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.

  • 1
    $\begingroup$ "very low concentration"? Then that's slightly acidic water and there's not much difference what acid is there. $\endgroup$
    – Mithoron
    Oct 12, 2016 at 15:48

2 Answers 2


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).

  • 1
    $\begingroup$ @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, 2016 at 14:41
  • 3
    $\begingroup$ 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, 2016 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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.