Acid and Base Equilibria
Chapter 16
PRE-REQUISITES:
What do you already know
about ACIDS?
1) Acids Taste?
Review Chapter 4.1 (Electrolytes)
Review Chapter 4.3 (Acid-Base Reactions)
2) Acids dissolve active metals, usually
liberating ___________.
Memorize 7 Strong Acids (Table 4.2)
Memorize the Strong Bases (Also Table
4.2)
3) Acids are corrosive – they dissolve
compounds that are otherwise hard to
dissolve.
Review Equilibrium (Ch. 15)
4) Acids turn litmus paper ______________.
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What do you already know
about BASES?
ARRHENIUS Acids and Bases
Definitions:
1) Bases Taste?
Arrhenius ACID:
Any compound that releases H+
when dissolved in H2O.
2) Bases dissolve oil and grease.
3) Bases are slippery to the touch.
4) Bases react with many metal ions to form
precipitates.
Arrhenius BASE:
Any compound that releases OH!
when dissolved in H2O.
5) Bases turn litmus paper ________________
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Need a new definition of acids
and bases based on
BRØNSTED - LOWRY
ACIDS AND BASES
Proton Transfer
BrØnsted ACID:
Any compound capable of donating an
H+ ion.
BrØnsted BASE:
Any compound capable of accepting a
H+ ion.
Hydronium ion
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CONJUGATE ACID-BASE PAIRS
Is it weak, strong or negligible?
Differ only by the presence or absence
of a proton (H+).
Conjugate Acid
Conjugate Base + H+
Examples: Conjugate Acid
1.
The conjugate of a strong acid is a
______________. (Example: Cl! is the
conjugate base of HCl). Spectators
have negligible acidity/basicity.
2.
The conjugate acid of OH! (strong
base) is _________ (negligible acidity).
Also true for strong bases H- and CH3-.
3.
The conjugate of a weak acid is a
_________________.
Conjugate Base
Note:
The stronger the acid, the weaker its
conjugate base.
Compound
Conjugate
Strong Acid
Strong Base
The weaker the acid, the stronger its
conjugate base.
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Weak Base
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Auto-ionization of water:
Water electrolyzes slightly to produce H+ and
OH! reversibly.
H2O(l)
H+ (aq) + OH- (aq)
What is the equilibrium constant expression
for this process?
What is the [H+] of pure water at 25°C?
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Kw is the Ion Product Constant
for water
The pH Scale
NOTE:
Kw is constant even when [H+] and [OH!]
are not equal.
1. What is the pH of pure water at 25
°C?
Calculate [H+] in a 0.05 M Ca(OH)2 solution.
2. What is the [H+] of an HCl solution
that has a pH of 2.34?
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pH and Concentration
Negative Log Scale is Useful
for Many Small Numbers:
pOH =
pKw =
How are pH and pOH Related?
Kw =
Take log of all quantities:
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Measuring pH
Compare the effect of
concentration on pH
!pH Meter
Solution
!Indicators
H(Ind)
Ind- + H+
[H+]
1
1.0 x 10-7 M
2
2.0 x 10-7 M
pH
Many biological systems involve proton
transfer reactions; the rate of reaction
depends upon [H+]
What effect will a large change in pH
have on a biological system?
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Which Bulbs Light Up?
Relationships to Remember
Kw =
2.5
Bulb Wattage
7.5
25
40
Distilled H2O
Tap H2O
pH =
NaCl(aq)
1M HCl(aq)
1M CH3COOH
pOH=
sugar(aq)
CH3OH
pKw =
What is required for the bulb to light up?
pH + pOH =
CONDUCTIVITY: ability to conduct electricity
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STRONG ACIDS
WEAK ACIDS
DISSOCIATE COMPLETELY to form:
PARTIALLY DISSOCIATE to form :
Weak acids are weak electrolytes
Strong acids are strong electrolytes
[H+]final = [HA]initial = CHA
[H+]final < [HA]initial
*** If the analytical concentration, CHA, is less
than 10!6 M, then the autoionization of water
must be considered.
Which one of the following is not a strong acid?
1.
2.
3.
4.
HNO3
HF
HClO3
HClO4
5. HCl
6. HBr
7. HI
8. H2SO4
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Examples:
Concentrations are calculated using the
equilibrium constant.
Keq= Ka =
19
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How do you find [H+] for a weak acid?
It is an equilibrium problem!
Ka is the acid dissociation constant
REVIEW:
General Approach to Equilibrium
Constant Problems
1. Write the balanced reaction.
2. Write the general form for Keq.
3. Set up a data table (ICE).
4. Substitute equilibrium
concentrations into the
expression for Keq and solve.
Quadratic Equation:
2
ax + bx + c = 0
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"b ± b 2 " 4ac
x=
2a
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!
!
22
Calculation of [H+] for acids
Calculations Continued
What is the [H+] of 0.10 M HI?
What is the pH of 0.10 M acetic acid?
What is the [H+] of 0.10 M acetic acid?
Ka = 1.8 x 10-5
What is the % dissociation?
[H ] x100%
[HA ]
+
Percent dissociation =
+
!
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POLYPROTIC ACIDS
What are the concentrations of H+,
HCO3!, and CO32! in 1 x 10!3 M H2CO3?
Capable of donating more than one
proton.
What happens to Ka with each successive
dissociation?
H2CO3(aq)
HCO3!(aq)
H+ (aq) + HCO3!(aq)
Ka1 = 4.3 x 10!7
H+ (aq) + CO32!(aq)
Ka2 = 5.6 x 10!11
H3PO4 (aq)
H2PO4!(aq)
HPO42!(aq)
H+(aq) + H2 PO4!(aq)
Ka1 = 7.5 x 10!3
H+ (aq) + HPO42!(aq)
Ka2 = 6.2 x 10!8
H+(aq) + PO43!(aq)
Ka3 = 4.2 x 10!13
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Distribution curve for phosphoric
acid (H3PO4)
In H2CO3(aq) solution, what are the
conjugate acids and bases present?
1.00
Rank them in order of:
a) increasing acid strength.
0.50
0.00
b) increasing base strength.
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NaOH is added to a solution of H3PO4
until the pH reaches 12. Which species
are present in the solution?
27
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WEAK BASES
STRONG BASES
PARTIALLY DISSOCIATE WATER to form :
KNOW:
! Group I and II hydroxides are
strong bases (EXCEPT Mg and Be).
! Arrhenius bases donate OH!.
! BrØnsted bases accept H+
Weak bases are weak electrolytes
Examples:
[OH!]final < [B]initial
Base Hydrolysis:
Strong Bases DISSOCIATE
COMPLETELY to form:
Bases react REVERSIBLY with water to form OH!
ions
What is the equilibrium constant for base
hydrolysis?
Keq = Kb =
Strong bases are strong electrolytes.
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WEAK BASES
Base Hydrolysis
" Weak bases can be neutral
Example: NH3, amines (NR3)
NH3(aq) + H2O(l)
Base Hydrolysis
What is the pH of 0.1 M NH3?
Kb = 1.8 x 10-5
NH4+(aq) + OH!(aq)
" Weak bases can be anions (any
ion derived from a weak acid)
Example: F!, NO2!, CH3COO!
F!(aq) + H2O(l)
HF(aq) + OH!(aq)
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How is Ka related to Kb?
Things to KNOW
What is the Conjugate Acid/ Base pair for acetic acid?
Acid Dissociation
HA + H2O(l)
Acid:
H3O+(aq) + A!(aq)
Equilibrium constant = Ka
+
Ka =
CH3COOH
H+ + CH3COO!
Base: CH3COO! + H2O
CH3COOH + OH-
------------------------------------------------------H2O
H+ + OH-
"
[H ][A ]
[HA]
Ka =
Kb =
Base Hydrolysis
!B
+ H2O(l)
HB+(aq)
+
OH!(aq)
Equilibrium constant = Kb
+
Kb =
Ka x Kb =
"
[HB ][OH ]
[B]
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!
Ka x Kb =
33
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Conjugate acid/base
Strength
When we add two reactions together, we
multiply their equilibrium constants.
H-F + OH!
F!
+ H2O
For conjugate acid-base pairs:
Ka x Kb = Kw = 1 x 10!14
Weaker acid
Ka = 10!14
Stronger acid
6.9 x 10!4
Stronger
base
Weaker base
Kb = 1.4 x 10!11
• The conjugate of a weak acid is a weak base
(and vice versa)
• The conjugate of a strong acid is a spectator ion
(example: Cl! is the conjugate base of HCl).
Larger Ka means smaller Kb
Stronger the acid, weaker the base
• The conjugate acid of OH! (strong base) is water.
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What are the Spectator Ions?
Strong Acids
Acid-Base Properties of Aqueous
Salt Solutions
Conjugate Base
Salts dissociate completely in H2O
CH3COONa "
NH4Cl
"
NaCl "
Conjugate Acids of Strong Bases (Arrhenius):
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What is the pH of a solution formed
by dissolving these salts in water?
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Salt Solutions
When a salt is added to water will the
solution be acidic or basic?
1. Salts dissociate completely in H2O
2. Does the cation act as an acid?
(donate a proton)? If so what is Ka?
3. Does the anion act as a base?
(hydrolyze water?) If so what is Kb?
Finding the pH of an aqueous
salt solution
What is the pH of 0.12 M CH3COONa?
Ka (CH3COOH) = 1.8 x 10!5
Hydrolysis: a cation or anion reacts
with H2O to form H+(aq) or OH!(aq)
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Formation of Salt Solutions
Acid + Base " Salt + water
Salt Solutions
Will a salt be acidic or basic?
HCl + NaOH " NaCl + H2O
1.
Na+ is the conj ACID of NaOH
Salt derived from a strong acid and a
strong base
Neutral solution (pH = 7)
Cl! is the conj BASE of HCl
Example: NaCl (from NaOH and HCl)
CH3COOH +NaOH " CH3COONa + H2O
2.
CH3COO! is the conj. BASE of CH3COOH
Salt derived from a weak acid and a
strong base
Basic solution (pH > 7)
Na+ is the conj ACID of NaOH
Examples:
HCl + NH3 " NH4Cl
NaClO (NaOH and HClO)
ClO! (aq) + H2O
NH4+ is the conj ACID of NH3
(CH3COO)2Ba (Ba(OH)2 and CH3COOH)
Cl! is the conj BASE of HCl
CH3COO!(aq) + H2O
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HClO (aq) + OH!(aq)
41
CH3COOH(aq) +OH!(aq)
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Salt Solutions
Finding the pH of a salt solution
3. Salt derived from a strong acid and a
weak base
1. Is it a salt? Dissociates
Acidic solution (pH <7)
completely in H2O
Example: NH4Cl (NH3 and HCl)
NH4+ + H2O
2. Does the cation act as an acid?
NH3 + H3O+
(donate a proton)? If so use Ka
4. Salt derived from a weak acid and a
weak base
3. Does the anion act as a base?
(hydrolyze water?) If so use Kb
pH depends on acid/base involved
Example: NH4CN (NH4+ and CN!)
# What is the pH of 0.02 M KN3?
Ka (HN3) = 1.9 x 10!5
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Summary: Salt Solutions
Acid Strength
XY(s) + H2O(l) " X+(aq) + Y!(aq)
Larger Ka = Stronger Acid
What if you don’t know Ka?
X+ yields a neutral pH solution if it is the
conjugate acid of a strong base.
Examples: Na+, K+
H-X
X = Cl-, CH3COO-, NO3-, etc.
X+ yields an acid (low pH) if it is the
conjugate acid of a weak base.
Examples: NH4+
How easily does H+ break away from X-?
Weaker H-X bond = Stronger Acid
Y- yields a neutral pH solution if it is the
conjugate base of a strong acid.
Examples: Cl!, Br!, NO3!
What does HX bond strength depend on?
Y- yields a basic (high pH) solution if it is
the conjugate base of a weak acid.
Examples: CH3COO!, OCl!, F!, CN!
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H
X
H
X
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• Remember the Order for
Electronegativity
Correlation of Acid Strength with
Structure: Polarization of H#X
For Our New Clambakes
Bring Internationally-Shipped
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Acid strength increases with
increasing polarization of
H#X bond
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Acid Strength
Draw the Lewis Structure
Acid strength increases with:
1) Increasing electronegativity of the
central atom
Oxyacids
Many acids consist of a central atom with
several attached oxygen atoms. These are
called oxyacids.
Examples:
HOClO3
HOCl > HOBr > HOI
HOClO2
2) Increasing oxidation state of the
central atom
HOClO3 > HOClO2 > HOClO > HOCl
HOClO
HOCl
General rule for uncharged oxyacids HxEOy:
If y-x > 2 then strong (H2SO4, HNO3,…)
If
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< 2 then weak (H2CO3, HBrO, HNO2,…)
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Inductive effect stabilizes
the conjugate base
Sample Problem
Of the following, __________
is the strongest acid.
Ka
A. F3C-COOH
B. Cl3C-COOH
C. Br3C-COOH
D. Br2ClC-COOH
E. H3C-COOH
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CH3COOH
1.8 x 10!5
CH2ClCOOH
1.4 x 10!3
CHCl2COOH
3.3 x 10!2
CCl3COOH
2.0 x 10!1
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Broader Definition of Acids and Bases:
Lewis Bases Coordinate to
Metal Cations
LEWIS ACIDS
Any substance that can accept
_________________________
•Have positive charge
•Need more electron density
Ni2+(aq) + 6NH3(aq) " Ni(NH3)62+(aq)
Examples of Lewis Acids:
Highly charged transition metal cations:
2+
Group III cations:
and compounds:
H3N
Smaller group II cations:
H3N
NH3
Ni
NH3
NH3
NH3
Compounds with Incomplete Octet:
LEWIS BASES
Any substance that can donate
_______________________
•Have lone pair electrons
•May be neutral or anionic
Examples of Lewis Bases:
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LEWIS CATIONS
To compare acidity of Lewis acids, first
compare charge. If charge is the same then
compare size.
Charge/Size Ratios
Metal Ion
Charge/Ionic radius
Na+
1.0
+
Li
1.5
Ca2+
2.1
Mg2+
3.1
2+
Zn
2.7
Cu2+
2.8
3+
Al
6.7
3+
Cr
4.8
Fe3+
4.7
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HYDRATION
Hydration
WHY do metal ions behave like acids?
H
:
Mz+
$-
:O
Hydrolysis is a reaction that dissociates water.
$+
H
Metal ions attract the lone pairs on the oxygen in
water molecules. This is a Lewis acid – Lewis
base reaction – the metal ion is the Lewis acid.
Hydration increases with increasing charge/size
ratio of the metal ions.
M(H2O)n-1(OH)(z-1)+ + H+
Fe(H2O)63+
Fe(H2O)5(OH)2+ + H+
The hydration of the metal ion provides a
source of protons (this is an example of
hydrolysis):
Al(H2O)63+
Fe(H2O)63+
Zn(H2O)42+
Ag(H2O)2+
Hydrated metal ions have acidic character which
increases with increasing charge/size ratio of
the metal ions.
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M(H2O)nz+
57
Ka=1.2x 10-5
Ka=6.7x 10-3
Ka=3.3x 10-10
Ka=1.2x 10-12
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Sample problem
Sample Problem
What is the concentration of
phosphate ions in a 2.5 M
solution of phosphoric acid? (The
pH of the solution is 0.87.)
Which of the following salts will
give the most basic solution
when dissolved in water?
1. KBrO4
The acid-dissociation constants of
phosphoric acid (H3PO4) at 25oC
are
2. KBrO3
3. KBrO2
Ka1 = 7.5 x 10!3
Ka2 = 6.2 x 10!8
Ka3 = 4.2 x 10!13
A.
B.
C.
D.
E.
4. KBrO
5. KClO4
1.9 x 10!19 M
1.0 x 10!6 M
0.13 M
6.2 x 10!8 M
4.2 x 10!13 M
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ACIDS
Acid/Base SALTS Review
React with water to form H3O+ ions.
2 types
Molecule containing ionizable protons
CH3COOH, HF, HNO3
1) Which one of the following salts
would have a basic aqueous
solution?
1. KF
2. Al(NO3)3
3. NaI
4. NH4Br
Cations/Lewis Acids.
Non-metal cations ex. NH4+
Metal cations (Lewis acids)
2) Arrange the following in the order
of increasing base strength:
NO2!
NO3!
PO43!
BASES
React with water to form OH!
Molecules that contain OH!
NaOH, Ba(OH)2
HSO4!
3) Which of the following cannot act
as a Lewis base?
1. Cl!
2. OH!
3. CN!
4. NH3
5. H+
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Anions/Lewis Bases
Hydrolyze water to form OH!
Example: CH3COO!, F!
Molecules with extra lone pairs(Lewis
Bases)
Examples: H2O, NH3
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SALTS
MA
•
YOU SHOULD KNOW
GIVEN
Mn+ + An- in water
If Mn+ has high charge/size ratio then acidic
•
If A! is conjugate base of weak acid then basic
•
If A! is conjugate base of strong acid then
neutral
FIND
pH
pOH
[H+] or [OH-]
List of acids
List of pKa’s or Ka’s
Ka or pKa and [HX]
pH and [HX]
[H+], [OH-],
pH
Weaker /Stronger
Weaker /Stronger
pH, [H+], [OH-]
Ka
Recall that a small Ka " high pKa, and
both mean weak acid and not much
dissociation.
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