Expt 005 -- Classifying Strong Acids and Bases By Reactions

Strong acids and strong bases may be classified on the basis of their reactions with pH indicators, metals, and iron(III).

Safety

The acids and bases are corrosive. Several are toxic. Barium compounds are particularly toxic. The indicators are toxic. Wear goggles and apron. Handle the chemicals with caution. Wash spills with water. Wash hands after the experiment.

Procedure

1. Arrange a matrix of solutions to test on an acetate sheet. (See below.)

   

2. Place a small piece of pH test paper (4 mm x 4 mm) on each circle in the third row of the matrix.
3. Clean a small piece of magnesium ribbon with sandpaper. Cut it into small pieces. (Share cut pieces rather than discard unused Mg). Place a small piece of magnesium metal on each circle in the fourth row of the matrix.
4. Place 2 drops of HCl on the first column in rows 1,2,5, and six. Continue adding acids or bases to each column in the matrix.
5. Add 1 drop of 0.5 % phenolphthalein to each test solution in the first row of the matrix. Note and record observations.

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6. Add the acid or base solution to be tested to each piece of pH paper.
7. Use the standard on the container to assign pH. Note and record observations.

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8. Add 1 drop of 0.1 % bromthymol blue to each test solution in the second row of the matrix. Note and record observations.

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9. Add the acid or base solution to be tested to each piece of pH paper.
10. Use the standard on the container to assign pH. Note and record observations.

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11. Add the acid or base solution to be tested to each piece of magnesium metal to each test solution in the fourth row of the matrix. Note and record observations.

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12. Add 1 drop of 0.1 M Fe(NO₃)₃ to each test solution in the fifth row of the matrix. Note and record observations.

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13. Test the conductivity of each test solution in the sixth row of the matrix.

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14. Note the relative rate of flashing as well as the intensity. Dip the electrodes in water and rinse with a wash bottle between measurements. Dim the lights or use black tubing over the LED to observe. Note and record observations.
    1. HCl

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    2. NaOH

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    3. H₂SO₄

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    4. HNO₃

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    5. KOH

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    6. Ba(OH)₂

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    7. H₂O

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15. Rinse the acetate sheet into a disposal jar provided by the instructor.
16. Wash hands.

Questions

1. For those solutions where high conductivity is observed, write an equation that describes the production of ions.
2. Suggest an explanation for the color changes with the pH indicators and the pH paper.
3. Write an equation to describe any reactions observed with magnesium metal.
4. Write an equation to describe any reactions observed with iron(III) nitrate.
5. Explain the feature that strong acids have in common. Explain the feature that all strong bases have in common.
6. Water seems anomalous. Explain why water seems anomalous, and what an explanation may be.

Handout

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 005 Classifying Strong Acids and Bases By Reactions

Watch the movies. Carefully record observations of the reactions just as you would in the laboratory.

Answer the questions.

Curriculum-

The is an excellent experiment to introduce the topic of acids and bases.

Safety-

The acids and bases are corrosive. Several are toxic. Barium compounds are particularly toxic. The indicators are toxic. Wear goggles and apron. Handle the chemicals with caution. Wash spills with water. Wash hands after the experiment.

Time-

Teacher Preparation: 30 minutes

Class Time: 40 minutes

Materials-

• 1.5 cm magnesium ribbon
• Prepare 0.5 mL of indicator for each group of students.
  • 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 % phenolphthalein -- dissolve 0.5 g phenolphthalein in 60 mL 95 % ethanol, and add enough distilled water to bring the final volume to 100 mL.
  • 0.1 M Fe(NO₃)₃ -- (Dissolve 4.04 g of Fe(NO₃)₃´9H₂O in a mixture of 20 mL water and 10 drops 3 M HNO₃. Add enough water to make 100 mL of solution.)
• Prepare 1.0 mL of acid or base for each group of students.
  • 0.2 M NaOH -- (Dissolve 0.800 g of NaOH in enough water to make 100 mL of solution.)
  • 0.2 M KOH -- (Dissolve 1.122 g of KOH in enough water to make 100 mL of solution.)
  • 0.2 M HCl -- (Dissolve 6.7 mL of 3 M HCl in enough water to make 100 mL of solution.)
  • 0.2 M H₂SO₄ -- (Dissolve 6.7 mL of 3 M H₂SO₄ in enough water to make 100 mL of solution.)
  • 0.2 M HNO₃ -- (Dissolve 6.7 mL of 3 M HNO₃ in enough water to make 100 mL of solution.)
  • 0.15 Ba(OH)₂ -- (Dissolve 4.732 g of Ba(OH)₂´8H₂O in enough water to make 100 mL of solution.)
• pH paper (one piece)
• acetate sheet
• conductivity tester (see Lab Hints below)

Disposal-

Treat the disposal jar with 5 g Na₂SO₄. Wait one day. Filter. Discard the solid with ordinary solid trash. Neutralize the filtrate (with vinegar or sodium carbonate). Discard the resulting solution at the sink.

Lab Hints-

• Disconnect the battery during storage to minimize shorting-out problems.
• After many years of trying different devices, we have reached the conclusion that the simplest device works best. When tested over excellent conductors, the diode may burn out. This has not happened often in classroom experience, however. Construct an apparatus using a 9-volt battery and a blinking LED (light emitting diode). (See a schematic diagram below.)

  

• To build the electrical conductivity apparatus suitable for use by students (Radio Shack^® catalog numbers are provided):
  • Blinking LED 276-030 (or 276-036)
  • 4 cm length black 7-mm i.d. rubber tubing
  • 9-volt battery clip 270-325
  • 9-volt battery
  • 40-cm length of two-conductor, 24-gauge, ribbon wire 278-755
  • (soldering iron, solder, razor or sharp knife, wire cutters, wire strippers)
  1. Construct the electrodes from a 10-cm length of ribbon wire. Use a razor or sharp knife to remove about 10 mm of insulation from both sides of the end the length of ribbon wire.

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  2. The insulation is removed to expose the wires so that they can come into direct contact with the solutions. Insulation is left between the wires to keep them from touching one another thereby preventing a "short circuit." For reproducible measurements the distance and area of the exposed wires must be constant.

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  3. Solder the red wire from the battery clip to the long lead of the LED.

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  4. Solder the short lead of the LED to one wire of the electrode ribbon. Solder the black wire from the battery clip to the other wire in the ribbon/electrode assembly.

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  5. Place a short length of black rubber tubing over the LED to improve visibility when faint flashes are emitted.
  6. Note the increased visibility in a light room with the tubing in place.

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Answers-

Q1. For those solution where high conductivity is observed, write an equation that describes the production of ions.

A1. HCl --> H⁺ + Cl^-

HNO₃ --> H⁺ + NO₃^-

H₂SO₄ --> 2 H⁺ + SO₄^2-

NaOH --> OH^- + Na⁺

KOH --> OH^- + K⁺

Ba(OH)₂ --> 2 OH^- + Ba²⁺

Q2. Suggest an explanation for the color changes with the pH indicators and the pH paper.

A2. The indicators show a basic color with NaOH, KOH, and Ba(OH)₂. They give an acid color with HCl, HNO₃, and H₂SO₄. Water has no effect on the indicators or paper.

Q3. Write an equation to describe any reactions observed with magnesium metal.

A3. Mg + 2 H⁺ --> H₂ + Mg²⁺

Q4. Write an equation to describe any reactions observed with iron(III) nitrate.

A4. Fe³⁺ + 3 OH^- --> Fe(OH)₃

Q5. Explain the feature that strong acids have in common. Explain the feature that all strong bases have in common.

A5. All of the strong acids have H⁺. All of the strong bases have OH^-.

Q6. Water seems anomalous. Explain why water seems anomalous, and what an explanation may be.

A6. Water seems to be able to give both ions, H⁺ and OH^-. This suggests that, when these ions are together, they react to form something that is not ionized. (In fact, water is a weak electrolyte, not a strong electrolyte; it is only partly broken up into ions.)

Handout Ans.-

Reference-

This activity was modified from Activity 1 of the Acids and Bases module of ChemSource.

Key Words 1-

acid, base, strong acid, strong base, precipitate, pH, dissociation, ionization, conductivity,

Elements-

H O Cl S N Ba Na K Mg Fe