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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. $\endgroup$ Apr 19 '17 at 20:35
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    $\begingroup$ The gun powder was thoroughly studied by Liebig and coworkers in the beginning of the $20$th century. It was shown that, in absence of air the reaction goes as Nilay Ghosh states, producing $\ce{K2S}$ as main solid stuff. But in the presence of air, $\ce{K2S}$ is quickly oxidized. With few oxygen, it is transformed into sulfur + $\ce{K2O}$ (quickly transformed into $\ce{K2CO3}$). And if more oxygen is available, $\ce{K2S2O3}$ is produced. And with still more oxygen, the final product is $\ce{K2SO4}$ $\endgroup$
    – Maurice
    Jul 19 '21 at 20:14
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If we talk about the reaction of only sulfur and potassium nitrate, then the reaction yields potassium oxide, sulfur dioxide and nitrogen.

$$\ce{4KNO3 + 5S -> 2K2O + 5SO2 + 2N2}$$

However, when carbon is added in the form of charcoal, then the resulting mixture is called gunpowder. The overall chemical reaction of gunpowder is as follows:

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

$$\ce{10 KNO3 + 3 S + 8 C -> 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2}$$

There are plenty of questions in chem.SE related to gunpowder:

  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. How to write the balanced equation of the reaction of potassium nitrate, carbon, and sulfur?
  5. What are the reaction products of sulfur and potassium nitrate?
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  • $\begingroup$ i believe OP is trying to make sulfuric acid. this is too irrelevant $\endgroup$
    – Francis L.
    Jul 19 '21 at 21:00
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    $\begingroup$ @FrancisL. Where did you find sulfuric acid? I only see SO3 being mentioned. $\endgroup$ Jul 20 '21 at 2:26

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