Relative reactivity of oxohalogenate ions

In this experiment several oxohalogenate ions XOn– are tested for their reactivity in aqueous solution at room temperature. Here, X is one of Cl, Br, or I. The value of n can be one of 1, 2, 3, or 4.

Seven different chemicals, based on the XOn– anion are added to a solution of hydrazine dihydrochloride and the reaction is observed. The picture below shows the last reaction (for IO4–).

  

 

Required chemicals:

  • hydrazine dihydrochloride

  • calcium hypochlorite (X = Cl, n = 1)

  • sodium chlorite (X = Cl, n = 2)

  • potassium chlorate (X = Cl, n = 3)

  • potassium bromate (X = Br, n = 3)

  • potassium iodate (X = I, n = 3)

  • potassium perchlorate (X = Cl, n = 4)

  • potassium metaperiodate (X = I, n = 4)

  • sodium hydroxide (for cleanup)

  • bleach (for responsible cleanup after the experiment, destroying any hydrazine which may possibly be left over after the experiment)

Required equipment:

  • set of test tubes

  • pasteur pipette

Safety:

  • Hydrazine and its compounds are carcinogens. Avoid exposure to these compounds.

  • Bromates seem to have carcinogenic properties as well, this is why their use in food is banned in most western countries. Avoid exposure.

  • All the oxohalogenate compounds are strong oxidizers.

  • Some of the reactions in this experiment are very violent. They are safe in the amounts, used in this experiment, but do not scale up the reactions.

Disposal:

  • Dump all liquid and solid waste in a glass beaker, which contains 10 ml or so of a 10% solution of sodium hydroxide. Rinse the test tubes with this solution as well and pour the solution back in the beaker. This destroys any iodine and other free halogen, which may be left over after the experiment.
  • Add 10 ml of household bleach to the beaker with the waste and the 10% solution of sodium hydroxide. This destroys any hydrazine left (if any).
  • The 20 ml of waste now can be flushed down the drain with a lot of water and the beaker and test tubes can be rinsed as well with water.

 

 

 
 

 

Procedure for performing the experiment

Dissolve approximately 2 grams of hydrazine dihydrochloride in 5 ml of water.

Divide this liquid over 7 test tubes, assuring that the liquid does not wet the walls of the test tubes. Use a pasteur pipette for transferring the liquid to all the test tubes.

Put all test tubes next to each other, in such a way that they can't tumble over each other or fall down.

Next, to each of the test tubes, add one chemical. Use a small amount, at most pea-sized chunks.

 

The result of the experiment

The picture below shows the result after adding the chemicals to the solution of hydrazine dihydrochloride. From left to right:

  • calcium hypochlorite

  • sodium chlorite

  • potassium chlorate

  • potassium bromate

  • potassium iodate

  • potassium perchlorate

  • potassium metaperiodate

 

  

 

It is nice to see the difference of the reactivity of the different oxohalogenates. Calcium hypochlorite reacts very violently, sodium chlorite also reacts violently, but a short induction period is needed. Potassium chlorate does not react. Potassium bromate reacts violently, the reaction becomes more violent after a few seconds, due to heating up of the mix. Potassium iodate is even more violent than potassium bromate. Potassium perchlorate does not react and the most violent of all is potassium periodate. This gives an extremely violent almost explosion-like whoop. A video demonstrates the different reactions and clearly shows the differences in the violence of the reactions. Download size of the video is almost 6 MByte.

The test tubes with potassium chlorate and potassium perchlorate were heated after the above experiment. The mix with potassium chlorate reacts vigorously, giving a colorless gas, when it is near boiling and the reaction is self-sustained, no further heating is necessary. The mix with perchlorate cannot be brought to reaction, not even by boiling the liquid.

 

 

Discussion of results

In this experiment, hydrazine is oxidized mainly to nitrogen. Hydrazine is a strong reductor and reacts violently with many oxidizers, even at room temperature and in dilute solutions. This makes this compound suitable for a comparison between different oxidizers.

This experiment does not reveal a single pattern for the reactivity of the oxohalogenate ions. For chlorine, the reactivity decreases for increasing n. Hypochlorite reacts at once, chlorite needs a short induction time, chlorate needs heating to near boiling and perchlorate does not react at all.

For fixed n, the reactivity seems to increase when going through Cl, Br, I. Bromate is much more violent than chlorate, iodate again is more violent than bromate, although the difference is not that large. Periodate is the most violent of all, unfortunately no test could be done with perbromate.

 

There are two different properties for the oxohalogenate ions

  • thermodynamic oxidative strength
  • kinetic ease of reaction.

A high thermodynamic oxidative strength does not mean that they are fast acting oxidizers. Kinetically, the iodine-based ions seem to be the fastest, easiest oxidizing, while thermodynamically, the bromine-based ions are the strongest oxidizer.

This experiment is working in aqueous solution. In solid pyrotechnic mixes at temperatures of 1000 ēC or so, the situation is totally different. Iodates then only are moderate oxidizers, not capable of making powerful compositions with most reductors, while perchlorates, chlorates and bromates are capable of making very powerful compositions.
   

 

   

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