17.2 Titrations and Titration Curves

Introduction to Titrations

• Titrations are experiments involving the mixing of acids and bases to analyze equilibria:

  • Three Main Types of Titrations:

    • Strong Acid with Strong Base

    • Strong Acid with Weak Base

    • Weak Acid with Strong Base

  • Weak Acid-Weak Base titrations are not covered.

  • At least one of the reactants must be strong.

Buffer Solutions vs Titrations

• Buffer Solutions: Mix an acid and a base to create a controlled pH environment.

  • Use the Henderson-Hasselbalch equation directly, without assuming a reaction.

  • No reaction needs to be stated.

• Titrations: Requires writing out a reaction.

  • Understanding the differences helps prevent confusion.

Recognizing Titrations

• Key characteristics of titrations:

  • At least one participant is a strong acid or strong base.

  • Examples include:

    • Strong Acid-Strong Base: HCl + NaOH

    • Strong Acid-Weak Base: HCl + Ammonia

    • Weak Acid-Strong Base: Acetic Acid + NaOH

• Buffer formation can occur during titrations but involves different calculations.

Understanding the Equivalence Point

• The Equivalence Point:

  • Happens when equal amounts (in moles) of acid and base have been mixed.

  • Allows the use of the equation: Molarity of Acid × Volume of Acid = Molarity of Base × Volume of Base.

• In Strong Acid-Strong Base Titration:

  • Expected pH at equivalence point is 7.

  • Reaction leads to completion, producing only water and a negligible salt.

pH at Different Titration Points

• Strong Acid-Strong Base: pH equals 7 at equivalence point.

• Strong Acid-Weak Base: pH will be less than 7 (around 4-7) because the resulting product forms an acidic salt (e.g., NH4Cl).

  • Strong acid promotes a lower pH.

• Weak Acid-Strong Base: pH will be greater than 7 (around 7-10) due to a basic product (e.g., sodium fluoride, NaF).

  • Resulting salt acts as a weak base, increasing pH.

Titration Curves

• Graphing pH vs. Volume of NaOH added:

  • Weak Acid-Strong Base Titration:

    • Curve starts low, shows a significant increase through the equivalence point.

    • Displays a buffer region prior to reaching equivalence point, giving clues about the pKa.

  • Strong Acid-Strong Base Titration:

    • Starts low, trend more flat initially, reaches equivalence point sharply at pH 7.

Half-Equivalence Point Concept

• The Half-Equivalence Point occurs where the concentrations of weak acid and its conjugate base are equal.

  • At this point, pH = pKa of the weak acid.

• Determined during weak acid-strong base titrations, this marks the buffer region where the solution can resist pH changes.

Titrating Polyprotic Acids

• Polyprotic acids have multiple dissociable protons (H+ ions) and thus multiple equivalence points:

  • Example: H3PO4 (Phosphoric Acid) has three dissociable protons leading to three equivalence points on the titration curve.

  • Each equivalence point corresponds to one proton donation, requiring careful analysis and identification.

Conclusion

• Titration Calculations:

  • Focus on recognizing the type of titration (strong vs weak) to determine calculations needed.

• Essential calculations based on five types:

  • Strong Acid-Strong Base,

  • Weak Acid-Weak Base,

  • Buffers, etc.