I've heard that oxidising sulfur with potassium nitrate yields $\ce{SO3}$ gas, but can't find a definitive answer anywhere.

Is sulfur trioxide produced? More generally, what's the full reaction?

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    $\begingroup$ There is nothing definitive about it. Oxidizing S with KNO3 is a pretty violent reaction, and it may involve a lot of things. $\endgroup$ – Ivan Neretin Apr 19 '17 at 20:35

Generally a mixture of sulfur, potassium nitrate with charcoal or an organic carbon compound is used in gunpowder mixture.

$$\ce{3C + S + 2KNO3 → 3CO2 + N2 + K2S}$$


There are many questions in chem.SE related to gunpowder and explosives which are all based on this reaction. You can find them of help.

  1. What is the oxidation mechanism of gunpowder?
  2. What causes KNO3 to decompose into KNO2 and Oxygen in gunpowder?
  3. Why can sulfur act like a catalyst?
  4. Write the chemical equation of this reaction:

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