94 Relative strengths of acids and bases: effect of molecular structure

(a) Identify the strong Brønsted-Lowry acids and strong Brønsted-Lowry bases.

(b) List those compounds in (a) that can behave as Brønsted-Lowry acids with strengths lying between those of H₃O⁺ and H₂O.

(c) List those compounds in (a) that can behave as Brønsted-Lowry bases with strengths lying between those of H₂O and OH⁻.

3. Household ammonia is a solution of the weak base NH₃ in water. List, in order of descending concentration, all of the ionic and molecular species present in a 1 M aqueous solution of this base.

4. Explain why the ionization constant, K_a, for H₂SO₄ is larger than the ionization constant for H₂SO₃.

Answer(s): The oxidation state of the sulfur in H₂SO₄ is greater than the oxidation state of the sulfur in H₂SO₃.

5. Explain why HI is a stronger acid than HF.

6. Gastric juice, the digestive fluid produced in the stomach, contains hydrochloric acid, HCl. Milk of Magnesia, a suspension of solid Mg(OH)₂ in an aqueous medium, is sometimes used to neutralize excess stomach acid. Write a complete balanced equation for the neutralization reaction, and identify the conjugate acid-base pairs.

Answer(s): \(\begin{array}{cccccc}\text{Mg}{\left(\text{OH}\right)}_{2}\left(s\right)+& \text{2HCl}\left(aq\right)& \phantom{\rule{0.2em}{0ex}}\rightarrow\phantom{\rule{0.2em}{0ex}}& {\text{Mg}}^{2+}\left(aq\right)+& 2{\text{Cl}}^{\text{−}}\left(aq\right)+\phantom{\rule{0.2em}{0ex}}& {\text{2H}}_{2}\text{O}\left(l\right)\\ \text{BB}& \text{BA}& & \text{CB}& \text{CA}& \end{array}\)

7. Consult the Table of Kb values to determine if CH₃NH₂ or (CH₃)₂NH, is the stronger base? Which conjugate acid, \({\left({\text{CH}}_{3}\right)}_{2}{\text{NH}}_{2}{}^{\text{+}}\) or \({\text{CH}}_{3}{\text{NH}}_{3}{}^{\text{+}}\), is the stronger acid?

Answer(s): The stronger base or stronger acid is the one with the larger K_b or K_a, respectively. In these two examples, they are (CH₃)₂NH and \({\text{CH}}_{3}{\text{NH}}_{3}{}^{\text{+}}.\)

8. Predict which acid in each of the following pairs is the stronger and explain your reasoning for each.

(a) H₂O or HF

(b) B(OH)₃ or Al(OH)₃

(c) \({\text{HSO}}_{3}{}^{\text{−}}\) or \({\text{HSO}}_{4}{}^{\text{−}}\)

(d) NH₃ or H₂S

(e) H₂O or H₂Te

9. Predict which compound in each of the following pairs of compounds is more acidic and explain your reasoning for each.

(a) \({\text{HSO}}_{4}{}^{\text{−}}\) or \({\text{HSeO}}_{4}{}^{\text{−}}\)

(b) NH₃ or H₂O

(c) PH₃ or HI

(d) NH₃ or PH₃

(e) H₂S or HBr

Answer(s): (a) \({\text{HSO}}_{4}{}^{\text{−}};\) higher electronegativity of the central ion. (b) H₂O; oxygen is more electronegative than nitrogen.  In addition, we know that NH₃ is a base and water is neutral, or we can compare K_a values. (c) HI; PH₃ is weaker than HCl (acidity increases as you move across a row); HCl is weaker than HI (acid strength increases as you move down a period). Thus, PH₃ is weaker than HI. (d) PH₃; in binary compounds of hydrogen with nonmetals, the acidity increases for the element lower in a group. (e) HBr; in a period, the acidity increases from left to right; in a group, it increases from top to bottom. Br is to the right and below S, so HBr is the stronger acid.

10. Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.

(a) acidity: HCl, HBr, HI

(b) basicity: H₂O, OH⁻, H⁻, Cl⁻

(c) basicity: Mg(OH)₂, Si(OH)₄, ClO₃(OH) (Hint: Formula could also be written as HClO₄.)

(d) acidity: HF, H₂O, NH₃, CH₄

11. Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.

(a) acidity: NaHSO₃, NaHSeO₃, NaHSO₄

(b) basicity: \({\text{BrO}}_{2}{}^{\text{-}},\) \({\text{ClO}}_{2}{}^{\text{-}},\) \({\text{IO}}_{2}{}^{\text{-}}\)

(c) acidity: HOCl, HOBr, HOI

(d) acidity: HOCl, HOClO, HOClO₂, HOClO₃

(e) basicity: \({\text{NH}}_{2}{}^{\text{-}},\) HS⁻, HTe⁻, \({\text{PH}}_{2}{}^{\text{-}}\)

(f) basicity: BrO⁻, \({\text{BrO}}_{2}{}^{\text{-}},\) \({\text{BrO}}_{3}{}^{\text{-}},\) \({\text{BrO}}_{4}{}^{\text{-}}\)

Answer(s): (a) NaHSeO₃ < NaHSO₃ < NaHSO₄; in polyoxy acids, the more electronegative central element—S, in this case—forms the stronger acid. The larger number of oxygen atoms on the central atom (giving it a higher oxidation state) also creates a greater release of hydrogen atoms, resulting in a stronger acid. As a salt, the acidity increases in the same manner. (b) \({\text{ClO}}_{2}{}^{\text{-}}<{\text{BrO}}_{2}{}^{\text{-}}<{\text{IO}}_{2}{}^{\text{-}};\) the basicity of the anions in a series of acids will be the opposite of the acidity in their oxyacids. The acidity increases as the electronegativity of the central atom increases. Cl is more electronegative than Br, and I is the least electronegative of the three. (c) HOI < HOBr < HOCl; in a series of the same form of oxyacids, the acidity increases as the electronegativity of the central atom increases. Cl is more electronegative than Br, and I is the least electronegative of the three. (d) HOCl < HOClO < HOClO₂ < HOClO₃; in a series of oxyacids of the same central element, the acidity increases as the number of oxygen atoms increases (or as the oxidation state of the central atom increases). (e) \({\text{HTe}}^{\text{-}}<{\text{HS}}^{\text{-}}<<\phantom{\rule{0.2em}{0ex}}{\text{PH}}_{2}{}^{\text{-}}<{\text{NH}}_{2}{}^{\text{-}};\) \({\text{PH}}_{2}{}^{\text{-}}\) and \({\text{NH}}_{2}{}^{\text{-}}\) are anions of weak bases, so they act as strong bases toward H⁺. \({\text{HTe}}^{\text{-}}\) and HS⁻ are anions of weak acids, so they have less basic character. In a periodic group, the more electronegative element has the more basic anion. (f) \({\text{BrO}}_{4}{}^{\text{-}}<{\text{BrO}}_{3}{}^{\text{-}}<{\text{BrO}}_{2}{}^{\text{-}}<{\text{BrO}}^{\text{-}};\) with a larger number of oxygen atoms (that is, as the oxidation state of the central ion increases), the corresponding acid becomes more acidic and the anion consequently less basic.

12. The active ingredient formed by aspirin in the body is salicylic acid, C₆H₄OH(CO₂H). The carboxyl group (−CO₂H) acts as a weak acid. The phenol group (an OH group bonded to an aromatic ring) also acts as an acid but a much weaker acid. List, in order of descending concentration, all of the ionic and molecular species present in a 0.001-M aqueous solution of C₆H₄OH(CO₂H).

Answer(s): \(\left[{\text{H}}_{2}\text{O}\right]\phantom{\rule{0.2em}{0ex}}>\left[{\text{C}}_{6}{\text{H}}_{4}\text{OH}\left({\text{CO}}_{2}\text{H}\right)\right]\phantom{\rule{0.2em}{0ex}}>{\text{[H}}^{\text{+}}\text{]}\phantom{\rule{0.2em}{0ex}}\text{0}>{\text{[C}}_{6}{\text{H}}_{4}\text{OH}{\left({\text{CO}}_{2}\right)}^{\text{-}}\right]\gg \left[{\text{C}}_{6}{\text{H}}_{4}\text{O}{\left({\text{CO}}_{2}\text{H}\right)}^{\text{-}}\right]\phantom{\rule{0.2em}{0ex}}>\left[{\text{OH}}^{\text{-}}\right]\)