# Is there any kind of reaction with two types of reactants (gases) and one product (also gas) such that the total volume after the reaction increases?

I would like to think of the following equation:

$$\ce{aA(g) + bB(g) -> cC(g)}$$

where $$a + b < c$$ and $$\ce{A},$$ $$\ce{B}$$ and $$\ce{C}$$ are different gases.

Is there such chemical reaction?

• I think extractions and ire synthesized isobutane from 2 separate gases but he couldn’t correctly synthesize it. You might want to check his channel out on YouTube if your interested. He might of made it correctly because he made multiple videos on it and I didn’t see all of the videos. Feb 6 '20 at 2:21
• There are plenty of reactions like this, especially if you go beyond STP, so it would be nice if you also specify the conditions. Feb 6 '20 at 5:08
• Burning of nearly any hydrocarbon would do, provided that it returns water in vapor form (which of course it does). Feb 6 '20 at 5:12
• @IvanNeretin Burning hydrocarbons won't satisfy one of his conditions, which is that the reaction produces a single product. Feb 6 '20 at 5:19
• @theorist Oops, I missed that one. Well, then all of a sudden there are not that many reactions, and those which are still there look highly unnatural and contrived, like O3+C3O2. Feb 6 '20 at 6:12

This is a fun question. The essence of what makes it challenging is that you're doing a synthesis -- combining two different species into one -- yet ending up with more particles, not (as is typical) less. This limits the number of possible reactions significantly. Here, however, is one that meets the conditions:

Octasulfur ($$\ce {S_8}$$) boils at 444.6 $$^\circ \text{C}$$ at standard pressure. So, above that temperature, one could have:

$$\ce{ S_{8(g)} + 4O_{2(g)}-> 8SO_{(g)}},$$

where $$1+4 < 8$$.

Of course, if octasulfur were combusted with oxygen, it's likely that many other species of sulfur oxides (i.e., compounds of the form $$\ce {S_xO_y}$$) would be produced as well.

• @Theorist : In the vapor phase, sulfur is not made of S8 molecules, but S2. Feb 6 '20 at 15:02
• @Maurice Do you have a reference to support that? According to aip.scitation.org/doi/abs/10.1063/1.1734241 [J. Berkowitz, Equilibrium Composition of Sulfur Vapor, J. Chem. Phys. 39, 275 (1963)]: "Sulfur vapor, as well as...have been studied mass‐spectrometrically. The results indicate the presence of all possible S$_n$ molecules between S$_2$ and S$_8$ in measurable concentration between room temperature and the boiling point of sulfur." Given this, one should also find S$_8$ just above the bp. And, of course, the proposed rxn could also take place below the bp. Feb 7 '20 at 1:57
• @Maurice This is not true, see doi.org/10.1007/b12405. Reading the abstract should be enough to find out that S₈ is one of the dominating forms in gaseous phase at the temperatures up to 1000 °C. Later, in the chapter it is mentioned that "the ratio of the partial pressures calculated by Detry et al. for saturated sulfur vapor at a temperature of 300 °C is as follows: S₈:S₇:S₆:S₅:S₄:S₃:S₂ = 1:0.35:0.44:0.026:0.001:0.001:0.007." Also, see this table for 500 °C. The theorist's answer looks perfectly fine to me. Feb 7 '20 at 3:54

Decomposition of ozone:

$$\ce{O3 + O3 -> 3 O2}$$

Technically, not 3 different gases, but the introduction of isotopes may be judged as in compliant (and yes, contrived).