CHM 2046C Module 11 Sample Exam Answers

Part H: Discussion Questions Chapter 14 10 points

1. Explain the difference between an Arrhenius, Bronsted-Lowry, and Lewis Acid or (Arrhenius, Bronsted-Lowry, and Lewis Bases)?

Arrhenius acid – a material that produces H⁺ ions in aqueous solutions.

Bronsted-Lowry acid – a material that can donate a proton to another substance

Lewis Acid – a material that accepts an electron pair in a chemical reaction

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2. Write equations to show how the HCO₃ ^1- ion can act either as an acid or a base (likewise for HPO₄ ^2- , HSO₄ ^1- or H₂PO₄ ^1-):

The hydrogen carbonate ion acting as an acid:

HCO₃ ^1-(aq) + HOH ßà H₃O¹⁺ (aq) + CO ^2- (aq)

The hydrogen carbonate ion acting as a base:

HCO₃ ^1-(aq) + HOH ßà H₂CO₃ (aq) + OH^1- (aq)

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3. In ionization reactions of an acid, base, or water one of the reactant molecules is water. But in the K_a, K_b, and K_w expressions, the concentration of water is not included in the calculation. Why?

The K_a, K_b, and K_w are special ionization constants which incorporate the concentration of water into the constant because it is not a variable in the ionization expressions. A constant times a constant is a constant which is either K_a, K_b, or K_w.

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4. What is the approximate molar concentration of water at room temperature?

At room temperature the density of water is 1.0 g/mL. Therefore one liter of water, which is 1000 mL, is 1000 grams. 1000 grams divided by the molar mass of water, 18.0 g/mol, is 55.5 moles. The molarity of water is 55.5 mol/liter or 55.5 M

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5. Why are salts from the neutralization of a strong acid and a weak base test acidic with litmus paper (Likewise a salt from the neutralization a weak acid and strong base test basic with litmus paper)?

A strong acid is nitric acid, HNO₃. A weak base is ammonium hydroxide, NH₄OH. The salt form from the neutralization of these two would be ammonium nitrate, NH₄NO₃. If I dissolved ammonium nitrate in water I get the following:

NH₄NO₃(s) + HOH à NH₄¹⁺(aq) + NO₃^1-(aq)

NH₄¹⁺⁽aq) + 2HOH ß à NH₄OH* (aq) + H₃O⁺¹ (aq)

* [ Ammonium hydroxide, NH₄OH (aq) is really NH₃ (aq) + HOH (l) ]

NO₃^1-(aq) + HOH à No reaction (salt of a strong acid)

Therefore the solution is acidic, since hydronium ions are formed from the hydrolysis of the ammonium ion forming a weak base ammonium hydroxide and nitrate ion does not hydrolyze because it comes from the salt of a strong acid.

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6. Why are salts from the neutralization of a strong acid and a strong base test neutral (or a pH of 7.0) with a pH meter?

The usual example given is Sodium chloride, table salt or NaCl. The salt is made from the neutralization of a strong acid, hydrochloric acid or HCl, and a strong base Sodium hydroxide or NaOH. The following is the hydrolysis reactions of the salt, sodium chlorides, when it is dissolved in water:

NaCl(s) + HOH à Na¹⁺(aq) + Cl^1-(aq)

Na¹⁺⁽aq) + HOH à No reaction (salt of a strong base)

Cl^1-(aq) + HOH à No reaction (salt of a strong acid)

Therefore the solution is Neutral because neither ion hydrolyzes in water and the pH is determine by the only ionization reaction in the solution which is the ionization of pure water which tests 7.00 with a pH meter.