The burning of coal releases significant quantities of sulfur dioxide (SO2), a n

Question

The burning of coal releases significant quantities of sulfur dioxide (SO2), a nasty-smelling, gaseous, environmental pollutant: an acidic anhydride that causes acid deposition. One method of reducing sulfur dioxide emissions from coal-burning power plants involves "scrubbing", passing the flue gases (byproducts of burning) through a wet slurry of calcium carbonate (limestone). The sulfur dioxide gas interacts with water to make aqueous sulfurous acid (H2SO3). The acidic solution reacts with the powdered limestone (CaCO3) to sequester sulfur dioxide as the solid calcium sulfite (CaSO3). Ultimately, the calcium sulfite gets converted to calcium sulfate (CaSO4) through further oxidation.

SO2 (g) + H2O (l) = H2SO3 (aq)

The sulfurous acid interacts with the limestone to produce water, carbon dioxide, and calcium sulfite.

CaCO3 (s) + H2SO3 (aq) = CaSO3 (s) + CO2 (g) + H2O (l)

a) Write the dissociation of limestone and list its Ksp value.

b) Write the two dissociation steps for sulfurous acid and list the corresponding Ka values.

c) The two liberated protons from sulfurous acid react with the carbonate ion in solution to produce water and carbon dioxide allowing the calcium ion to precipitate with the sulfite ion.

Ca2+ (aq) + SO32- (aq) = CaSO3 (s)

Show the two-step process that converts the carbonate ion into carbon dioxide and water. (Remember to invert the K values when you reverse the individual steps to get new K values.) List the new values.

d) A Ksp value for calcium sulfite found on the internet is 6.8E-8. Write the "association" reaction of calcium ion precipitating with sulfite ion and list the new K value.

e) With these six equations, show how they add up to give the overall reaction. (List all of the reactants and all of the products, then cancel like terms from either side of the equation by striking through like terms.)

CaCO3 (s) + H2SO3 (aq) = CaSO3 (s) + H2O (l) + CO2 (g)

f) With the six constants listed what is the equilibrium constant for the overall process?

g) Show by calculation that the transposition reaction between calcium carbonate and sulfite is not favored relative to that of calcium sulfite and carbonate ion.

i) CaCO3 (s) + SO32- (aq) = CaSO3 (s) + CO32- (aq)

K = ?

ii) CaSO3 (s) CO32- (aq) = CaCO3 (s) SO32- (aq)

K = ?

Obviously acidifying the carbonate ion and eliminating from the very hot solution through the expulsion of carbon dioxide allows this reaction to precede to precipitation.

Explanation / Answer

Acid rain is often associated with the burning of large quantities of poor quality coal in huge power plants. Because there is a prevailing wind that generally blows in the same direction (southwest toward the northeast) ponds and lakes down wind of the power plants may collect the acid rain and natural limestone in the lake bottoms may have limited capacity to neutralize the acids. The lakes can then become too acidic to support the native plants, fish and animals that inhabit their waters. Poor quality coal contains higher levels of sulfur that may be exhausted to the atmosphere where reactions with moisture convert it to sulfuric acid (H2SO4). Modern power plants are cleaner today because they use higher grades of coal or use special processes to reduce the sulfur content of coal or their exhaust stream.

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