CHAPTER 15
AP CHEMISTRY
COMMON ION EFFECT
• If you have HC2H3O2(aq) <=> H+ (aq) + C2H3O2-(aq)
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What would happen if you added the salt NaC2H3O2?
Addition of C2H3O2- would cause the system to shift to the
left
This is called the COMMON-ION EFFECT
Page 682 example
BUFFER
• Prepared by adding both the weak acid HB and its
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conjugate base B- to water
[H+] = ka[HB] = ka nHB
[B-]
nBEffect of adding strong acids or bases to a buffer
HB(aq) + OH-(aq) ---> H2O(l) + B-(aq)
B-(aq) + H+(aq) ---> HB(aq)
H+ and OH- ions are consumed so the pH is not affected
Buffered solutions are most effective when concentrations
of H+ and base B- are about the same
Read chapters 19 and 20 notes are due the first day of
fourth term
CONTINUED
• Henderson-Hasselbach equation is used in
biology to calculate the pH of the buffer
• pH = pka+ log [base]
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[acid]
ACID-BASE TITRATION
• Read and study pages 699-711
• Titration curve
– Graph of pH to concentration
• strong acid - strong base page 697
• H+(aq) + OH-(aq) <=> H2O(l) k = 1/kw = 1/(1
X 10-14)
– pH = 7, change rapidly as you near the end
point
CONTINUED
• Weak acid-strong base
• HA(aq) + OH-(aq) <=> A-(aq) + H2O(l) k =1/kb
• pH > 7 at end point, change occurs slowly at end
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point
Strong acid-weak base
H+ (aq) + A-(aq) <=> HA(aq) k = 1/ka
pH < 7, at end point, change occurs slowly at end
point
Titration of polyproptic acids
When neutralization steps are separated the
substance will show a titration curve with multiple
equivalence points
INDICATORS
• HIn(aq) <=> H+(aq) + In-(aq)
In = indicator
• HIn and In- have different colors, the wavelengths that
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bounce back are different when hydrogen is attached
Color of a solution depends on the concentration ratio
[HIn] = [H+]
[In- ]
ka
Endpoint occurs when [H+] = ka
About pH 5 for methyl red, pH 7 for bromothymol blue,
and pH 9 for phenolphthalein
Read pages 711-716
SOLUBILITY EQUILIBRIA
• If Q(ion product) is compared with the
ksp, the following can be said
• Q > ksp ppt occurs until Q = ksp
• Q = ksp equilibrium (saturation has
occured)
• Q < ksp solid dissolves until Q = ksp
• AgCl(s) <=> Ag+ (aq) + Cl- (aq)
• Slightly soluble
ksp found
on page 718
• ksp = [Ag+][Cl-]
CONTINUED
• the solubility of slightly soluble salts
containing basic anions will increase as
the [H+] increases
• Looking at the solubility of a salt
• In pure water it is the same as the
reaction
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CONTINUED
• Complex ions
– Metal ions with Lewis bases bonded to it
page 958, 731-737
• Amphoterism
– Behavior of water molecules around a
metal ion
• Read pages 724-730