The Self-Made Tapestry Phần 10 ppt

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Appendix 2

Oscillating Chemical Reactions

There is a variety of reliable oscillatory chemical reactions described in the chemistry literature,

including many accessible recipes in books intended for teaching or for a general readership. One of the

most striking in terms of the colour change is the iodate/iodine/peroxide oscillator . The recipe that I

have tested for myself is as follows:

Solution A: 200 ml of potassium iodate (KIO3) solutionmade by adding 42.8 g KIO3 and 80 ml of

2M sulphuric acid to distilled water to make a total volume of 1 litre.

Solution B: 200 ml of malonic acid/manganese sulphate (MnSO4) solutionmade by adding 15.6 g

malonic acid and 4.45 g MnSO4 to distilled water to a total of 1 litre.

Solution C: 40 ml of 1% starch solutionmade by adding a slurry of 'soluble' starch to boiling water.

Solution D: 200 ml of 100 vol. (about 30%) hydrogen peroxide (H2O2) solution.

Mix solutions A, B and C together in a conical flask and then initiate the reaction by adding solution D.

Mix well using a magnetic stirrer. After a minute or two the solution, which is initially blue (owing to

the formation of iodine, which reacts with starch to form a blue compound), turns a pale yellow (as the

iodine intermediate disappears), and then abruptly blue again to begin another cycle. The colour

changes persist for about 15–20 min, but finally run out of steam because some of the initial reagents

are consumed in each cycle and not replenished.

After a few minutes the mixture begins to bubble, as carbon dioxide gas is generated from the oxidation

of malonic acid.

If the mixture is not stirred, the colour changes still take place but grow from filamentary patches

throughout the solution.

It is important that the malonic acid solution is not prepared too far in advanceit begins to decompose

over the course of several weeks.

The most famous oscillatory reaction is the Belousov-Zhabotinsky reaction, for which various recipes

are available in the literature. Here's one that I have seen work:

Solution A: 400 ml of 0.5M malonic acid (52.1 g malonic acid in a litre of water).

Solution B: 200 ml of 0.01M cerium(IV) sulphate (Ce(SO4)2) in 6M sulphuric acid.

Solution C: 0.25M potassium bromate (KBrO3) (41.8 g KBrO3 in 1 litre of water).

Mix solutions A and B in a magnetically stirred conical flask, and then add solution C to initiate the

reaction. After about 3 min, the solution starts to alternate between colourless and yellow. The

oscillations last for 10–15 min.

This is the colour change that Belousov first saw; but it can be made more dramatic by adding 1 ml of

an indicator called ferroin (iron tris(phenanthroline)), which makes the solution change between blue

and a purplish red. The chemistry behind these oscillations is described in Chapter 3.

I have taken these recipes from the chemical demonstrations leaflet of the chemistry department of

University College, London, and am extremely grateful to Graeme Hogarth and Andrea Sella for help in

performing these experiments and those in the following two appendices.

There are many other oscillating reactions, and variants of these two recipes, to be found in:

B.Z. Shakhashiri (1985). Chemical Demonstrations: A Handbook for Teachers of Chemistry. University

of Wisconsin Press, Madison.

H.W. Roesky & K. Möckel (1996). Chemical Curiosities. VCH, Weinheim.

L.A. Ford (1993). Chemical Magic. Dover, New York.

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Appendix 3

Chemical Waves in the BZ Reaction

The target patterns of the inhomogeneous Belousov-Zhabotinsky (BZ) reaction always looked to me so

extraordinary that I found it hard to believe they would be easy to reproduce. I was thrilled to find,

when I tried the reaction first-hand, that this was not the case. This is a recipe that seems very reliable:

Solution A: 2 ml sulphuric acid + 5 g sodium bromate (NaBrO3) in 67 ml water.

Solution B: 1 g sodium bromide (NaBr) in 10 ml water.

Solution C: 1 g malonic acid in 10 ml water.

Solution D: 1 ml of ferroin (25 mM phenanthroline ferrous sulphate).

Solution E: 1 g Triton X-100 (a kind of soap) in 1 litre of water.

Put 6 ml of solution A into a Petri disk about 3 inches in diameter, add 1–2 ml of solution B and 1 ml of

solution C. The solution turns a brownish colour as bromine is produced. Make sure you do not inhale

deeply over the dishbromine is noxious! After a minute or so the brown colour will disappear. Once the

solution has become clear, add 1 ml of solution D (which will turn the liquid red) and a drop of solution

E. Swirl the Petri dish gently to mix the solutions (it will turn blue as you do so, but then quickly back

to red), then leave to stand. Gradually, blue spots will appear in the quiescent red liquid, and these will

slowly expand as circular wave fronts. New wave fronts will be initiated behind the expanding waves.

Typically there will be one to a dozen or so separate target-wave centres, and the blue fronts annihilate

one another as they collide.

This reaction is most impressively seen when the dish is placed on an overhead projector (see above).

The heat of the projector will warm the solution and accelerate the wave fronts somewhat. After some

time, bubbles (of carbon dioxide) will start to appear. These can begin to obscure or disrupt the pattern,

but you can get rid of them and restart the process by swirling the solution around a little.

This recipe is taken from the chemical demonstrations leaflet of the chemistry department of University

College, London.

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Appendix 4

Liesegang Bands

This is a wonderful experiment, but takes several days. The bands are zones of precipitation of an

insoluble compound, which occur at intervals down a column filled with a gel, through which one of the

reagents of the precipitation reaction diffuses from above.

You can use a burette as the column (about 1-cm diameter), although ideally a glass tube without

gradation markings is best. The recipe I have used involves the reaction between cobalt chloride and

ammonium hydroxide, which precipitates bluish bands of cobalt hydroxide. The cobalt chloride is

dispersed in a gelatin gel:mix 1.5 g of fine-grained gelatin and 1 g of hydrous cobalt chloride

(CoCl2.6H2O) with 25 ml of distilled water and heat to boiling point for five minutes. Then transfer this

mixture immediately to the glass column, cover the top of the column with plastic film, and allow to

stand for 24h to set at room temperature (22°C).

Then add 1.5 ml of concentrated ammonia solution to the top of the solidified gel using a pipette. Cover

the tube again and leave it to stand.

After several days, the bands begin to appear down the column. They are closely spacedabout a

millimetre apart, although the spacing is not constant (see p. 62). You have to get on eye level with the

bands to see them clearly, but they should be sharp and well defined (see figure).

This recipe is taken from:

R. Sultan and S. Sadek (1996). Patterning trends and chaotic behaviour in Co2+/NH4OH Liesegang

systems. Journal of Physical Chemistry 100, 16912.

References to other systems are given in Henisch (1988) (see Bibliography: Waves).