Salts and Hydrolysis

1

• Salts and Hydrolysis

• Salts are simply ionic compounds.

• Salts can be formed by

• A metal reacting with a non-metal.

2 Na(s) Cl2(g) ? 2 NaCl(s)

• An acid reacting with a base.

KOH(aq) HCl(aq) ? H2O(l) KCl(aq)

• An acid reacting with a metal.

Mg(s) 2 HCl(aq) ? H2(g) MgCl2(aq)

• Salts often include polyatomic ions

eg
Na3PO4 NH4Cl CaSO4
2

• 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) CO32-(aq)
dissociation first, then
Hydrolysis causes some OH- to be produced.
? a solution of Na2CO3 is actually basic.
3
eg
NH4Cl ammonium chloride
NH4Cl(s) ? NH4(aq) Cl-(aq)
then
? a solution of NH4Cl is actually acidic.
4
eg
NaHCO3 sodium bicarbonate
NaHCO3(s) ? Na(aq) HCO3-(aq)

• Does HCO3- act as an acid or a base?

• Compare KA with KB

KA 5.6 x 10-11
2.3 x 10-8
KA(H2CO3)
? HCO3- acts as a base.
? a solution of NaHCO3 is basic.

• 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.

5
General Rules for Hydrolysis

• Cations ( ions) of strong bases do not
  hydrolyze.eg groups I II metal ions (Na,
  K, Ca2)

• Anions (- ions) of strong acids do not
  hydrolyze.eg Cl-, Br-, ClO4-, NO3-, and I-

• 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

• 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.