Lecture 52 Acids and Bases II

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Lecture 52 - Acids and Bases II
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Strengths of Acids and Bases

• acid dissociation constant, Ka
• HCl(aq) H2O(l) ? H3O(aq) Cl-(aq)

H3O Cl-
Ka
HCl H2O
(H2O is constant)
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Strengths of Acids and Bases

• more negative pKa stronger acid

pKa -log10(Ka)
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Strengths of Acids and Bases

• big Ka (more negative pKa) strong acid
• Acid Ka pKa
• HCl 106 -6

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Strengths of Acids and Bases

H3O Cl-
Ka
(gt106)

HCl
H3O Cl-
i.e. HCl lt
106
Difficult to measure exactly
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Strengths of Acids and Bases

• big Ka (more negative pKa) strong acid
• Acid Ka pKa
• HCl 106 -6
• HNO3 30 -1.5
• HSO4- 0.011 1.96
• HF 6.6 x 10-4 3.18
• CH3COOH 1.8 x 10-5 4.74

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e.g. 1.0 M HNO3(aq)

• Find HNO3(aq)
• HNO3(aq) H2O(l) ? H3O(aq) NO3-(aq)

1. Originally, HNO3 1.0 M 2. At equilibrium,
HNO3 1.0 - x
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e.g. 1.0 M HNO3(aq)

• 2. At equilibrium, HNO3 1.0 - x
• 3. At equilibrium, H3O NO3- x

x2
4. Thus,
30
(1-x)
5. x 0.97
6. HNO3 1 - 0.97 0.03 M
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Strengths of Acids and Bases

• base dissociation constant, Kb
• NH3(aq) H2O(l) ? NH4(aq) OH-(aq)

NH4 OH-
Kb
(H2O is constant)
NH3
pKb -log10(Kb)
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Strengths of Acids and Bases

• big Kb (more negative pKb) strong base
• Base Kb pKb
• NaOH big big (negative)
• CH3NH2 4.4 x 10-4 3.36
• NH3 1.8 x 10-5 4.74
• C5H5N 1.7 x 10-9 8.77

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• CH3NH2
• methyl amine or amino methane
• C5H5N
• pyridine

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The Special Case of Water

• H2O(l) H2O(l) ? H3O(aq) OH-(aq)
• Kw (Ka Kb) H3O(aq) OH-(aq)
• 1.0 x 10-14 at 25oC
• (true in any aqueous solution)
• (pKa pKb pKw 14)

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The Special Case of Water

• in pure water,
• H3O OH- 1.0 x 10-7 M

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Strengths of Binary Acids

• e.g. HX acids, X a halogen
• larger bond energy weaker acid
• HI gt HBr gt HCl gt HF

bond energy increases
acid strength decreases
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Strengths of Oxoacids

• A. HOX acids
• if X is electronegative, acid is stronger
• H - O - X

electrons pulled this way by X
this bond is weakened !
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Strengths of Oxoacids

• A. HOX acids
• if X is electronegative, acid is stronger
• HOI lt HOBr lt HOCl

EN (X) increases
acid strength increases
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Strengths of Oxoacids

• B. HXOn acids
• with more oxygens, acid is stronger
• H-O-Cl lt H-O-Cl-O lt H-O-Cl-O lt H-O-Cl-O

acid strength increases
Ka 108
Ka 2.9 x 10-8
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Strengths of Carboxylic Acids

• R - C - O - H

R (almost) anything often written RCOOH
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Strengths of Carboxylic Acids
O
O


CH3 - C - O - H
CH3(CH2)3 - C - O - H
Pentanoic Acid (Valeric Acid)
Ethanoic Acid (Acetic Acid)
Ka 1.5 x 10-5
Ka 1.8 x 10-5
i.e. little effect of HC chains
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Strengths of Carboxylic Acids
ClCH2CH2COOH
ICH2CH2COOH
3-Iodopropanoic Acid Ka 8.3 x 10-5
3-Chloropropanoic Acid Ka 1.0 x
10-4
(Cl more electronegative than I)
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Strengths of Carboxylic Acids
ClCH2CH2COOH
CH3CH2ClCOOH
3-Chloropropanoic Acid Ka 1.0 x
10-4
2-Chloropropanoic Acid Ka 1.4 x
10-3
(Cl closer to the acidic hydrogen)
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Strengths of Carboxylic Acids
CH3CHCl2COOH
CH3CH2ClCOOH
2,2-Dichloropropanoic Acid Ka
8.7 x 10-3
2-Chloropropanoic Acid Ka 1.4 x
10-3
(more Cl in molecule)
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Organic Bases

• - are amines (N-containing)
• - include many drugs
• - react with strong acids to form water soluble
  salts

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Organic Bases
Procaine (solubility 0.5g/100mL)
HCl(aq)

H
Cl-
(Novocaine, solubility 100 g/100 mL)