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The Barking Dog chemistry demonstration is based on an exothermic reaction between nitrous oxide or nitrogen monoxide and carbon disulfide. Ignition of the mixture in a long tube results in a bright blue chemiluminescent flash, accompanied by a characteristic barking or woofing sound.
Materials for the Barking Dog Demonstration
- Stoppered glass tube containing N2O (nitrous oxide) or NO (nitrogen monoxide or nitric oxide). You can prepare and collect nitrous oxide or nitrogen monoxide yourself.
- CS2, carbon disulfide
- Lighter or match
How to Perform the Barking Dog Demonstration
- Unstopper the tube of nitrous oxide or nitrogen monoxide to add a few drops of carbon disulfide.
- Immediately re-stopper the container.
- Swirl the contents around to mix the nitrogen compound and carbon disulfide.
- Light a match or lighter. Unstopper the tube and ignite the mixture. You can throw a lit match into the tube or use a long-handled lighter.
- The flame front will move rapidly, creating a bright blue chemiluminescent flash and a barking or woofing sound. You can re-light the mixture a few times. After the demonstration is performed, you can see sulfur coating the inside of the glass tube.
This demonstration should be prepared and performed inside a fume hood by a person wearing safety goggles. Carbon disulfide is toxic and has a low flash point.
What Is Happening in the Barking Dog Demonstration?
When the nitrogen monoxide or nitrous oxide is mixed with carbon disulfide and ignited, a combustion wave travels down the tube. If the tube is long enough you can follow the progression of the wave. The gas ahead of the wavefront is compressed and explodes at a distance determined by the length of the tube (which is why when you re-ignite the mixture, the 'barking' sounds in harmonics). The bright blue light that accompanies the reaction is one of the few examples of a chemiluminescent reaction that occurs in the gas phase. The exothermic decomposition reaction between nitrogen monoxide (oxidizer) and carbon disulfide (fuel) forms nitrogen, carbon monoxide, carbon dioxide, sulfur dioxide and elemental sulfur.
3 NO + CS2 → 3/2 N2 + CO + SO2 + 1/8 S8
4 NO + CS2 → 2 N2 + CO2 + SO2 + 1/8 S8
Notes about the Barking Dog Reaction
This reaction was performed by Justus von Liebig in 1853 using nitrogen monoxide and carbon disulfide. The demonstration was so well-received that Liebig performed it a second time, although this time there was an explosion (Queen Therese of Bavaria received a minor wound on the cheek). It's possible the nitrogen monoxide in the second demonstration was contaminated with oxygen, to form nitrogen dioxide.
There is also a safer alternative to this project that you can do with or without a lab.