Ch. 7 - Acids and Bases cont
Transcript of Ch. 7 - Acids and Bases cont
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Week 10 CHEM 1310 - Sections L and M 1
Ch. 7 - Acids and Bases cont.
Last Week Arrhenius vs. Bronsted-Lowry Acids and Bases Conjugate Acid-Base Pairs Water: Acid or Base pH and Related Simple Calculations
Today Acid Strength More pH Calculations Bases (more detail) Polyprotic Acids (more detail)
Week 10 CHEM 1310 - Sections L and M 2
Determining Acid StrengthVia Ka Value
The larger the Ka value the stronger the acid
Week 10 CHEM 1310 - Sections L and M 3
Determining Acid StrengthVia Position of Equilibrium
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Strong Acidsfully dissociate!
Before equilibrium At equilibrium
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Week 10 CHEM 1310 - Sections L and M 4
Determining Acid Strength
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
[HA]0 ≈ [H+]equilibriumfor
Strong Acids
Before equilibrium At equilibrium
Via Equilibrium [H+] vs. Original [HA]
Week 10 CHEM 1310 - Sections L and M 5
Determining Acid StrengthVia Position of Equilibrium
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Weak Acidsdo not fullydissociate!
Before equilibrium At equilibrium
Week 10 CHEM 1310 - Sections L and M 6
Determining Acid Strength
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
[HA]0 >> [H+]equilibriumfor
Weak Acids!
Before equilibrium At equilibrium
Via Equilibrium [H+] vs. Original [HA]
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Week 10 CHEM 1310 - Sections L and M 7
Determining Acid StrengthVia Strength of A- vs. H2O
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Strongacid
Weak Conjugate
Base
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Weakacid
Strong Conjugate
Base
Week 10 CHEM 1310 - Sections L and M 8
PRS Question
1. Weak Acid
2. Strong Acid
3. HF is not an acid
4. Cannot be determined from information given
Is HF a strong or a weak acid?
Acid Formula Ka
Hydrochloric Acid HCl 1 x 107
Hydrofluoric Acid HF 7.2 x 10-4
Week 10 CHEM 1310 - Sections L and M 9
PRS Question
1. Weak Acid
2. Strong Acid
3. HF is not an acid
4. Cannot be determined from information given
Is HF a strong or a weak acid?
The larger the Ka valuethe stronger the acid
Acid Formula Ka
Hydrochloric Acid HCl 1 x 107
Hydrofluoric Acid HF 7.2 x 10-4
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Week 10 CHEM 1310 - Sections L and M 10
pH: Strong vs. Weak Acids
pH = -log [H+]
For a strong acid: [HA] ≈ [H+] = 1.0 M
Thus, pH = -log (1.0) = 0
What is the pH of a 1.0M HCl solution?
Acid Formula Ka
Hydrochloric Acid HCl 1 x 107
Hydrofluoric Acid HF 7.2 x 10-4
Week 10 CHEM 1310 - Sections L and M 11
pH: Strong vs. Weak Acids
pH = -log [H+]What is the pH of a 1.0M HF solution?
HF H+ + F-
Ka =[H+] x [F-]
[HF]Ka =
X2
[HF]0 - X
[HF]0 >> X
Acid Formula Ka
Hydrochloric Acid HCl 1 x 107
Hydrofluoric Acid HF 7.2 x 10-4
Week 10 CHEM 1310 - Sections L and M 12
pH: Strong vs. Weak Acids
pH = -log [H+]What is the pH of a 1.0M HF solution?
HF H+ + F-
Ka =X2
1 - X
[HF]0 >> X
Let X = [H+] = [F-]
X = √Ka = √7.2 x 10-4 X = 0.0268
Ka = X2
Acid Formula Ka
Hydrochloric Acid HCl 1 x 107
Hydrofluoric Acid HF 7.2 x 10-4
pH = -log (0.0268) = 1.6
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Week 10 CHEM 1310 - Sections L and M 13
Another Sample Problem
A solution is prepared by dissolving 0.56-gof benzoic acid (C6H5COOH; Ka = 6.4x10-5)into enough water to make 1 L.Calculate the pH of the solution.
Determine the MW of benzoic acid Write the chemical equation describing the
dissolution of this acid Is this a weak acid?
Solve problem on board!
Week 10 CHEM 1310 - Sections L and M 14
PRS Question
Which of the following Ka values belongs tothe strongest acid?
1. 6.6 x 10-4
2. 4.6 x 10-4
3. 9.1 x 10-8
4. 3.0 x 10-8
5. Cannot be determined from the info given
Week 10 CHEM 1310 - Sections L and M 15
PRS Question
Which of the following Ka values belongs tothe strongest acid?
1. 6.6 x 10-4
2. 4.6 x 10-4
3. 9.1 x 10-8
4. 3.0 x 10-8
5. Cannot be determined from the info given
The stronger the acid,the higher the Ka
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Week 10 CHEM 1310 - Sections L and M 16
PRS Question
Which of the following pKa values belongs tothe strongest acid?
1. 9.25
2. 7.46
3. 3.40
4. 1.92
5. Cannot be determined from the info given
Week 10 CHEM 1310 - Sections L and M 17
PRS Question
Which of the following pKa values belongs tothe strongest acid?
1. 9.25
2. 7.46
3. 3.40
4. 1.92
5. Cannot be determined from the info given
The stronger the acid,the higher the Ka
and the lower the pKa!
Week 10 CHEM 1310 - Sections L and M 18
Percent DissociationHA(aq) + H2O(l) H3O+(aq) + A-(aq)
% Dissociation = [A-][HA]
x 100
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Week 10 CHEM 1310 - Sections L and M 19
Bases and Base Strength
B (aq) + H2O (l) ↔ BH+ (aq) + OH– (aq)B = Base
Kb is the Basicityconstant
b
2
[BH ][OH ]K
[B][H O]
[BH ][OH ]
[B]
+ -
+ -
=
=
b 10 bpK log K= -
Week 10 CHEM 1310 - Sections L and M 20
Polyprotic Acids
Ka1 > Ka2 > Ka3
Week 10 CHEM 1310 - Sections L and M 21
Polyprotic Acids
Sulfuric acid (H2SO4) is unique Ka1 = 100 STRONG ACID Ka2 = 1.2 x 10-2 WEAK ACID
H2SO4 (aq) → H+ (aq) + HSO4– (aq) Ka1 ≈ 1.0x102
HSO4– (aq) ↔ H+ (aq) + SO4
2– (aq) Ka2 ≈ 1.2x10-2
4a1
a2
K 10
Kª