ACID-BASE THEORIES

Bronsted-Lowry Theory Chemists identified a need to define acids and bases in a way that was independent of H2O In 1923, Johannes Brønsted and Thomas Lowry independently proposed new definitions for acids and bases that focused on proton transfer.

Bronsted-Lowry Acids and bases A Brønsted-Lowry acid is defined as a proton donor: it can give an H+ (proton) to another molecule. A Brønsted-Lowry base is defined as a proton acceptor: it can receive an H+ (proton) to another molecule. H+(aq) + OH-(aq)  H2O(l) Acid Base PD PA

HCl(g) + H2O(ℓ) → H3O+(aq) + Cl−(aq) Which substance is the B-L Acid HCl(g) + H2O(ℓ) → H3O+(aq) + Cl−(aq) Which substance is the B-L Acid? Which substance is the B-L Base? C6H5OH + NH2− → C6H5O− + NH3

Al(H2O)63+ + H2O → Al(H2O)5(OH)2+ + H3O+ Which substance is the B-L Acid? Which substance is the B-L Base?

In the reaction between ammonia and water, the reaction does not go to completion Instead, the reverse reaction occurs as well and the two processes cancel out any additional change (this is the reason for the ‘double arrow’) In this reaction, the NH4+ ion is donating a proton to the OH- ion, which is accepting it. So, NH4+ is a proton donor (acid), while OH- is accepting it (base)

The reverse reaction is also an acid-base reaction – accepting and donating protons We have two sets of similar substances that differ only by a proton (H+) with one being an acid and one being a base.

Conjugate acid-base Pairs The two sets of similar species are known as conjugate acid-base pairs

Example Identify the conjugate acid-base pairs (CH3)3N + H2O ⇄ (CH3)3NH+ + OH– NH2– + H2O ⇄ NH3 + OH–