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) + H₂PO₄^2–(aq)

acid base conj. acid conj. base

CO₃^2–(aq) + H₂O(l) →← HCO₃^–(aq) + OH^–(aq)

acid base conj. acid conj. base

Acid Strength

In aqueous solutions this is measured by equilibrium constants

HA(aq) + H₂O(l)→← H₃O⁺(aq) + A^–(aq)

pKa = –logK_a

A larger K_a (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