Chapter 8.1, Problem 8.6Q

(a)

Interpretation Introduction

Interpretation:

The conjugate acid-base pairs for the reversible reaction of phosphoric acid with water have to be identified.

Concept Introduction:

Bronsted-Lowry acid:  According toBrønsted-Lowry theory, an acid is a proton donor.

Bronsted-Lowry base:  According toBrønsted-Lowry theory, a base is a proton acceptor.

When a Brønsted-Lowry acid donates a proton to a Brønsted-Lowry base, the base that

accepts the proton becomes a conjugate acid.  Contrastingly, the acid that donated the

proton becomes a conjugate base.

    $\begin{array}{l}\text{HX}\text{+}\text{Y }\stackrel{}{\rightleftharpoons }{\text{HY}}^{\text{+}}\text{+}{\text{X}}^{\text{-}}\\ \text{Acid}\text{Base}\text{Conjugate}\text{acid}\text{Conjugate}\text{base}\\ \\ {\text{HY}}^{\text{+}}\text{and}\text{Y}\text{are}\text{Conjugate}\text{acid-base}\text{pair}\\ \\ \text{HX}\text{and}{\text{X}}^{\text{-}}\text{are}\text{Conjugate}\text{acid-base}\text{pair}\end{array}$

Explanation of Solution

Since phosphoric acid is a Bronsted-Lowry acid, it donates proton when it reacts with water.  The chemical reaction between phosphoric acidand water and conjugate acid-base pairs are given by

    $\begin{array}{l}{\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}\text{+}{\text{H}}_{\text{2}}\text{O}\stackrel{}{\rightleftharpoons }{\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}\text{+}{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \\ {\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}\text{+}{\text{H}}_{\text{2}}\text{O}\stackrel{}{\rightleftharpoons }{\text{HPO}}_{\text{4}}{}^{\text{2-}}\text{+}{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \\ {\text{HPO}}_{\text{4}}{}^{\text{2-}}\text{+}{\text{H}}_{\text{2}}\text{O}\stackrel{}{\rightleftharpoons }{\text{PO}}_{\text{4}}{}^{\text{3-}}\text{+}{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\\ \\ {\text{H}}_{\text{3}}{\text{PO}}_{\text{4}}\text{and}{\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}\text{conjugate}\text{acid-base}\text{pair}\\ \\ {\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}{}^{\text{-}}\text{and}{\text{HPO}}_{\text{4}}{}^{\text{2-}}\text{conjugate}\text{acid-base}\text{pair}\\ \\ {\text{HPO}}_{\text{4}}{}^{\text{2-}}\text{and}{\text{PO}}_{\text{4}}{}^{\text{3-}}\text{conjugate}\text{acid-base}\text{pair}\\ \\ {\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\text{and}{\text{H}}_{\text{2}}\text{O}\text{are}\text{conjgate}\text{acid-base}\text{pair}\end{array}$

(b)

Interpretation Introduction

Interpretation:

The conjugate acid-base pairsfor the reversible reaction ofmethyl amine with water have to be identified.

Concept Introduction:

Refer to part a.

Explanation of Solution

Since methyl amine is a Bronsted-Lowry base, it accepts proton when it reacts with water. The chemical reaction between methylamineand water and conjugate acid-base pairs are given by

    $\begin{array}{l}{\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}\text{+}{\text{H}}_{\text{2}}\text{O}\stackrel{}{\rightleftharpoons }{\text{CH}}_{\text{3}}{\text{NH}}_{\text{3}}{}^{\text{+}}\text{+}{\text{OH}}^{\text{-}}\\ \\ {\text{CH}}_{\text{3}}{\text{NH}}_{\text{3}}{}^{\text{+}}\text{and}{\text{CH}}_{\text{3}}{\text{NH}}_{\text{2}}\text{are}\text{conjugate acid-base pair}\\ \\ {\text{H}}_{\text{2}}\text{O}\text{and}{\text{OH}}^{\text{-}}\text{are}\text{conjugate acid-base pair}\end{array}$