INChO 2016 problem 1.8

Each Problem in this question paper is like a comprehension, whose each subpart picks up where the previous left. This makes reading the previous subparts nearly mandatory for solving Problem 1.8, in order to gain proper context, sorry for that! I've copied the most relevant parts here for this subpart though

Problem 1 in question paper, Answer key

Subpart 8 of Problem 1:

Water vapour in this air condenses on the tank walls. H2S in the air then slowly dissolves in these droplets. Since oxygen is available in immediate vicinity of the droplets (because of high surface area to volume ratio as compared to sewage below), sulphur oxidizing bacteria (SOB) start accumulating in these droplets. In this new aqueous system, SOB oxidize sulphide species and produce sulphuric acid. The cement part in concrete walls can be represented as $\ce{(CaO)3(Al2O3)}$, which is attacked by sulphuric acid. Write the balanced reaction(s) responsible for sulphuric acid production in the water droplets and its possible reaction/s with the cement in concrete.

My attempt:

For the formation of sulphuric acid, I thought this:


which seems a good enough redox reaction for me. (Sulphide oxidised to sulphate, whereas the oxygen got reduced).

This acid then formed should react with both $\ce{CaO}$ and $\ce{Al2O3}$ in the cement, because the former's a good base, while the latter's weakly basic and amphoteric anyway. Thus the second reaction should be:


My query:

Neither of my answer's matches with the given answer, which is:

$$\ce{HS-(aq) + 2O2 + H2O -> SO4^{-2} (aq) + H3O+}$$


$$\ce{CaO(s) + H2SO4(aq) -> CaSO4(s) + H2O}$$

I hope I've been clear enough, please help me understand where I went wrong. Thank you!

  • $\begingroup$ $\ce{H2S}$ on dissolving in water would give the bisulphide ion and hydronium ion. This is what makes aq. $\ce{H2S}$ slightly acidic. So, you just need to balance $\ce{HS^-\to SO_4^{2-}}$. Again, the sulphate ion as we are dealing with a dilute aqueous medium. $\endgroup$ – Eashaan Godbole Jan 2 '18 at 13:37
  • $\begingroup$ As for the further reactions with the concrete, I suppose the $\ce{CaO}$ would react "faster" as it is basic compared to the amphoteric $\ce{Al2O3}$. The $\ce{H2SO4}$ concentrations due to the sulphur-oxidising bacteria would be quite low. Maybe the sulphuric acid is not enough to quench the $\ce{CaO}$. $\endgroup$ – Eashaan Godbole Jan 2 '18 at 13:44
  • $\begingroup$ @EashaanGodbole Yes, that could be a possibility. $\endgroup$ – Gaurang Tandon Jan 2 '18 at 14:10

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