CHM 501 Lecture

Acids/Bases

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

The definition is not solvent or phase dependent; fairly universal.

Acid Strength

In aqueous solutions this is measured by equilibrium constants

HA(aq) + H₂O(l) H₃O⁺(aq) + A^–(aq)

pK_a = –logK_a

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

A gas phase scale would be simpler:

B(g) + H⁺(g) HB⁺(g) H = –proton affinity = –PA

Periodic trends of PA: increases to the right on the Periodic Table; increases down the Periodic Table

To account for properties in solution, write a Born–Haber cycle:

B(g)	+	H+(g)		HB+(g)	–PA
↓ +H2O(l)		↓ +H2O(l)		↓ +H2O(l)	solvation energies
B(aq)	+	H+(aq)		HB+(aq)	1/Ka

Use the Born equation to estimate solvation energies.

Structural aspects of Bronsted 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

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

Examples:

HClO₄ pKa ~ 8–3(5) = –7
H₃PO₄ pKa₁ ~ 8–5 = 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) + 2H₂O(l) HCO₃^–(aq) + H₃O⁺(aq)

SO₃(g) + OH^–(aq) HSO₄^–(aq)

Basic oxides: ionically bound oxides

BaO(s) + H₂O(l) Ba²⁺(aq) + 2OH^–(aq)

CaO(s) + 2H₃O⁺(aq) Ca²⁺(aq) + 3H₂O(l)

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

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

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