Photochemical Reaction of Hydrogen and Chlorine
Publication information:
Abstract
What it Shows
Different colors of LED light illuminate a small corked test tube with no reaction. Ultraviolet light causes an explosion that shoots the cork across the room.
Video of our demo in action https://youtu.be/NN82GoBG98s
Another video of our demo in action https://youtu.be/tJhVy1x9X2c
How it Works
Using a power supply, 8M hydrochloric acid solution is electrolyzed to produce a 1:1 ratio of hydrogen gas (H2) and chlorine gas (Cl2) according to the following equation:
HCl (aq) → ½ H2 (g) + ½ Cl2 (g)
The electrochemical half reactions with their standard reduction potentials are:
2H+(aq) + 2e- → H2 (g) 0V
2Cl-(aq) → Cl2 (g) + 2e- -1.36 V
Though the electrolysis is not run at standard conditions, the overall standard cell potential is -1.36 V, negative indicating a nonspontaneous reaction, requiring electricity to drive the reaction. Bubbling can be seen at the graphite electrodes when the applied voltage is approximately (insert voltage here).
The reaction that causes the explosion of the cork out of the test tube is the reverse of the electrolysis reaction, forming hydrogen chloride (HCl) gas.
½ H2 (g) + ½ Cl2 (g) → HCl(g)
According to Shakhashiri, the mechanism is a chain reaction starting with the breaking of the Cl-Cl bond:
Cl2 + hν → 2Cl·
Cl· + H2 → HCl + H·
H· + Cl2 → HCl + Cl·
The reaction is initiated by light, but not light of just any wavelength. A series of colored light is applied to the test tube, increasingly higher in energy until the light is of sufficient energy to break the Cl-Cl bonds. The H-H bond energy is 436 kJ/mol and the Cl-Cl bond energy is 243 kJ/mol. These energies correspond to a photon energy of 13.6 eV (275 nm) and 2.51 eV (492 nm) respectively.
Safety
Wear eye protection, gloves, and a lab coat when doing this demonstration as the hydrochloric acid is very corrosive. In particular, HCl will react with gold so make sure to remove gold jewelry that may be exposed. Chlorine gas is irritating to the eyes and skin and is very reactive, so run the electrolysis reaction in a fume hood. The amount of HCl produced by the reaction is minimal but is a toxic irritating gas. Ear protection should be worn because the noise from the explosion can cause a ringing in the ears. Tell the students to cover their ears. As an extra precaution, wrap the test tube with clear tape, though we have never known of an instance where the test tube has shattered. You must use wood corks rather than rubber in this experiment to avoid shattering the test tube.
Setting it Up
The gas mixture is made by the electrolysis of 8M hydrochloric acid. Run the reaction in a fume hood. Place about 500 mL of HCl in a 500 mL side arm flask. The electrodes are two graphite rods. They are stuck through a two hole rubber stopper which is placed firmly on the flask. Between 2-3 cm of the graphite rods should be sticking above the stopper to attach the alligator clips.
To the side arm of the flask, attach about 8-10 cm of Tygon tubing with a diameter of 1/2 inch. Attach a two way Teflon connector. Then attach about 30 cm of tubing to the other end. Place a right angle of glass tubing on the end of the tubing. Note: The connector is not necessary - a single length of tubing could be used in place if you don’t have the connector. Rubber tubing could be used instead as well, but just keep in mind that you will have to replace the tubing each time as chlorine gas will degrade it and cause cracks.
Setting up the water bath:
A water bath allows for filling the tubes under water. Fill a trough approximately 30 cm x 10 cm x12 cm with distilled water (tap water could be used if distilled water is not available). Our trough is glass, but you could use any sort of glass or plastic container, provided that it has enough space for the test tubes.
Place two quartz test tubes into the water bath, making sure to fill the tubes completely without any air bubbles which may poison the reaction. Also add 4-5 wood corks to get saturated by water. It is important that the corks are wood rather than rubber because it requires too much force to pop out the rubber cork and the test tube could break instead. You can use regular test tubes instead - wrap them with clear packing tape as a safety precaution.
Place the end of the tubing with the glass tubing into one of the test tubes.
Preparing the gas:
In order for a pure mixture of chlorine and hydrogen gases to be collected, all oxygen must be purged from the flask, tubing, and water in the trough. Oxygen will poison the reaction because chlorine radicals will react with the oxygen and inhibit the reaction between chlorine and hydrogen. Therefore, it is essential to run the reaction for a period of time before performing the experiment. We have determined that electrolyzing about 0.5 mol of HCl will clear out the oxygen. Depending on how much time you have before the demonstration, set the current according to the following guidelines:
Performing the next day: Set the current to 0.32A and run overnight in a fume hood. In the morning, increase the current to 1.5A. Right before collecting the gas, turn up the current to 3A.
Performing in 6-8 hours: Set the current to 0.32A and run overnight in a fume hood. In the morning, increase the current to 1.5A. Right before collecting the gas, turn up the current to 3A.
Performing in a few hours: Set the current to 3A and collect the gas at 3A. Warning: Sometimes this is not sufficient to clear all the oxygen and the reaction sometimes doesn’t occur.
Collecting the gas:
Making sure that the mouth of the test tube stays under water, fill the tube with gas. As soon as it’s filled, keep it upright under water and push the test tube down firmly onto a cork. Remove it and wrap the test tube in a paper towel in the unlikely event that the light sets off the reaction.
Performing the demonstration:
On the end of the lecture bench, set up a ring stand with a three finger tube holder at a 45* angle. Beside it array the coin cell LED flashlights from red to UV. Next to the red and green lights place the bright red and green flashlights. Provide hearing protection in addition to eye protection for the demonstrator.
Fill the tube with hydrogen and chlorine right before the demonstration. Dim the board and overhead lights, and place the tube in the holder with the cork pointing away from you. Tighten the holder gently.
Starting with the red LED, proceed through the color sequence to the UV, which will go bang and shoot the cork across the front of the hall. Use the bright flashlight to show that more light of inadequate frequency will not initiate the reaction.
Cleaning up:
Turn off the power supply, remove the alligator clips from the electrodes, and disconnect the tube from the connector. Take off the glass plate and let the whole setup sit in the fume hood for a day to clear out the residual chlorine. The flask with the acid will now appear greenish because of chlorine gas dissolved in the solution. Store this flask with the acid in a safe acid storage area for next time. Rinse the trough, tubing, and test tubes with water before putting away. Keep the corks.
Next time you do the demo: Rather than throwing away the acid in the flask, pour out some acid, leaving about 250 mL, and add enough concentrated acid (about 250 mL of 12 M HCl) to the flask to bring the molarity back up to 8M. We believe that the concentration of the acid is approximately 4M after the reaction has been run.
Comments
We have also used a slide projector, as recommended in Shakhashiri to set off the reaction.
References
* Richard Schwenz and Lynn Geiger. "Photon-Initiated Hydrogen-Chlorine Reaction." Journal of Chemical Education 76.4 (1999): 470.
* Shakhashiri, Bassam Z. "Chemical Demonstrations: A Handbook for Teachers of Chemistry." Madison: The University of Wisconsin Press, 1983. ( http://books.google.com/books?id=0rx6Cjx_l-AC&lpg=PP1&pg=PA121#v=onepage&q&f=false)
Full text
What it Shows
Different colors of LED light illuminate a small corked test tube with no reaction. Ultraviolet light causes an explosion that shoots the cork across the room.
Video of our demo in action https://youtu.be/NN82GoBG98s
Another video of our demo in action https://youtu.be/tJhVy1x9X2c
How it Works
Using a power supply, 8M hydrochloric acid solution is electrolyzed to produce a 1:1 ratio of hydrogen gas (H2) and chlorine gas (Cl2) according to the following equation:
HCl (aq) → ½ H2 (g) + ½ Cl2 (g)
The electrochemical half reactions with their standard reduction potentials are:
2H+(aq) + 2e- → H2 (g) 0V
2Cl-(aq) → Cl2 (g) + 2e- -1.36 V
Though the electrolysis is not run at standard conditions, the overall standard cell potential is -1.36 V, negative indicating a nonspontaneous reaction, requiring electricity to drive the reaction. Bubbling can be seen at the graphite electrodes when the applied voltage is approximately (insert voltage here).
The reaction that causes the explosion of the cork out of the test tube is the reverse of the electrolysis reaction, forming hydrogen chloride (HCl) gas.
½ H2 (g) + ½ Cl2 (g) → HCl(g)
According to Shakhashiri, the mechanism is a chain reaction starting with the breaking of the Cl-Cl bond:
Cl2 + hν → 2Cl·
Cl· + H2 → HCl + H·
H· + Cl2 → HCl + Cl·
The reaction is initiated by light, but not light of just any wavelength. A series of colored light is applied to the test tube, increasingly higher in energy until the light is of sufficient energy to break the Cl-Cl bonds. The H-H bond energy is 436 kJ/mol and the Cl-Cl bond energy is 243 kJ/mol. These energies correspond to a photon energy of 13.6 eV (275 nm) and 2.51 eV (492 nm) respectively.
Safety
Wear eye protection, gloves, and a lab coat when doing this demonstration as the hydrochloric acid is very corrosive. In particular, HCl will react with gold so make sure to remove gold jewelry that may be exposed. Chlorine gas is irritating to the eyes and skin and is very reactive, so run the electrolysis reaction in a fume hood. The amount of HCl produced by the reaction is minimal but is a toxic irritating gas. Ear protection should be worn because the noise from the explosion can cause a ringing in the ears. Tell the students to cover their ears. As an extra precaution, wrap the test tube with clear tape, though we have never known of an instance where the test tube has shattered. You must use wood corks rather than rubber in this experiment to avoid shattering the test tube.
Setting it Up
The gas mixture is made by the electrolysis of 8M hydrochloric acid. Run the reaction in a fume hood. Place about 500 mL of HCl in a 500 mL side arm flask. The electrodes are two graphite rods. They are stuck through a two hole rubber stopper which is placed firmly on the flask. Between 2-3 cm of the graphite rods should be sticking above the stopper to attach the alligator clips.
To the side arm of the flask, attach about 8-10 cm of Tygon tubing with a diameter of 1/2 inch. Attach a two way Teflon connector. Then attach about 30 cm of tubing to the other end. Place a right angle of glass tubing on the end of the tubing. Note: The connector is not necessary - a single length of tubing could be used in place if you don’t have the connector. Rubber tubing could be used instead as well, but just keep in mind that you will have to replace the tubing each time as chlorine gas will degrade it and cause cracks.
Setting up the water bath:
A water bath allows for filling the tubes under water. Fill a trough approximately 30 cm x 10 cm x12 cm with distilled water (tap water could be used if distilled water is not available). Our trough is glass, but you could use any sort of glass or plastic container, provided that it has enough space for the test tubes.
Place two quartz test tubes into the water bath, making sure to fill the tubes completely without any air bubbles which may poison the reaction. Also add 4-5 wood corks to get saturated by water. It is important that the corks are wood rather than rubber because it requires too much force to pop out the rubber cork and the test tube could break instead. You can use regular test tubes instead - wrap them with clear packing tape as a safety precaution.
Place the end of the tubing with the glass tubing into one of the test tubes.
Preparing the gas:
In order for a pure mixture of chlorine and hydrogen gases to be collected, all oxygen must be purged from the flask, tubing, and water in the trough. Oxygen will poison the reaction because chlorine radicals will react with the oxygen and inhibit the reaction between chlorine and hydrogen. Therefore, it is essential to run the reaction for a period of time before performing the experiment. We have determined that electrolyzing about 0.5 mol of HCl will clear out the oxygen. Depending on how much time you have before the demonstration, set the current according to the following guidelines:
Performing the next day: Set the current to 0.32A and run overnight in a fume hood. In the morning, increase the current to 1.5A. Right before collecting the gas, turn up the current to 3A.
Performing in 6-8 hours: Set the current to 0.32A and run overnight in a fume hood. In the morning, increase the current to 1.5A. Right before collecting the gas, turn up the current to 3A.
Performing in a few hours: Set the current to 3A and collect the gas at 3A. Warning: Sometimes this is not sufficient to clear all the oxygen and the reaction sometimes doesn’t occur.
Collecting the gas:
Making sure that the mouth of the test tube stays under water, fill the tube with gas. As soon as it’s filled, keep it upright under water and push the test tube down firmly onto a cork. Remove it and wrap the test tube in a paper towel in the unlikely event that the light sets off the reaction.
Performing the demonstration:
On the end of the lecture bench, set up a ring stand with a three finger tube holder at a 45* angle. Beside it array the coin cell LED flashlights from red to UV. Next to the red and green lights place the bright red and green flashlights. Provide hearing protection in addition to eye protection for the demonstrator.
Fill the tube with hydrogen and chlorine right before the demonstration. Dim the board and overhead lights, and place the tube in the holder with the cork pointing away from you. Tighten the holder gently.
Starting with the red LED, proceed through the color sequence to the UV, which will go bang and shoot the cork across the front of the hall. Use the bright flashlight to show that more light of inadequate frequency will not initiate the reaction.
Cleaning up:
Turn off the power supply, remove the alligator clips from the electrodes, and disconnect the tube from the connector. Take off the glass plate and let the whole setup sit in the fume hood for a day to clear out the residual chlorine. The flask with the acid will now appear greenish because of chlorine gas dissolved in the solution. Store this flask with the acid in a safe acid storage area for next time. Rinse the trough, tubing, and test tubes with water before putting away. Keep the corks.
Next time you do the demo: Rather than throwing away the acid in the flask, pour out some acid, leaving about 250 mL, and add enough concentrated acid (about 250 mL of 12 M HCl) to the flask to bring the molarity back up to 8M. We believe that the concentration of the acid is approximately 4M after the reaction has been run.
Comments
We have also used a slide projector, as recommended in Shakhashiri to set off the reaction.
References
* Richard Schwenz and Lynn Geiger. "Photon-Initiated Hydrogen-Chlorine Reaction." Journal of Chemical Education 76.4 (1999): 470.
* Shakhashiri, Bassam Z. "Chemical Demonstrations: A Handbook for Teachers of Chemistry." Madison: The University of Wisconsin Press, 1983. ( http://books.google.com/books?id=0rx6Cjx_l-AC&lpg=PP1&pg=PA121#v=onepage&q&f=false)