Expt 008 -- Bronsted/Lowry Acids and Bases

The relative acidity of various salt solutions is determined by dissolving the crystals on pH paper and approximating the pH according to the color scale provided.

Safety

Some of the chemicals are toxic. Wear goggles and apron. Do not ingest any chemicals. Wash up spills immediately. Wash hands after the experiment.

Procedure

1. Place one piece of each type of pH paper used on the plastic sheet Place a few small crystals of each solid to be tested spaced at distances of about 2 cm on the pH paper strips.

   !!!Click here to See Picture.

2. Add 2-3 drops of distilled water from a plastic transfer pipet to each small pile of crystals.

   !!!Click here to See Movie.

3. Estimate the pH using the color charts provided.

   !!!Click here to See Movie.

4. Record the observations in a data table.

   !!!Click here to See Picture.

   !!!Click here to See Picture.

5. For each ion studied, write a balanced equation for the reaction with water.

Questions

1. Order these acids in terms of strength: CH₃COOH, HCl and HCO₃^-. Justify this ordering.
2. Fats react with bases to form water soluble products. 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.

Handout

Name ___________________________ Class ________

Teacher__________________________

SmallScale 008 Bronsted/Lowry Acids and Bases

Makeup students should record data from the pictures.

Sample Data Table:

Salt	Approx. pH	Rel. Acid/Base	Hydrolysis (yes or no)
FeCl3
Al2(SO4)3
Na3PO4
Na2SO4
NaCH3COO
NH4Cl
(NH4)2SO4
NaNO3
NaCl
Na2CO3

Curriculum-

This experiment fits in during discussion of acid, bases, and especially salts.

Safety-

Some of the chemicals are toxic. Wear goggles and apron. Do not ingest any chemicals. Wash up spills immediately. Wash hands after the experiment.

Time-

Teacher Preparation: 10 minutes

Class Time: 40 minutes

Materials-

• 1 plastic sheets (acetate)
• universal pH paper with color chart
• solid samples of the some of the following salts (stored in microcentrifuge tubes), (0.1g per group)
• Na₂CO₃•H₂O (or Na₂CO₃)
• NaCl
• Na₂SO₄
• NH₄Cl
• NaC₂H₃O₂•3H₂O ( NaCH₃COO•3H₂O)
• Na₃PO₄•12H₂O
• Al₂(SO₄)₃•18H₂O
• FeCl₃•6H₂O
• (NH₄)₂SO₄
• NaNO₃
• NaHCO₃

Disposal-

Dispose of the soiled pH papers with ordinary solid trash.

Lab Hints-

• In most parts of the United States, 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. Distilled water often is acidic because of dissolved carbon dioxide. Boiling removes the dissolved gas.
• The number of salt samples tested may be varied. Other salts may be substituted for the ones suggested in this lab.

Background-

• 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.
• A different result obtains for weak acids and bases.

  • 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 OHÑ and are acidic; water solutions of NaCH₃COO contain an excess of OHÑ 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.
• For 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. In a sense, each acid or base is being compared in terms of strength to water. 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.
• When the conjugate acid and base of both ions in a salt are weak, then the outcome depends upon which substance is stronger. Solutions of (NH₄)₂CO₃ are alkaline -- HCO₃^- is weaker as an acid than is NH₃ as a base. NH₄CH₃COO is a special case -- both ions are equally strong, and the solution is neutral.
• Based upon the results 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 a very great extent, to a slight extent, or to no measurable extent. (For some substances, such as NaH or KNH₂, a great degree of reaction is observed.)
• Water acts as a leveler of the acid and base. (The solvent acts as a leveler.)
• For the acid in a pair,
  • CH₃COOH + H₂O --> H₃O⁺ + CH₃COO^-
  • K_a = ([H⁺][CH₃COO^-])/[CH₃COOH]
• For the base in a pair,
  • CH₃COO^- + H₂O --> CH₃COOH + OH^-
  • K_b = ([OH^-][CH₃COOH])/[CH₃COO^-]
• and the product of these is
  • K_a x K_b =(([H⁺][CH₃COO^-])/[CH₃COOH] ) x (([OH^-][CH₃COOH])/[CH₃COO^-]) = [H⁺][OH^-]
• This product is known as the K_w for water, so
  • K_a x K_b = K_w.

Data Table-

Salt	Approx. pH	Rel. Acid/Base	Hydrolysis (yes or no)
FeCl3	4	acid	yes
Al2(SO4)3	4	acid	yes
Na3PO4	10	base	yes
Na2SO4	7	neutral	no
NaCH3COO	8	base	yes
NH4Cl	5	acid	yes
(NH4)2SO4	6	acid	yes
NaNO3	7	neutral	no
NaCl	7	neutral	no
Na2CO3	10	base	yes

Data Analysis-

Interpretation of Data:

Acid	Conjugate Base	Extent of Reaction
Na+	NaOH	none
K+	KOH	none
NH4+	NH3	slight
Al(H2O)63+	Al(H2O)5OH2+	some
Fe(H2O)63+	Fe(H2O)5OH2+	some

Base	Conjugate Acid	Extent of Reaction
CO32-	HCO3-	some
Cl-	HCl	none
NO3-	HNO3	none
SO42-	HSO4-	none
CH3COO-	CH3COOH	slight
PO43-	HPO42-	some

Answers-

Q1. Order these acids in terms of strength: CH₃COOH, HCl and HCO₃^-. Justify the ordering.

A1. According to the experimental results CH₃COOH is weaker than HCl. Water solutions of NaCl are neutral. Water solutions of NaCH₃COO are alkaline. HCO₃^- is weaker than CH₃COOH because, according to the pH paper results, solutions of Na₂CO₃ are more alkaline than solutions of NaCH₃COO.

Q2. Commercial cleaning products usually contain Na₂CO₃ or Na₃PO₄. Explain this formulation.

A2. 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.

Q3. NaAl(SO₄)₂ is mixed with NaHCO₃ in some baking powder formulations. Write equations to show how CO₂ is produced.

A3. Al(H₂O)₆³⁺ + H₂O --> Al(H₂O)₅(OH)²⁺ + H₃O⁺

HCO₃^- + H₃O⁺ --> CO₂ + 2 H₂O

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

Key Words 1-

acid, base, pH, salt, hydrolysis, Bronsted acid, Bronsted base, Bronsted/Lowry

Elements-

Na K N C S Al Fe P Cl