17.2-17.3b: CHEM 2

Introduction to Titrations and Titration Curves

• Chad's Prep aims to reduce stress in learning science.

• This lesson focuses on conceptual understanding of titrations and titration curves.

• Future lessons will cover pH calculations in depth.

• Types of titration curves discussed:

  • Strong acid/strong base

  • Weak acid/strong base

  • Weak base/strong acid

Key Terminology

• Analyte: the substance whose concentration is being measured, usually the acid.

• Titrant: the substance (base/acid) used to titrate the analyte, which is standardized to know its exact concentration.

Strong Acid-Strong Base Titration

• Example:

  • Analyte: HCl (strong acid)

  • Titrant: NaOH (strong base)

• The purpose is to determine the concentration of the analyte (HCl).

  • Initial pH is low; it rises with additions of NaOH.

  • A steep increase in pH occurs near the inflection point (equivalence point).

Inflection Point and Equivalence Point

• Inflection Point: where the slope of the curve changes significantly (indicating an equivalence point).

• Equivalence Point: when the moles of HCl equal the moles of NaOH (1:1 ratio).

• At this point, strong acid and strong base are fully neutralized, yielding NaCl (neutral salt) and water, resulting in a pH of 7.

Role of the Indicator

• Indicators are substances that change color at a specific pH range to indicate the endpoint of a titration.

• Phenolphthalein: a common indicator that changes color near pH 7 (from colorless to pink).

Weak Acid-Strong Base Titration

• Example:

  • Analyte: HF (weak acid)

  • Titrant: NaOH (strong base)

• pH changes rapidly initially, exhibiting a steeper slope near the equivalence point compared to strong acid-strong base titration curve.

• At the equivalence point, the pH will be above 7 due to the formation of a basic salt (e.g., NaF) which hydrolyzes.

Half Equivalence Point

• Halfway to the equivalence point (1:1 ratio of weak acid to conjugate base) is the half equivalence point.

• This point is essential as pH equals pKa here.

Weak Base-Strong Acid Titration

• Example:

  • Analyte: NH3 (weak base)

  • Titrant: HCl (strong acid)

• The equivalence point is below 7, creating an acidic solution, as the salt (NH4Cl) produced is acidic.

Titration Curves Summary

• Four Critical Regions for Titration Curves:

  1. Before titration - concentration of the acid/base (weak or strong) determines the initial pH.

  2. Mid-point (before equivalence) - contains both weak acid and its conjugate base; can utilize Henderson-Hasselbalch equation.

  3. At equivalence point - determines resultant pH based on the active species present.

  4. Beyond equivalence point - excess strong acid/base dictates pH calculation.

Practical Calculations Overview for Titrations

• For Strong Acid/Strong Base:

  • Before EQ: use pH of the H+(aq) concentration.

  • At EQ: pH is neutral (7).

  • After EQ: calculate p