CHM 401

 

Structural aspects of Brønsted-Lowry acids:

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

M(H2O)nm+(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 OpE(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)