BrønstedSäure

BrønstedSäure, commonly referred to as a Brønsted acid, is a substance that donates a proton (H+) to a Brønsted-Lowry base in an acid–base reaction. The concept, named after Johannes Nicolaus Brønsted and Thomas Lowry (1923), defines acids and bases by proton transfer rather than solely by solvent effects.

In a Brønsted-Lowry framework, an acid HA transfers a proton to a base B, producing the conjugate base A− and the conjugate acid BH+. The strength of a Brønsted acid is indicated by its tendency to donate protons, quantified by the dissociation constant (Ka) or pKa. In aqueous solution, strong Brønsted acids such as HCl, HNO3, and H2SO4 dissociate completely, whereas weak Brønsted acids like acetic acid (CH3COOH) dissociate only partially.

Acid strength is influenced by factors that stabilize the conjugate base, including electronegativity, resonance, and charge delocalization, as well as inductive effects and solvent polarity. Many Brønsted acids are polyprotic, capable of donating more than one proton (e.g., H2SO4, H3PO4). It is important to note that acidity is solvent-dependent; pKa values are defined for a specific solvent and can vary with the medium.

Relation to other theories: Brønsted acidity is a specific case of acid–base chemistry based on proton transfer, and while many Brønsted acids are also Lewis acids, not all Lewis acids function as Brønsted acids.