Extreme reactivity of chlorosulfonic acid

Chlorosulfonic acid is a chemical, which is used in industry on a large scale. In the chemical lab it is much less common and this is for a good reason. The compound is extremely corrosive and very reactive with almost every other chemical, including water and many common solvents and salts. In this experiment, this extreme reactivity is demonstrated in a beautiful way.

Chlorosulfonic acid is extremely corrosive. With water it reacts explosively. It eats through skin in a fraction of a second. There will be NO time to rinse away the acid or wipe off the acid! Avoid contact with the human body! If you are not confident in being capable of handling this chemical safely, then do not perform this experiment.

 

Required chemicals:

  • chlorosulfonic acid
  • potassium permanganate
  • potassium dichromate
  • sodium sulfite (metabisulfite or thiosulfate also is suitable) for cleaning up
  • dilute sulfuric acid for cleaning up

Required equipment:

  • big flask
  • glass pipette and balloon for conveniently transferring corrosive liquids

         

Safety:

  • Chlorosulfonic acid is extremely corrosive (see above).
  • Potassium permanganate is a strong oxidizer, but in this experiment it is not used in a way, which introduces risk of accidental exothermic oxidizing reactions.
  • Potassium dichromate is a known carcinogen on inhalation. It also may act as a strong sensitizer in some people. The fumes, produced in the reaction with potassium dichromate are carcinogenic, avoid exposure to these.

Disposal:

  • The waste can be reduced with a solution of sodium sulfite or a solution of sodium thiosulfate and can be flushed down the drain with a lot of water.

 

 

Production of a mushroom cloud

In this experiment, potassium permanganate (KMnO4) is mixed with chlorosulfonic acid (HSO3Cl). When these chemicals are mixed, then a violent reaction starts. Here it is described how this reaction can be performed safely on a small scale.

Take a small spatula of potassium permanganate and put this in the dry flask. Assure that the solid is on a small pile. A round bottom flask is most convenient, because that makes collecting all solid at a single spot easy, just by swirling the flask a little bit.

Put the flask in a clamp, such that its contents can be observed well.

Using a glass pipette and a balloon, take 0.1 to 0.2 ml of chlorosulfonic acid from its bottle. Do not pour the acid and do not add more acid than the mentioned amount! Pouring the acid introduces a high risk of accidents and spills. Using larger amounts of acid makes cleaning up the waste very troublesome, due to its extremely violent reaction with water!

Drip the liquid on the solid potassium permanganate.

When the liquid touches the solid, a brown/green fume is produced. It is not entirely clear, whether this is a gas, or smoke. Most likely, this is a gas, which also has white fumes of the acid mixed with it. For a short time, more of the dense brown/green gas/fume is produced, and then suddenly, there is a small explosion, in which brown smoke is produced. This brown smoke forms a mushroom cloud, which quickly moves upwards. The pictures below show what happens.

   

   

The brown smoke later moves through the flask and thicker flakes of the solid fall back towards the bottom. Below follow a few links of high quality movies. One at real speed, one at half speed and one at 20% of real speed. Especially the latter nicely shows the formation of the mush room cloud  and the formation of flakes, which fall back to the bottom:

          Video of reaction at 100% of real speed

          Video of reaction at 50% of real speed

          Video of reaction at 20% of real speed


For cleaning up, prepare a solution of a few hundreds of mg of sodium sulfite in appr. 20 ml of water and add this to the flask. Swirl in order to get rid of all brown stains and let stand for an hour or so, covered with a petri dish, to allow all fumes to settle and dissolve in the water. The stains can be removed very easily with such a solution. After that, the flask can be cleaned further by rinsing with a lot of water. The waste can be flushed down the drain without problems for the environment.


 

Orange clouds in a flask.

With potassium dichromate (K2Cr2O7)  instead of potassium permanganate it also is possible to produce a volatile compound in a violent reaction. In this reaction, the color of the fumes is beautiful. Be careful though, the fumes from the reaction with potassium dichromate are much more toxic than the fumes, produced in the reaction with potassium permanganate. They contain hexavalent chromium, which is a known carcinogen. Here it is described how this reaction can be performed safely on a small scale.

Take a spatula of potassium dichromate and put this in the dry flask. A few hundreds of mg of crystals is sufficient. As in the previous experiment, assure that the solid is on a small pile, covering a small area.

Put the flask in a clamp, such that its contents can be observed well.

Using a glass pipette and a balloon, take 0.2 to 0.3 ml of chlorosulfonic acid from its bottle. Do not pour the acid and do not add more acid than the mentioned amount! Pouring the acid introduces a high risk of accidents and spills. Using larger amounts of acid makes cleaning up the waste very troublesome, due to its extremely violent reaction with water!

Drip the liquid on the solid potassium dichromate.

In this experiment, bright orange/red fumes are produced. These fumes consist of a complicated mix of chromyl chloride, fumes from the chlorosulfonic acid and the remarkable compound chromyl chlorosulfonate, dissolved in chlorosulfonic acid. All of these chemicals together appear as an orange/red gas/fume.

   

   

The orange fumes in this reaction are much more uniform than the smoke in the reaction with potassium permanganate. The fume slowly fills the flask. There also is formed more orange gas during a few minutes. After the initial violent reaction, the liquid at the bottom keeps on releasing orange gas for several minutes. After 10 minutes, the flask is filled with a dense orange gas, and the fumes have mostly settled at the glass.

     

These fumes are quite dense and can be poured out of this flask. The picture shows that inside the flask there is a gas and not a fume and that the constituents of the fume (liquid and/or solid matter) have settled in the lower part of the flask on the glass wall. When the gas mix from the flask is poured out in air, then the orange gas can be observed, but there also is formation of new fumes. That is most likely, due to presence of HCl in the gas mix and due to hydrolysis of the orange CrO2Cl2-vapor in the flask in contact with water vapor in the air.

Below follow a few videos:

          Video of reaction at 100% of real speed

          Video of reaction at 20% of real speed

          Video of pouring the vapor from the flask


For cleaning up, prepare a solution of half a gram or so of sodium sulfite in appr. 50 ml of dilute sulfuric acid (5% acid is OK) and add this to the flask. Swirl in order to dissolve all of the orange/red solid, sticking to the glass, and let stand for an hour or so, covered with a petri dish, to allow all fumes and vapor to settle and dissolve in the dilute acid. The solution in the flask will turn green. After that, the flask can be cleaned further by rinsing with a lot of water. If sufficient sodium sulfite is used (the liquid must be purely green), then the waste can be flushed down the drain without problems for the environment. Trivalent chromium is not particularly toxic and the small amount, used in this experiment, is no problem at all.


 

Discussion of results

Chlorosulfonic acid reacts with many acids, having OH-groups, by replacing the OH-group by a Cl-atom, itself being converted to sulfuric acid. An example is the reaction with HNO3 (which has structure O2N-OH).

   O2NOH + HOSO2Cl → O2NCl + HOSO2OH

With salts of oxoacids, however, a similar reaction occurs, with formation of the metal bisulfate. With potassium permanganate this leads to the following reaction, written in a somewhat unconventional form, which better shows the structure of the reactants and products at the moment of reaction (one of the resonance extremes of permanganate ion is O3MnO–):

   O3MnOK + HOSO2Cl → O3MnCl + HOSO2OK

The volatile compound permanganyl chloride, O3MnCl, usually written as MnO3Cl, is very unstable and it easily explosively decomposes to MnO2, O2 and Cl2. At the start, fumes and vapor of permanganyl chloride are formed, but soon, the formed permanganyl chloride decomposes nearly explosively, releasing a lot of gas and dark brown solid MnO2, which appears as a brown mushroom cloud.
 

With potassium dichromate the reaction is more complicated. With hexavalent chromium, a similar reaction can occur as with permanganate, but chlorosulfonic acid also can form chlorosulfonates with hexavalent chromium. The first reaction is:

   KO-CrO2-O-CrO2-OK + HOSO2Cl → KO-CrO2-O-CrO2-Cl + HOSO2OK

This compound, however, is not stable, and immediately breaks down into different compounds and through different intermediate steps, the following compounds are formed:

  • CrO2Cl2 (chromyl chloride, deep orange/red vapor)
  • KOCrO2Cl (potassium chlorochromate, an orange/red solid)
  • CrO2(OSO2Cl)2 (chromyl chlorosulfonate, dissolved in excess HSO3Cl)
  • there might also be mixed chloride chlorosulfonate chromyl

All of these compounds appear in the red/orange gas/fumes. Finally, when all liquid and solid matter has settled, only CrO2Cl2 remains present in the gas phase.

 
   

 

   

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