Weak vs Strong Acids

We can represent an acid generally as $\mathrm{HA}$. Here, $\mathrm{H}$ is the hydrogen that can form an $\mathrm{H^+}$ ion, while $\mathrm{A}$ represents the remainder of the acid. When the acid separates into ions, the general change is:

This separation into ions is called dissociation. Acid strength tells us the degree of dissociation: the proportion of the dissolved acid particles that have separated into ions.

Strong acids dissociate completely

A strong acid completely dissociates in aqueous solution. Essentially all its dissolved acid particles separate into ions, so the equation is written with a single arrow:

Hydrochloric acid, nitric acid and sulfuric acid are examples of strong acids. Complete dissociation does not mean that the solution must contain a large amount of acid; it describes what happens to the acid particles that are present.

Weak acids dissociate partially

A weak acid only partially dissociates in aqueous solution. Some acid particles form ions, while others remain undissociated:

The reversible arrow shows that both the forward and reverse changes occur. Eventually a dynamic equilibrium is established: acid particles continue to dissociate and ions continue to recombine, but their overall amounts remain constant. This does not mean that the quantities on the two sides are equal.

Ethanoic acid, citric acid and carbonic acid are examples of weak acids. For example, ethanoic acid can be represented as:

Connecting dissociation to hydrogen ions and pH

At the same acid concentration, a strong acid has a greater degree of dissociation than a weak acid. It therefore produces a higher concentration of $\mathrm{H^+}$ ions and has a lower pH.

The equal-concentration condition matters. Acid strength and acid concentration describe different things:

• strong or weak describes how completely the acid dissociates;
• concentrated or dilute describes how much acid is dissolved in a given volume.

A strong acid can therefore be dilute, and a weak acid can be concentrated. You cannot always decide which of two solutions has the lower pH merely from the labels “strong” and “weak” if their concentrations differ.