3.1.12.1 - Brønsted-Lowry Acid-Base Equilibria

Acids, bases and proton transfer

Brønsted-Lowry definitions are based on what a substance does to a proton, not on whether the solution happens to have a particular pH. In any acid-base reaction, one species loses a proton and another gains it.

If a species donates $\ce{H+}$, it is acting as an acid in that reaction. Hydrogen chloride is a familiar example because it can pass a proton to another species.

A base must have a way to accept that proton, often by using a lone pair of electrons. In the reaction below, ammonia accepts a proton from hydrogen chloride:

Here, $\ce{HCl}$ is the acid because it donates a proton. $\ce{NH3}$ is the base because it accepts that proton to form $\ce{NH4+}$.

The diagram below shows proton transfer from hydrogen chloride to ammonia, with the lone pair on nitrogen allowing ammonia to accept the proton.

[DIAGRAM: asset_name: 1.12.1 - Bronsted-Lowry Acid-Base Equilibria in Aqueous Solution - Diagram 1; asset_slug: 1.12.1 - Bronsted-Lowry Acid-Base Equilibria in Aqueous Solution - Diagram 1; recommended_method: matplotlib_exact_plot; description: Ammonia-HCl proton-transfer mechanism: lone pair on nitrogen, curly arrow to H, second curly arrow from H-Cl bond to Cl; NH4+ and Cl- products, delta charges.]

A quick reliable way to classify the species is to compare them before and after the reaction. If a substance has lost $\ce{H+}$, it behaved as the acid. If it has gained $\ce{H+}$, it behaved as the base.

Proton transfer in aqueous solution

In aqueous solution, the proton is not present as a bare particle moving around on its own. It is attached to water, so chemists often write $\ce{H+(aq)}$ as a convenient shorthand for $\ce{H3O+(aq)}$.

When hydrogen chloride dissolves in water, water accepts a proton:

Water is therefore acting as a base in this reaction. That is a good reminder that the label "acid" or "base" depends on the role played in a particular reaction.

Water can also do the opposite and donate a proton. With ammonia:

This time, water behaves as an acid because it loses a proton to $\ce{NH3}$. Ammonia is the base because it accepts the proton. The production of $\ce{OH-}$ follows naturally from water having donated $\ce{H+}$.

A useful habit is to track just one proton. In the first equation, the proton moves from $\ce{HCl}$ to $\ce{H2O}$. In the second, it moves from $\ce{H2O}$ to $\ce{NH3}$. Once you can see that movement, identifying acid and base becomes much more reliable.

Why acid-base reactions can be equilibria

Some proton-transfer reactions are reversible, so the forward and reverse reactions both matter.