Acids & Bases

Hydrolysis

Lesson 7

Hydrolysis

Hydrolysis of a salt is…• The reaction between water and the cation or

anion ( or both) that make up the salt in order to produce either an acidic or basic solution.

Hydrolysis

Hydrolysis of a salt is…• All salts are considered 100% ionized in water.• We will only consider the reactions between

the ions and water.– Ignore any reactions between the ions

themselves.

Hydrolysis

Not all ions will react with water.

The ones that do not are called “spectator ions”

Memorize these spectator ions!

Spectator ions

• NaOH

• HCl

Spectator Ions

Spectator Cations: alkali metals (group 1 on periodic table) + alkaline earth metals ( group 2 on periodic table)– Column I and column II

Spectator Anions: the first 5 anions found on the top right of the tableClO4, I, Br, Cl, NO3 memorize them!

HSO4 is not a spectator because it is a weak acid.

So what to do with salts now?

• Determine ions that make them up• Discard any spectators• Remaining ions will act as acids if found on the

left side of table (acids), or they will act as bases if found on right side of the table.

NaCl

NH4Cl

NaF

Fe(H20)6Cl3

Al(H2O)5(OH)(NO3)2

AmphiproticIons

Acids or Bases?

Amphiprotic Ions H3PO4 Acid

H2PO4- Amphiprotic

HPO42- Amphiprotic

PO43- Base

 Amphiprotic ions have a Ka and a Kb.

If the Ka > Kb, then the ion is an acid.

If the Kb > Ka, then the ion is a base.

Is H2PO4- acid or basic

acid

1. Is H2PO4- an acid or base when in water? Write the predominate

hydrolysis equation for this ion.  

Ka (H2PO4-) = 6.2 x 10-8

Kb (H2PO4-) = Kw

Ka(H3PO4)

= 1 x 10-14

7.5 x 10-3

= 1.3 x 10-12

Ka > Kb Acid

H2PO4- + H2O ⇄ HPO4

2- + H3O+

Is HPO42- acid or basic

basic

2. Is HPO42- an acid or base when in water? Write the hydrolysis

equation for this ion.  

Ka (HPO42-) = 2.2 x 10-13

 Kb (HPO4

2-) = Kw

Ka(H2PO4-)

= 1 x 10-14

6.2 x 10-8

= 1.6 x 10-7

Kb > Ka base

HPO42- + H2O ⇄ H2PO4

- + OH-

3. Show by calculation if NH4F is an acid or base when in water.

NH4F → NH4+ + F-

acid baseNeither is a spectator; neither will cross off!What will the salt solution be?

Ka(NH4+) = 5.6 x 10-10

Kb (F-) = = KwKa(HF)

= 1 x 10-14

3.5 x 10-4

= 2.9 x 10-11

Ka > Kb acid

TRY Show by calculation if NH4CH3COO is an acid or base when in water.

NH4CH3COO → NH4+ + CH3COO-

acid base

Ka(NH4+) = 5.6 x 10-10

Kb (CH3COO-) = = KwKa(CH3COOH)

= 1 x 10-14

1.8 x 10-5

= 5.6 x 10-10

Ka = Kb neutral pH = 7.00

Homework

• Hebden 148– #69 (top row), #70 (top row)– # 71, 72, 73.