Exercises

1. We have seen an introductory definition of an acid: An acid is a compound that reacts with water and increases the amount of hydronium ion present. In the chapter on acids and bases, we saw two more definitions of acids: a compound that donates a proton (a hydrogen ion, H⁺) to another compound is called a Brønsted-Lowry acid, and a Lewis acid is any species that can accept a pair of electrons. Explain why the introductory definition is a macroscopic definition, while the Brønsted-Lowry definition and the Lewis definition are microscopic definitions.
2. Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:
   1. [latex]{\text{CO}}_{2}+{\text{OH}}^{-}\longrightarrow {\text{HCO}}_{3}{}^{-}[/latex]
   2. [latex]\text{B}{\left(\text{OH}\right)}_{3}+{\text{OH}}^{-}\longrightarrow \text{B}{\left(\text{OH}\right)}_{4}{}^{-}[/latex]
   3. [latex]{\text{I}}^{-}+{\text{I}}_{2}\longrightarrow {\text{I}}_{3}{}^{-}[/latex]
   4. [latex]{\text{AlCl}}_{3}+{\text{Cl}}^{-}\longrightarrow {\text{AlCl}}_{4}{}^{-}[/latex] (use Al-Cl single bonds)
   5. [latex]{\text{O}}^{2-}+{\text{SO}}_{3}\longrightarrow {\text{SO}}_{4}{}^{2-}[/latex]
3. Write the Lewis structures of the reactants and product of each of the following equations, and identify the Lewis acid and the Lewis base in each:
   1. [latex]{\text{CS}}_{2}+{\text{SH}}^{-}\longrightarrow {\text{HCS}}_{3}{}^{-}[/latex]
   2. [latex]{\text{BF}}_{3}+{\text{F}}^{-}\longrightarrow {\text{BF}}_{4}{}^{-}[/latex]
   3. [latex]{\text{I}}^{-}+{\text{SnI}}_{2}\longrightarrow {\text{SnI}}_{3}{}^{-}[/latex]
   4. [latex]\text{Al}{\left(\text{OH}\right)}_{3}+{\text{OH}}^{-}\longrightarrow \text{Al}{\left(\text{OH}\right)}_{4}{}^{-}[/latex]
   5. [latex]{\text{F}}^{-}+{\text{SO}}_{3}\longrightarrow {\text{SFO}}_{3}{}^{-}[/latex]
4. Using Lewis structures, write balanced equations for the following reactions:
   1. [latex]\text{HCl}\left(g\right)+{\text{PH}}_{3}\left(g\right)\longrightarrow [/latex]
   2. [latex]{\text{H}}_{3}{\text{O}}^{+}+{\text{CH}}_{3}{}^{-}\longrightarrow [/latex]
   3. [latex]\text{CaO}+{\text{SO}}_{3}\longrightarrow [/latex]
   4. [latex]{\text{NH}}_{4}{}^{+}+{\text{C}}_{2}{\text{H}}_{5}{\text{O}}^{-}\longrightarrow [/latex]
5. In dilute aqueous solution HF acts as a weak acid. However, pure liquid HF (boiling point = 19.5 °C) is a strong acid. In liquid HF, HNO₃ acts like a base and accepts protons. The acidity of liquid HF can be increased by adding one of several inorganic fluorides that are Lewis acids and accept F^– ion (for example, BF₃ or SbF₅). Write balanced chemical equations for the reaction of pure HNO₃ with pure HF and of pure HF with BF₃. Write the Lewis structures of the reactants and products, and identify the conjugate acid-base pairs.
6. The simplest amino acid is glycine, H₂NCH₂CO₂H. The common feature of amino acids is that they contain the functional groups: an amine group, –NH₂, and a carboxylic acid group, –CO₂H. An amino acid can function as either an acid or a base. For glycine, the acid strength of the carboxyl group is about the same as that of acetic acid, CH₃CO₂H, and the base strength of the amino group is slightly greater than that of ammonia, NH₃.
   1. Write the Lewis structures of the ions that form when glycine is dissolved in 1 M HCl and in 1 M KOH.
   2. Write the Lewis structure of glycine when this amino acid is dissolved in water. (Hint: Consider the relative base strengths of the –NH₂ and [latex]-{\text{CO}}_{2}{}^{-}[/latex] groups.)
7. Boric acid, H₃BO₃, is not a Brønsted-Lowry acid but a Lewis acid.
   1. Write an equation for its reaction with water.
   2. Predict the shape of the anion thus formed.
   3. What is the hybridization on the boron consistent with the shape you have predicted?

Show Selected Answers

1.

(a)

(b)

(c)

(d)

(e)

4.

(a)

(b) [latex]{\text{H}}_{3}{\text{O}}^{+}+{\text{CH}}_{3}{}^{-}\longrightarrow {\text{CH}}_{4}+{\text{H}}_{2}\text{O}[/latex]

(c) [latex]\text{CaO}+{\text{SO}}_{3}\longrightarrow \text{CaSO}4[/latex]

(d) [latex]{\text{NH}}_{4}{}^{+}+{\text{C}}_{2}{\text{H}}_{5}{\text{O}}^{-}\longrightarrow {\text{C}}_{2}{\text{H}}_{5}\text{OH}+{\text{NH}}_{3}[/latex]

 

5. [latex]{\text{HNO}}_{3}\left(l\right)+\text{HF}\left(l\right)\longrightarrow {\text{H}}_{2}{\text{NO}}_{3}{}^{+}+{\text{F}}^{-}[/latex]; [latex]\text{HF}\left(l\right)+{\text{BF}}_{3}\left(g\right)\longrightarrow {\text{H}}^{+}+{\text{BF}}_{4}[/latex]

7. (a) [latex]{\text{H}}_{3}{\text{BO}}_{3}+{\text{H}}_{2}\text{O}\longrightarrow {\text{H}}_{4}{\text{BO}}_{4}{}^{-}+{\text{H}}^{+}[/latex];

(b) First, form a symmetrical structure with the unique atom, B, as the central atom. Then include the 32e^– to form the Lewis structure:

Because there are four bonds and no lone pair (unshared pair) on B, the electronic and molecular shapes are the same—both tetrahedral.

(c) The tetrahedral structure is consistent with sp³ hybridization.