Chapter 15, Problem 1PE

Which of the following are conjugate acid-base pairs? Describe why the others are not true conjugate acid-base pairs: $\left(\text{a}\right){\text{ H}}_{\text{3}}{\text{PO}}_{\text{4}}{\text{ and H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}\text{, }\left(\text{b}\right){\text{ HI and H}}^{\text{+}}\text{, }\left(\text{c}\right){\text{ NH}}_{\text{2}}{}^{\text{-}}{\text{and NH}}_{\text{3}}\text{, }\left(\text{d}\right){\text{ HNO}}_{\text{2}}{\text{ and NH}}_{\text{4}}{}^{\text{+}}\text{, }\left(\text{e}\right){\text{ CO}}_{\text{3}}{}^{\text{2-}}{\text{and CN}}^{\text{-}}\text{, }\left(\text{f}\right){\text{ HPO}}_{\text{4}}{}^{\text{2-}}{\text{and H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}$. (Hint: Recall that conjugate acid-base pairs must differ by one ${\text{H}}^{\text{+}}$.)

Interpretation Introduction

Interpretation:

The conjugate acid-base pairs are to be identified and the reason for those which are not acid-base pairs is to be described.

Concept Introduction:

There are various concepts which describe acids and bases, such as Arrhenius concept, Bronsted-Lowry concept, Lewis concept.

According to Bronsted-Lowry concept, acids are the substances which provide ${\text{H}}^{+}$ in the solution. These are ${\text{H}}^{+}$ donors, such as HCl, HBr, while those substances which accept ${\text{H}}^{+}$ are termed as bases, such as ${\text{NH}}_3,{\text{ Cl}}^{-}$.

The species formed after donation of ${\text{H}}^{+}$ by a Bronsted acid is called its conjugate base. The species formed after accepting ${\text{H}}^{+}$ by a Bronsted base is called its conjugate acid. The acid corresponding to its conjugate base and the base corresponding to its conjugate acid are termed as conjugate acid-base pairs.

Answer to Problem 1PE

Solution:

a) This is a true conjugate acid-base pair.

b) ${\text{I}}^{-}$ is the conjugate base of HI.

c) This is a true conjugate acid-base pair.

d) ${\text{NO}}_2{}^{-}$ is the conjugate base of ${\text{HNO}}_2$ and ${\text{NH}}_3$ is the conjugate acid of ${\text{NH}}_4{}^{+}$.

e) ${\text{HCO}}_3{}^{-}$ is the conjugate acid of ${\text{CO}}_3{}^{2-}$ and HCN is the conjugate acid of ${\text{CN}}^{-}$.

f) This is a true conjugate acid-base pair.

Explanation of Solution

a) ${\text{H}}_3{\text{PO}}_4$ and ${\text{H}}_2{\text{PO}}_4{}^{-}$

${\text{H}}_3{\text{PO}}_4$ differs from ${\text{H}}_2{\text{PO}}_4{}^{-}$ only by ${\text{H}}^{+}$. The dissociation occurs as follows:

${\text{H}}_3{\text{PO}}_4\to {\text{ H}}_2{\text{PO}}_4{}^{-}{\text{ + H}}^{+}$.

${\text{H}}_3{\text{PO}}_4$ acts a Bronsted acid and ${\text{H}}_2{\text{PO}}_4{}^{-}$ as its conjugate base. So, these two form a conjugate acid-base pair.

b) $\text{HI}$ and ${\text{H}}^{+}$

$\text{HI}$ acts as Bronsted acid and dissociates as follows:

$\text{HI}\to {\text{ I}}^{-}{\text{ + H}}^{+}$

${\text{I}}^{-}$ is the conjugate base of HI. As $\text{HI}$ and ${\text{I}}^{-}$ differ only by ${\text{H}}^{+}$ and form a conjugate acid-base pair, ${\text{HI and H}}^{+}$ do not form a conjugate acid-base pair.

c) ${\text{NH}}_2{}^{-}$ and ${\text{NH}}_3$

${\text{NH}}_2{}^{-}$ acts as a Bronsted base and accepts ${\text{H}}^{+}$ to form ${\text{NH}}_3.$ The conversion occurs as follows: ${\text{NH}}_2{}^{-}{\text{ + H}}^{+}\to {\text{NH}}_3$

${\text{NH}}_3$ is the conjugate acid of ${\text{NH}}_2{}^{-}$. Hence, these two form a conjugate acid-base pair.

d) ${\text{HNO}}_2$ and ${\text{NH}}_4{}^{+}$

${\text{HNO}}_2$ acts as Bronsted acid and dissociates as follows:

${\text{HNO}}_2\to {\text{ NO}}_2{}^{-}{\text{ + H}}^{+}$

So, ${\text{NO}}_2{}^{-}$ is the conjugate base of ${\text{HNO}}_2$. However, ${\text{NH}}_4{}^{+}$ is not a part of the above equation. So, ${\text{HNO}}_2{\text{ and NH}}_4{}^{+}$ do not form a conjugate acid-base pair.

e) ${\text{CO}}_3{}^{2-}$ and ${\text{CN}}^{-}$

${\text{CO}}_3{}^{2-}$ acts as a Bronsted base and accepts ${\text{H}}^{+}$ to form ${\text{HCO}}_3{}^{-}.$ The conversion occurs as follows:

${\text{CO}}_3{}^{2-}{\text{ + H}}^{+}\to {\text{HCO}}_3{}^{-}$

${\text{HCO}}_3{}^{-}$ is the conjugate acid of ${\text{CO}}_3{}^{2-}$. However, ${\text{CN}}^{-}$ is not a part of the above equation. So, ${\text{CO}}_3{}^{2-}{\text{ and CN}}^{-}$ do not form a conjugate acid-base pair.

f) ${\text{HPO}}_4{}^{2-}$ and ${\text{H}}_2{\text{PO}}_4{}^{-}$

${\text{HPO}}_4{}^{2-}$ differs from ${\text{H}}_2{\text{PO}}_4{}^{-}$ only by ${\text{H}}^{+}$. Here, ${\text{HPO}}_4{}^{-}$ acts as a Bronsted base. The conversion occurs as follows:

${\text{HPO}}_4{}^{2-}{\text{+H}}^{+}\to {\text{H}}_2{\text{PO}}_4{}^{-}$.

${\text{HPO}}_4{}^{-}$ acts a Bronsted acid and ${\text{H}}_2{\text{PO}}_4{}^{-}$ as its conjugate base. So, these two form a conjugate acid-base pair.