Expt 009 -- Carbonic Acid, Bicarbonate Ion, Carbonate Ion, and Carbon Dioxide

Carbon dioxide gas is prepared by heating dry ice in a plastic transfer pipet. Using a stream of this gas, reactions with pH indicator solutions and calcium ion solutions are studied.

Safety

• Dry ice is so cold that it causes burns -- tissue damage. Handle powdered dry ice with extreme care. Use thermal gloves and goggles when filling the pipet with dry ice.
• Lime water is toxic and caustic. Ammonia and the indicators used are toxic. Wear goggles and apron. Know the location of the eye wash; review use of the eye wash before starting the experiment. Wash hands after completing the experiment.

Procedure

1. Prepare nine wells of a 24-well plate as follows. In one row of three wells, place 30 drops of limewater (saturated Ca(OH)₂) in well. Create three pairs of adjacent wells. Place 30 drops of distilled water and 2 drops of 1 M NH₃ into each well. Add 2 drops of 0.5% phenolphthalein to one pair, 2 drops of litmus solution to a second pair, and 2 drops of bromthymol blue to a third well. (Other indicators may be recommended by the teacher.)

   !!!Click here to See Picture.

2. The arrangement will look something like this:

   30 Ca(OH)2
   30 Ca(OH)2	30 H2O	30 H2O	30 H2O
   	2 NH3	2 NH3	2 NH3
   	2 Ind1	2 Ind2	2 Ind3
   30 Ca(OH)2	30 H2O	30 H2O	30 H2O
   	2 NH3	2 NH3	2 NH3
   	2 Ind1	2 Ind2	2 Ind3

3. Use a plastic transfer pipet. Cut the top from the bulb of the pipet.

   !!!Click here to See Movie.

4. Select a one-hole stopper which fits the bulb of the pipet.

   !!!Click here to See Picture.

5. Scoop crushed dry ice into the bulb of the pipet, and stopper with a one-hole stopper.

   !!!Click here to See Movie.

   !!!Click here to See Picture.

6. If the chunks of dry ice become too large to fit into the pipet, call the teacher to have the chunks powdered.

7. Place a small piece of tape over the hole or hold a finger or thumb on the hole to force the CO₂ through the solutions.

8. Holding the pipet bulb in one hand, place the tip of the stem below the surface of liquid in the first limewater well. Bubble carbon dioxide into the well until a reaction becomes evident.

   !!!Click here to See Movie.

9. Repeat this procedure with the second limewater well, but this time continue adding CO₂ until a second reaction process becomes evident.

   !!!Click here to See Movie.

10. Keep one indicator well of each pair as a control. Bubble carbon dioxide into the second well of a pair. Note and record any evidence of reaction. Phenolphthalein, litmus, and bromthymol blue are seen reacting during the movie.

    !!!Click here to See Movie.

    !!!Click here to See Picture.

Questions

1. Based upon the color changes observed for the pH indicators, write a balanced equation to describe the reaction of CO₂ with water.

2. CaCO₃ is insoluble, but Ca(HCO₃)₂ is soluble. On the basis of these formulas and facts, write balanced chemical equations to describe the reactions of CO₂ with limewater, Ca(OH)₂.

3. Moist caves in limestone-rich (i.e., CaCO₃-rich) regions are filled with stalactites and stalagmites -- marble like structures coming down from the cave's ceiling and rising up from the cave's floor. (Marble also has the formula CaCO₃.) "Live" caves are moist and nearly always show drops of liquid coming from the stalactites. On the basis of the reactions studied, explain how these structures might form, beginning with the dissolving of limestone.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 009 Carbonic Acid, Bicarbonate Ion, Carbonate Ion, and Carbon Dioxide

1. Record all reactions from the pictures and movies. Note any color changes observed and the significance of the color change. Note that NH₃ is a base.

   Limewater reactions

   Indicator reactions

2. Answer the questions using the observations from the movies and pictures.

Curriculum-

This topic can be included when studying acids and bases, precipitate formation, reactions of carbon dioxide, carbon dioxide as an environmental influence, and/or chemical equilibrium.

Safety-

• The biggest potential safety problem is during the handling of the dry ice. The instructor should powder the dry ice, and provide only powdered dry ice to students for their use.

• Dry ice is so cold that it causes burns -- tissue damage. Handle dry ice with extreme care. Use thermal gloves and goggles when crushing the dry ice or filling the pipet with dry ice.

• Lime water is toxic and caustic. Ammonia and the indicators used are toxic. Wear goggles and apron. Know the location of the eye wash; review use of the eye wash before starting the experiment. Wash hands after completing the experiment.

Time-

Teacher Preparation: 30 minutes (Saturated limewater should be prepared 2 weeks before the experiment and shaken occasionally.)

Class Time: 40 minutes

Materials-

• Dry ice (If dry ice unavailable, see Experiment 006 for an alternate procedure; setup one well to generate CO₂ and bubble into all chambers as shown in Experiment 006.)

• 0.5 mL of 1 M NH₃ -- Place 33 mL of 3 M NH₃ in a beaker and add enough distilled water to bring the total volume to 100 mL.

• 5 mL of Limewater (saturated Ca(OH)₂) -- Place 100 mL of distilled water in a 250-mL bottle. Add 1.0 g Ca(OH)₂. Cover and seal the cap tightly to prevent access to atmospheric CO₂. Shake daily for 2 weeks.

• 0.3 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.

• 0.3 mL of 0.5% phenolphthalein -- dissolve 0.5 g phenolphthalein in 60 mL 95% ethanol, and add enough distilled water to bring the final volume to 100 mL.

• 0.3 mL of 0.5% litmus -- purchase this as 0.5% solution.

• Other indicators with color changes in the general range of pH 5 to 9 may be used.

• 24-well plate

• one hole stopper +modified plastic delivery pipet with a wide stem

Disposal-

Allow unused dry ice to sublime in a well-ventilated area.

Lab Hints-

• Obtain dry ice from an ice house, a packing house, a grocery store, or a hospital (among possible sources). Dry ice can be manufactured from a CO₂ fire extinguisher by discharging the gas through a piece of cheese cloth. Recharging fire extinguishers is expensive, so this should be done as a last resort and only when other fire extinguishers remain available to deal with emergencies.

• Dry ice is provided in blocks. The easiest way to prepare powdered dry ice is to wrap a chunk (about 10 cm x 10 cm x 10 cm) in a strong towel or piece of canvas and twist the cloth creating a sort of ball with a handle. Then smash the ball repeatedly on a hard surface -- such as a concrete floor or sidewalk. A rubber mallet or other hammer also can be used, but wrapping the dry ice in a strong cloth is the key to success. Discarded bath or hand towels work very well for smashing the blocks of dry ice and storing the crushed dry ice.

• Store the dry ice in a Styrofoam cooler. Keep thermal gloves handy to handle the dry ice. Wrap the powdered dry ice set out for students loosely in several layers of cloth.

Observations-

• As the dry ice warms in the experimenter's hand, bubbles can be produced under the surface of a liquid. These bubbles presumably consist of CO₂.

• When bubbled through Ca(OH)₂, a white precipitate forms that dissolves when additional gas is passed through the solution.

• The indicators change colors from the basic color to the acidic color.

Answers-

Q1. Based upon the color changes observed for the pH indicators, write a balanced equation to describe the reaction of CO₂ with water.

A1. CO₂ produces an acid:

CO₂ + H₂O --> H₂CO₃ --> H⁺ + HCO₃^-

The acid reacts with hydroxide ion from the ammonia:

NH₃ + H₂O --> NH₄⁺ + OH^-

H⁺ + OH^- --> H₂O

As the H⁺ continues to grow, the indicator changes color:

In^- + H⁺ --> HIn

Q2. CaCO₃ is insoluble, but Ca(HCO₃)₂ is soluble. On the basis of these formulas and facts, write balanced chemical equations to describe the reactions of CO₂ with limewater, Ca(OH)₂.

A2. The CO₂ first produces CaCO₃:

Ca(OH)₂ + CO₂ --> CaCO₃ + H₂O

In the presence of excess CO₂, bicarbonate forms:

CaCO₃ + H₂O + CO₂ --> Ca²⁺ + 2 HCO₃^-

Q3. Moist caves in limestone-rich (i.e., CaCO₃-rich) regions are filled with stalactites and stalagmites -- marble-like limestone structures coming down from the cave's ceiling and rising up from the cave's floor. (Marble also has the formula CaCO₃.) "Live" caves are moist and nearly always show drops of liquid coming from the stalactites. On the basis of the reactions studied, explain how these structures might form, beginning with the dissolving of limestone.

A3. CO₂ from rain dissolves limestone:

CaCO₃ + H₂O + CO₂ --> Ca²⁺ + 2 HCO₃^-

The solution permeates into the cave. As the drops come down from the roof of the cave, water evaporates and produces limestone:

Ca²⁺ + 2 HCO₃^- --> CaCO₃ + H₂O + CO₂

This solid grows down from the top of the cave toward the bottom of the cave. As liquid drips from these downward-growing spires, it lands on the same point directly beneath the spire where evaporation also leads to crystal growth:

Ca²⁺ + 2 HCO₃^- --> CaCO₃ + H₂O + CO₂

Key Words 1-

acids, bases, carbonates, acid anhydrides, crystal formation, gas, equilibrium, acid rain

Elements-

C Ca