SO(g) is emitted by coal-fired power plants burning high-sulfur coal. This can c

Question

SO(g) is emitted by coal-fired power plants burning high-sulfur coal. This can combine with water in the atmosphere to form the acid H_2SO_3. If the SO(g) reacts with reactive oxygen species in the air to make SO/ ), what is the acid that results from combination of this gas with water? a. Which of these acids is stronger, and why? b. Would it be easy to measure the pKa for the first deprotonation of these acids in water? Why or why not? c Suggest a way of measuring the relative acidities of these two acids in solution, even it the solution is not aqueous.

Explanation / Answer

1. When SO2(g) reacts with reactive O2 species in air, we get SO3(g) gas which upon combination with H2O gives H2SO4, sulfuric acid.

a. H2SO4 is a stronger acid as it has 4 oxygens attached to S, which makes the conjugate base of H2SO4 more stable by stabilizing the -ve charge present on the ion once it looses H+ from acid as opposed to three oxygen in H2SO3 acid.

b. Both the acids readily loose its one H+ to form conjugate base species in aqueous medium. Here it donates a H+ to water to form H3O+ and HSO3- and HSO4- species. The pKa for the first deprotonation is 100% and thus easy to calculate pKa values from initial concentration of acids present in solution.

c. Relative acidity of the two H2SO3 abd H2SO4 acids can be measured starting with a very dilute concentration of the acids in solution and measuring the dissociation percentages of each under certain fixed reaction conditions.

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