Carbon Dioxide as an Acid

Description

Dry ice is used as a source of carbon dioxide. The carbon dioxide changes the color of indicator solutions prepared in very dilute ammonia. The carbon dioxide may be used to form a precipitate of calcium carbonate. Continued addition of carbon dioxide causes the calcium carbonate to redissolve. This experiment may be performed as a demonstration or as a student experiment on the microscale.

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Improper handling of dry ice can lead to burns.

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Handle the dry ice cautiously. Do not touch with bare fingers; wear gloves, or wrap in a towel.

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This experiment is usually performed as a demonstration. It may be performed by students using small scale carbon dioxide generators.

1. Wear gloves. Fill the bulb of a slit thin stem plastic transfer pipet with powdered dry ice.
2. Use plastic tape to tape the seal the slit once the bulb is filled.
3. Insert the stem of the carbon dioxide generator into a small test tube containing a 4 mL of distilled water, a drop of suitable indicator, and a drop of 1 M ammonia.
4. Note evidence for reaction.
5. Place two test tubes containing 2 mL lime water in a small beaker. Pass carbon dioxide through one solution until a marked turbidity is noticed. Pass carbon dioxide through the other solution for several minutes.
6. Continue passing carbon dioxide through the second tube after the turbidity forms until a second change occurs. Compare the turbidity of the tube with excess CO₂ to the turbidity of the first tube.

Instructor Demonstration

1. Place 1.5 L of distilled water into each of four 2-L, tall-form, clear, colorless vessels. Use either conventional glass, water, or the plastic vessels just described. Add about 10 mL of 1 M ammonia to each vessel.
2. Squirt about 1 mL of 1% ethanolic phenolphthalein solution into each of the two vessels on the left. Note any color change.
3. Squirt about 1 mL of 1% bromthymol blue indicator solution into each of the two vessels on the right. Note any color change.
4. Stir each mixture with a rod. Wipe the rod dry with a paper towel between the second and third solutions.
5. Use a glove. Add some chunks of dry ice to the two center vessels.
6. Note evidence of reaction.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 107 Carbon Dioxide as an Acid

Watch the movies.

1. Predict the outcome of using a mixture of phenolphthalein and bromthymol blue indicators.
2. Explain why the basic ammonia solution becomes acidic when CO₂ is added.
3. Write a balanced equation for the reaction of CO₂ and water.
4. Write a balanced equation for the acid-base ionic equilibrium a CO₂ solution.

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Purpose

To demonstrate the acid properties of carbon dioxide.

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• cooler, gloves, mallet, towel, hot pads
• dry ice
• scoopula
• 4 2-L clear, colorless plastic soda-pop bottle
• sharp knife with long blade
• 5 mL 1% ethanolic phenolphthalein
• 5 mL 1% bromthymol blue indicator
• 50 mL 1 M ammonia
• distilled water
• stirring rod
• Microscale (per student, in addition to above):
• razor blade or scalpel (for teacher use)
• 6 mL saturated calcium hydroxide (lime water)
• Beral pipet
• 5 cm vinyl tape
• 6 13x100-mm test tube
• 1 150-mL beaker (to support test tubes)

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• Dry ice is needed for this experiment. The expansion of gaseous carbon dioxide under high pressure will produce dry ice as a result of cooling upon expansion. This is inefficient and expensive. Ice companies often sell dry ice for reasonable prices.
  • Dry ice is usually produced in cubes about 25 cm on an edge. When you purchase some dry ice, the dealer will use a band saw to cut the ice to a rectangular block of the desired weight and will usually give it to you wrapped in paper. The block will be fragile. Break the block into large pieces in its paper wrapper. Transfer to a small styrofoam chest.
  • Wear a glove. Remove the desired amount to the center of an old bath towel or piece of canvas. Wrap the ice. Smash it against the floor until the dry ice breaks into a powder.
• Prepare the tall form vessels and/or the slit plastic pipets for the students. Remove the label from a colorless 2-L plastic soda bottle. Use a sharp knife to cut the top off the bottle. The bottom will serve as a reaction vessel.
• Use a razor blade or scalpel to make a slit in the bulb of a transfer pipet parallel to the axis of the stem. Prepare these bulbs for the students.

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Obtaining dry ice: 15-90 minutes

Teacher preparation: 10-30 minutes

Presentation: 15-20 minutes

Student experiment: 30 minutes

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Improper handling of dry ice can lead to burns. Cuts are possible when constructing the plastic tall form vessels and slitting the pipets.

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• Handle the dry ice cautiously. Do not touch with bare fingers; wear gloves, or wrap in a towel.
• Use caution with sharp knives and razors. Do not permit students to use the knives or razors.

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• Allow dry ice to sublime of its own accord in a well ventilated area.
• The solutions may be disposed of safely at the sink. Store the plastic vessels for reuse.

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Presentation Question:

• Predict the outcome of using a mixture of phenolphthalein and bromthymol blue indicators.
  • The pink and blue give a purplish color. As the carbon dioxide reacts, the pink is discharged leaving a blue. The bromthymol blue changes (blue through green to yellow) are then observed.

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Carbon dioxide reacts with lime water to produce solid calcium carbonate:

CO₂ + Ca²⁺ + 2 OH^- CaCO₃ + H₂O

In excess carbon dioxide, the solid calcium carbonate dissolves forming bicarbonate ions:

CaCO₃ + H₂O + CO₂ Ca²⁺ + 2 HCO₃^-

The formation of stalactites and stalagmites in limestone caves is attributed to this process. Water containing calcium and bicarbonate ions is called "temporary hard" water. Hard water contains dissolved cations that interfere with soaps by forming insoluble precipitates. Calcium ion is such an interfering ion. When boiled, temporary hard water releases carbon dioxide, forms insoluble calcium carbonate, and is thereby "softened."

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1. The pink and blue give a purplish color. As the carbon dioxide reacts, the pink is discharged leaving a blue. The bromthymol blue changes (blue through green to yellow) are then observed.
2. CO₂ dissolves in water to produce an acidic species, H₂CO₃, which reacts with the base in the water.
3. CO₂ + H₂O H₂CO₃
4. H₂CO₃ H⁺ + HCO₃^-

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• acid
• carbon dioxide
• CO₂
• dry ice

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