Chapter 16 (March 28)

  • Concept of pH Stability

    • Adding certain salts to water will not change the pH of the solution.
    • Salts made from strong acids and strong bases do not affect pH as they generate weak conjugate ions.

  • Example with Sodium Nitrate

    • Sodium nitrate (NaNO3) dissolved in pure water remains at a pH of 7.
    • Sodium ion (Na+) is the conjugate acid of sodium hydroxide (NaOH), a strong base, making it too weak to react with water.
    • Nitrate ion (NO3-) is the conjugate base of nitric acid (HNO3), also strong, and is too weak to act with water.

  • Conjugate Acid-Base Pairs

    • Importance of identifying if ions come from weak or strong acids/bases.
    • Ions conjugate to strong acids or bases do not react further with water to change pH.
    • Example: Sodium acetate (NaC2H3O2)
    • Na+ (from NaOH - strong base) does not react.
    • Acetate ion (C2H3O2-) comes from acetic acid (weak), can react with water.
    • Acts as a base, generating OH- in solution, making it basic.

  • Acidic or Basic Solutions from Salts

    • Acidic solutions originate from salts with ions where at least one is the conjugate of a weak acid.
    • Chloride (Cl-) is the conjugate of strong HCl, therefore it is weak.
    • Ammonium (NH4+) comes from weak base ammonia (NH3) and can react, generating H3O+, making the solution acidic.

  • Examples of Ions Affecting pH

    • Small, highly charged metal ions (e.g., Al3+) form complex ions with water.
    • These complexes can release H+ ions, generating hydronium ions (H3O+), yielding acidic solutions.

  • Key Ions and Their Behavior

    • Iron (Fe3+), Zinc (Zn2+), Copper (Cu2+), and others show similar behavior in forming acidic solutions due to strong charge interactions.
    • Nitrate ions (NO3-) from strong acids will not change pH when in presence of weak conjugate acids.

  • Calculations to Predict pH

    • Determine if solution will be acidic or basic by identifying ions from salts.
    • Use ICE tables to analyze equilibrium, providing concentrations for calculations.
    • Example of sodium acetate:
    • Kb calculated from Ka of acetic acid, to find OH- concentration, followed by pOH then pH.

  • Further Practical Examples

    • Discussing the concept of calculating pH based on the dominant ion effect in solutions.
    • Strong acids or bases always dominate the pH determination.
    • Influential factors in salt solutions, particularly when both conjugate acids and bases are present.

  • Practice Sessions

    • Thorough understanding of relationships between strong/weak acids/bases requires practice with various salts.
    • Multiple examples emphasized throughout the class, reinforcing learning.
    • Future practice problems to predict solution characteristics (acidic/basic/neutral).