What is a conjugate acid and base pair?

Jul 18, 2018

Conjugate acid-base pair are compounds which differ by ${H}^{+}$

Explanation:

Here's are two examples of conjugate acid-base pair.

The concept of conjugate acid-base pair is related to Bronsted-Lowry acid-base theory and according to this theory, acid is a proton $\left({H}^{+}\right)$ donor while base is a proton acceptor .

Let's focus on the first example, $C {H}_{3} C O O H$. It behaves as an acid because it donates a proton and becomes $C {H}_{3} C O {O}^{-}$. Imagine this:
$C {H}_{3} C O O H - {H}^{+} \to C {H}_{3} C O {O}^{-}$

The product that is formed ($C {H}_{3} C O {O}^{-}$) as a result of the acid, $C {H}_{3} C O O H$ donating its proton is called a conjugate base.

• acid: $C {H}_{3} C O O H$
• conjugate base: ($C {H}_{3} C O {O}^{-}$)

You might be wondering, why is $C {H}_{3} C O {O}^{-}$ called a conjugate base and not conjugate acid. Let's look at the reverse reaction. Going from $C {H}_{3} C O {O}^{-} \to C {H}_{3} C O O H$ will require $C {H}_{3} C O {O}^{-}$ to accept proton, making it's a "base".

For the second example, ${H}_{2} O$, it behaves as a base because it accepts proton (from $C {H}_{3} C O O H$) and becomes ${H}_{3} {O}^{+}$, which is the conjugate acid.

${H}_{2} O + {H}^{+} \to {H}_{3} {O}^{+}$

In this case,

• base: ${H}_{2} O$
• conjugate acid: ${H}_{3} {O}^{+}$

In short,

• conjugate base is formed when an acid donates a proton.
• conjugate acid is formed when a base accepts a proton.