Bronsted/Lowry Acids and Bases

Introduction

To determine the relative acidity of various salt solutions.

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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. Add 2-3 drops of distilled water from a Beral pipet to each small pile of crystals. Estimate the pH using the color charts provided.
2. Record your observation in the Data Table.
3. Complete the remaining entries of the Data Table.
   

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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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Sample Data Table:

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Safety

Some of the chemicals are toxic. Do not ingest any chemicals. Wash up spills immediately.

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1. 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.
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^-

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.

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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.

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 on the left side of these reactions. 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. 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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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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Some of the chemicals are toxic.

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• Each lab station should have its own set of dropper bottles.
• In most parts 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.
• 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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• plastic sheets (acetate)
• universal pH paper with color chart
• solid samples of the following salts in microcentrifuge tubes:
• Na₂CO₃•H₂O
• NaCl
• NaHCO₃
• NH₄Cl
• NaCH₃COO
• Na₃PO₄•12H₂O
• Al₂(SO₄)₃•18H₂O
• FeCl₃•6H₂O
• CuSO₄•5H₂O

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Interpretation of Data:

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

Base	Conjugate Acid	Extent of Reaction
CO32-	HCO3-	slight
Cl-	HCl	none
NO3-	HNO3	none
SO42-	HSO4-	none (is actually very slight)
CH3COO-	CH3COOH	slight
PO43-	HPO42-	slight

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Do not ingest any chemicals. Wash up spills immediately.

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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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Teacher preparation: 1 hour
Procedure: 20 minutes

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