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Acid-Base Equilibria
BLB 10th Chapter 16
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Examples of acids & bases
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Acids Bases
Sour (like vinegar) Bitter and slippery (like soap)
React with bases to neutralize them and form salts
React with acids to neutralize them and form salts
Change indicator colors in opposite direction from base (e.g. litmus blue to red)
Change indicator colors in opposite direction from acid (e.g. litmus red to blue)
Aqueous solutions conduct electricity
Aqueous solutions conduct electricity
Liberate hydrogen in reactions with active metals
React in aqueous solution with salts of heavy metals to form insoluble hydroxides or oxides
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16.1 Acids & Bases: A Brief Review
Arrhenius Definitions Acid – a substance that produces
hydrogen ions (H+) in water
HA → H+ + A-
Base – a substance that produces hydroxide ions (OH-) in water
BOH → B+ + OH-
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16.2 Brønsted-Lowry Acids & Bases
H+ (proton) in water:
H+ + H2O → H3O+
hydronium ionHydronium ion can hydrogen bond with
more water molecules to form large clusters of hydrated hydronium ions.
H+ and H3O+ are used interchangeably.
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16.2 Brønsted-Lowry Acids & Bases
Brønsted-Lowry definitions
acid – proton donor Neutral (HNO3), anionic (HCO3
-), cationic (NH4+)
Must have a removable (acidic) proton
base – proton acceptor Neutral (NH3), anionic (CO3
2-) Must have a lone pair of electrons
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Acid-Base Reactions
HCl(aq) + H2O(l) → H3O+ (aq) + Cl-(aq)
NH3(aq) + H2O(l) NH⇌ 4+(aq) + OH-(aq)
HCl(aq) + NH3(aq) → NH4+(aq) + Cl-(aq)
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Acid-base reaction in non-aqueous media:HCl(g) + NH3(g) → NH4Cl(s)
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amphiprotic – capable of behaving as a Brønsted acid and Brønsted base
amphoteric – capable of behaving as a Lewis acid and Brønsted base (17.5)
neutralization: acid + base → salt + water
Conjugate acid/base pairs – differ by a single proton
HA(aq) + H2O(l) → H3O+(aq) + A-(aq)
acid + base conj. acid + conj. base
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Relative Acid/Base Strength Strength is a measure of the ability of an
acid (or base) to donate (or accepts) a H+. Stronger acids donate H+ more readily.
Completely dissociate in water Conjugate bases have negligible tendency to
accept protons.
Weaker acids donate H+ less readily. Partially dissociate and establish equilibrium Conjugate bases have some tendency to accept
protons.
The stronger an acid, the weaker its conjugate base and vice versa.
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p. 672
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Acid/base reactions proceed from the stronger acid-base pair to the weaker acid-base pair.
Common strong acids (p. 679):
HClO4, HClO3, H2SO4, HI, HBr, HCl, HNO3
Monoprotic acid – capable of donating only one H+
Polyprotic acid – capable of donating more than one H+
Common strong bases (p. 680):
M(OH)n, where M = group I (n=1) & II (n=2) metals, except Be
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Acid/Base Reactions
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16.3 The Autoionization of Water
H2O(l) + H2O(l) H⇌ 3O+(aq) + OH-(aq) Kw = [H3O+][OH-] = [H+][OH-] = 1.0 x 10-14 (@ 25°C)
Kw – ion-product constant (or dissociation constant)
Pure water is neutral. Thus,
[H3O+] = [OH-] = 1.0 x 10-7 M @ 25°C
For an aqueous solution:
[H3O+] > [OH-] acidic
[H3O+] = [OH-] neutral
[H3O+] < [OH-] basic
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Working with Kw
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16.4 The pH Scale pH represents a solution’s acidity (@
25°C. 0 ← 7 → 14acid neutral base
See Table 16.1, p. 678 for summary. See Figure 16.5, p. 679 for examples. pH = −log[H3O+] = −log[H+]
[H3O+] = 10-pH
pOH = −log[OH-] pH + pOH = 14[OH-] = 10-pOH
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p. 676
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More common chemicals
Chemical pH
BasicWindex 10.57
Bleach 9.58
Neutral Tap water* 7.46
Acidic
Alka Seltzer (in tap water) 6.43
Distilled water** 6.37
Flat Coke 2.60
Toilet bowl cleaner 1.04
6.0 M HCl −0.29
*CaCO3 CO3- + H2O ⇌ HCO3
- + OH-
**CO2 + H2O → H2CO3
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pH calculations
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More about pH
pH does not necessarily indicate strength.
Measuring pH pH meters Acid-base indicators
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p. 679
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16.5 Strong Acids and Bases
Strong acids & bases completely ionize.
[HA]0 = [H3O+] → pH
[MOH]0 = [OH-] → pOH → pH
2[M(OH)2]0 = [OH-] → pOH → pH
H3O+ is the strongest acid that can exist in water. (produced by all acids in water)
OH- is the strongest base that can exist in water. (produced by all bases in water)
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pH problems
End Test #1 material
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16.6 Weak Acids & 16.7 Weak Bases
Weak acids & bases do not completely ionize.
Weak acids establish an equilibrium in aqueous solution.
HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
HA(aq) ⇌ H+(aq) + A-(aq) They do not readily donate or accept H+’s. [HA]0 ≠ [H3O+]
[MOH]0 ≠ [OH-]
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HA(aq) + H2O(l) H⇌ 3O+(aq) + A-(aq)HA(aq) H⇌ +(aq) + A-(aq)
Ka ↑ acid strength ↑For polyprotic acids: Ka1 >> Ka2 >> Ka3
pKa = −log[Ka] pKa ↑ acid strength↓
Weak Acids & Acid-dissociation Constant
][
]][[
][
]][[ 3
HA
AH
HA
AOHKa
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From p. 682 + more in Appendix D, p. 1115-1116
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Weak Bases & Base-dissociation Constant
Weak bases establish an equilibrium in aqueous solution.
B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)
][
]][[
B
OHBHKb
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From p. 691 + more in Appendix D, p. 1115-1116
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% Dissociation (or ionization)
% dissociation decreases as concentration increases (p. 686)
%100][
%100%
0
xHA
x
xionconcentratinitial
ddissociateamountondissociati
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Weak acid/base Problems1) Ka (or Kb) from equilibrium pH2) pH from Ka (or Kb)
1. Identify as weak acid or base.
2. Write the chemical equilibrium.
3. Write the equilibrium constant expression.
4. Set up concentration table. (Ch. 15.5)
5. Solve for x.
6. Check with 5% rule. If greater than 5%, use quadratic equation. (type 2 only)
7. Complete problem.
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The pH of a 0.10 M solution of propanoic acid (CH3CH2CO2H) is 2.94. Calculate the Ka for propanoic acid.
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Calculate the pH of a 1.0 M HF solution.
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Calculate the pH of a 0.0010 M HF solution.
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Calculate the pH of a 0.20 M solution of triethylamine N(CH2CH3)3.
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16.8 Relationship between Ka and Kb
For a conjugate acid/base pair:
Ka x Kb = Kw (derivation p. 693)
pKa + pKb = pKw = 14.00
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16.9 Acid-Base Properties of Salt Solutions
Salt – ionic compound Salts dissolve in water to produce ions. Ions can also affect the pH. Hydrolysis – reaction between an ion
and water to produce H3O+ or OH-
F-(aq) + H2O(l) ⇌ HF(aq) + OH-(aq)
NH4+(aq) + H2O(l) H⇌ 3O+(aq) + NH3(aq)
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Which ions will undergo hydrolysis, i.e. react with water and affect the pH of the solution?
Anion: Conjugate base of a weak acid ► basic Conjugate base of a monoprotic strong
acid ► neutral Cation:
Conjugate acid of a weak base ► acidic Group I & II metal ions ► neutral
(exceptions Be2+ and Mg2+ ► acidic) Other metal ions ► acidic
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Cation + Anion ►Acidic, basic, or neutral?
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16.10 Acid-Base Behavior and Chemical Structure
Binary Acids (HX) As bond strength increases, acid strength
decreases.
Group: size of X ↑ acid strength ↓ Period: electronegativity of X ↑ acid strength↑
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Oxyacids – acidic H attached to an oxygen atom Same # of OH groups and O atoms: central
atom electronegativity ↑ acid strength ↑
HClO > HBrO > HIO
Same central atom, Y: # O atoms ↑ acid strength ↑
HClO4 > HClO3 > HClO2 > HClO
Carboxylic acids – contain −COOH or CO2H # electronegative atoms ↑ acid strength ↑
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Oxyacids
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16.11 Lewis Acids and Bases
Lewis acid – electron-pair acceptor e--poor compounds Metal ions
Lewis base – electron-pair donor Amines, NR3
Ligands (see chapter 24.1)
Every Brønsted base is a Lewis base, but not vice versa.
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Lewis acid & base examples