AP Chemistry Unit 8.6 Molecular Structure of Acids and Bases

Molecular Structure of Acids and Bases

  • For X-H bonds there are two factors for acidity in binary compounds
    1. Bond Strength (between H and other atom): low = strong acid bcuz H can easily dissociate
      • Compare bond dissociation energies
    1. Bond Polarity (high → weak acid)
      • With H: The greater the difference in electronegativity between these two elements, the more polar the bond will be → more polar bond = stronger bond → weaker acid
  • For X-H bonds, acid strength increases going down a column because the electronegativity of the elements bonded to hydrogen decreases
    • Greater electronegativity of central atom = weaker acid


  • Acid that has oxygen, hydrogen, and at least another element
    • The Hydrogen is always bonded to Oxygen
  • With oxyacids, acid strength increases with an increase in the number of oxygen atoms
    • Why? Oxygens are very electronegative → causes the electron density to be greater and more pulled towards the oxygen side which weakens the bond between H and other atom
  • Compare compounds with same number of oxygens but diff elements → more electronegative element = compound will have greater electron density → stronger acid
  • For oxyacids, acid strength decreases going down a group because the electronegativity of the central atom decreases


  • Base that has more negative charge (-) → more strongly attracts H+ = stronger base

Mixture of Acids

  • The process is the same: determine the major species and the stronger (bigger Ka) will dominate
  • If both acids are weak → the acid with the larger Ka is slightly stronger → when calculating pH only need to focus on (make ICE table) for dominant acid
  • Strong acid + weak acid → focus on strong acid
  • Strong acid + strong acid → have to do both

Complex Ions

  • Complex ion: a charged species consisting of a metal ion surrounded by ligands → produces an acidic solution
    • the higher the charge on the metal ion, the stronger the acidity of the hydrated ion.
  • Ligand: a Lewis base
    • Common ligands 
    • Coordination number: The number of ligands attached to a metal ion



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