acid/base equilibria chapter 16. models of acids and bases arrhenius concept: acids produce h + in...
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Acid/Base Equilibria
Chapter 16
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Models of Acids and Bases
Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion.
Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors.
HCl + H2O Cl + H3O+
acid base
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A Brønsted–Lowry acid…
…must have a removable (acidic) proton.
A Brønsted–Lowry base…
…must have a pair of nonbonding electrons.
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What Happens When an Acid Dissolves in Water?
Water acts as a Brønsted–Lowry base and abstracts a proton (H+) from the acid.
As a result, the conjugate base of the acid and a hydronium ion are formed.
© 2012 Pearson Education, Inc.
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Conjugate Acid/Base Pairs
HA(aq) + H2O(l) H3O+(aq) + A(aq) conj conj conj conj
acid 1 base 2 acid 2 base 1
conjugate base: everything that remains of the acid molecule after a proton is lost.
conjugate acid: formed when the proton is transferred to the base.
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Acid Dissociation Constant (Ka)
HA(aq) + H2O(l) H3O+(aq) + A(aq)
Ka3H O A
HA
H A
HA
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Acid Strength
- Its equilibrium position lies far to the right. (HNO3)
- Yields a weak conjugate base. (NO3)
Strong Acid:
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Acid and Base Strength
Strong acids are completely dissociated in water. Their conjugate bases are
quite weak.
Weak acids only dissociate partially in water. Their conjugate bases are
weak bases.
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Strong Acid - Weak Acid
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HA
(a)
H+ A–
HA
(b)
H+ A–
Before dissociationAfter dissociation,at equilibrium
HA
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Strong AcidsYou will recall that the seven strong acids
are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4.
These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.
For the monoprotic strong acids,
[H3O+] = [acid].
© 2012 Pearson Education, Inc.
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Strong AcidsHCl – Hydrochloric Acid
HBr – Hydrobromic Acid
HI – Hydroiodic Acid
HNO3 - Nitric Acid
H2SO4 – Sulfuric Acid
HClO3 – Chloric Acid
HClO4 – Perchloric Acid
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Acid Strength(continued)
- Its equilibrium lies far to the left. (CH3COOH)
- Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO)
Weak Acid:
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Strong Acid Weak Acid14_1577
(a) (b)
H+H+
H+
H+
H+
H+
H+H+
H+
H+
H+
H+
H+
A-
A-
A-
A-
A-
A-
A-
A- A-
A-
A-
A- B-
HBHB
HBHB
HB HB
HB
HBHBHB
HB
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Relativeacid strength
Relativeconjugatebase strength
Veryweak
Weak
Strong
Verystrong
Veryweak
Weak
Strong
Verystrong
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Table 14.2 Values of Ka for Some Common Monoprotic Acids
Formula Name Value of Ka*
HSO4 Hydrogen sulfate ion 1.2 x 10 2
HClO2 Chlorous acid 1.2 x 10 2
HC2H2ClO2 Monochloracetic acid 1.35 x 10 3
HF Hydrofluoric acid 7.2 x 10 4
HNO2 Nitrous acid 4.0 x 10 4
HC2H3O2 Acetic acid 1.8 x 10 5
[Al(H2O)6]3+ Hydrated aluminum(III) ion 1.4 x 10 5
HOCl Hypochlorous acid 3.5 x 10 8
HCN Hydrocyanic acid 6.2 x 10 10
NH4 Ammonium ion 5.6 x 10 10
HOC6H5 Phenol 1.6 x 10 10
Incr
ea
sin
g a
cid
str
en
gth
*The units of Ka are mol/L but are customarily omitted.
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Water as an Acid and a Base
Water is amphoteric (it can behave either as an acid or a base).
H2O + H2O H3O+ + OH
conj conj acid 1 base 2 acid 2 base 1
Kw = 1 1014 at 25°C
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Autoionization of WaterAs we have seen, water is amphoteric.
In pure water, a few molecules act as bases and a few act as acids.
This is referred to as autoionization.
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Ion Product Constant
The equilibrium expression for this process is
Kc = [H3O+] [OH]
This special equilibrium constant is referred to as the ion product constant for water, Kw.
At 25C, Kw = 1.0 1014
© 2012 Pearson Education, Inc.
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The pH Scale
pH log[H+]
pH in water ranges from 0 to 14.
Kw = 1.00 1014 = [H+] [OH]
pKw = 14.00 = pH + pOH
As pH rises, pOH falls (sum = 14.00).
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[H+] pH
10–14 14
10–13 13
10–12 12
10–11 11
10–10 10
10–9 9
10–8 8
10–6 6
10–5 5
10–4 4
10–3 3
10–2 2
10–1 1
1 0
Acidic
Neutral
Basic
1 M NaOH
Ammonia (Householdcleaner)
BloodPure water
Milk
VinegarLemon juice
Stomach acid
1 M HCl
10–7 7
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How Do We Measure pH?
For less accurate measurements, one can use
Litmus paper
“Red” paper turns blue above ~pH = 8
“Blue” paper turns red below ~pH = 5
Or an indicator.
© 2012 Pearson Education, Inc.
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How Do We Measure pH?
For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.
© 2012 Pearson Education, Inc.
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Solving Weak Acid Equilibrium Problems
- List major species in solution.
- Choose species that can produce H+ and write reactions.
- Based on K values, decide on dominant equilibrium.
- Write equilibrium expression for dominant equilibrium.
- List initial concentrations in dominant equilibrium.
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Solving Weak Acid Equilibrium Problems (continued)
- Define change at equilibrium (as “x”).
- Write equilibrium concentrations in terms of x.
- Substitute equilibrium concentrations into equilibrium expression.
- Solve for x the “easy way.”
- Verify assumptions using 5% rule.
- Calculate [H+] and pH.
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Percent Dissociation (Ionization)
% dissociationamount dissociated( )initial concentration( )
100% MM
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Polyprotic Acids
. . . can furnish more than one proton (H+) to the solution.
H CO H HCO ( )
HCO H CO ( )
2 3 3 a
3 32
a
1
2
K
K
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Acids and Bases
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Bases
“Strong” and “weak” are used in the same sense for bases as for acids.
strong = complete dissociation (hydroxide ion supplied to solution)
NaOH(s) Na+(aq) + OH(aq)
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Strong BasesLiOH
Lithium Hydroxide
NaOH Sodium
Hydroxide
KOH Potassium Hydroxide
RbOH Rubidium Hydroxide
CsOH Cesium
Hydroxide
Ba(OH)2 Barium
Hydroxide
Sr(OH)2 Strontium Hydroxide
Ca(OH)2 Calcium
Hydroxide
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Bases(continued)
weak = very little dissociation (or reaction with water)
H3CNH2(aq) + H2O(l) H3CNH3+(aq) + OH(aq)
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Acid-Base Properties of Salts
Cation AnionAcidic
or Basic Exampleneutral neutral neutral NaCl
neutral conj base ofweak acid
basic NaF
conj acid ofweak base
neutral acidic NH4Cl
conj acid ofweak base
conj base ofweak acid
depends onKa & Kb
values
Al2(SO4)3
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Problem
An unknown salt is either NaF, NaCl or NaOCL. When 0.050 moles of the salt is dissolved in 0.500 L of solution, the pH of the solution is 8.08. What is the identity of the salt? Show Calculations.
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Table 14.2 Values of Ka for Some Common Monoprotic Acids
Formula Name Value of Ka*
HSO4 Hydrogen sulfate ion 1.2 x 10 2
HClO2 Chlorous acid 1.2 x 10 2
HC2H2ClO2 Monochloracetic acid 1.35 x 10 3
HF Hydrofluoric acid 7.2 x 10 4
HNO2 Nitrous acid 4.0 x 10 4
HC2H3O2 Acetic acid 1.8 x 10 5
[Al(H2O)6]3+ Hydrated aluminum(III) ion 1.4 x 10 5
HOCl Hypochlorous acid 3.5 x 10 8
HCN Hydrocyanic acid 6.2 x 10 10
NH4 Ammonium ion 5.6 x 10 10
HOC6H5 Phenol 1.6 x 10 10
Incr
easi
ng a
cid
stre
ngth
*The units of Ka are mol/L but are customarily omitted.
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Problem
Sorbic acid (C5H7COOH) is a weak monoprotic acid with Ka = 1.7 x10-5. Its salt (potassium sorbate) is added to cheese to inhibit the formation of mold.
What is the pH of a solution containing 9.50 g of potassium sorbate in 2.00 L of solution?
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Problem – 16.90
Trisodium phosphate (Na3PO4) is available in hardware stores as TSP and is used as a cleaning agent. The label on a box of TSP warns that the substance is very basic (caustic or alkaline). What is the pH of a solution containing 35.0 g of TSP in a liter of solution?
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Structure and Acid-Base Properties
Two factors for acidity in binary compounds:
- Bond Polarity (high is good)
- Bond Strength (low is good)
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14_326 Cl O H
Electron density
Cl O H
Electron density
O
Cl O H
Electron density
O
O
Cl O H
Electron density
O
OO
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Oxides
Acidic Oxides (Acid Anhydrides):
- OX bond is strong and covalent.
SO2, NO2, CrO3
Basic Oxides (Basic Anhydrides):
- OX bond is ionic.
K2O, CaO
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Lewis Acids and Bases
Lewis Acid: electron pair acceptor
Lewis Base: electron pair donor
Al3+ + 6H
OH
Al OH
H6
3+
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Problem 16.86
Predict whether aqueous solutions of the following are acidic, basic, or neutral:
a. CrBr3
b. LiCl
c. K3PO4
d. (CH3NH3)Cl
e. KHSO4
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