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I'm preparing a chemistry show where I end the show with yellow powder bang.

The powder is a mixture of:

$$\ce{K2CO3 + KNO3 + S} $$ My question is, what's the reaction between these chemicals?

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    $\begingroup$ Not sure about this, but the nitrate might oxidise the sulphur to sulphur dioxide or trioxide. $\endgroup$ Commented Mar 18, 2015 at 13:28
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    $\begingroup$ It seems that nobody really knows about the role of the carbonate, but there's a nice video in the Free Range Chemistry series of the RSC. $\endgroup$ Commented Mar 18, 2015 at 16:09
  • $\begingroup$ It looks like a variation on a gunpowder mixture which is usually sulfur, KNO3 and charcoal $\endgroup$
    – bon
    Commented Mar 18, 2015 at 18:34
  • $\begingroup$ Yellow powder is said to be unpredictable, so it is probably better to use a different bang. $\endgroup$
    – permeakra
    Commented Mar 19, 2015 at 12:19

2 Answers 2

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Here's just a guess, but I do have some confidence that I'm right:
$\ce{KNO3 + S -> K2SO4 + K2SO3 + K2S + K2O + SO2 + N2}$

$\ce{K2CO3 ->[\Delta] K2O + CO2}$
The first, unbalanced equation is the reaction between potassium nitrate and sulfur, producing quite some gas as product, and a variety of potassium compounds left behind, with the composition depending on your formulation of the powder. Of course there might be some sulfur trioxide produced, but the major oxide of sulfur should be the dioxide. In the second equation is the thermal decomposition of potassium carbonate releasing carbon dioxide. This is the part that I'm not very sure of, but it may be to release more gas to give your reaction more puff. However, IIRC the decomposition requires a very high temperature. In contrast, bicarbonate has a much lower decomposition temperature and also release more gas(carbon dioxide and water), but the products would likely be absorbed by potassium oxide to form hydroxide and carbonate, so bicarbonate might not help much.

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Most likely, a mixture of potassium polysulfides is formed first. Possibly, a further more complex reaction takes place between the nitrate ion, carbonate ion and sulfide that forms small amounts of tetrasulfur tetranitride, disulfur dinitride and sulfites, evolving nitrogen and sulfur dioxide. The remaining solids are clearly (just look at their possible structure) more reactive than simple polysulfides and ought to be responsible for the final high speed decomposition of the mixture. On a side note, the less costly way to determine if sulfur nitrides are actually present would be to pass the sample under a Raman spectrometer.

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