How would $\ce{H2SO4}$ react to heat? I assume that it would form toxic sulfur (di/tri)oxide, but I can't find any information on this.

I'm concerned about fire hazards specifically, so assume very intense heating and very low concentration.

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    $\begingroup$ "very low concentration"? Then that's slightly acidic water and there's not much difference what acid is there. $\endgroup$ – Mithoron Oct 12 '16 at 15:48

Sulfuric acid, $\ce{H2SO4}$, having an enthalpy of formation of $-814\ \mathrm{kJ/mol}$, is quite stable and won't decompose easily.

According to A Kinetic Study of the Decomposition of Spent Sulfuric Acids at High Temperature, Dominique Schwartz, Roger Gadiou, Jean-François Brilhac, Gilles Prado, and Ginès Martinez:

The decomposition of $\ce{H2SO4}$ to $\ce{H2O}$ and $\ce{SO3}$ is predominant between $400$ and $700\ \mathrm K$. The formation of a small amount of gaseous sulfuric acid can be observed. Above $673\ \mathrm K$, the equilibrium constant of the reaction R1 becomes higher than 1 and increases rapidly.

$$\ce{H2SO4 <=> H2O + SO3}\tag{R1}$$

The second process is the reduction of sulfur trioxide to $\ce{SO2}$. This endothermic reaction needs a high temperature to take place, the equilibrium constant of the reaction R2 being higher than 1 above $1050\ \mathrm K$.

$$\ce{SO3 <=> SO2 + \dfrac12O2}\tag{R2}$$

So, you only have to worry when the temperature reaches $400\ \mathrm K$, or $127\ \mathrm{^\circ C}$ (false precision).

  • $\begingroup$ Thanks ! According to this page : reference.com/science/hot-wood-fire-902305ee9dfd05a4 , a simple wood fire can go to 900 °F, which amounts to around 500 °C, which is higher than the temperature in your conclusion. Also, I thought SO3 and SO2 are very toxic. Does all that mean that a fire occurring near sulfuric acid could indeed become dangerous to people nearby (aside from the obvious fire burns) ? $\endgroup$ – Hey Oct 11 '16 at 14:32
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    $\begingroup$ @YdobEmos Usually available sulfuric acid is dissolved in water (except sulfuric acid in which case there is little water), so the temperature would be limited by the water. $\endgroup$ – DHMO Oct 11 '16 at 14:41
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    $\begingroup$ In aqueous solution, you also have to worry about the water boiling and spitting acid at you. A speck of hot sulphuric acid on a mucus membrane is not a pleasant experience. $\endgroup$ – OrangeDog Oct 11 '16 at 17:36

First, you don't need to worry about fire: sulphuric acid is non-flammable.

Around $1000$ kelvin, this reaction would occur:

$$\ce{2H2SO4 <=> 2SO2 + 2H2O + O2}$$

This is industrially used for the production of hydrogen gas, and the total reaction cycle is called the sulphur-iodine cycle.


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