Beauty of chemistry

This web page describes a few simple experiments, brought in a special way. The experiments are the following:

• Dissolving copper metal in concentrated nitric acid. This reaction is filmed at high speed (1000 fps) and when this is done, then many interesting details are revealed. The final product of the reaction also allows making beautiful pictures with remarkable color contrasts.
• Dissolve iodine pentoxide in a concentrated solution of hydrazine dihydrochloride. This extremely violent and exothermic reaction also is filmed at 1000 fps.
• Use the exothermic reaction between chlorine gas and red phosphorus to set off a flash mix. The combination of different reactions into one experiment makes this a particularly impressive demonstration, which can easily be done and can be carried out safely.
  

Nitric acid cannot be obtained anymore in the EU, but it can fairly easily be distilled from sodium nitrate or potassium nitrate and sulfuric acid in an all glass distillation apparatus. Another option is to mix sodium bisulfate monohydrate (common pH-minus for swimming pools) and sodium nitrate and distill off nitric acid from this mix. Only a small quantity is needed. I distilled a little yellow fuming nitric acid (appr. 30 ml) and diluted this for the experiment to appr. 60% by weight.

Required chemicals:

• copper metal (e.g. from flexible electrical wire, from which the plastic isolation is removed)
  
• nitric acid (to be prepared from sodium nitrate and sulfuric acid), HNO₃
  
• iodine pentoxide, I₂O₅
  
• hydrazine dihydrochloride, N₂H₄·2HCl
  
• calcium hypochlorite (swimming pool chlorinator), Ca(OCl)₂
  
• dilute hydrochloric acid
• aluminium powder
• red phosphorus
  
• potassium perchlorate (can be substituted by potassium nitrate)
  

Required equipment:

• erlenmeyer
  
• test tubes
  

Safety:

• All of these experiments produce fumes or gases, which should not be inhaled. Quantities are small, so there is not a really high risk, but the experiments should be carried out at least in a well-ventilated room. Even better is the use of a fume hood.
• Nitric acid is corrosive. Distilling the acid also must be done with care. Be absolutely sure not to get any of the hot acid on your skin or eyes. Wear appropriate protection.
• Iodine pentoxide is corrosive and also is a strong oxidizer, which gives nasty stains on skin (it is reduced to iodine by grease and cells from the skin).
  
• Hydrazine and its compounds are suspected carcinogens. Avoid exposure. In this experiment, however, the hydrazine will not make it into the air. Excess iodine is expelled and this assures total destruction of any hydrazine, which might be released into the air.
• Red phosphorus is very flammable.

Disposal:

• The waste of the dissolved copper should be neutralized with a little sodium bicarbonate, until fizzling stops and then be treated as heavy metal waste. The waste of the other experiments can be flushed down the drain with a lot of water. Any iodine-waste from the second experiment can be neutralized easily with some sodium sulfite before it is flushed away.
  

Bubbling of copper metal in concentrated (not fuming) nitric acid

This is a simple experiment. A piece of copper is added to concentrated nitric acid and the reaction is watched. It is a very well-known experiment, which also is demonstrated in many high schools, but this classic remains very interesting and when viewed at 40x slow motion, a whole new world opens up on how the reaction starts and how the bubbles move through the liquid.

Pour a few ml of concentrated nitric acid (appr. 60% by weight) in a test tube. Home-made nitric acid (distilled from sodium nitrate), usually is more concentrated. This must be diluted with some water for a smooth and fast reaction.

Heat the nitric acid somewhat. It should not be boiling hot, a temperature of 60 °C or so is fine. No need to be precise with this, a good guideline is having the test tube just a little too hot to touch it for longer than a second.

Make the copper wire into a small ball and then let it fall into the acid.

As soon as the copper falls into the acid, a violent reaction sets in. A video at real speed can be downloaded here. The video at 40x slow motion is much more impressive. Below follow three images of the formation of the bubbles, immediately after adding the copper to the acid.

In a few tens of seconds all of the copper is dissolved and a bright blue solution is obtained.

The final color contrast is really striking:

Reaction between iodine pentoxide and hydrazine

The reagents of this experiment are less common, but the experiment itself is simple again. A piece of solid iodine pentoxide is dropped into a concentrated solution of hydrazine dihydrochloride. This results in an extremely violent and exothermic reaction in which nitrogen and iodine are formed. So much heat is produced, that a large part of the iodine escapes as a plume of purple gas.

Prepare a few ml of a nearly saturated solution of hydrazine dihydrochloride. The liquid cools down a little bit when the solid is dissolved. There is no need to heat the liquid in this experiment.

Drop a piece of solid iodine pentoxide in the cold solution of hydrazine dihydrochloride.

As soon as the piece of iodine pentoxide falls in the solution, the reaction starts. Immediately, a big bubble of nitrogen is formed and even in this short time from the start of the reaction (just a few tens of ms), a clearly visible purple color of iodine vapor can be observed. This picture was made with a high speed camera at 1000 fps.

The reaction itself is quite interesting to watch, even without high speed camera. A big plume of purple iodine vapor escapes from the test tube:

A video of this reaction is available in normal time, showing the plume of iodine, and a close-up video of this reaction at 40x slow motion shows the formation of bubbles and iodine in the test tube.

Ignition of flash powder by chlorine gas

This experiment combines several reactions into an impressive experiment, which also works very well as a demonstration experiment. It is well-known that red phosphorus reacts exothermically with halogens. With chlorine the reaction is so exothermic, that the red phosphorus ignites. A mix of aluminium powder and potassium perchlorate (a flash mix) burns extremely fast and hot, with a white flash. The flash mix, however, is somewhat hard to ignite. The heat of burning red phosphorus, however, is sufficient to ignite the flash mix, if the phosphorus is sufficiently finely divided throughout the mix. So, for this experiment, a tiny amount of red phosphorus is mixed with aluminium and finely powdered potassium perchlorate. Simply immersing this mix in chlorine gas sets off the mix.

Note 1: A mix of red phosphorus and potassium perchlorate can be handled fairly safely, as long as the potassium perchlorate does not contain any chlorate impurity. For this experiment only a few tens of mg are mixed, so the risk is small anyway.

Note 2: In the EU, potassium perchlorate and sodium perchlorate cannot be obtained anymore. Potassium nitrate can be used instead, but the reaction is somewhat less spectacular. The flashes with the nitrate are less intense. For this experiment, I prepared a little potassium perchlorate from waste of other experiments. Perchloric acid can be obtained legally in the EU and I used that for other experiments on transition metal complexes and the waste of these experiments I kept and from that I made a small quantity of potassium perchlorate (this is the only perchlorate which can easily be made from that waste in a pure state, due to its low solubility in cold water). Ammonium perchlorate also can be obtained legally and that can also be used to make small quantities of potassium perchlorate. Using ammonium perchlorate instead of potassium perchlorate does not give good results. The mix with ammonium perchlorate burns much slower. If you really need to use a substitute, then use potassium nitrate. Do not use chlorate!

Prepare a little amount of flash mix, with some phosphorus mixed in as well. In order to do so, mix a spatula full of finely powdered potassium perchlorate with half a spatula full of aluminium powder and a tiny amount (10% to 20% of the amount of aluminium) of finely powdered dry red phosphorus. Carefully swirl the mix in a petri dish or an aluminium holder for a tea light. Do not grind! The precise ratio of chemicals is not critical. 

Pour 20 ml or so of dilute hydrochloric acid in an erlenmeyer.

Add a teaspoon of granular calcium hypochlorite to the acid. Adding 10 ml of concentrated bleach also works. Use of TCCA is not recommended, because of foaming. Wait till the erlenmeyer is full of chlorine gas. Avoid inhaling the gas!

Using a metal spatula, pour some of the powdered flash mix with phosphorus into the chlorine gas in the erlenmeyer.

When the finely powdered mix is immersed in the chlorine gas, then the exothermic reaction between the chlorine and the red phosphorus in the mix sets off the flash mix and this gives a really spectacular effect. Sometimes you even get mushroom clouds, like in a nuclear explosion. The following pictures were taken with intervals of 10 ms between two of them.

Another set of pictures was made, also with 10 ms intervals between images. In that series, there are many sparks, and the mix used in that experiment burns even hotter.

 
 
 
 
 
 
 
 


Two videos can be downloaded of this reaction: video1 and video2.

Discussion of results

In the first experiment, copper reacts with nitric acid, giving copper(II) ions in solution and nitrogen dioxide gas. Under the conditions in this experiment (quite warm and also quite concentrated acid), the main gaseous product, produced in the reaction between copper metal and nitric acid is the brown NO₂ gas. The reaction equation is:

    Cu  + 4 HNO₃  →  Cu²⁺  +  2 NO₃^–  +  2 NO₂  +  2 H₂O

    3 Cu  + 8 HNO₃  →  3 Cu²⁺  +  6 NO₃^–  +  2 NO  +  4 H₂O

In practice, none of the above reactions occurs exclusively, there will always be a combination of both reactions, at the start, when the acid is still concentrated, mainly the first one and at the end mainly the second one. So, this reaction between copper metal and nitric acid is not really suitable to make pure NO₂ or pure NO, one gets a mix of these gases.

In the second experiment, iodine pentoxide reacts with hydrazine. In solid hydrazine dihydrochloride, the hydrazine is protonated twice, present as an ion ⁺H₃N-NH₃⁺. In solution, this ion decomposes and when the solid salt is dissolved in water, then the compound ionizes as follows:

    N₂H₄·2HCl   →  N₂H₅⁺  +  H⁺  +  2 Cl^–

The solution becomes strongly acidic. It contains nearly no free hydrazine. Hydrazine is a strong reductor, regardless of whether it is protonated or not. Iodine pentoxide, on the other hand, is a strong and very facile oxidizer, and hence, when these two compounds are brought in contact with each other a very violent and exothermic reaction occurs:

    3 N₂H₅⁺  +  I₂O₅   →  3 N₂  +  2 I^–  + 5 H⁺  +  5 H₂O  + heat

When there is excess iodine pentoxide, then any iodide reacts with iodine pentoxide to iodine in the presence of acid. This explains the formation of all the iodine vapor. On the solid piece of iodine pentoxide, which also is covered with a lot of gaseous nitrogen, the amount of hydrazine quickly is used up and any iodide ion then is quickly oxidized to iodine:

    10 I^–  +  I₂O₅  +  10 H⁺  →  6 I₂  +  5 H₂O

Due to the heat, produced in the reaction with hydrazine, the iodine partially evaporates and escapes as purple gas. The rest remains in solution as brown iodine and some is precipitated as nearly black solid.

In the third experiment, phosphorus reacts with chlorine, producing heat, and this in turn ingites the flash mix of aluminium and potassium perchlorate:

    2 P  +  3 Cl₂   →  2 PCl₃  + a lot of heat

The heat is sufficient to ignite the flash mix:

    8 Al  +  3 KClO₄   →  4 Al₂O₃  +  3 KCl  +  much more heat

The reaction between aluminium and potassium perchlorate produces a really large amount of energy, leading to white hot particles and emission of a lot of light. The loose powder, falling freely, does not really give an explosion, but a finely powdered intimate mix of aluminium and potassium perchlorate, when in a pile on a spoon, can explode, due to self-confining, once it is ignited.
The phosphorus in the mix makes it somewhat less energetic. In the first set of images, one can see that the light has a somewhat yellowish cast. This is because a little too much phosphorus was added to the mix. In the second set of images, one can clearly see that a somewhat more carefully prepared flash mix has even brighter light.

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