A simple oscillating reaction

In this experiment, students witness a clear example of an oscillating reaction, as bromate ions oxidise malonic acid to carbon dioxide in the presence of manganese(II) ions as a catalyst. The reaction mixture oscillates in colour between red-brown and colourless, with a period of about 20 seconds.

Oscillating reactions can be both dramatic and are worth including at an open day to stimulate an interest in chemistry. This particular demonstration is fairly straightforward to set up, works reliably, and is based on the Belousov–Zhabotinsky reaction.

The demonstration itself takes about ten minutes, but needs rather more time to set up.

Equipment

Apparatus

• Eye protection (goggles)
• Disposable gloves (preferably nitrile) (optional)
• Beaker, 1 dm³
• Magnetic stirrer (optional – see note 7 below)
• Weighing boats or watch-glasses, x3
• Balance, reading to 0.1 g

Chemicals

• Concentrated sulfuric(VI) acid (CORROSIVE), 75 cm³
• Propane-1,3-dioic (malonic) acid (HARMFUL), 9 g
• Potassium bromate(V) (TOXIC, OXIDISING), 8 g
• Manganese(II) sulfate-1-water (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 1.8 g
• Deionised or distilled water, 750 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) throughout, and consider using disposable gloves.
• Concentrated sulfuric(VI) acid, H₂SO₄(l), (CORROSIVE) – see CLEAPSS Hazcard HC098a.
• Propane-1,3-dioic (malonic) acid, HOOCCH₂COOH(s), (HARMFUL) – see CLEAPSS Hazcard HC036B. 
• Potassium bromate(V), KBrO₃(s), (TOXIC and OXIDISING) – see CLEAPSS Hazcard HC080. 
• Manganese(II) sulfate-1-water, MnSO₄.H₂O (HARMFUL) – see CLEAPSS Hazcard HC060. 
• The use of a magnetic stirrer is optional but highly recommended since the use of a glass stirring rod will detract from the colour changes occurring during the demonstration. Those of a cautious disposition might like to try out the demonstration in private first, before submitting it to a public demonstration.

Procedure

Before the demonstration

1. Place 750 cm³ deionised/distilled water in the beaker.
2. Slowly, and with stirring, add 75 cm³ concentrated sulfuric acid carefully. The mixture will heat up to about 50 °C.
3. Allow the diluted acid to cool back to room temperature. This will take some time.
4. Weigh out separately 9 g of propane-1,3-dioic (malonic) acid, 8 g of potassium bromate(V) and 1.8 g of manganese(II) sulfate-1-water on weighing boats or watch-glasses.

The demonstration

1. Place the beaker of dilute sulfuric acid on a magnetic stirrer and stir the solution fast enough for a vortex to form.
2. Add the malonic acid and potassium bromate(V).
3. When these have dissolved, add the manganese(II) sulfate and observe what happens.
4. A red colour should develop immediately. This will disappear after about one minute.
5. Thereafter the colour will oscillate from red to colourless with a time period of about 20 seconds for a complete oscillation. This will continue with a gradually increasing time period for over ten minutes – long enough for most audiences to lose interest.

Teaching notes

A white background helps to make the colour changes more vivid.

A member of the audience with a stopwatch could be asked to time the oscillation. The time period of 20 seconds mentioned above refers to an ambient temperature of about 20 °C. If the temperature is higher than this then the time period drops accordingly.

The reaction will NOT work if tap water is used instead of deionised water. Chloride ions, via the addition of a pinch of potassium chloride or dilute hydrochloric acid, will immediately stop the oscillations. The use of clean apparatus is therefore essential.

The reaction mixture can be washed down the sink with plenty of tap water after the demonstration.

The theory of oscillating reactions is complex and not fully understood. However, this particular process is an example of a class of processes known as Belousov-Zhabotinsky (BZ) reactions. The overall reaction is usually given as:

3CH₂(COOH)₂(aq) + 4BrO₃^–(aq) → 4Br^–(aq) + 9CO₂(g) + 6H₂O(l)

The oxidation of the malonic acid by the bromate(V) ions is catalysed by manganese(II) ions, and manganese(III) ions are produced as intermediates.

Some references claim that the red colour is due to molecular bromine which could be produced via the following two steps:

Br^–(aq) + BrO₃^–(aq) + 2H⁺(aq) → HBrO₂ + HBrO(aq)

Br^–(aq) + HBrO(aq) + H⁺(aq) → Br₂(aq) + H₂O(l)

However, other detailed studies of the processes occurring give a variety of colourless bromate ions and bromic acid molecules as intermediates, rather than bromine itself, so it is therefore possible that the red colour is due to something else, maybe the transient existence of Mn³⁺ ions which are known to be red/purple in colour.

The colour oscillation is brought about by two autocatalytic steps, which are highly complex in nature and have been the cause of several advanced research projects over the past 30 years or so. Some web references are given below.

For the needs of the likely target audience, an analogy with predator-prey relationships might be one way to give students some idea of what is going on. For example, a population of rabbits (analogous to the red manganese(III) ions) will increase rapidly (exponentially) if there is plenty of food (reactants). However, the plentiful supply of rabbits will stimulate a rapid increase in the fox population (another intermediate that reacts with the manganese(III) ions) which will then deplete the rabbits. Lacking rabbits, the foxes will die, bringing us back to square one, ready for a rapid increase in rabbits and so on.

Further information

• For detailed information about the reaction, refer to Chemistry and Mathematics of the Belousov–Zhabotinsky Reaction in a School Laboratory.
• A resource from the Lycée Faidherbe de Lille provides more information about oscillating reactions and chemical waves.