Chemistry 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)

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)

K_a = [H₃O⁺]_e[A^–]_e                      [HA]_e 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

Structural Aspects of Brønsted/Lowry Acids

1) Aqua acids: water coordinated to a central (usually cationic) species

M(H₂O)_n^m+(aq) + H₂O(l) → ← M(H₂O)_n-1(OH)^(m-1)+(aq) + H₃O⁺(aq)

d block metals, some Group 2 metals; not Group 1 metals

ionic models work reasonably well to describe the bonding in these types of acids

2) Hydroxy acids: only OH bound to a central atom

H₄SiO₄ = Si(OH)₄ generally p block central atoms

3) Oxoacids: ionizable H from an OH but also has one or more =O groups

HClO₄; H₂CrO₄ both p block and d block examples known

Pauling's rules: write the acid structure as O_pE(OH)_q

E = central atom

1. pKa₁ ~ 8 – 5p

2. each subsequent pKa increases by ~ 5 units

HClO₄     pKa ~ 8 – 5(3) = –7

H₃PO₄    pKa₁ ~ 8 – 5(1) = 3 (2.1 obs); pKa₂ ~ 8 (7.2 obs); pKa₃ ~ 13 (11.8 obs)

Oxides: when dissolved in water, can give either an acid or a base

Acidic oxides: covalently bound oxides

CO₂(g) + 2 H₂O(l) → ← HCO₃^–(aq) + H₃O⁺(aq)

SO₃(g) + OH^–(aq) → ← HSO₄^–(aq)

Basic oxides: ionically bound oxides

BaO(s) + H₂O(l) → Ba²⁺(aq) + 2 OH^–(aq)

CaO(s) + 2 H₃O⁺(aq) → Ca²⁺(aq) + 3 H₂O(l)

Amphoteric oxides: act as either acid or base; at the line of covalent or ionic bonding

Ga₂O₃(s) + 6 H₃O⁺(aq) + 3 H₂O(l) → ← 2 [Ga(H₂O)₆]³⁺(aq)

Ga₂O₃(s) + 2 OH^–(aq) + 3 H₂O(l) → ← 2 [Ga(OH)₄]^–(aq)