Lecture on Acids and Bases: Resonance Structures and Periodic Trends

Introduction

• Connecting topics and models to explain acid and base behavior in solution.
• Focus on acids, applicable to bases as well.

Factors Affecting Acid Strength

1. Bond Length

   • Longer bonds are weaker and easier to break.
   • Example: Acid halides (HF, HCl, HBr, HI) show increasing bond length and decreasing bond strength in this order.
   • Order of Strength: HI > HBr > HCl > HF (where HF is weakest)
   • Definitions of strong vs. weak acids depend on percent dissociation in water.

2. Electronegativity Difference

   • Larger difference leads to more polar bonds and higher partial positive charge on hydrogen.
   • Examples:
     • NH3 (0.8 difference) < H2O (1.2) < HF (1.8)
   • Ranking in terms of acidity: HF > H2O > NH3
   • In water, HF is acidic, H2O is neutral, NH3 is basic.

3. Resonance Stabilization of Conjugate Base

   • Conjugate base stability affects acid strength.
   • Example: Oxoacids (HClO, HBrO, HIO) show stronger acids with more electronegative halogens.
   • Order of Stability: Cl > Br > I
   • More electronegative halogen stabilizes the conjugate base better.

Oxoacids and Resonance

• Oxoacids: Stronger acids have more resonance structures.
• Example:
  • NO3- vs NO2-
  • NO3- has more resonance structures, making HNO3 stronger than HNO2.

Application

• Compare conjugate bases for perchloric, chloric, chlorous, and hypochlorous acids.
• Example Series:
  • HClO4 and HClO3 are strong acids.
  • HClO2 and HClO are weaker.
  • Requires comparison of CLO4-, CLO3-, etc.

Key Takeaways

• Strong acids dissociate more in water, not necessarily more reactive or dangerous.
• Electronegativity differences affect bond polarity and acid strength.
• Resonance stabilization is crucial for determining the stability of conjugate bases and thus acid strength.