Chapter 16 L3 Titration of Weak Acid or Base Solutons

Overview of Titration Concepts

• Titration is a process used to determine the concentration of a solution by reacting it with a solution of known concentration.

• For weak acids with strong bases (or vice versa), the pH at the equivalence point does not equal 7.

Neutralization Reaction

• Example: Propanoic acid (weak acid) reacts with sodium hydroxide (strong base).

• Reaction produces propanoate (a weak base).

• At the equivalence point, all propanoic acid is converted into propanoate.

Equivalence Point & pH Calculation

• The equivalence point is reached when equivalent amounts of acid and base have reacted.

• For weak acid/strong base titrations:

  • pH at equivalence point > 7 (basic solution).

• Hydrolysis of propanoate in water contributes to basicity.

Midpoint of Titration

• At the halfway point (half the number of moles of base added), we have equal concentrations of the weak acid and its conjugate base.

• Use the Henderson-Hasselbalch equation:

  • pH = pKa + log([base]/[acid]).

  • When [base] = [acid], pH = pKa.

• Example Calculation for Propanoic Acid:

  • Given Ka, calculate pH at the midpoint:

    • pH = 4.88 (equal to pKa since concentrations are equal).

Validating Assumptions

• Assumption: x is small for using Henderson-Hasselbalch.

• Percent ionization calculated as:

  • % Ionization = (x / [initial]) * 100%

  • If percent ionization < 5%, assumption is valid.

pH at the Equivalence Point

• For a strong base/weak acid titration, the pH calculation involves determining the concentration of the resultant weak base from the reaction.

• Example at equivalence point with propanoate:

  • Must calculate Kb to solve for OH− concentration:

    • Kb = Kw / Ka = 7.75 x 10^-10.

  • Determine concentration from the final volume and equilibrium expression.

Titration Beyond Equivalence Point

• If overshooting titration by adding excess base, Evaluate the amounts left (weak base from the weak acid and strong base).

• Setup ICE table for equilibrium involving strong base and weak base:

  • Calculate new concentrations and pH based on excess hydroxide.

Titration Curves Comparison

• Initial pH: Strong acids low, weak acids higher.

• Equivalence Point pH: Strong acid/strong base is 7; weak acid/strong base > 7.

• Buffer Capacity: Greater in weak acid/strong base titrations.

• Volume of Titrant: Affects shape and slope of curve in different reactions.

steps for Calculating pH at the Equivalence Point

1. Identify the Reaction:Write the balanced chemical equation for the acid-base reaction.

2. Determine the Products:Identify the products formed at the equivalence point (usually a salt and water).

3. Find the Weak Base or Weak Acid:At the equivalence point, determine if the resultant solution contains a weak acid or a weak base.

4. Calculate Kb or Ka:Use the relationship between Ka and Kb to find the relevant constant.[ Kb = \frac{K_w}{K_a} ]or[ Ka = \frac{K_w}{K_b} ]where ( K_w ) is the ion product of water (1.0 x 10^-14 at 25°C).

5. Calculate Concentration of the Weak Base or Weak Acid:Determine the concentration of the weak base or weak acid at the equivalence point based on the initial concentrations and total volume.

6. Set Up Kb or Ka Expression:Use the Kb or Ka expression for the species present at equilibrium:

   • For weak bases:[ K_b = \frac{[OH^-]^2}{[base]} ]

   • For weak acids:[ K_a = \frac{[H^+]^2}{[acid]} ]

7. Calculate [OH⁻] or [H⁺]:Solve for the hydroxide or hydrogen ion concentration using the equilibrium expression.

8. Calculate pOH or pH:

   • If you calculated [OH⁻]:[ pOH = -\log[OH^-] ][ pH = 14 - pOH ]

   • If you calculated [H⁺]:[ pH = -\log[H^+] ]