Lab Summary

This project simulates one environmental measure used by coal-burning industries to reduce the presence of acid rain in our atmosphere, while at the same time incorporates basic concepts in chemistry appropriate for a first year laboratory. Such concepts include pH, acid-base reactions, relative acidities, neutralization reactions, and solubility.

A pollutant abatement process used by coal-burning industries known as "scrubbing" involves decreasing the amount of sulfur dioxide gas emitted into the atmosphere by forcing the coal combustion gases to interact with a base such as calcium carbonate. In this way, the sulfur dioxide produced from the combustion of coal is converted into calcium sulfite:

SO₂(g) + CaCO₃(s) CaSO₃(s) + CO₂(g)

To simulate this process in a laboratory setting appropriate for the first-year laboratory, the student first observes the increase in acidity gaseous SO₂ brings about in rainwater by burning a small amount of sulfur and then bubbling the SO₂ gas produced through distilled water. The student then performs additional experiments in which the SO₂(g) is allowed to interact with four different reagents before it is bubbled through the distilled water. The student tests the pH of the water before and after each reaction in order to observe the effectiveness of each reagent at neutralizing the SO₂ gas and then makes a conclusion about which have potential to be good SO₂ scrubbers.

Hazards:

The hazards associated with this project arise mainly from the corrosive nature of potassium hydroxide used as one of the four scrubbing reagents. Inhalation of dust from the sulfur, calcium carbonate, and calcium sulfate could cause irritation of the eyes and upper respiratory tract, so it is strongly advised that all work be done in a fume hood. Contact of all chemicals with eyes and other mucous membranes should be avoided. Proper eye protection should be worn at all times.

Part One. The increase in rainwater acidity brought about by SO₂(g)

• Two side-arm test tubes are connected to one another using rubber stoppers with bored holes, glass tubing, and Tygon tubing. A small amount of sulfur is placed in one side-arm test tube and a small volume of distilled water is placed in the second, which is connected to an aspirator.

  The sulfur is burned using direct heat from a Bunsen burner flame. The partial vacuum produced by the aspirator pulls air over the sulfur and forces the gas produced into the second test tube, where it is bubbled through distilled water. Using pH paper, the student tests the pH of the water both before and after the reaction to observe the increase in acidity of the water brought about by the reaction between SO₂(g) and H₂O(l) to produce sulfurous / sulfuric acid. The instructor may wish to provide a short discussion on how this reaction occurs and why an increase in acidity in the distilled water is observed.

Part Two. Exploration of four reagents as potential SO₂ scrubbers.

• SO₂ gas is allowed to interact with calcium carbonate (CaCO₃), calcium sulfate (CaSO₄), sodium carbonate (Na₂CO₃), and potassium hydroxide (KOH) before it is bubbled through the distilled water.

  The student determines which reagents act effectively as SO2 scrubbers and will explain their observations in terms of the specific reactions that occur.

  Typical Student Results: Scrubbing ability of four different reagents

  Reagent	pH of distilled water before reaction	pH of distilled water after reaction	Conclusion on scrubbing ability
  CaCO3(s) (ground calcite)	5	1-2	Ineffective
  CaCO3 (aqueous slurry)	5	3-4	Ineffective
  CaCO3 (aqueous slurry with stirring)	5	5	Effective
  CaSO4 (s)	5	1-2	Ineffective
  CaSO4 (aqueous slurry)	5	2-3	Ineffective
  Na2CO3 (aq)	5	5	Effective
  KOH (s)	5	5	Effective
  KOH (aq)	5	5	Effective

  
  
  • Interaction of SO₂(g) with calcium carbonate (CaCO₃)

    • A small amount of sulfur is placed in the first test tube, and the third test tube is filled with a small volume of distilled water and connected to an aspirator. Calcium carbonate is placed in the middle test tube in one of three forms:

      1. ground calcite (particle size no smaller than 1 mm)
      2. an aqueous calcium carbonate slurry
      3. an aqueous calcium carbonate slurry with constant stirring

      The student tests the pH of the distilled water in the third test tube before and after each reaction in order to observe the effectiveness of CaCO₃ as a scrubber and to observe how the reaction is affected by the type of CaCO₃ used. The instructor should provide a provide a brief discussion on the factors affecting heterogeneous reactions, including why little or no reaction between the SO₂(g) and the ground calcite would be observed. Armed also with a knowledge of solubilities, the student should be able to explain why constant stirring of the aqueous CaCO₃ slurry is beneficial in the reaction between the SO₂(g) and the calcium carbonate in the aqueous slurry. The instructor should assist the student in writing an equation for the specific neutralization reaction that occurs between CaCO₃ and SO₂ and why the calcium carbonate can act effectively as a scrubber.

  • Interaction of SO₂(g) with calcium sulfate (CaSO₄)

    • Using the same procedure as outlined above, the student observes the scrubbing ability of calcium sulfate in two different forms:

      1. Drierite
      2. an aqueous calcium carbonate slurry

      The instructor should assist the student in understanding why a neutralization reaction between SO₂(g) and the calcium sulfate reagent does not occur.

  • Interaction of SO₂(g) with sodium carbonate (Na₂CO₃)

    • The scrubbing ability of sodium carbonate is observed by placing a small volume of a 1M solution of Na₂CO₃ in the middle test tube and running the reaction as carried out with the two previous reagents. The student should be assisted by the instructor in writing and understanding the neutralization reaction for this interaction.

  • Interaction of SO₂(g) with potassium hydroxide (KOH)

    • The scrubbing ability of potassium hydroxide is observed by using KOH in two different forms:

      1. KOH pellets
      2. 1M KOH solution

      The same procedure used for testing the other three reagents is once again employed. The student should be assisted by the instructor in writing and understanding the neutralization reaction for this interaction, which is given below:

      SO₂(g) + 2KOH(aq) → K₂SO₃(aq) + H₂O

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