The barking dog reaction is a classic chemistry demonstration that raises interest in science and illustrates combustion, an exothermic reaction, and physical chemistry concepts. The demonstrator ignites a mixture of carbon disulfide and nitrogen oxide, causing a bright blue chemiluminescent flash and a characteristic barking sound. The target audience is a high school or first year college chemistry class, although the reaction also entertains a general audience. Here is how to do the barking dog reaction and a look at its chemistry.
Barking Dog Reaction Materials
The reaction requires carbon disulfide, a nitrogen oxide, and a long cylindrical glass tube:
- Stoppered tube filled with either nitrous oxide (N2O) or nitric oxide (NO)
- Carbon disulfide (CS2)
Performing the Barking Dog Reaction
- Set the tube in a fume hood.
- Uncap the glass tube and add a few drops of carbon disulfide.
- Stopper the container and swirl the contents to mix them.
- Dim the lights.
- Light the lighter, uncap the tube, and ignite the vapor. Either use a long handled lighter or else use long tongs and drop a lit match into the tube.
- You can reignite the vapor a few times and repeat the demonstration.
Once lit, the flame front moves down the tube. As it does, the reaction produces a bright blue flash and woofing or barking sound. The diameter and length of the tube determines the sound of the bark. Yellow sulfur coats the inside of the tube as the reaction proceeds.
How the Barking Dog Reaction Works
Igniting the mixture of carbon disulfide with nitrous or nitric oxide initiates a combustion reaction. The products of the reaction are elemental sulfur, carbon dioxide, and nitrogen gas:
8 N2O + 4 CS2 → S8 + 4 CO2 + 8 N2 (for nitrous oxide)
8 NO + 4 CS2 → S8 + 4 CO2 + 4 N2 (for nitric oxide)
However, other reactions also occur, producing carbon monoxide (CO) and sulfur dioxide (SO2).
The reaction proceeds as a wave traveling down the tube. In a long tube, it’s easy to follow the reaction progress. Slowing a video of the reaction in a short tube also shows the effect. The wave front compresses gas ahead of it, eventually causing it to explode. The harmonics of the sound of the explosion cause the barking sound. The gas compression also explains why you can re-light the mixture multiple times.
The blue light comes from chemiluminescence. Usually, you see chemiluminescence in liquids, as in a glow stick. The barking dog reaction is one of only a few reactions that displays gas phase chemiluminescence.
German scientist Justus von Liebig gets credit for revolutionizing chemistry education. He incorporated the chemistry lab as we recognize it today into the educational setting. The popularity of his barking dog reaction led him to perform the demonstration for the Bavarian royal family in April of 1853. Unfortunately, Liebig’s glass container shattered and sprayed shards of glass over himself and the royal family. No one was seriously injured, but Queen Therese received a minor cut on her cheek.
One explanation for Liebig’s mishap in his reaction between carbon disulfide and nitric oxide is that the nitrogen monoxide might have been contaminated with oxygen. Over time nitric oxide reacts with oxygen in air, forming nitrogen dioxide. Nitrogen dioxide is a powerful oxidizing agent.
Don’t repeat Liebig’s mistake! Perform the demonstration a safe distance from the audience, just in case something goes awry. The sound and light from the reaction are easily observable, so this isn’t a problem.
The barking dog reaction is a chemistry demonstration for the experienced chemistry teacher or lab technician. As with any chemistry demonstration, wear safety goggles and a lab coat. Carbon disulfide is flammable and toxic, so perform this demonstration within a fume hood. In any case, the end result is sulfur, which has an offensive odor.
- Brock, William H. (1997). Justus Von Liebig: The Chemical Gatekeeper. Cambridge, England: Cambridge University Press. ISBN 9780521524735.
- Jensen, William B. (2018). “Justus von Liebig – Chemistry Mentor and Teacher”. Preceptors in Chemistry. 6: 111-147. doi:10.1021/bk-2018-1273.ch006
- Seabourne, Ché Royce; Maxwell, George; Wallace, James (2006). “Taming the Barking Dog”. Journal of Chemical Education. 83 (5): 751. doi:10.1021/ed083p751