AP Chemistry Unit 8 Review: Acids and Bases

Introduction

• Presenter: Jeremy Krug
• Focus: AP Chemistry Unit 8
• Topics: Acids and Bases

pH and pOH

• Definitions:
  • pH = -log[hydronium ion concentration]
  • pOH = -log[hydroxide ion concentration]
• Relationship:
  • At 25°C, [H₃O⁺][OH⁻] = 1 x 10⁻¹⁴ (Kw)
  • pH + pOH = 14
  • pH = pOH implies neutral solution (pH and pOH both 7.00 at 25°C)

Effect of Temperature

• Higher temperatures increase Kw value, lowering pH of pure water
• Pure water remains neutral despite pH changes with temperature

Strong Acids and Bases

• Strong Acids:
  • Ionize completely; pH = -log[acid concentration]
  • Example: 0.010 M nitric acid, pH = 2.00
• Strong Bases:
  • Similar calculation using pOH
  • Group 1 and 2 hydroxides are strong bases
  • Example: 0.010 M calcium hydroxide, pOH from -log[0.020], pH = 12.30

Weak Acids and Bases

• Equilibrium Reactions:
  • Weak acids dissociate reversibly, defined by equilibrium constant Ka
  • pKa = -log[Ka]
  • Weak bases react with water (Kb), pKb = -log[Kb]
• ICE Box Calculations:
  • Used for equilibrium problems
  • Solve for x, ignore small x for simplification
  • Calculate percent dissociation

Neutralization Reactions

• Strong Acids and Bases:
  • Net ionic equation: always same; results in neutral pH (7 if equimolar)
• Weak Acid/Base with Strong Acids/Bases:
  • Results in buffer if weak acid/base in excess
  • Treated as strong acid/base problem if strong counterpart in excess
• Weak Acid with Weak Base:
  • Compare magnitudes of Ka and Kb

Acid-Base Titrations

• Titration Curve:
  • Volume of titrant (x-axis) vs pH (y-axis)
  • Equivalence point: moles of base = moles of acid
  • Half equivalence point: pH = pKa of weak acid
• Polyprotic Acids:
  • Number of inflection points = number of acidic hydrogens
  • Estimate first and second Ka

Strength of Acids and Bases

• Strong acids dissociate more completely, weak conjugate bases
• Strong bases have weak conjugate acids
• Consider electronegativity and oxygen in organic acids

Indicators

• pKa corresponds to pH point where indicator changes color
• Choose indicator matching pH of equivalence point

Buffers

• Definition:
  • Mixture of weak acid and its conjugate base
  • Resist pH changes
• Henderson-Hasselbalch Equation:
  • Used to calculate pH of a buffer
  • Buffer capacity related to concentration

Solubility and pH

• Solubility of ionic compounds can depend on pH
• Example: Magnesium carbonate's solubility increases with decreasing pH

Conclusion

• Join Jeremy Krug for review of Unit 9 on Thermodynamics and Electrochemistry