# 14.1 Brønsted-lowry acids and bases  (Page 4/14)

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${\text{2H}}_{2}\text{O(}l\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{+}\left(aq\right)+{\text{OH}}^{\text{−}}\left(aq\right)$

The ion product of water, K w is the equilibrium constant for the autoionization reaction:

${K}_{\text{w}}=\left[{\text{H}}_{3}{\text{O}}^{\text{+}}\right]\left[{\text{OH}}^{\text{−}}\right]=1.0\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}1{0}^{-14}\phantom{\rule{0.2em}{0ex}}\text{at}\phantom{\rule{0.2em}{0ex}}25\phantom{\rule{0.2em}{0ex}}\text{°C}$

## Key equations

• K w = [H 3 O + ][OH ] = 1.0 $×$ 10 −14 (at 25 °C)

## Chemistry end of chapter exercises

Write equations that show NH 3 as both a conjugate acid and a conjugate base.

One example for NH 3 as a conjugate acid: ${\text{NH}}_{2}{}^{\text{−}}+{\text{H}}^{\text{+}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{3};$ as a conjugate base: ${\text{NH}}_{4}{}^{\text{+}}\left(aq\right)+{\text{OH}}^{\text{−}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{3}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)$

Write equations that show ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$ acting both as an acid and as a base.

Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:

(a) ${\text{H}}_{3}{\text{O}}^{\text{+}}$

(b) HCl

(c) NH 3

(d) CH 3 CO 2 H

(e) ${\text{NH}}_{4}{}^{\text{+}}$

(f) ${\text{HSO}}_{4}{}^{\text{−}}$

(a) ${\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{\text{+}}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right);$ (b) $\text{HCl}\left(l\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{\text{+}}\left(aq\right)+{\text{Cl}}^{\text{−}}\left(aq\right);$ (c) ${\text{NH}}_{3}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{\text{+}}\left(aq\right)+{\text{NH}}_{2}{}^{\text{−}}\left(aq\right);$ (d) ${\text{CH}}_{3}{\text{CO}}_{2}\text{H}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{\text{+}}\left(aq\right)+{\text{CH}}_{3}{\text{CO}}_{2}{}^{\text{−}}\left(aq\right);$ (e) ${\text{NH}}_{4}{}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{\text{+}}\left(aq\right)+{\text{NH}}_{3}\left(aq\right);$ (f) ${\text{HSO}}_{4}{}^{\text{−}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}^{+}\left(aq\right)+{\text{SO}}_{4}{}^{\text{2−}}\left(aq\right)$

Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry acid:

(a) HNO 3

(b) ${\text{PH}}_{4}{}^{\text{+}}$

(c) H 2 S

(d) CH 3 CH 2 COOH

(e) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$

(f) HS

Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:

(a) H 2 O

(b) OH

(c) NH 3

(d) CN

(e) S 2−

(f) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$

(a) ${\text{H}}_{2}\text{O}\left(l\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right);$ (b) ${\text{OH}}^{\text{−}}\left(aq\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}\text{O}\left(l\right);$ (c) ${\text{NH}}_{3}\left(aq\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{4}{}^{\text{+}}\left(aq\right);$ (d) ${\text{CN}}^{\text{−}}\left(aq\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{HCN}\left(aq\right);$ (e) ${\text{S}}^{2-}\left(aq\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{HS}}^{\text{−}}\left(aq\right);$ (f) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}\left(aq\right)+{\text{H}}^{\text{+}}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{PO}}_{4}\left(aq\right)$

Show by suitable net ionic equations that each of the following species can act as a Brønsted-Lowry base:

(a) HS

(b) ${\text{PO}}_{4}{}^{\text{3−}}$

(c) ${\text{NH}}_{2}{}^{\text{−}}$

(d) C 2 H 5 OH

(e) O 2−

(f) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$

What is the conjugate acid of each of the following? What is the conjugate base of each?

(a) OH

(b) H 2 O

(c) ${\text{HCO}}_{3}{}^{\text{−}}$

(d) NH 3

(e) ${\text{HSO}}_{4}{}^{\text{−}}$

(f) H 2 O 2

(g) HS

(h) ${\text{H}}_{5}{\text{N}}_{2}{}^{\text{+}}$

(a) H 2 O, O 2− ; (b) H 3 O + , OH ; (c) H 2 CO 3 , ${\text{CO}}_{3}{}^{\text{2−}};$ (d) ${\text{NH}}_{4}{}^{\text{+}},$ ${\text{NH}}_{2}{}^{\text{−}};$ (e) H 2 SO 4 , ${\text{SO}}_{4}{}^{\text{2−}};$ (f) ${\text{H}}_{3}{\text{O}}_{2}{}^{\text{+}},$ ${\text{HO}}_{2}{}^{\text{−}};$ (g) H 2 S; S 2− ; (h) ${\text{H}}_{6}{\text{N}}_{2}{}^{2+},$ H 4 N 2

What is the conjugate acid of each of the following? What is the conjugate base of each?

(a) H 2 S

(b) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$

(c) PH 3

(d) HS

(e) ${\text{HSO}}_{3}{}^{\text{−}}$

(f) ${\text{H}}_{3}{\text{O}}_{2}{}^{\text{+}}$

(g) H 4 N 2

(h) CH 3 OH

Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:

(a) ${\text{HNO}}_{3}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{\text{+}}+{\text{NO}}_{3}{}^{\text{−}}$

(b) ${\text{CN}}^{\text{−}}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{HCN}+{\text{OH}}^{\text{−}}$

(c) ${\text{H}}_{2}{\text{SO}}_{4}+{\text{Cl}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}\text{HCl}+{\text{HSO}}_{4}{}^{\text{−}}$

(d) ${\text{HSO}}_{4}{}^{\text{−}}+{\text{OH}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{SO}}_{4}{}^{\text{2−}}+{\text{H}}_{2}\text{O}$

(e) ${\text{O}}^{2-}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}2{\mathrm{OH}}^{\text{−}}$

(f) ${\left[\text{Cu}{\left({\text{H}}_{2}\text{O}\right)}_{3}\left(\text{OH}\right)\right]}^{\text{+}}+{\left[\text{Al}{\left({\text{H}}_{2}\text{O}\right)}_{6}\right]}^{3+}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\left[\text{Cu}{\left({\text{H}}_{2}\text{O}\right)}_{4}\right]}^{2+}+{\left[\text{Al}{\left({\text{H}}_{2}\text{O}\right)}_{5}\left(\text{OH}\right)\right]}^{2+}$

(g) ${\text{H}}_{2}\text{S}+{\text{NH}}_{2}{}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{HS}}^{\text{−}}+{\text{NH}}_{3}$

The labels are Brønsted-Lowry acid = BA; its conjugate base = CB; Brønsted-Lowry base = BB; its conjugate acid = CA. (a) HNO 3 (BA), H 2 O(BB), H 3 O + (CA), ${\text{NO}}_{3}{}^{\text{−}}\left(\text{CB}\right);$ (b) CN (BB), H 2 O(BA), HCN(CA), OH (CB); (c) H 2 SO 4 (BA), Cl (BB), HCl(CA), ${\text{HSO}}_{4}{}^{\text{−}}\left(\text{CB}\right);$ (d) ${\text{HSO}}_{4}{}^{\text{−}}\left(\text{BA}\right),$ OH (BB), ${\text{SO}}_{4}{}^{\text{2−}}$ (CB), H 2 O(CA); (e) O 2− (BB), H 2 O(BA) OH (CB and CA); (f) [Cu(H 2 O) 3 (OH)] + (BB), [Al(H 2 O) 6 ] 3+ (BA), [Cu(H 2 O) 4 ] 2+ (CA), [Al(H 2 O) 5 (OH)] 2+ (CB); (g) H 2 S(BA), ${\text{NH}}_{2}{}^{\text{−}}\left(\text{BB}\right),$ HS (CB), NH 3 (CA)

Identify and label the Brønsted-Lowry acid, its conjugate base, the Brønsted-Lowry base, and its conjugate acid in each of the following equations:

(a) ${\text{NO}}_{2}{}^{\text{−}}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{HNO}}_{2}+{\text{OH}}^{\text{−}}$

(b) $\text{HBr}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{\text{+}}+{\text{Br}}^{\text{−}}$

(c) ${\text{HS}}^{\text{−}}+{\text{H}}_{2}\text{O}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}\text{S}+{\text{OH}}^{\text{−}}$

(d) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}+{\text{OH}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{HPO}}_{4}{}^{\text{2−}}+{\text{H}}_{2}\text{O}$

(e) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}+\text{HCl}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{PO}}_{4}+{\text{Cl}}^{\text{−}}$

(f) ${\left[\text{Fe}{\left({\text{H}}_{2}\text{O}\right)}_{5}\left(\text{OH}\right)\right]}^{2+}+{\left[\text{Al}{\left({\text{H}}_{2}\text{O}\right)}_{6}\right]}^{3+}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{[Fe}{\left({\text{H}}_{2}\text{O}\right)}_{6}\right]}^{3+}+{\left[\text{Al}{\left({\text{H}}_{2}\text{O}\right)}_{5}\left(\text{OH}\right)\right]}^{2+}$

(g) ${\text{CH}}_{3}\text{OH}+{\text{H}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{CH}}_{3}{\text{O}}^{\text{−}}+{\text{H}}_{2}$

What are amphiprotic species? Illustrate with suitable equations.

Amphiprotic species may either gain or lose a proton in a chemical reaction, thus acting as a base or an acid. An example is H 2 O. As an acid:
${\text{H}}_{2}\text{O}\left(aq\right)+{\text{NH}}_{3}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{4}{}^{\text{+}}\left(aq\right)+{\text{OH}}^{\text{−}}\left(aq\right).$ As a base: ${\text{H}}_{2}\text{O}\left(aq\right)+\text{HCl}\left(aq\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+{\text{Cl}}^{\text{−}}\left(aq\right)$

State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species:

(a) H 2 O

(b) ${\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}$

(c) S 2−

(d) ${\text{CO}}_{3}{}^{\text{2−}}$

(e) ${\text{HSO}}_{4}{}^{\text{−}}$

State which of the following species are amphiprotic and write chemical equations illustrating the amphiprotic character of these species.

(a) NH 3

(b) ${\text{HPO}}_{4}{}^{\text{−}}$

(c) Br

(d) ${\text{NH}}_{4}{}^{\text{+}}$

(e) ${\text{ASO}}_{4}{}^{\text{3−}}$

amphiprotic: (a) ${\text{NH}}_{3}+{\text{H}}_{3}{\text{O}}^{\text{+}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{4}\text{OH}+{\text{H}}_{2}\text{O},$ ${\text{NH}}_{3}+{\text{OCH}}_{3}{}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{NH}}_{2}{}^{\text{−}}+{\text{CH}}_{3}\text{OH};$ (b) ${\text{HPO}}_{4}{}^{\text{2−}}+{\text{OH}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{PO}}_{4}{}^{\text{3−}}+{\text{H}}_{2}\text{O},$ ${\text{HPO}}_{4}{}^{\text{2−}}+{\text{HClO}}_{4}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}{\text{PO}}_{4}{}^{\text{−}}+{\text{ClO}}_{4}{}^{\text{−}};$ not amphiprotic: (c) Br ; (d) ${\text{NH}}_{4}{}^{\text{+}};$ (e) ${\text{AsO}}_{4}{}^{\text{3−}}$

Is the self ionization of water endothermic or exothermic? The ionization constant for water ( K w ) is 2.9 $×$ 10 −14 at 40 °C and 9.3 $×$ 10 −14 at 60 °C.

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