Acid-Base Equilibrium

We discussed acids and bases briefly on this page and also discuss it in a more biological context on this page. However, in this sub-section of Chemical Equilibrium, we're going to entirely tackle the concept of equilibrium when it comes specifically to acids and bases.

There were many definitions of what an acid and base are before 1923. Svante Arrhenius believed acids were compounds that produce H⁺ ions when dissolved in water and bases were compounds produced hydroxide(OH^-) ions when dissolved in water.

However, in 1923, a more generalized description of Arrhenius' framework was proposed by Johannes Brønsted and Thomas Lowry. They stated that when an acid donates a proton(H⁺), the species that remains is known as a conjugate base. Similarly, when a base accepts a proton, the species that remains is known as a conjugate acid.

Essentially, a Brønsted-Lowry acid is a species that donates a proton and a Brønsted-Lowry base is a species that accepts a proton.

Let's use the example above to illustrate what we're discussing. Water(H₂O) is the acid in this reaction with our base of ammonia(NH₃). Ammonia takes the proton from the water, which donates it. The remaining species left of the water is known as the conjugate base, which is just the water molecule without one of its hydrogen atoms, which makes sense since the water molecule donated a hydrogen cation/proton. Similarly, the remaining species left of ammonia is the conjugate acid of ammonium ion, which is just ammonia with an extra hydrogen. Again, this makes sense because ammonia is the base so it accepted the proton that the water donated to it.

Notice that this reaction is reversible. If the reverse reaction occurred, the ammonium ion(NH₄⁺) would be the acid donating a proton to the base, which is the hydroxide ion(OH^-).

The reaction we've just showed you is a classic acid-base reaction and is really all there is to these reactions. It's an acid donating a proton(H⁺) to a base, which accepts it. The remaining species are the acid without the proton, known as the conjugate base, and the base with the proton, known as the conjugate acid.

*Note that water is generally considered amphoteric, meaning it can be considered an acid and a base, depending on the chemical reaction.