• Note: stronger acid/base = less stable

X-H Acids

For X-H acids, there are two factors for acid strength

  1. Bond Strength (between H and other atom): low = strong acid bcuz H can easily dissociate
  • Compare bond dissociation energies 2. Bond Polarity (high → weak acid)
  • The greater the difference in electronegativity between these two elements, the more polar the bond will be → more polar bond → stronger X-H 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 other element = 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 O

  • Compare compounds with same number of oxygens but diff elements → more electronegative element = compound will have greater electron density → weaker O-H bond → stronger acid
  • For oxyacids, acid strength decreases going down a group because the electronegativity of the central atom decreases


  • Base with more electronegative element = weaker base b/c will be a poorer proton acceptor
  • Base with larger Kb value (stronger base) = weaker/more stable conjugate acid = conj acid has less electronegative element

Determining the pH of a 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