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Autoionization of Water

Calculating with H+. Autoionization of Water H 2 O reacts with itself, some H 2 O acts as an acid and others act as bases H 2 O (l) + H 2 O (l)  Keq = [H 3 O + ][OH - ] or Keq = [H + ][OH - ] At 25 o C, Keq = 1 x 10 -14

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Autoionization of Water

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  1. Calculating with H+ • Autoionization of Water • H2O reacts with itself, some H2O acts as an acid and others act as bases • H2O(l) + H2O(l) • Keq = [H3O+][OH-] • or Keq = [H+][OH-] • At 25oC, Keq = 1 x 10-14 • Even if solution is acidic or basic, Kw is still 1 x 10-14 • If we know the H+ concentration, we can determine the [OH-] • As [H+] goes up, [OH-] goes down • Example – Determine the [H+] if • [OH-] = 1 x 10-8M • [OH-] = 6.3 x 10-9 M H3O+(aq) + OH-(aq) Same as H3O+ = Kw ionization constant for water [H+] = 1 x 10-6 M [H+] = 1.59 x 10-6

  2. 1 x 10-3 M 1 x 10-11 M H+ OH- B. pH 1. Definition “power of hydrogen ions” pH = -log [H+] For water pH = 7 Low pH indicates a higher H+ concentration 0-7 acidic 7 neutral 7 – 14 basic Change in pH of +1 indicates a 10x change in the concentration pH 2 = 0.01 M Acid pH = 3 1 x 10-7 M 1 x 10-7 M H+ OH- pH 1 = 0.1 M

  3. 2. Other scales pOH -log [OH-] Since [H+][OH-] = 1 x 10-14 pH + pOH = 14 3. pH of strong acids and bases Strong acids HCl HNO3 HI HBr HClO3 HClO4 (H2SO4) All these break apart completely If we know the molarity of the acid, we know the [H+] Example – The pH of a 0.025 M HCl [H+] = 0.025 M pH = -log (0.025) pH = 1.60 H2SO4 is more complex

  4. Bases All group 1 hydroxides Heavy group 2 hydroxides Break apart completely in water. We can calculate the [H+] from the [OH-] Example – Calculate the pH of a 0.015 M NaOH solution [OH-] = 0.015 M Example – Calculate the pH of a 0.025 M Ca(OH)2 solution Since there are 2 OH- in the formula, [OH-] = 0.050 M NaOH KOH Ca(OH)2 Ba(OH)2 pOH = -log 0.015 pOH = 1.82 pH + pOH = 14 pH + 1.82 = 14 pH = 12.18 pOH = -log 0.050 pOH = 1.30 pH + pOH = 14 pH + 1.30 = 14 pH = 12.70

  5. 4. Other basic compounds a. Metal oxides In water, these become bases, especially Na2O and CaO CaO  Ca+2 + O-2 O-2 + H2O  2 OH- b. Nitrides and hydrides Also create bases Na3N  3 Na+ + N-3 N-3 + 3 H2O  NH3 + 3 OH- LiH  Li+ + H- H- + H2O  H2 + OH-

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