D ISSOCIATION OF W ATER H 2 O + H 2 O H 3 O 1+ + OH 1- The equilibrium expression is products over reactants. K = [H 3 O 1+ ] [OH 1- ] / [H 2 O] [H 2 O] The molarity for the water is a constant at any specific temperature. So K [H 2 O] [H 2 O] = [H 3 O 1+ ] [OH 1- ] The quantity on the right hand side of the equation is formally defined as Kw. The numerical vale for Kw is different at different temperatures. At 25oC Kw = 1.014 x 10-14 Kw = K[H 2 O] [H 2 O] or Kw = [H 3 O 1+ ] [OH 1- ]
D ISSOCIATION OF W ATER Equilibrium constants exist then for both acid dissociation and base. (K a and K b ) The higher the K a, the stronger the acid and the higher the K b, the stronger the base. K a and K b are related by the previous equation. K w = K a K b
D ISSOCIATION OF W ATER As K a gets larger the strength of the acid gets higher, but K b must fall. Therefore the stronger the acid, the weaker the conjugate base.
D ISSOCIATION OF W ATER It can now be said that the conjugate base (acid) of a weak acid (base) is a weak base (acid) and the conjugate base (acid) of a strong acid (base) is a worthless base (acid).
D ISSOCIATION OF W ATER The strength of an acid/base is usually given as a pK a value. As pK a is inversely related to K a, the higher the K a (the stronger the acid), the lower the pKa value. The same is true of bases.
C ALCULATING P H -log [H+] Power of Hydronium (Hydrogen) P[OH-] = - log [OH]
P H IN S OLUTIONS OF S TRONG A CIDS AND S TRONG B ASES Strong acids Certain acids are known as strong acids. These are acids that fully ionize when placed in water: HA + H 2 O A - + H 3 O + Goes to completion and thus Ka = [A-][H3O+]/[HA] = infinity Some common strong acids are: HCl, hydrochloric acid HBr, hyrdobromic acid HI, hydroiodic acid H 2 SO 4, sulfuric acid HNO 3, nitric acid HClO 4, perchloric acid
P H IN S OLUTIONS OF S TRONG A CIDS AND S TRONG B ASES Strong Bases Certain bases are known as strong bases. These are bases that fully ionize when placed in water. Some common strong bases are: LiOH, lithium hydroxide NaOH, sodium hydroxide KOH, potassium hydroxide Ca(OH) 2, calcium hydroxide Sr(OH) 2, strontium hydroxide Ba(OH) 2, barium hydroxide Alkaline earth oxides. Lime (CaO)