Choose the two correct options from A-F, why isn't it commercially suitable to dissolve $\ce{SO3}$ directly in water to give concentrated sulfuric acid instead of using the contact process? The major reaction of the contact process are

$$\ce{S + O2 -> SO2}$$ $$\ce{2 SO2 + O2 <=>2 SO3}$$ $$\ce{SO3 + H2SO4 -> H2S2O7}$$ $$\ce{H2S2O7 + H2O -> 2H2SO4}$$

  • A. To reduce the hazards of spillage
  • B. Because the density of the product is too high
  • C. To minimise transport costs of large volumes
  • D. Because the last step is too expensive.
  • E. Because the last step of the process is too exothermic.
  • F. An aerosol of the sulfuric acid rapidly fills the container.

The solution is C and F, but I think it should be A and D. Is it because to make gas react with $\ce{H2O}$, too much pressure is needed and it will be expensive? Can someone explain why each choice is false and true?


2 Answers 2


Part of the difficultly with this question is that it is asking about what is commercially viable as opposed to what is merely possible. It is, of course, quite possible to dissolve SO$_3$ in water, but doing so creates a runaway chemical reaction. So let's go through each of the points and explain why they don't make commercial sense.

(A) The contact process produces SO$_3$, as well as several other noxious gasses anyway, so they is no reduced risk as regards spillage.

(B) The density of the product is totally irrelevant, as you get the same product in both cases.

(C) Sulphur is a solid at standard temperature and pressure (STP), so one can transport a large amount using a standard container and transport vehicle. As a gas at STP, one can transport far less (many millions of moles less) SO$_3$ at STP for the same size container, so an unfeasibly large container would be needed unless a highly pressurized or very low temperature container is used. Either method is considerably more expensive, so less commercially viable.

(D) The last step of the contact process is to react oleum (H$_2$S$_2$O$_7$) with water. This is in fact relatively safe to do if done slowly and water is extremely cheap, so the process is not expensive

(E) The reaction of oleum and water is exothermic. However, in practice water is added relatively slowly to the oleum (both of which are controllable liquids), which controls the process.

(F) As said earlier, the reaction of water with SO$_3$ gas cannot be easily controlled. It runs away with itself, and the water starts to boil. This throws up lots of small droplets (an aerosol) of conc. sulfuric acid. This is highly dangerous, can increase the pressure in the reaction chamber, and also makes it difficult to cheaply extract the product from the reaction chamber.

Therefore, on economic grounds, the answer is indeed C and F.

  • $\begingroup$ $\ce{SO3}$ is not a gas at STP $\endgroup$
    – permeakra
    Dec 4, 2016 at 20:55

A is false because even if you are doing it by contact process the hazard of spillage would be the same.

B is false because the product in both processes are the same. I don't know that there is a problem with high density material.

C is true because sulfur is solid and SO3 is gas and gases can occupy a large volume when same no. of moles are taken.

D is false because if the 2nd and 3rd step have same condition requiring high pressure to force its equilibrium forward according to Le Chattelier Principle

E is false because SO3(g) + H2O(l) ---> H2SO4(aq), ......ΔH = -227.8 kJ S(s)+O2(g)-->SO2(g), ......ΔH = -296.8 kJ ( even more exothermic )

F is true as it is only other option left given that two answers have to be selected and also it is a fact which might be explainable, but I cannot explain with tools in my hand at present.


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