fill 3/4 of the plastic bowl)
  • [If water is distilled] A little table salt (~1gram) or any other highly soluble, neutral salt (to allow complete circuit).
  • A thin, flat - bottomed, clear plastic bowl that is big enough to hold the two test tubes or a clear plastic cup
  • Two completely made  of metal thumbtacks that are completely made of electric conductors (eg. steel, copper, etc)
  • Two wooden splints (for Part II)
 
Procedure:

Preparation

No specific preparation is needed for this demonstration.

Demonstration

Part I: Water Electrolysis

Plastic Bowl

Cut the plastic cup to half of its original height if necessary to be used as a plastic bowl. Nail the thumbtacks from the bottom of the plastic bowl ideally with the same distance from the center. The distance between the two thumbtacks should match with the distance between the two electrodes of the 9V battery.

Carefully fill 3/4 of the plastic bowl with water.

Put a little (about 1gram) table salt (or any other highly soluable, neutral salt) into the plastic bowl. Dissolve the salt completely with water using a stir rod (to introduce ions in solution to allow complete circuit).

Demonstration:

Place the plastic bowl with water and nailed thumbtacks directly on top of the electrodes of a 9V battery. The plastic bowl might need support to be prevented from falling.

Completely fill one test tube with water, cover the openning with the thumb, flip the test tube upside down and insert it to the water in the plastic bowl. Remove the covering of the test tube and place it on top of the tip of one of the thumbbacks. Let the test tube sit on the plastic bowl and make sure gas that comes from the tip of the thumback is being collected by the test tube.

Repeat the same procedure to place another test tube on top of the other thumbback tip.


Part II: Gas Testing

H2
The side has more bubbles producing over a given period of time and fills up faster
Prepare a lighted splint using Bunsen burner. Remove the test tube from the plastic bowl and quickly insert a lighted splint inside the test tube.
O2
The side has fewer bubbles producing over a given period of time and fills up slower
Prepare a dimmed splint using Bunsen burner. Remove the test tube from the plastic bowl and quickly insert a dimmed splint inside the test tube.

Observation and Discussion

Part I: Water Electrolysis

  • What happens immediately after the plastic bowl setup is placed on top of the battery terminals?
  • Which side produces more bubbles over a given period of time?
  • How else can you tell if one side produces more gas over the same period of time?
  • What is the approximate ratio of gas produced in the two test tubes?
  • What does the ratio tell us?

Part II: Gas Testing

  • What can you observe from the "Gas Testing" experiments?
  • What do phenomenons observed in these experiments tell us?
  • Estimate the chemical formula [with the correct stoichiometry ratio] of water based on what you learn from both parts of the demonstration.
  • Write the balanced chemical equation for the overall reaction.

Advanced

  • Signs of the terminals are clearly labeled on the 9 V battery. Match the sign with the product producing in each test tube [terminal]. Write the Oxidation-Reduction half equations for each side [hint: one type (two possibilities) of ion is formed as product at anode].
  • Which direction do electrons flow in the overall reaction?
  • Can distilled water be directly used in this water electrolysis demonstration?
 
Notes:

Water electrolysis is a decomposition reaction since water is the only reactant in the reaction. Gas release can be seen soon after the demonstration is set up, with greater volume of gas on one side than the other, indicating that there are more of one type of element than the other in a water molecule [volume of gas on the side with a greater product yield to the side with a smaller product yield should be 2:1]. Part II of the experiment suggests the two products are H2 and O2, and more H2 gas is produced than O2 over the same period of time. With the information gathered from Part I and II of the demonstration, the chemical equation of water electrolysis can be written as below:

$\text{2H}_2\text{O}_{(l)} \to \text{2H}_2_{(g)} + \text{O}_2_{(g)}$

The greater volume of gas produced [H2] can be found on top of the negative terminal meaning that H species in water gains electrons to form H2 gas [reduction]. Similarly, O2 is formed by O species in water losing electrons [Oxidation]. The two half reactions for each terminal can be written as following:

Reduction at cathode:

$\text{2H}^{+}_{(aq)} + \text{2e}^{-} \to \text{H}_2_{(g)}$

Oxidation at anode:

$\text{2H}_2\text{O}_{(l)} \to \text{4H}^{+}_{(aq)} + \text{2O}_2_{(g)} + \text{4e}^{-}$

or

Reduction at cathode:

$\text{2H}_2\text{O}_{(l)} + \text{2e}^{-} \to \text{H}_2_{(g)} + \text{2OH}^{-}_{(aq)}$

Oxidation at anode:

$\text{4OH}^{-}_{(aq)} \to \text{O}_2_{(g)} + \text{2H}_2\text{O}_{(l)} + \text{4e}^{-}$


Video of this specific demonstration is viewable here.

Related videos can be view here:

 
Disposal:

Dead batteries should be recycled or disposed properly

Plastic bowl and thumbtack should be recycled or reused

No specific concern for disposing leftover water

 
Difficulty:No specific experience required
Preparation Time:2 minutes Demonstration Time: 1 minutes
Availability of Materials:Local grocery store
Cost of materials:$3
Last Updated:Wed 15 Jun 2011 14:54:42 EDT Viewed:160070 times viewed
Source:Darren Fix, Yan Luo, November 28, 2010, ScienceFix.com