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 '16 at 15:48

Sulfuric acid, $\ce{H2SO4}$, having an enthalpy of formation of $-814$ 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 $700$ K. The formation of a small amount of gaseous sulfuric acid can be observed. Above $673$ 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 that 1 above 1050 K.

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

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

| improve this answer | |
  • $\begingroup$ 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
  • 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 '16 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 '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.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.