Intersection 11
11/12/05
Reading: 16.8 (p794-800) 17.2 (p 828-836)
Equilibrium Representations
Water Projects…Now What?
Outline
• Strong vs. Weak Acids and Bases• Which H’s are acidic? Which groups are basic?• Periodic Trends and Acid Strength• What affects the pH of a solution?• Polyprotic acids• Concept Questions• Acid/Base Titration: A Closer Look
Strong vs. Weak
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Vocabulary
Strong (16 definitions):
Weak (10 definitions):
Favorable reaction..
ionizing freely in solution
ionizing only slightly in solution
Strong Reaction
exothermic
spontaneous
product favored
goes to completion
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Strong Acids An acid that dissociates completely (the equilibrium is shifted all of the way to
its conjugate base and hydronium ion) is said to be a strong acid.
HCl(aq) + H2O(l) → H3O+
(aq) + Cl-(aq)
acid conj. base
An acid that does not dissociate completely (an equilibrium is established in solution between the acid, its conjugate base, and hydronium ion) is said to be a weak acid.
HClO2(aq) + H2O(l) ↔ H3O+
(aq) + ClO2- (aq)
acid conj base
Ka = ([H3O+][ClO2
-]) / [HClO2]
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Strong BasesA base that dissociates completely (the equilibrium is shifted all of
the way to its conjugate acid and hydroxide) is said to be a strong base.
NaOH(aq) + H2O(l) → OH-(aq) + Na+
(aq) + H2O(l)
base conj. acid
A base that does not dissociate completely (an equilibrium is established in solution between the base, its conjugate acid, and hydroxide) is said to be a weak base.
(CH3)3N(aq) + H2O(l) ↔ (CH3)3NH+(aq) + OH-
(aq) base conj. acid
Kb = ([(CH3)3NH+][OH-]) / [(CH3)3N]
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Strong Acid
Weak Acid
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There are six strongly dissociating acids:HCl HNO3
HBr HClO4
HI H2SO4
There are also five bases that dissociate completely in solution (strong):LiOH Ca(OH)2
NaOH Ba(OH)2
KOH You should commit the strong acids and bases to memory.Appendix F in your text book lists Ka and Kb values for many weakly
dissociating acids and bases.
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Problem 1
Trimethylamine (CH3)3N has a Kb of 6.5 x10-5. Write out its chemical reaction with water: What is the [OH-] of a 0.010 M solution of triethylamine? What is the pOH? What is the pH?
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Which H’s are acidic?What groups are basic?
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What kinds of hydrogen atoms (protons) are acidic?
H-halogen (HF, HCl, HBr, HI)
H2O
H2S (Ka1 = 8.9x10-8)
Oxoacids (H-polyatomic ions) (H2CO3, HNO3, etc.) HCN
O
C
HO
C
CC
O
OH
OH
H
H
OH
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Organic Acids
CR
O
OH
+ H2O
CR
O
O-
+ H3O+
RCOOH
CR
O-
O
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Bases
OH
R3N
H2N CH C
CH3
OH
O
H2N CH C
CH2
OH
O
CH2
C
OH
O
HN
C OH
O
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Periodic Trends and Acid Strength
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Groups 7 and Period 2
Acid strength: HI > HBr > HCl> HF
HF>H2O >> H3N >>> CH4
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Oxoacids I
HOY + H2O H3O+ + OY-
Since a negatively charged ion must be formed, it will be most stable when Y is an atom that is most effective at attracting electron density to itself, thereby stabilizing the negative charge. So the more electronegative atom as Y, yields a stronger acid
Acid strength: HOI < HOBr < HOCl
Y = I, Br, Cl
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Oxoacids II
What trend do you see and why?
Name Hypochlorous Chlorous Chloric Perchloric
Formula HOCl HOClO HOClO2 HOClO3
Experimental Ka
3.2 x10-8 1.3 x10-2 1x102 2 x107
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What affects the pH?
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Question 1
• How could you change the pH of a solution of acetic acid (CH3COOH)?
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Question 2
Apatite, Ca5(PO4)3OH is the mineral in teeth.Ca5(PO4)3OH(s) 5 Ca+2(aq) + 3PO4
-4(aq) + OH-(aq)
Sour milk contains lactic acid. Not removing sour milk from the teeth of young children can lead to tooth decay. Use chemical principles to explain why.
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Can salts affect the pH of a solution?
Ca(OH)2 Na(CH3COO)
NH4Cl
NaNO3
Ksp = 7.9x10-6
Kb NH3 = 1.8 x10-5
Ka CH3COOH = 1.8 x10-5
NH4+ + H2O NH3 + H3O+
NaH2PO4
Na2HPO4
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Name ions found in salts that would not affect the pH:
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Question 3
A carbonated beverage is left open to the atmosphere. Will the pH change?
CO2(aq) + H2O(l) ↔ H2CO3(aq)
H2CO3 + H2O(l) ↔ H3O + + HCO3¯ Ka1 = 4.2 × 10-7
HCO3¯ + H2O(l) ↔ H+ + CO32¯ Ka2 = 4.8 × 10 -11
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