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Chapter 14 Acids and Bases
14.1 Nature of Acids and Bases
A. Acids –
B. Bases –
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C. Arrhenius Definition
1. acid –
2. base –
3. Acid-base reaction involving Arrhenius acids and bases:
4. Some substances are bases that don’t have an OH- in their formulas, so a
new definition was needed.
B. BrØ nsted-Lowry Definition
1. acid -
2. base –
3. Acid-Base reaction involving BrØnsted-Lowry acids and bases:
HCO3- (aq) + HF (aq) ⇄
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4. Conjugate acid-base pair –
5. Example: Find the conjugate acid/base pairs in the following:
a. HNO2 (aq) + H2O (l) ⇄ H3O+ (aq) + NO2- (aq)
b. NH3 (aq) + H2O (l) ⇄ NH4+ (aq) + OH- (aq)
c. H2PO4- (aq) + CO32- (aq) ⇄ HCO3- (aq)+ HPO42-(aq)
14.2 Acid Strength
A. Acid Dissociation Constant (Ka) –
1. Strong acids
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2. Weak acids
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5 3. The smaller the Ka,
Table of Monoprotic Acids -
4. Example: Using the figure above, rank the acids HX, HY, and HZ in order
from the strongest to the weakest acid.
14.9
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4.9/10 Predicting Acid/Base strengths
A. Periodic Trends for Binary Acids (H-X)
1. Factors determining the relative acid strength
B. Periodic Trends for Oxyacids (H-O-Y)
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C. Number of Oxygens in an oxoacid (HO)mYOn
D. Ex: In each of the following pairs of acids, which is the stronger acid?
1. H2Te or H2Se
2. . H2TeO3 or H2SeO3
3. H2TeO3 or H2TeO2
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14.6 Base Strength
A. Base Dissociation Constant (Kb) –
1. Strong bases
2. Weak bases
3. The smaller the Kb,
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9 4. Example: Identify the following as strong acid, base, or weak acid, base?
CH3NH2
HOCH2CH2NH2
KOH
H2SeO4
HI
HClO-
B. Conjugate acid-base pairs and acid-base strength
1. Strong acids have
2. Strong bases have
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C. Water
1. Water is amphoteric –
2. Autoionization of water –
3. Ion Product constant for water (Kw):
4. From experiments of pure water at 25°C, [OH-] = [H3O+] =
5. Both ions are
6. What happens when either OH- or H3O+ are added to the system?
7. Three possible situations:
a. [H3O+] = [OH-]
b. [H3O+] > [OH-]
c. [H3O+] < [OH-]
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11 8. Ex: Calculate [H3O+] in a solution at 25°C that has [OH-] = 6.7 x 10-2. Is
the solution acidic or basic?
9. Ex: What is the [H3O+] and [OH-] of 0.33M HNO3 at 25°C? Is the solution
acidic or basic?
10. Ex: At 5°C, the value of Kw is 1x10-15 at 0°C. Calculate the
concentrations of ions at 0°C. Is the ionization of water endothermic or
exothermic?
11. Kw = Ka x Kb for a conjugate acid-base pair
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12 12. Ex: At 25°C, the Ka for acetic acid is 1.8 x 10-5, what is the Kb for the
acetic acid’s conjugate base?
14.3 The pH Scale
A. p Scale used to represent small values
1. In aqueous solutions, [H3O+] =
2. pH =
sig figs for logarithms –
3. pOH =
4. pH changes by 1 for ever power of 10 change in [H+]
5. Ex: Calculate the pH and pOH for the following solutions at 25°C.
a. 1.2 x 10-3 M H+
b. 1.5 x 10-6 M OH-
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13 6. Another useful equation:
7. Ex: The pH of a sample of stomach acid is 2.20 at 25°C. Calculate the
pOH, [H+], and [OH-] for the sample.
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14 14.4/14.6 Calculating the pH of Strong Acid and Base Solutions
A. Since strong acids and bases completely dissociate, the pH can easily be found.
1. Ex: What is the [H3O+], [OH-], pH, and pOH of 0.25M HCl at 25 °C?
2. Ex: What is the [H3O+], [OH-], pH, and pOH of 0.50 M NaOH at 25 °C?
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15 14.5 Calculating the pH of Weak Acid/Weak Base Solutions
A. Since weak acids DO NOT completely dissociate,
1. Ex: A solution of nicotinic acid, HC6H4NO2 is 0.012M and has a pH of 3.39
at 25°C. What is the Ka? What percent has dissociated?
Step 1: Write equation.
Step 2: Make an ICE table.
Assumptions to make math easier:
a. [H3O+] from auto ionization of water
b. Weak acid has a small Ka
Step 3: Solve for Ka
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Step 4: If any assumptions were made, check that these assumptions are valid.
2. Ex: A 0.10 M solution of nicotinic acid is prepared at 25°C. What are
the equilibrium concentrations of each substance? What is the percent
dissociation? Ka = 1.4 x 10-5
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17 2. Ex: What is the pH (at 25°C) of a solution made by dissolving a 0.325 g
tablet of aspirin (HC9H7O4) in water? The acid is monprotic and the Ka =
3.3 x 10-4 at 25°C.
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18 B. Polyprotic Acids – can lose 2 or more protons
1. Examples –
a. Sulfuric Acid (strong acid)
b. Carbonic Acid (weak acid)
2. In general –
3. Ex: Ascorbic acid (vitamin C) is H2C6H6O6. What is the pH of a 0.10M
solution? Ka1 = 7.9 x 10-5 and Ka2 = 1.6 x 10-12.
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C. Weak Bases
1. Ex: What is the pH of a 0.0075 M solution of morphine (C17H19NO3) at
25°C? Kb = 1.6 x 10-6 at 25°C.
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14.8 Acid-Base Properties of Salts
A. Salt (MX)
B. Cations from salts (Mx+)
C. Anions from salts (X-)
D. Examples
1. Predict whether the following aqueous salt solution will be acidic, basic or
neutral
a. NaCl
b. NaC2H3O2
c. NH4Cl
d. NH4CN (Ka for NH4 is 5.5 x 10-10 and Kb for CN- is 1.6 x 10-5).
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2. Calculate the pH of NH4Cl. The Ka for NH4 is 5.5 x 10-10
14.11 Lewis Acids and Bases –
A. Lewis acid –
B. Lewis Base
C. Examples
1. Na2O (aq) + SO3 (g) → Na2SO4
2. BF3 (aq) + NH3 (aq) → BF3NH3
3. HCl + NH3 → NH4+
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