Chemistry » Acid-Base and Redox Reactions » Acids And Bases Continued

Conjugate Acid-Base Pairs Continued

Conjugate acid-base pairs

Look at the reaction between hydrochloric acid and ammonia to form ammonium and chloride ions:

$$\text{HCl}(\text{aq}) + \text{NH}_{3}(\text{g})$$ $$\leftrightharpoons$$ $$\text{NH}_{4}^{+}(\text{aq}) + \text{Cl}^{-}(\text{aq})$$

Looking at the forward reaction (i.e. the reaction that proceeds from left to right), the changes that take place can be shown as follows:

• $${\color{red}{\text{HCl(aq)}}} \to {\text{Cl}}^{-}{\text{(aq)}} + {\color{darkgreen}{{\text{H}}^{+}{\text{(aq)}}}}$$

• $${\color{blue}{\text{NH}_{3}{\text{(g)}}}} + {\color{darkgreen}{\text{H}^{+}{\text{(aq)}}}} \to {\text{NH}_{4}^{+}{\text{(aq)}}}$$

• In the forward reaction, HCl is a proton donor ($${\color{red}{\textbf{acid}}}$$) and $$\text{NH}_{3}$$ is a proton acceptor ($${\color{blue}{\textbf{base}}}$$).

Looking at the reverse reaction (i.e. the reaction that proceeds from right to left), the changes that take place are as follows:

• $${\color{blue}{\text{Cl}^{-}{\text{(aq)}}}} + {\color{darkgreen}{{\text{H}}^{+}{\text{(aq)}}}} \to {\text{HCl(aq)}}$$

• $${\color{red}{\text{NH}_{4}^{+}{\text{(aq)}}}} \to {\text{NH}_{3}{\text{(g)}}} + {\color{darkgreen}{{\text{H}}^{+}{\text{(aq)}}}}$$

• In the reverse reaction, the chloride ion ($$\text{Cl}^{-}$$) is the proton acceptor ($${\color{blue}{\textbf{base}}}$$) and the ammonium ion ($$\text{NH}_{4}^{+}$$) is the proton donor ($${\color{red}{\textbf{acid}}}$$).

In the forward reaction $$\color{red}{\text{HCl}}$$ donates a proton ($$\text{H}^{+}$$) to form $$\color{blue}{\text{Cl}^{-}}$$. In the reverse reaction $$\color{blue}{\text{Cl}^{-}}$$ accepts a proton to form $$\color{red}{\text{HCl}}$$. $$\color{blue}{\text{Cl}^{-}}$$ is the $$\color{blue}{\text{conjugate base}}$$ of the $$\color{red}{\text{acid HCl}}$$. So $$\text{HCl}$$ and $$\text{Cl}^{-}$$ are a $${\color{orange}{\textbf{conjugate acid-base pair}}}$$.

Similarly, in the forward reaction $$\color{blue}{\text{NH}_{3}}$$ accepts a proton ($$\text{H}^{+}$$) to form $$\color{red}{{\text{NH}}_{4}^{+}}$$. In the reverse reaction $$\color{red}{{\text{NH}}_{4}^{+}}$$ donates a proton to form $$\color{blue}{\text{NH}_{3}}$$. $$\color{red}{\text{NH}_{4}^{+}}$$ is the $$\color{red}{\text{conjugate acid}}$$ of the $$\color{blue}{\text{base NH}_{3}}$$. So $$\text{NH}_{3}$$ and $$\text{NH}_{4}^{+}$$ are a $${\color{purple}{\textbf{conjugate acid-base pair}}}$$.

The reaction between ammonia and water is another example:

Definition: Conjugate acid-base pair

A conjugate acid-base pair contains two compounds that differ only by a hydrogen ion ($$\text{H}^{+}$$) and a charge of $$\text{+1}$$.

Example: Conjugate Acid-Base Pairs

Question

Determine the conjugate acid-base pairs for the following reaction:

$$\text{HNO}_{3}(\text{aq}) + \text{OH}^{-}(\text{aq})$$ $$\rightleftharpoons$$ $$\text{NO}_{3}^{-}(\text{aq}) + \text{H}_{2}\text{O}(\text{l})$$

Step 1: Which reactant is an acid and which is a base?

$$\text{HNO}_{3}$$ is nitric acid. It donates a proton in the forward reaction:

$$\text{HNO}_{3}(\text{aq})$$ $$\to$$ $$\text{NO}_{3}^{-}(\text{aq}) + \text{H}^{+}(\text{aq})$$

$$\text{OH}^{-}$$ accepts a proton in the forward reaction and is therefore the base:

$$\text{OH}^{-}(\text{aq}) + \text{H}^{+}(\text{aq})$$ $$\to$$ $$\text{H}_{2}\text{O}(\text{l})$$

Step 2: Which product is the conjugate base of the acid?

Nitric acid ($$\text{HNO}_{3}$$) donates a proton to become $$\text{NO}_{3}^{-}$$. In the reverse reaction $$\text{NO}_{3}^{-}$$ accepts a proton to become $$\text{HNO}_{3}$$. Therefore $$\text{NO}_{3}^{-}$$ is the conjugate base of $$\text{HNO}_{3}$$.

$$\text{NO}_{3}^{-}(\text{aq}) + \text{H}^{+}(\text{aq})$$ $$\to$$ $$\text{HNO}_{3}(\text{aq})$$

Step 3: Which product is the conjugate acid of the base?

$$\text{OH}^{-}$$ accepts a proton to become $$\text{H}_{2}\text{O}$$. In the reverse reaction $$\text{H}_{2}\text{O}$$ donates a proton to become $$\text{OH}^{-}$$. Therefore $$\text{H}_{2}\text{O}$$ is the conjugate acid of $$\text{OH}^{-}$$.

$$\text{H}_{2}\text{O}(\text{l})$$ $$\to$$ $$\text{OH}^{-}(\text{aq}) + \text{H}^{+}(\text{aq})$$