Colored gases - chlorine and chlorine dioxide

In this experiment two colored gases are made. This experiment nicely demonstrates the difference in intensity of the color of the two gases, while the colors themselves are fairly close to each other.

This experiment involves the formation of a very unstable gas. When the experiment is done carelessly, this may result in an explosion. Carefully follow the instructions below and do not scale up and do not attempt to concentrate the gas in a confined space.

Required chemicals:

• calcium hypochlorite, or bleach with 10% active chlorine, the cheapest stuff there is, without additives

• sodium chlorate

• concentrated hydrochloric acid, approximately 30% by weight

• dilute hydrochloric acid, approximately 10% by weight

Required equipment:

• test tubes

• clamps
• rubber stopper

Safety:

• bleach is corrosive and caustic;
• hydrochloric acid is corrosive, the concentrated acid gives off fumes, which are very noxious;
• the gases produced are very noxious and this experiment must be done outside or in a very well ventilated area, standard kitchen exhaust is not sufficient;
• chlorine dioxide is a very unstable compound when pure or when at high pressure. Do not attempt to collect large amounts of this gas in a confined space. The gas is notoriously explosive and can explode, apparently without any reason, when its partial pressure exceeds approximately 1 atm.
• sodium chlorate is a strong oxidizer;
• calcium hypochlorite is a strong oxidizer.

Disposal:

• The waste can be flushed down the drain with a lot of water

Procedure for performing the experiment

Put a spatula full of calcium hypochlorite or approximately 2 ml of bleach in a test tube and add two to three ml of dilute hydrochloric acid. Do this slowly and carefully, as the reaction can be quite vigorous with liquid foaming out of the test tube. Let reaction proceed and when bubbling ceases, loosely stopper the test tube with a rubber stopper. A light green gas can be observed.

Put a spatula full of sodium chlorate in a test tube and add a few ml of concentrated hydrochloric acid. The liquid will start bubbling and foaming. The reaction, however is less vigorous than with the calcium hypochlorite. An intensely colored yellow/green gas mixture can be observed. The liquid becomes deep yellow. Do not stopper this test tube, as this might result in a too high partial pressure of chlorine dioxide.

The green gas is chlorine, Cl₂, and the deep yellow/green gas is impure ClO₂. Both gases are toxic and corrosive and one should be careful not to breathe these gases. Luckily, the warning level of these gases is good. Even breathing a tiny amount of these gases already is experienced as a very unpleasant event.

The following picture shows both test tubes. The left one contains fairly pure chlorine gas. Only a faint green color can be observed, because of the fact that only a thin layer of gas is viewed. The right one contains a mixture of approximately 50% chlorine and 50% chlorine dioxide. It is remarkable how intense the yellow color is for even this thin layer of gas.

The following picture shows a close up of the right test tube with the evolution of the chlorine/chlorine dioxide mixture. The solid at the bottom is the sodium chlorate added, partially converted to sodium chloride, which is almost insoluble in concentrated hydrochloric acid. The liquid is deep yellow as well. This is due to dissolved chlorine dioxide.

Discussion of results

Ordinary household bleach contains hypochlorite ions, ClO^–, together with chloride, Cl^–, and hydroxide, OH^–. It is produced by electrolysis and constantly mixing of brine or by passing chlorine through a solution of sodium hydroxide. When this is added to hydrochloric acid, then the following net reaction occurs (the actual mechanism behind the reaction is quite complex):

  ClO^– + Cl^– + 2H⁺ → Cl₂(g) + H₂O

With solid calcium hypochlorite a similar reaction occurs.

The reaction of chlorate with concentrated hydrochloric acid is very complex. Two possible idealized 'extremes' can be given for the reaction equation. The net real reaction will be a linear combination of these two extremes, resulting in a gaseous mixture of approximately 50% Cl₂ and 50% ClO₂ and a higher percentage of ClO₂ in the liquid (this gas dissolves better in water than does Cl₂).  The exact ratio cannot easily be determined and depends on the concentration of the acid and on the temperature. Pure chlorine or pure chlorine dioxide can, however, never be obtained with the reaction between a chlorate and hydrochloric acid.

The two idealized 'extremes' for the reaction equation are.

  ClO₃^– + 5Cl^– + 6H⁺ → 3Cl₂(g) + 3H₂O

  5ClO₃^– + Cl^– + 6H⁺ → 6ClO₂(g) + 3H₂O

A reasonable approximation of the net reaction which occurs in reality is:

  2ClO₃^– + 2Cl^– + 4H⁺ → Cl₂(g) + 2ClO₂(g) + 2H₂O

The exact stoichiometry of the reaction, however, can deviate significantly from the last equation, but it always will be a linear combination of the two idealized extremes.

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