CHM 401


Acid/Base Reactions

Acid-base reactions are atom transfer reactions; these are definition dependent; we use them in an attempt to classify reactivity and aid us in predicting reactions.


Brønsted/Lowry definition

Acid: H+ donor

Base: H+ acceptor

A/B reaction: H+ transfer

Definition is not solvent or phase dependent; fairly universal

H2O(l) H+(aq) + OH(aq)     not B/L A/B as written

2 H2O(l) H3O+(aq) + OH(aq)     now is a B/L reaction

H3PO4(aq) + H2O(l) H3O+(aq) + H2PO42–(aq)

acid base conj. acid conj. base

CO32–(aq) + H2O(l) HCO3(aq) + OH(aq)

base acid conj. acid conj. base

Acid Strength

In aqueous solutions this is measured by equilibrium constants

HA(aq) + H2O(l) H3O+(aq) + A(aq)

pKa = –logKa

A larger Ka (smaller pKa) is a stronger acid

This is a so-so measure of extent of reaction but breaks down at higher concentrations; modified by activity coefficients; tells us nothing about structure or the role of the solvent

Differentiating solvent: allows relative acid strength to be measured

Leveling solvent: two or more acids give the same acid strength

Water is differentiating for organic acids but leveling for HCl, HBr, HI