Further Aspects of Equilibria

Kw - Dynamic Product of Water

• Kw varies with temperature.
• At room temperature, Kw = 1 x 10^-14 mol² dm^-6.
• Kw derived from Kc expression.
• Water dissociates into H+ and OH-.
• Equilibrium lies to the left due to weak dissociation of water.
• Assumption: concentration of water constant.

pH Calculations

• pH = -log10[H+].
• [H+] = 10^-pH.
• Conversion from pH to [H+] and vice versa.

Example Problem: Calculate pH

1. [H+] = 5.32 x 10^-4 mol/dm³.
2. Use pH equation.
3. Calculate using calculator.

Example Problem: Calculate [H+]

1. pH = 10.5.
2. Use [H+] equation.
3. Calculate using calculator.

pH of Strong Acids

• Strong acids completely dissociate in solution.
• [H+] = [Acid].
• Calculate pH using the pH equation.

pH of Strong Bases

1. Calculate [OH-].
2. Use Kw to find [H+].
3. Convert [H+] to pH using the pH equation.

Example Problem: Sodium Hydroxide

1. [OH-] = 0.05 mol/dm³.
2. Find [H+] using Kw.
3. Calculate pH.

Ka - Dissociation Constant

• Ka: Equilibrium constant for weak acids.
• Lower Ka = lower [H+] and [A-] = equilibrium lies to the left.
• Ka values can be converted to pKa.
• As pKa increases, acid strength decreases.

Example Problem: Ethanoic Acid

1. Ka given, calculate [H+].
2. Write equilibrium and Ka expressions.
3. Solve for [H+], then calculate pH.

Indicators in Titration

• Indicators are weak acids changing color on proton donation.
• Titration curves show pH changes during acid-base titration.
• Indicator must change color at the endpoint of the titration.
• Example: Bromothymol indicator for strong acid-strong base titration.
• Different titration curves for different combinations (strong/weak acids/bases).
• No sharp fall in weak acid-weak base titration, so no suitable indicator for endpoint.

Titration Curve Analysis

• Strong base - strong acid titration: Sharp fall from pH 10.5 to 3.5.
• Weak base - strong acid titration: Highest pH = 11, lowest = 1.
• Weak acid - weak base titration: No sharp fall.