Chemistry 144
Dr K de Villiers
Weak Acids & Bases
CONTACT SESSION
Starter discussion:
Important concepts:
1. Extent of ionization
𝐻𝐻𝐶𝐶𝐶𝐶(𝑎𝑎𝑎𝑎)
𝐻𝐻𝑁𝑁𝑁𝑁2 (𝑎𝑎𝑎𝑎)
𝐻𝐻 + (𝑎𝑎𝑎𝑎) + 𝐶𝐶𝐶𝐶 − (𝑎𝑎𝑎𝑎)
𝐻𝐻 + (𝑎𝑎𝑎𝑎) + 𝑁𝑁𝑁𝑁2 − (𝑎𝑎𝑎𝑎)
Chemistry 144
Weak Acids & Bases
Dr K de Villiers
2. Ka and pKa
𝑯𝑯𝑨𝑨(𝒂𝒂𝒂𝒂) + 𝑯𝑯𝟐𝟐 𝑶𝑶 ⇌ 𝑯𝑯𝟑𝟑 𝑶𝑶+ (𝒂𝒂𝒂𝒂) + 𝑨𝑨− (𝒂𝒂𝒂𝒂)
[𝐻𝐻 + (𝑎𝑎𝑎𝑎)][𝐴𝐴− (𝑎𝑎𝑎𝑎)]
𝐾𝐾𝑎𝑎 =
[𝐻𝐻𝐻𝐻(𝑎𝑎𝑎𝑎)]
Large(r) Ka?
Small(er) Ka?
3. Kb and pKb
𝐵𝐵(𝑎𝑎𝑎𝑎) + 𝐻𝐻2 𝑂𝑂(𝑙𝑙) ⇌ 𝐵𝐵𝐻𝐻 + (𝑎𝑎𝑎𝑎) + 𝐻𝐻𝑂𝑂− (𝑎𝑎𝑎𝑎)
[𝐵𝐵𝐻𝐻 + (𝑎𝑎𝑎𝑎)][𝐻𝐻𝑂𝑂− (𝑎𝑎𝑎𝑎)]
𝐾𝐾𝑏𝑏 =
[𝐵𝐵(𝑎𝑎𝑎𝑎)]
Large(r) Kb?
Small(er) Kb?
𝑨𝑨− (𝒂𝒂𝒂𝒂) + 𝑯𝑯𝟐𝟐 𝑶𝑶(𝒍𝒍) ⇌ 𝑯𝑯𝑯𝑯(𝒂𝒂𝒂𝒂) + 𝑯𝑯𝑯𝑯− (𝒂𝒂𝒂𝒂)
[𝐻𝐻𝐻𝐻(𝑎𝑎𝑎𝑎)][𝐻𝐻𝐻𝐻− (𝑎𝑎𝑎𝑎)]
𝐾𝐾𝑏𝑏 =
[𝐴𝐴− (𝑎𝑎𝑎𝑎)]
Chemistry 144
Dr K de Villiers
Weak Acids & Bases
4. Ion product of water, Kw
𝐾𝐾𝑤𝑤 = [𝐻𝐻 + ][𝑂𝑂𝐻𝐻 − ] = 1.0 × 10−14 (𝑎𝑎𝑎𝑎 25 ℃)
𝟏𝟏𝟏𝟏. 𝟎𝟎𝟎𝟎 = 𝒑𝒑𝒑𝒑 + 𝒑𝒑𝒑𝒑𝒑𝒑
𝐾𝐾𝑎𝑎 × 𝐾𝐾𝑏𝑏 = [𝐻𝐻 + (𝑎𝑎𝑎𝑎)][𝐻𝐻𝐻𝐻− (𝑎𝑎𝑎𝑎)] = 𝐾𝐾𝑊𝑊
𝟏𝟏𝟏𝟏. 𝟎𝟎𝟎𝟎 = 𝒑𝒑𝑲𝑲𝒂𝒂 + 𝒑𝒑𝑲𝑲𝒃𝒃
Large(r) Ka?
5. Acid-base conjugate pairs
Chemistry 144
Dr K de Villiers
6. Salt solutions
𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺 = 𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪+ + 𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨−
NO EFFECT
Cation
+
ACIDIC (↓ pH)
NO EFFECT
Anion¯
BASIC (↑ pH)
Weak Acids & Bases
Chemistry 144
Dr K de Villiers
Weak Acids & Bases
IN CLASS QUESTION
Determine the pH of a 0.20 M solution of NH4Cl (Kb (NH3) = 1.8 × 10-5).
Before you do the calculation, do you expect the pH to be acidic, neutral or basic? Why?
I
C
E