42H2O(l) + 59kJ H3O+(aq) + OH-(aq) Ionization of Water2H2O(l) + 59kJ H3O+(aq) + OH-(aq)Since reaction is endothermic: At higher temperatures ______________are favoured and Kw is _____________er. At lower temperatures ______________ are favoured and Kw is _____________er.
5Ionization of Water Always: [H3O+] [OH-] = Kw At 250C only: [H3O+] [OH-] = 1.00 x 10-14
6[H3O+] & [OH-] in Neutral Water At 25oC: (NOTE: Assume T = 25oC unless otherwise noted) [H3O+] [OH-] = 1.00 x 10-14and [H3O+] = [OH-] if water is neutralSubstitute [H3O+] for [OH-]: [H3O+] [H3O+] = 1.00 x 10-14 [H3O+]2 = 1.00 x 10-14 √[H3O+] = √1.00 x = x MAlso [OH-] = [H3O+] = 1.00 x M
7[H3O+] & [OH-] in Neutral Water Example Given: Kw at 600C = 9.55 x Calculate [H3O+] & [OH-] at 600C.
8[H3O+] & [OH-] in Acids and Bases 2H2O(l) H3O+(aq) + OH-(aq)Add acid, H3O+ increases, so equilibrium shifts LEFT and [OH-] decreases.Add base, [OH-] increases, so the equilibrium shifts LEFT and [H3O+] decreases.
9[H3O+] & [OH-] in Acids and Bases Ex. Find the [OH-] in M HCl. Ex. Find [H3O+] in M NaOH.
10[H3O+] & [OH-] in Acids and Bases Ex. Find [H3O+] in M Ba(OH)2 .Ex. Calculate [OH-] in M HNO3 at 600C.Kw at 600C = 9.55 x 10-14Hebden Textbook Page 127 Questions #28-30
11pH Shorthand method of showing acidity (or basicity/alkalinity) pH= -log[H3O+]If [H3O+] = 1.0 x 10-7pH = -log (1.0 x 10-7 ) = 7
12Converting [H3O+] to pH Ex. Find the pH of 0.030 M HCl. **In pH or pOH notation, only the digits after the decimal are significant digits. The digits before the decimal come from the “non-significant” power of 10 in the original [H3O+].
13Converting [H3O+] to pHEx. Find the pH of M NaOH at 250 C. Ex. Find the pH of neutral water at 250 C.
14pH Scale In neutral water pH = 7.0 In acid solution pH < 7.0 In basic solution pH > 7.0
15Converting pH to [H3O+][H3O+] = antilog (-pH) Ex.) If pH = , find [H3O+]. Ex.) If pH = calculate [H3O+].
16Logarithmic Nature of pH A change of 1 pH unit a factor of 10 in [H3O+] (or acidity).How many times more acidic is pH 3 than pH 7?Natural rainwater pH ~ 6Extremely acidic acid rain pH ~ 3So, acid rain is 1000 times more acidic than natural rain water!
18pOH Ex. Calculate the pOH of 0.0020M KOH. Ex. Find the pH of the same solution.Notice: pH + pOH = 14.00
19Relation of pH to pOH where pKw = -log Kw (definition of pKw) Since... [H3O+] [OH-] = Kw-log[H3O+ ] + -log [OH- ] = -log KwpH pOH = pKwwhere pKw = -log Kw(definition of pKw)TRUE AT ALL TEMPERATURES!
20Relation of pH to pOH Specifically at 250C: Kw = 1.00 x 10-14 pKw = -log (1.00 x 10-14)pKw =pH + pOH =TRUE AT 25°C!
21pH and pOH Ex. Find the pH of 5.00 x 10-4 M LiOH (250C). Ex. Find the pOH of M HBr (250C).See pOH scale & pH scale on page 140 of Hebden Textbook.
22pH and pOHWhen not at 250C: Ex. At 600C Kw = 9.55 x Find the pH of neutral water at 600C.
23pH and pOH Is pH always 7.00 in neutral water?_________ At higher temperatures:2H2O + heat H3O+ + OH-[H3O+] > 1.0 x so pH < 7[OH-] > 1.0 x so pOH < 7
24Summary of pH and pOHThese are very important? Make sure you study these!In neutral water pH = pOH at any temperaturepH & pOH = 7.00 at 250C onlyAt lower temps pH and pOH are > 7At higher temps pH and pOH are < 7At any temperature: pH + pOH = pKwAt 250C: pH + pOH =Hebden Textbook Pages Questions #49-53, 55-57