Brønsted-Lowry theory of acids and bases — common mistakes

Examiners are very strict on the use of reaction arrows. Use $\to$ for strong acid/base dissociation to show the reaction goes to completion. Use $⇌$ for weak acid/base dissociation to show an equilibrium is established. Using the wrong arrow will lose you marks.

Strength refers to the degree of dissociation: a strong acid fully dissociates in water, while a weak acid only partially dissociates. Concentration refers to the amount of acid dissolved per unit volume (e.g., in mol dm⁻³). You can have a dilute solution of a strong acid (e.g., 0.001 mol dm⁻³ HCl) or a concentrated solution of a weak acid (e.g., 10 mol dm⁻³ CH₃COOH).

A neutral hydrogen atom (H) consists of one proton in its nucleus and one electron. When it loses its single electron to form a hydrogen ion (H⁺), all that remains is the proton. Therefore, in the context of acid-base chemistry, 'proton' and 'H⁺' are used interchangeably.

Yes. The definition of a Brønsted-Lowry acid is a proton donor. Any species with a proton it can donate can act as an acid. For example, the hydrogencarbonate ion, $HCO_3^-$, can donate its proton to form the carbonate ion, $CO_3^{2-}$. In this case, the negative ion $HCO_3^-$ is acting as an acid.

In aqueous solutions, a bare proton ($H^+$) does not exist on its own. It is highly reactive and immediately bonds to a water molecule. The resulting species, $H_3O^+$, is called the hydronium ion. For simplicity in A-Level Chemistry, we often write $H^+(aq)$, but it's important to understand that this is a shorthand for $H_3O^+(aq)$.