slide 1 of 52 16-3 the self-ionization of water and the ph scale
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Slide 1 of 52
16-3 The Self-Ionization of Water and the pH Scale
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Slide 2 of 52
Ion Product of Water
Kc= [H2O][H2O]
[H3O+][OH-]
H2O + H2O H3O+ + OH-
base acidconjugate
acid
conjugate
base
KW= Kc[H2O][H2O] = = 1.010-14[H3O+][OH-]
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Slide 3 of 52
pH and pOH
The potential of the hydrogen ion was defined in 1909 as the negative of the logarithm of [H+].
pH = -log[H3O+] pOH = -log[OH-]
-logKW = -log[H3O+]-log[OH-]= -log(1.010-14)
KW = [H3O+][OH-]= 1.010-14
pKW = pH + pOH= -(-14)
pKW = pH + pOH = 14
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Slide 4 of 52
pH and pOH Scales
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Slide 5 of 52
16-4 Strong Acids and Bases
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Slide 6 of 52
16-5 Weak Acids and Bases
Lactic Acid Glycine
General Chemistry: Chapter 16 Prentice-Hall © 2007
Acetic Acid
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Slide 7 of 52
Acetic Acid
Weak Acids
Ka= = 1.810-5
[CH3CO2H]
[CH3CO2-][H3O+]
pKa= -log(1.810-5) = 4.74
General Chemistry: Chapter 16 Prentice-Hall © 2007
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Slide 8 of 52
Weak Bases
Kb= = 4.310-4
[CH3NH2]
[CH3NH3+][HO-]
pKb= -log(4.210-4) = 3.37
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Slide 9 of 52
Table 16.3 Ionization Constants of Weak Acids and Bases
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Slide 10 of 52
Determining a Value of KA from the pH of a Solution of a Weak Acid. Butyric acid, HC4H7O2 (or CH3CH2CH2CO2H) is used to make compounds employed in artificial flavorings and syrups. A 0.250 M aqueous solution of HC4H7O2 is found to have a pH of 2.72. Determine KA for butyric acid.
HC4H7O2 + H2O C4H7O2 + H3O+
Ka = ?
EXAMPLE 16-5
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Slide 11 of 52
HC4H7O2 + H2O C4H7O2 + H3O+
Initial conc. 0.250 M 0 0
Changes -x M +x M +x M
Equilibrium (0.250-x) M x M x MConcentration
EXAMPLE 16-5
Solution:
For HC4H7O2 KA is likely to be much larger than KW. Therefore assume self-ionization of water is unimportant.
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Slide 12 of 52
Log[H3O+] = -pH = -2.72
HC4H7O2 + H2O C4H7O2 + H3O+
[H3O+] = 10-2.72 = 1.910-3 = x
[H3O+] [C4H7O2-]
[HC4H7O2] Ka=
1.910-3 · 1.910-3
(0.250 – 1.910-3)=
Ka= 1.510-5 Check assumption: Ka >> KW.
EXAMPLE 16-5
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Slide 13 of 52
Percent Ionization
HA + H2O H3O+ + A-
Degree of ionization =[H3O+] from HA
[HA] originally
Percent ionization =[H3O+] from HA
[HA] originally 100%
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Slide 14 of 52
Percent Ionization
Ka =[H3O+][A-]
[HA]
Ka =n
H3O+ A-n
HAn
1
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