CM4106 Chemical Equilibria & Thermodynamics

Lesson 3 (Part 2)Additional Aspects of Acid-Base Equilibria(Topic 3.6 – 3.9)

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Indicators

Indicator pH = pKln pH range and colour change

Phenolphthalein

9.10Pink

8.30Colourless

↔ 10.0Pink

Litmus 6.50Purple

5.00Red

↔ 8.00Blue

Methyl Orange 3.70Orange

2.90Red

↔ 4.60Yellow

Bromothymol Blue

7.00Green

6.00Yellow

↔ 7.50Blue

Acid-Base Titration

Choose indicator where the pH range at which colour change occurs is close to the pH at equivalence point.

Choice of Indicator

Suitable

Suitable

Not Suitable

Choose indicator where the pH range at which colour change occurs is close to the pH at equivalence point.

A suitable indicator has a colour change range that lies within the change of pH equivalence point.

Acid-Base TitrationStrong Acid – Strong Base Strong Acid – Weak Base

1. sigmoidal shape2. large pH Δ around eq. pt.3. pH at eq. pt. = 7.04. Indicator: most indicators

1. “buffer region”2. smaller pH Δ around eq. pt.3. pH at eq. pt. < 7.04. Indicator: Methyl Orange

xBuffer Region

pH < 7Salt hydrolysis

pOH = pKb

Acid-Base TitrationWeak Acid – Strong Base Weak Acid – Weak Base

1. “buffer region”2. No steep pH Δ around eq. pt.3. No suitable indicator

1. “buffer region”2. smaller pH Δ around eq. pt.3. pH at eq. pt. > 7.04. Indicator: phenolphthalein

x

At Equivalence PtKa = Kb pH = 7Ka > Kb pH < 7Ka < Kb pH > 7

Buffer Region

pH > 7Salt hydrolysispH = pKa

1. Initial Pt Initial pOH [OH-] due to dissociation of base ICE Table

2. Buffering Region

3. Equivalence pt Volume of acid Concentration of base used At equivalence pt, all acid and base are

completely neutralized to give salt and H2O; determine [salt] =[conj. acid]pH [H+] due to hydrolysis of salt

4. Half equivalence pt pOH pKb

Acid-Base Titration Strong Acid – Weak Base

x

Buffer RegionpH < 7Salt hydrolysis

pOH = pKb1.

[base]

[salt]logKOH b pp

[base]

[salt]logKOH b pp

5. After Equivalence PtLow pH due to excess strong acid added

1. Initial Pt Initial pH [H+] due to dissociation of acid ICE Table

2. Buffering Region

3. Equivalence pt Volume of base Concentration of acid used At equivalence pt, all acid and base are completely neutralized to give salt and H2O; determine [salt] =[conj. base]pOH [OH-] due to hydrolysis of salt

4. Half equivalence pt pH pKa

Acid-Base Titration Weak Acid – Strong Base

x

Buffer Region

pH > 7Salt hydrolysispH = pKa

1.

[acid]

[salt]logKH a pp

[acid]

[salt]logKH a pp

5. After Equivalence PtHigh pH due to excess strong base added

pH of an acidic buffer pH of an alkaline buffer

pH at the point of maximum buffering capacity of an acidic

buffer

pH at the point of maximum buffering capacity of an

alkaline buffer

pH = pKa pOH = pKb

[base]

[salt]logKOH b pp

[acid]

[salt]logKH a pp

Choice of Indicators

Strong acid – strong alkali

Any

Strong acid – weak alkali

Methyl Orange

Weak acid – strong alkali

Phenolphthalein

Weak acid – Weak alkali

None

pH at equivalence point (Give equation of hydrolysis reaction, if any)

eg. NaCl

HCl (aq) + NaOH (aq) → NaCl (aq) +

H2O (l)

7.0No hydrolysis

e.g. NH4Cl

NH3 + HCl (aq) NH4

+Cl- (aq)

Acidic pH (pH < 7)

NH4+ (aq) + H2O (l)

⇌ NH3 (aq) + H3O+ (aq)

e.g. CH3COONa

CH3COOH(aq) + NaOH(aq) →

CH3COONa(aq) + H2O (l)

Basic pH (pH > 7)

CH3COO- (aq) + H2O (l) ⇌ CH3COOH (aq) + OH- (aq)

Depends on Ka and Kb values of conjugate acid and

conjugate base in salt

Ka = Kb pH = 7Ka > Kb pH < 7Ka < Kb pH > 7