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 table ClO 4, I, Br, Cl, NO 3 memorize them! HSO 4 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

NH 4 Cl

NaF

Fe(H 2 0) 6 Cl 3

Al(H 2 O) 5 (OH)(NO 3 ) 2

Amphiprotic Ions Acids or Bases?

Amphiprotic Ions H 3 PO 4 Acid H 2 PO 4 - Amphiprotic HPO 4 2- Amphiprotic PO 4 3- 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 H 2 PO 4 - acid or basic acid

1. Is H 2 PO 4 - an acid or base when in water? Write the predominate hydrolysis equation for this ion. Ka (H 2 PO 4 - ) =6.2 x Kb (H 2 PO 4 - ) = Kw Ka(H 3 PO 4 ) =1 x x =1.3 x Ka > Kb Acid H 2 PO H 2 O ⇄ HPO H 3 O +

Is HPO 4 2- acid or basic basic

2. Is HPO 4 2- an acid or base when in water? Write the hydrolysis equation for this ion. Ka (HPO 4 2- ) =2.2 x Kb (HPO 4 2- ) = Kw Ka(H 2 PO 4 - ) =1 x x =1.6 x Kb > Ka base HPO H 2 O ⇄ H 2 PO 4 - +OH -

3.Show by calculation if NH 4 F is an acid or base when in water. NH 4 F→ NH 4 + +F - acidbase Neither is a spectator; neither will cross off! What will the salt solution be? Ka(NH 4 + )=5.6 x Kb (F - ) = = Kw Ka(HF) =1 x x =2.9 x Ka > Kbacid

TRYShow by calculation if NH 4 CH 3 COO is an acid or base when in water. NH 4 CH 3 COO→ NH 4 + +CH 3 COO - acidbase Ka(NH 4 + )=5.6 x Kb (CH 3 COO - ) = = Kw Ka(CH 3 COOH) =1 x x =5.6 x Ka = Kbneutral pH = 7.00

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