Bronsted/Lowry Acids and Bases

Description

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Some of the chemicals tested may be toxic.

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Do not ingest chemicals.

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1. Arrange pieces of red litmus, blue litmus and pH papers systematically on an acetate sheet.
2. For each group of papers, add 1 or 2 drops of the test solution to each paper. Note color changes for litmus. Use the pH chart to estimate a pH value for the pH paper. Repeat for all solutions to be tested.
3. The Hydrion B paper colors are Red, pH 1 to Blue pH 11 which are displayed on the standard at pH 1,3,5,7,9, and 11. The Hydrion B paper standards are displayed at pH 2,4,6,8, and 10.
4. Record your observations in the Data Table.

Alternate

Supply samples of solids in small vials; capped 1.5-mL plastic microcentrifuge tubes serve the purpose very well. Use a long strip of pH paper. Place one or two tiny crystals of the compound to be test every 3 cm on the strip. Moisten the crystal or crystals with one drop of distilled water. Estimate the pH from the pH chart; record the data.

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• Salts dissociate in water. Consider NaCl. In water, it consists of Na⁺(aq) and Cl^-(aq). Either of these may react with water in an acid base reaction:

  Na⁺ + H₂O NaOH + H⁺

  Cl^- + H₂O HCl + OH^-

  In fact, neither of these reactions takes place as written. Instead, the reverse of these reactions takes place.

• What about salts like NaCH₃COO and NH₄Cl?

  CH₃COO^- + H₂O CH₃COOH + OH^-

  NH₄⁺ + H₂O NH₄OH + H⁺

  In these cases, the products NH₄OH and CH₃COOH do not dissociate completely. These reactions do have a tendency to go as written, at least to some extent. As a result, water solutions of NH₄Cl contain an excess of H⁺ over OH^- and are acidic; water solutions of NaCH₃COO contain an excess of OH^- over H⁺, and are basic.

• In an older context, these were described as reactions with water, and the term hydrolysis was used to describe the reaction.
• Always, acids and bases in solution come in pairs called conjugates. For each acid, there is a conjugate base; for each base, there is a conjugate acid.

  Consider these reactions:

acid1....................................base1
Na⁺ + H₂O NaOH + H⁺
base2..............................acid2

acid1................................base1
Cl^- + H₂O HCl + OH^-
base2................................acid2

acid1......................................base1
NH₄⁺ + H₂O NH₄OH + H⁺
base2...................................acid2

acid1............................................base1
CH₃COO^- + H₂O CH₃COOH + OH^-
base2............................................acid2

Notice that water is always acting as either an acid or a base on the left side of these reactions. In a sense, each acid or base is being compared to water in terms of strength. In water solutions, then, the ions from a salt are compared to one another. In NaCl, neither Na⁺ nor Cl^- is strong compared to water. However, in NH₄Cl, the ammonium ion is stronger as an acid than is the Cl^- ion as a base. The result is that water solutions of this salt contain excess H⁺; they are acidic. Similarly, NaCH₃COO is alkaline.

• What would happen with NH₄CH₃COO? Well, it would depend upon which species reacted more strongly with water. As it turns out, both are equally strong, and, in this special case, the solution is neutral.
• Based upon the results you observed, for all ions studied, write formulas for the ion and its conjugate acid or base. Indicate whether the formation of the conjugate in water takes place to no measurable extent, to a slight extent., or to a very great extent. (For some substances, such as NaH or KNH₂, a great degree of reaction is observed.)

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Name ___________________________ Class ________

Teacher__________________________

DoChem 106 Bronsted/Lowry Acids and Bases

Interpretation of Data:

Closure Questions:

1. Order these acids in terms of strength: CH₃COOH, HCl and HCO₃^-. Justify your ordering.
2. Commercial cleaning products usually contain Na₂CO₃ or Na₃PO₄. Explain this formulation.
3. NaAl(SO₄)₂ is mixed with NaHCO₃ in some baking powder formulations. Write equations to show how CO₂ is produced.

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Include Handout

Name ___________________________ Class ________

Teacher__________________________

DoChem 106 Bronsted/Lowry Acids and Bases

1. Watch the movie and record the color changes from the picture at the end on the attached student handout. Answer all questions based on the movie and picture. Note the color change for NaCl on the hydrion paper is only wetting the paper; the solution is neutral. Use this color change as a reference for other observations.
2. Predict the outcome of testing solutions such as aluminum acetate, ammonium acetate, and ammonium carbonate.

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Purpose

To determine the relative acidity of various salt solutions.

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(for 10 students working in pairs)

• 5 plastic sheets (acetate)
• 5 vials red litmus paper
• 5 vials blue litmus paper (pH paper may be used)
• 5 vials universal pH paper with color chart (or rolls of pH paper provided in dispensers supplied with color codes)
• 5 0.1 M solutions of each of the following in small dropper bottles
• Na₂CO₃•H₂O (12.4 g/L)
• NaCl (5.85 g/L)
• NaNO₃ (8.5 g/L)
• (NH₄)₂SO₄ (13.2 g/L)
• NH₄Cl (5.35 g/L)
• NaCH₃COO (8.2 g/L)
• Na₂SO₄ (14.2 g/L)
• Na₃PO₄•12H₂O (40 g/L)
• Al₂(SO₄)₃•18H₂O (66.6 g/L)
• FeCl₃•6H₂O (27.0 g/L)
• Alternate: solid samples of compounds provided in sealed vials (such as capped, plastic, 1.5-mL microcentrifuge tubes); distilled water

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• Each lab station should have its own set of dropper bottles.
• Have students place a drop of solution directly on the litmus paper and record results. Test both red and blue litmus.
• In most regions of the country tap water and water that is run through a demineralizer will have a pH of less than 7.0. The acidity of the water may alter some results. Use pure bottled distilled water if available to prepare the solutions.
• The number of salt samples tested may be varied. Other salts may be substituted for the ones suggested in this lab.

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Teacher preparation: 1 hour

Class time: 30 minutes

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Some of the chemicals tested may be toxic.

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Select nontoxic chemicals. Do not ingest chemicals.

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The chemicals suggested in this experiment may be disposed of safely at the sink. Gather all used test papers for disposal with ordinary trash. Save the solutions in the dropper bottles for use in later classes.

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Presentation Question:

• Predict the outcome of testing solutions such as aluminum acetate, ammonium acetate, and ammonium carbonate.
  • Solutions of salts of weak acids and weak bases are more difficult to predict than are those of one weak electrolyte/ one strong electrolyte. The solution acidity depends upon which acid or base is stronger. If the weak acid is stronger than is the weak base, the solution is acidic.

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Closure Questions:

1. Order these acids in terms of strength: CH₃COOH, HCl and HCO₃^-. Justify your ordering.
2. Commercial cleaning products usually contain Na₂CO₃ or Na₃PO₄. Explain this formulation.
3. NaAl(SO₄)₂ is mixed with NaHCO₃ in some baking powder formulations. Write equations to show how CO₂ is produced.

Answers to Closure Questions:

1. According to the experimental results CH₃COOH is weaker than HCl. Water solutions of NaCl are neutral (to litmus). Water solutions of NaCH₃COO turn red litmus blue. HCO₃^- is weaker than CH₃COOH because, according to the pH paper results, solutions of Na₂CO₃ are more alkaline than solutions of NaCH₃COO.
2. CO₃^2- + H₂O HCO₃^- + OH^-

   PO₄^3- + H₂O HPO₄^2- + OH^-

   OH^- is among the active ingredients in cleaners; it reacts with fats, greases, and oils.

3. Al(H₂O)₆³⁺ + H₂O Al(H₂O)₅(OH)₂⁺ + H₃O⁺

   HCO₃^- + H₃O⁺ CO₂ + 2H₂O

   The aluminum salt acts as an acid source, and releases CO₂ from the bicarbonate.

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• Water acts as a leveler of the acid and base. (The solvent acts as a leveler.)

  For the acid in a pair,

  CH₃COOH + H₂O H⁺ + CH₃COO^-

  Ka = [H⁺][CH₃COO^-] / [CH₃COOH]

  For the base in a pair,

  CH₃COO^- + H₂O CH₃COOH + OH^-

  Kb = [OH^-][CH₃COOH] / [CH₃COO^-]

  and the product of these is

  Ka x Kb = { [H⁺][CH₃COO^-] / [CH₃COOH] } x

  { [OH^-][CH₃COOH] / [CH₃COO^-] }

  = [H⁺][OH^-]

• This product is known as the Kw for water, so
  • Ka x Kb = Kw.

Interpretation of Data:

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• Bronsted/Lowry acids and bases
• acid
• base
• litmus
• pH paper
• pH
• salt

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