Expt 006 -- Acid Rain Investigations

Carbon dioxide, sulfur dioxide, and nitrogen oxides, all of which may be found in rain water, are generated. Their reactions with indicators are studied.

Background

• Oxides of carbon, nitrogen, and sulfur enter the atmosphere. Burning of organic material -- as in the respiration of food by humans -- adds carbon dioxide to the air. The burning of fossil fuels (coal, oil) is a major source. Dissolved carbon dioxide makes the pH of most sources of distilled water about 5.6.
• Nitrogen oxides are produced when air is burned with fuels in internal combustion engines. Lightning strikes create enormous amounts of nitrogen oxides. These oxides end up as a natural fertilizer.
• Sulfur in coal and oil forms sulfur dioxide. This oxide is further oxidized to sulfur trioxide.
• When these oxides dissolve in water, the result is production of acids. The carbon dioxide and nitrogen oxide are used by plants. The sulfuric acid created when sulfur dioxide is oxidized and dissolved is not used by plants. When it falls on limestone or marble sources, a reaction uses up the acid. When it falls on granite (like sand), it remains. Environmental results are often dramatic and disastrous.

Safety

Perform this experiment as a demonstration. The acids 6 M HNO₃ and 3 M HCl are toxic and corrosive. The nitric acid stains skin. Sulfur dioxide and nitrogen oxides are toxic and noxious. The room must be well ventilated. Avoid inhaling these gases. Components of the buffers are toxic. Wear goggles and apron. Wash spills with water. Wash hands after the experiment.

Procedure

1. Prepare a solution by adding 3 mL of 0.05 M H₂SO₄ each to test tubes containing sand, marble chips, and limestone chalk. Measure the pH of each solution with pH paper.

   !!!Click here to See Movie.

2. Heat these mixtures in a water bath heated to 60-80 ºC while you are doing the demonstration below. After the demonstration, measure the pH of each solution with pH paper. Students should record the results.
3. This is suggested as an overhead projector demonstration.
4. Obtain a 24-well plate. The plate will be arranged as shown below.

   

5. Buffers are chemical systems that tend to maintain constant pH (see Experiment 011). Put 10 drops of 0.1 M HCl in the first 3 wells. Put 10 drops of pH 3 buffer into the first 3 wells of the 2nd row. Put 10 drops of pH 4 buffer into the first 3 wells of the 3rd row. Put 10 drops of pH 5 buffer into the first 3 wells of the 4th row.
6. Add 1 drop of bromthymol blue to each of the wells in the first column. Put 1 drop of methyl orange in each well of the second column. Place 2 drops of cabbage juice in each well of the third column.
7. Place 10 drops of water and 1 drop of bromthymol blue into each of wells 4, 5, and 6 in the second row. Place 10 drops of water and 1 drop of methyl orange into each of wells 4, 5, and 6 in the third row. Place 10 drops of water and 2 drops of cabbage juice into each of wells 4, 5, and 6 in the fourth row.
8. Prepare to do the reactions. Place a small piece of copper metal (turnings or fine strips rolled into a ball) in well-4 of row-1. Place a small amount (the size of a small pea) of NaHSO₃ in well-5 of row-1. Place a small piece of marble chip in well-6 of row-1.

   !!!Click here to See Picture.

9. Add 10 drops of 6 M HNO₃ to the copper metal in well-4 of row-1. Stopper the well with a gas collection tube (cut down stopper; plastic tube). Bubble the gas produced through each of the wells in the column containing the reacting well. Once the indicator changes are complete, add water to the well to dilute the acid and stop the production of toxic gas.

   !!!Click here to See Movie.

10. Ask the students to make observations. Compare any colors to those of the indicators in solutions of known pH. Comment on and collect these observations. (Appoint one student as recorder.)
11. Use a wash bottle to rinse the stopper and tube with water.
12. Add 10 drops of 3 M HCl to the marble chip in well-6 of row-1. Stopper the well with a gas collection tube (cut down stopper; plastic tube). Expect a short delay while the reaction starts.

    !!!Click here to See Movie.

13. Bubble the gas produced through each of the wells in the column containing the reacting well.

    !!!Click here to See Movie.

14. Ask the students to make observations. Compare any colors to those of the indicators in solutions of known pH. Comment on and collect these observations.
15. Use a wash bottle to rinse the stopper and tube with water.
16. Add 10 drops of 3 M HCl to the NaHSO₃ in well-5 of row-1. Stopper the well with a gas collection tube (cut down stopper; plastic tube). Bubble the gas produced through each of the wells in the column below the reacting well.

    !!!Click here to See Movie.

17. Ask the students to make observations. Compare any colors to those of the indicators in solutions of known pH. Comment on and collect these observations.

    !!!Click here to See Picture.

Questions

1. Write balanced equations for the combustion of C, N₂, and S₈.
2. Write equations for the reactions of CO₂, SO₂, and NO₂ with water.
3. Explain why regions with large deposits of CaCO₃ seem to withstand acid rain better than those surrounded by granite (which is modeled by SiO₂).

Handout

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 006 Acid Rain Investigations

Record the color of each indicator before and after the gas bubbles through the solutions. Estimate the pH from the buffer solutions in well 1 to 3

		Color before pH	Color after pH
NO2
	bromthymol blue
	methyl orange
	red cabbage
CO2
	bromthymol blue
	methyl orange
	red cabbage
SO2
	bromthymol blue
	methyl orange
	red cabbage
pH after heating
	sand
	marble chips
	limestone chalk

Curriculum-

Use when discussing acids and bases, oxidation, environmental chemistry, pH indicators, gases, or redox. This experiment works well in applied chemistry classes. This activity has been set up as a demonstration to use at the overhead projector. The topic of pH should be covered before this experiment is presented.

Safety-

Perform this experiment as a demonstration. The acids (6 M HNO₃ and 3 M HCl) are toxic and corrosive. The nitric acid stains skin. Sulfur dioxide and nitrogen oxides are toxic and noxious. The room must be well ventilated. Avoid inhaling these gases. Components of the buffers are toxic. Wear goggles and apron. Wash spills with water. Wash hands after the experiment.

Time-

Teacher Preparation: 20 minutes

Class Time: 20 minutes

Materials-

• 0.3 g copper metal
• 0.3 g NaHSO₃
• 2 g marble chip
• 2 g sand
• 2 g limestone-chalk optional
• 10 mL 0.05 M H₂SO₄ -- (Add 1.7 mL 3 M sulfuric acid (H₂SO₄) to enough water to make 100 mL solution.)
• 2 mL 0.1 M HCl --(Add 3.3 mL 3 M hydrogen chloride (HCl) to enough water to make 100 mL solution.)
• 0.5 mL of 0.1% bromthymol blue -- (Dissolve 0.1 g bromthymol blue in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
• 0.5 mL of methyl orange -- (Dissolve 0.1 g methyl orange in 100 mL of distilled water.)
• 1 mL of 6 M HNO₃ -- (Add 19 mL concentrated nitric acid to 20 mL of distilled water, and add enough distilled water to bring the total volume to 50 mL.)
• 1 mL of 3 M HCl -- (Use the stock solution directly as prepared.)
• Buffer Solutions
  • 6 mL Solution A: 0.2 M anhydrous boric acid and 0.05 M citric acid monohydrate. (Place 1.24 g H₃BO₃ and 1.05 g citric acid monohydrate in 60 mL of water. Dissolve. Add enough water to bring the final volume to 100 mL. Use a 100-mL volumetric flask if one is available.)
  • 2 mL Solution B: 0.1 M trisodium phosphate dodecahydrate (Place 3.80 g trisodium phosphate dodecahydrate, Na₃PO₄•12H₂O, in 60 mL of water. Dissolve. Add enough water to bring the final volume to 100 mL. Use a 100-mL volumetric flask if one is available.)
• Combining Ratios:

  pH	#drops A	# drops B
  2	39	0
  3	35	4
  4	31	8
  5	27	12

• cabbage juice -- (Place 50 g red cabbage and 50 mL of water in a blender. Blend for 3 minutes. Filter. Use the filtrate.)
• cotton swabs
• NaHCO₃ (baking soda for disposal)
• matches
• 24-well plate
• 3 13- x 100-mm test tubes
• 250-mL beaker
• ringstand
• ring
• gauze
• burner
• disposal jar
• gas collection tube See lab hints. (cut down # 0 1-hole stopper; place a rigid plastic tube in the hole; attach a flexible 15-20 cm plastic tube to the rigid plastic tube) (You may wish to prepare 3 of these to eliminate washing during the demonstration.)

Disposal-

Fill a large jar with water. Use a transfer pipet to suck the contents of each of the reaction cells from the 24-well plate, and squeeze the pipet to discharge the liquid into the disposal jar. Remove any unreacted solid from the wells, and discard that with ordinary solid trash. Discard the remaining liquids into the sink with running water. Neutralize the contents of the disposal jar with NaHCO₃, and then discard the solution at the sink with running water.

Lab Hints-

• This is suggested as an overhead projector demonstration.
• To prepare the gas collection tube, cut down a # 0 1-hole stopper to fit conveniently into the 24 well plate.

  !!!Click here to See Movie.

• Insert a rigid plastic tube in the hole. Attach a flexible 15-20 cm plastic tube to the rigid plastic tube.

  !!!Click here to See Picture.

• You may wish to prepare 3 of these to eliminate washing during the demonstration.
• The tubing must be washed between gases to prevent contamination and spurious results. Use a wash bottle or dip in a large beaker of water several times.

Answers-

Q1. Write balanced equations for the combustion of C, N₂, and S₈.

A1. C + O₂ --> CO₂; N₂ + 2 O₂ --> 2 NO₂; S₈ + 8 O₂ --> 8 SO₂.

Q2. Write equations for the reactions of CO₂, SO₂, and NO₂ with water.

A2. CO₂ + H₂O --> H⁺ + HCO₃^-; SO₂ + H₂O --> H⁺ + HSO₃^-;

2 NO₂ + H₂O --> HNO₂ + H⁺ + NO₃^-.

Q3. Explain why regions with large deposits of CaCO₃ seem to withstand acid rain better than those surrounded by granite (which is modeled by SiO₂).

A3. H⁺ + CaCO₃ --> Ca²⁺ + HCO₃^-; the limestone reacts (dissolves) and uses up the acid. The sand does not react.

Reference-

See D. N. Epp & R. Curtright, J. Chem. Educ., 1991, 68, 1034-1035.

Buffers:

Silberman, R. G. J. Chem. Educ. 1992, 69, A42-43.

Carmody, W. R. J. Chem. Educ. 1961, 38, 559

Key Words 1-

acid, oxidation, redox, acid rain, acid anhydride, pollution, pH, pH indicator

Elements-

Cu N O C S Cl H Na