Conductivity of Acids and Bases

Description

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• Free electrons (as in a metal), free ions, or some other mobile charge carrier must be available to sustain an electric current. A solution that contains ions will conduct electricity. The amount of electricity that will flow through a solution depends primarily on the numbers of ions present: the greater the number of ions, the greater the amount of electricity which will flow. This can be observed by using a conductivity apparatus. The amount of electricity flowing is indicated by the brightness of the light bulb, the LED, or the meter reading.
• A strong electrolyte ionizes to a large degree. When a strong electrolyte is also soluble, its solutions contain many ions, conduct electricity well, and cause the lamps in a conductivity apparatus to burn brightly. A weak electrolyte dissociates only slightly in solution, forms fewer ions, and causes the light bulb to glow dimly. A nonelectrolyte has very few ions, and the light bulb will not glow. In this experiment the conductivity apparatus allows us to distinguish between strong and weak acids or bases.
• Acids and bases react to form salts and water. Salts are nearly always strong electrolytes.

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There is danger of electrocution when using 110 volt electricity. Some of the chemicals may be toxic. Glass shards near the electrode base may cause cuts.

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Avoid ingesting or inhaling chemical substances. Provide adequate ventilation. Wash hands after the experiment. Student may only use the low voltage apparatus.

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Follow the same procedures described in DCExperiment 062 to study the conductivity of strong and weak acids and bases.

Student Experiment with Transformer

1. The low-voltage AC device may be used in place of the conventional conductivity apparatus. Set the meter to a scale which measures current.
2. Add indicator to a beaker containing 20 mL of distilled water and 3 mL of 0.02 M Ba(OH)₂.
3. Titrate the Ba(OH)₂ by the dropwise addition of H₂SO₄ from a thin stem pipet. Note the meter reading and any color or other changes in the beaker.
4. Slow the rate of addition when the meter reading approaches zero.

Teacher Demonstration only

1. As a teacher demonstration, set up the conventional conductivity apparatus with a ground fault interrupter.
2. Place 15 mL distilled water and 3 mL 0.02 M Ba(OH)₂ into a 50-mL beaker. Add a magnetic stirring bar. Start and adjust the stirrer. Add a few drops of methyl orange indicator.
3. Titrate the Ba(OH)₂ by the dropwise addition of H₂SO₄ from a thin stem pipet.
4. Note the brightness of the lamp and any color or other changes in the beaker. When the light is nearly extinguished, slow the rate of addition.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 104 Conductivity of Acids and Bases

Watch the movie.

1. Record the color changes and current as the reaction progresses for the student experiment.
2. Record the color changes and light intensity as the reaction progresses for the Demonstration.

1. State in your own words why some substances cause the light bulb to glow brightly, others dimly, yet others not at all.
2. When a strong electrolyte, HCl, is mixed with a strong electrolyte, NaOH, why does the solution test as a strong electrolyte?
3. Explain the sequence of light brightness measurements (or meter readings) observed during the titration of Ba(OH)₂ with H₂SO₄. Relate these to the color changes.

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Purpose

To illustrate that solutions of strong acids are better conductors of electricity than solutions of weak acids.

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• electrical conductivity apparatus
• 50 mL beakers (enough for the solutions tested)
• 50 mL of each of the following solutions:
• vinegar
• 0.1 M HCl
• acetic acid (HC₂H₃O₂ ) 5.7mL of 99.5% acid (glacial)/L
• household ammonia
• 0.1 M NaOH
• 0.1 M NaC₂H₃O₂
• 0.1 M NH₄C₂H₃O₂
• 0.1 M NH₄Cl
• 0.1 M NaCl
• 5 mL of 0.02 M H₂SO₄ (mix 20 mL of 0.1 M H₂SO₄ with 80 mL of water)
• 5 mL of 0.02 M Ba(OH)₂ (0.4 g/100 mL)
• 10 mL phenolphthalein solution 1g/50 ml ethanol, mix with 50 ml water
• 100 mL distilled water
• 1 25- or 50-mL buret

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• Perform this experiment as a demonstration rather than as a student experiment if only conventional apparatus, which uses a light bulb and 110 volts, is available. Use a ground fault interrupter with the conventional apparatus.
• See the lab hints section of DCExperiment 062 for detailed instructions and movies about using building the conductivity apparatus. Click here go to the experiment directly.

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Teacher preparation: 30 minutes

Demonstration: 20 minutes

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There is danger of electrocution when using 110 volt electricity. Some of the chemicals may be toxic. Glass shards near the electrode base may cause cuts.

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• Perform this experiment as a demonstration rather than as a student experiment if only conventional apparatus is available. Use a ground fault interrupter to guard against electrocution.
• Select substances for testing that are not toxic. Avoid ingesting or inhaling chemical substances. Provide adequate ventilation. Wash hands after the experiment.
• If electrodes are being made for the conventional apparatus, wrap the light bulb in a bag when shattering to avoid shards. (See DCexperiment 62.) Do not permit students to prepare the electrodes.

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• Treat barium hydroxide solutions with a slight excess of dilute sulfuric acid. Wait several days. Filter. Discard the solid with ordinary solid trash.
• Discard the filtrate and other acids and bases at the sink after neutralization.

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Closure Questions:

1. State in your own words why some substances cause the light bulb to glow brightly, others dimly, yet others not at all.
2. When a strong electrolyte, HCl, is mixed with a strong electrolyte, NaOH, why does the solution test as a strong electrolyte?
3. Explain the sequence of light brightness measurements (or meter readings) observed during the titration of Ba(OH)₂ with H₂SO₄ . Relate these to the color changes.

Answers to Closure Questions:

1. Bright lights are observed when there are many ions in solution. Dim lights are observed when there are few ions in solution. No lights result when there are no ions in solution.
2. The resulting salt, sodium chloride, is a strong electrolyte.
3. Ba(OH)₂ is a strong electrolyte, and the lights glow brightly. By itself, H₂SO₄ is a strong electrolyte. When added, these two substances react to produce BaSO₄. Barium sulfate, although a strong electrolyte, is only slightly soluble in water. As a result, by adding the sulfuric acid to the barium hydroxide, and insoluble salt forms and the number of ions in solution decreases. After the equivalent amount has been added, more sulfuric acid has nothing to react with, and the conductivity increases. The reactions are:

   H⁺ + OH^- --> H₂O

   Ba²⁺ + SO₄^2- --> BaSO₄(c)

   Methyl orange is red in acid and yellow in base. When the titration is started, the barium hydroxide causes an excess of hydroxide ions (base) to be present. As sulfuric acid is added, these hydroxide ions are used up. Once all of the hydroxide is used up, addition of more acid causes an excess of hydrogen ion to be present. The indicator color changes when this occurs.

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1. The solution is yellow initially and current is about 17. As the solution is titrated, the current decreases and the solution turns orange. Precipitate forms. The current drops to 0 at the endpoint. As excess acid is added the current increases again.
2. The solution is yellow initially and the light very bright. As the solution is titrated, the light dims and the solution turns orange. Precipitate forms. As excess acid is added the light brightens again.

See closure answers.

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• acid
• base
• conductor
• titration
• conductivity
• conductimetric titration

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