# Conjugate Acid-Base Pairs

## Conjugate acid-base pairs

Look at the reaction between hydrochloric acid and ammonia to form ammonium and chloride ions (again we have highlighted the different parts of the equation):

$${\color{red}{\text{HCl}}}\text{ (aq)} + {\color{blue}{\text{NH}_{3}}}\text{(aq)} \rightarrow {\color{blue}{\text{NH}_{4}^{+}}}\text{(aq)} + {\color{red}{\text{Cl}^{-}}}\text{(aq)}$$

We look at what happens to each of the reactants in the reaction:

$$\text{HCl (aq)} \rightarrow \text{Cl}^{-}\text{(aq)}$$ and

$$\text{NH}_{3}\text{(aq)} \rightarrow \text{NH}_{4}^{+}\text{(aq)}$$

We see that $$\text{HCl}$$ acts as the acid and $$\text{NH}_{3}$$ acts as the base.

But what if we actually had the following reaction:

$${\color{blue}{\text{NH}_{4}^{+}}}\text{(aq)} + {\color{red}{\text{Cl}^{-}}}\text{(aq)} \rightarrow {\color{red}{\text{HCl}}}\text{ (aq)} + {\color{blue}{\text{NH}_{3}}}\text{(aq)}$$

This is the same reaction as the first one, but the products are now the reactants.

Now if we look at the what happens to each of the reactants we see the following:

$$\text{NH}_{4}^{+}\text{(aq)} \rightarrow \text{NH}_{3}\text{(aq)}$$ and

$$\text{Cl}^{-}\text{(aq)} \rightarrow \text{HCl (aq)}$$

We see that $$\text{NH}_{4}^{+}$$ acts as the acid and $$\text{Cl}^{-}$$ acts as the base.

#### Tip:

Up to now you have looked at reactions as starting with the reactants and going to the products. For acids and bases we also need to consider what happens if we swop the reactants and the products around. This will help you understand conjugate acid-base pairs.

When $$\text{HCl}$$ (the acid) loses a proton it forms $$\text{Cl}^{-}$$ (the base). And that when $$\text{Cl}^{-}$$ (the base) gains a proton it forms $$\text{HCl}$$ (the acid). We call these two species a conjugate acid-base pair. Similarly $$\text{NH}_{3}$$ and $$\text{NH}_{4}^{+}$$ form a conjugate acid-base pair.

#### Tip:

The word conjugate means coupled or connected.

We can represent this as:

### Optional Activity: Conjugate acid-base pairs

Using the common acids and bases from the table in the previous lesson, pick an acid and a base from the list. Write a chemical equation for the reaction of these two compounds.

Now identify the conjugate acid-base pairs in your chosen reaction. Compare your results to those of your classmates.