Brønsted–Lowry Theory

The fundamental principle of this theory is that acid-base reactions are equilibrium processes involving the competition for protons. A reaction occurs when an acid encounters a base that has a higher affinity for the proton than the acid's own conjugate base.

Proton transfer is rarely a isolated event; it typically involves the breaking of a polar bond (like $H-Cl$ or $O-H$) and the simultaneous formation of a new bond with a lone pair on the base. This means the 'strength' of an acid or base is relative to the other species present in the system.

Every Brønsted–Lowry reaction involves two conjugate acid-base pairs. When an acid donates a proton, the remaining species is capable of re-accepting that proton, making it a conjugate base.

Conversely, when a base accepts a proton, the resulting species can donate that proton back, making it a conjugate acid. These pairs always differ by exactly one $H^+$ ion and the corresponding charge difference.

For a general reaction $HA + B \rightleftharpoons A^- + HB^+$, the pairs are $(HA, A^-)$ and $(B, HB^+)$. In any equilibrium, the stronger the acid, the weaker its conjugate base, and vice versa.

Certain substances possess the unique ability to act as either a Brønsted–Lowry acid or a base depending on the reaction conditions; these are termed amphiprotic species. Water ($H_2O$) is the most common example, acting as a base when reacting with stronger acids and as an acid when reacting with stronger bases.

To be amphiprotic, a molecule must have both a removable hydrogen atom (to act as an acid) and a lone pair of electrons (to act as a base). Common ions like hydrogen carbonate ($HCO_3^-$) and dihydrogen phosphate ($H_2PO_4^-$) also exhibit this dual behavior.

Identify the Transfer: When given a chemical equation, always track the movement of the $H$ atom. The species that has one fewer $H$ on the product side is the acid; the species with one more $H$ is the base.

Check the Charges: A common source of lost marks is failing to adjust the charge. If a neutral molecule loses $H^+$, the product must be $-1$. If a $-1$ ion gains $H^+$, the product becomes neutral.

Pairing Rule: Conjugate pairs must differ by exactly one proton. $H_2SO_4$ and $SO_4^{2-}$ are NOT a conjugate pair because they differ by two protons; the conjugate base of $H_2SO_4$ is $HSO_4^-$.

Protons vs. Electrons: Students often confuse Brønsted–Lowry (proton transfer) with Lewis theory (electron pair transfer). Remember: Brønsted–Lowry is strictly about the $H^+$ ion.

The 'Base' Misconception: Many assume a base must contain an $OH$ group. In this theory, any species with a lone pair (like $F^-$, $NH_3$, or $CH_3COO^-$) can be a base.

Ignoring the Solvent: In aqueous reactions, water is often a participant. Never assume water is just a spectator; it is frequently the acid or base in the reaction.