Question

Consider the reaction of acetic acid in water

${\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}{\mathbf{H}}_{\left(\mathbf{aq}\right)}+{\mathbf{H}}_{\mathbf{2}}{\mathbf{O}}_{\left(\mathbf{l}\right)}\rightleftharpoons {\mathbf{CH}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}{}^{-}{-}_{\left(\mathbf{aq}\right)}+{\mathbf{H}}_{\mathbf{3}}{{\mathbf{O}}^{+}}_{\left(\mathbf{aq}\right)},\mathbf{where}{\mathbf{K}}_{\mathbf{a}}=\mathbf{1}.\mathbf{8}\times {\mathbf{10}}^{-\mathbf{5}}.$

a. Which two bases are competing for the proton?

b. Which is the stronger base?

c. In light of your answer to part b, why do we classify the acetate ion $\left({\mathbf{CH}}_{\mathbf{3}}\mathbf{C}{{\mathbf{O}}_{\mathbf{2}}}^{-}\right)$asa weak base?

Use an appropriate reaction to justify your answer.

Short answer

If an acid has a higher K_avalue, it will be stronger as compared to another with a low value of K_a. Strong acids give a weak conjugate base.

1. ${\text{H}}_{2}OandC{H}_{3}CO{O}^{-}$
2. ${\text{CH}}_{3}CO{O}^{-}$is the weak base as compared to ${\text{H}}_{2}O$
3. The stronger the acid, the weaker its conjugate base will be.

Step-by-step solution

Reaction of Acetic Acid with Water

$$\begin{gathered} C{H}_{3}COO{H}_{\left(aq\right)}+H_2{O}_{\left(l\right)}\rightleftharpoons C{H}_{3}CO{O}^{-}+H_3{O^{+}}_{\left(aq\right)} \\ AcidBaseConjugateConjugate \\ baseacid \end{gathered}$$

Bases Competing for the Proton

The two bases competing for the proton in the given reaction are ${\mathbf{\text{H}}}_{\mathbf{2}}\mathbf{O}$ and conjugate base of acetic acid, i.e.,${\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{COO}}^{-}$ , because two are the species acting as the base in the given reaction.

Stronger Base out of H2O and CH3COO-  

Acetate ion, i.e., ${\text{CH}}_{3}CO{O}^{-}$ is the weak base as compared to ${\text{H}}_{2}O$ . This means acetate ion has a lesser tendency to accept a proton.

Reason for Acetate ion to be Weaker as Compared to  H2O

K_a value for acetic acid is given$1.8\times {10}^{-5}$, while for water it is $1.0\times {10}^{-14}$so, acetic acid is stronger acid as compared to water (this can be seen from the given equation as well).

The stronger the acid, the weaker its conjugate base will be.