Question 14.3: PREDICTING THE DIRECTION OF ACID–BASE REACTIONS If you mix e...
PREDICTING THE DIRECTION OF ACID–BASE REACTIONS
If you mix equal concentrations of reactants and products, which of the following reactions proceed to the right and which proceed to the left?
(a) H_{2}SO_{4}(aq) + NH_{3}(aq) \rightleftharpoons NH_{4}^{+}(aq) + HSO_{4}^{-}(aq)
(b) HCO_{3}^{-}(aq) + SO_{4}^{2-}(aq) \rightleftharpoons HSO_{4}^{-}(aq) + CO_{3}^{2-}(aq)
STRATEGY
To predict the direction of reaction, use the balanced equation to identify the acids and bases, and then use Table 14.1 to identify the stronger acid and the stronger base. When equal concentrations of reactants and products are present, proton transfer always occurs from the stronger acid to the stronger base.
TABLE 14.1 Relative Strengths of Conjugate Acid–Base Pairs
| Acid, HA | Base, A^{-} | ||
| Stronger acid
Weaker |
\left. \begin{matrix} HClO_{4}\\HCl\\H_{2}SO_{4}\\HNO_{3} \end{matrix} \right\}\begin{matrix} Strong acids:\\100\% dissociated\\in aqueous\\solution. \end{matrix}
H_{3}O^{+}
\left. \begin{matrix} HSO_{4}^{-}\\H_{3}PO_{4} \\HNO_{2}\\HF\\CH_{3}CO_{2}H\\H_{2}CO_{3} \\H_{2}S\\NH_{4}^{+} \\ HCN\\HCO_{3}^{-} \end{matrix} \right\}\begin{matrix}Weak acids:\\Exist in solution \\ as a mixture of\\HA, A^{–}, and H_{3}O^{+}. \end{matrix}
H_{2}O
\left. \begin{matrix} NH_{3}\\OH^{–}\\H_{2} \end{matrix} \right\}\begin{matrix} Very weak acids:\\Negligible tendency\\to dissociate . \end{matrix} |
\left. \begin{matrix} ClO_{4}^{–}\\Cl^{–}\\HSO_{4}^{–} \\NO_{3}^{-} \end{matrix} \right\}\begin{matrix} Very weak bases:\\ Negligible tendency \\to be protonated in\\aqueous solution. \end{matrix}
H_{2}O
\left. \begin{matrix} SO_{4}^{2–}\\H_{2}PO_{4}^{–} \\NO_{2}^{–}\\F^{–}\\ CH_{3}CO_{2}^{–}\\HCO_{3}^{–} \\HS^{–}\\NH_{3}\\CN^{–}\\ CO_{3}^{2-}\end{matrix} \right\}\begin{matrix} Weak bases:\\Moderate tendency \\ to be protonated in\\aqueous solution. \end{matrix}
OH^{-}
\left. \begin{matrix} NH_{2}^{–}\\O^{2–}\\H^{-} \end{matrix} \right\}\begin{matrix} Strong bases:\\100\% protonated in\\aqueous solution. \end{matrix} |
Weaker base
Stronger |
(a) In this reaction, H_{2}SO_{4} and NH_{4}^{+} are the acids, and NH_{3} and HSO_{4}^{-} are the bases.
According to Table 14.1, H_{2}SO_{4} is a stronger acid than NH_{4}^{+} and NH_{3} is a stronger base than HSO_{4}^{-}. Therefore, NH_{3} gets the proton and the reaction proceeds from left to right.
(b) HCO_{3}^{-} and HSO_{4}^{-} are the acids, and SO_{4}^{2-} and CO_{3}^{2-} are the bases. Table 14.1 indicates that HSO_{4}^{-} is the stronger acid and CO_{3}^{2-} is the stronger base. Therefore, CO_{3}^{2-} gets the proton and the reaction proceeds from right to left.
\underset{Weaker acid}{HCO_{3}^{-}(aq)} + \underset{Weaker acid}{SO_{4}^{2-}(aq)} → \underset{Stronger acid}{HSO_{4}^{-}(aq)} + \underset{Stronger acid}{CO_{3}^{2-}(aq)}