IX. Salts and Hydrolysis Salts are simply ionic compounds.

Salts can be formed by:

1. A metal reacting with a non-metal.2 Na(s) + Cl2(g) 2 NaCl(s)

2. An acid reacting with a base.KOH(aq) + HCl(aq) H2O(l) + KCl(aq)

3. An acid reacting with a metal.Mg(s) + 2 HCl(aq) H2(g) + MgCl2(aq)

Salts often include polyatomic ions:eg: Na3PO4 NH4Cl CaSO4

All salts are considered at least slightly soluble (recall Ksp) and many are highly soluble.

Many salts when dissolved in solution will change the pH of the solution – this is known as a hydrolysis reaction.

eg: Na2CO3 – sodium carbonate

Na2CO3(s) 2 Na+(aq) + CO3

2-(aq)

dissociation first, then…

CO32-

(aq) + H2O(l) HCO3-(aq)

+ OH-(aq)

Hydrolysis causes some OH- to be produced.

a solution of Na2CO3 is actually basic.

eg: NH4Cl – ammonium chloride

NH4Cl(s) NH4+

(aq) + Cl-(aq)

then…

NH4+

(aq) + H2O(l) NH3(aq) + H3O+

(aq)

a solution of NH4Cl is actually acidic.

eg: NaHCO3 – sodium bicarbonate

NaHCO3(s) Na+(aq) + HCO3

-(aq)

Does HCO3- act as an acid or a base?

HCO3-(aq) + H2O(l) H2CO3(aq)

+ OH-(aq)

a solution of NaHCO3 is basic.

Compare KA with KB:

KA = 5.6 x 10-11 KB = 1.0 x 10-14

4.3 x 10-7 = 2.3 x 10-8

KA(H2CO3)

HCO3- acts as a base.

In these examples notice that Na+ and Cl- do not react with water. They have no H+ to donate and no tendency to accept H+ from H2O.

General Rules for Hydrolysis:

1. Cations (+ ions) of strong bases do not hydrolyze.eg: groups I & II metal ions (Na+, K+, Ca2+…)

2. Anions (- ions) of strong acids do not hydrolyze.eg: Cl-, Br-, ClO4

-, NO3-, and I-

3. Any ion present on the KA table between strong acids & strong bases will hydrolyze. If it has no H+, you know it will act as a base.

eg: CO32-

(aq) + H2O(l) HCO3-(aq)

+ OH-(aq)

If it has H+, it is amphiprotic and you must determine whether it acts as an acid or a base.(compare its KA to its KB)

Note: NH4+ only acts as an acid.

HSO4- only acts as an acid.