acid and base equilibria what do you already know …courses.chem.psu.edu/chem112/fall/lecture...
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1CH112 LRSVDS ACIDS BASES Part 2
Acid and Base EquilibriaChapter 16
Review Chapter 4.1 (Electrolytes)
Review Chapter 4.3 (Acid-Base Reactions)
Memorize 7 Strong Acids (Table 4.2)
Memorize the Strong Bases (Also Table
4.2)
Review Equilibrium (Ch. 15)
PRE-REQUISITES:
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What do you already know
about ACIDS?
1) Acids Taste?
2) Acids dissolve active metals, usually
liberating ___________.
3) Acids are corrosive – they dissolve
compounds that are otherwise hard to
dissolve.
4) Acids turn litmus paper ______________.
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What do you already know
about BASES?
1) Bases Taste?
2) Bases dissolve oil and grease.
3) Bases are slippery to the touch.
4) Bases react with many metal ions to form
precipitates.
5) Bases turn litmus paper ________________
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Definitions:
Arrhenius ACID:
Any compound that releases H+
when dissolved in H2O.
Arrhenius BASE:
Any compound that releases OH!
when dissolved in H2O.
ARRHENIUS Acids and Bases
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Hydronium ion
Need a new definition of acidsand bases based on
Proton Transfer
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BrØnsted ACID:
Any compound capable of donating an
H+ ion.
BrØnsted BASE:
Any compound capable of accepting a
H+ ion.
BRØNSTED - LOWRYACIDS AND BASES
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CONJUGATE ACID-BASE PAIRSCONJUGATE ACID-BASE PAIRS
Differ only by the presence or absenceof a proton (H+).
Conjugate Acid Conjugate Base + H+
Examples: Conjugate Acid Conjugate Base
Note:The stronger the acid, the weaker its
conjugate base.
The weaker the acid, the stronger itsconjugate base.
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Is it weak, strong or negligible?
1. The conjugate of a strong acid is a______________. (Example: Cl! is theconjugate base of HCl). Spectatorshave 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
_________________.
Compound Conjugate
Strong Acid
Strong Base
Weak Base
9CH112 LRSVDS ACIDS BASES Part 2 10CH112 LRSVDS ACIDS BASES Part 2
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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NOTE:
Kw is constant even when [H+] and [OH!]
are not equal.
Calculate [H+] in a 0.05 M Ca(OH)2 solution.
Kw is the Ion Product Constant
for water
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1. What is the pH of pure water at 25°C?
2. What is the [H+] of an HCl solution
that has a pH of 2.34?
The pH Scale
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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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pH and Concentration
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Measuring pH
!pH Meter
!Indicators
H(Ind) Ind- + H+
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Compare the effect of
concentration on pH
pH[H+]Solution
2.0 x 10-7 M2
1.0 x 10-7 M1
Many biological systems involve proton
transfer reactions; the rate of reaction
depends upon [H+]
What effect will a large change in pHhave on a biological system?
17CH112 LRSVDS ACIDS BASES Part 2
Relationships to Remember
Kw =
pH =
pOH=
pKw =
pH + pOH =
18CH112 LRSVDS ACIDS BASES Part 2
Which Bulbs Light Up?
Distilled H2O
Tap H2O
NaCl(aq)
1M HCl(aq)
1M CH3COOH
sugar(aq)
CH3OH
What is required for the bulb to light up?
CONDUCTIVITY: ability to conduct electricity
Bulb Wattage2.5 7.5 25 40
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STRONG ACIDS
DISSOCIATE COMPLETELY to form:
Strong acids are strong electrolytes
[H+]final = [HA]initial = CHA
*** 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. HNO3 5. HCl
2. HF 6. HBr
3. HClO3 7. HI
4. HClO4 8. H2SO4
20CH112 LRSVDS ACIDS BASES Part 2
PARTIALLY DISSOCIATE to form :
Weak acids are weak electrolytes
[H+]final < [HA]initial
Examples:
Concentrations are calculated using theequilibrium constant.
Keq= Ka =
WEAK ACIDS
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Ka is the acid dissociation constant
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How do you find [H+] for a weak acid?It is an equilibrium problem!
REVIEW:
General Approach to EquilibriumConstant Problems
1. Write the balanced reaction.
2. Write the general form for Keq.
3. Set up a data table (ICE).
4. Substitute equilibriumconcentrations into theexpression for Keq and solve.
!
x ="b ± b
2" 4ac
2a
!
ax2
+ bx + c = 0
Quadratic Equation:
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What is the [H+] of 0.10 M HI?
What is the [H+] of 0.10 M acetic acid?
Ka = 1.8 x 10-5
Calculation of [H+] for acids
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Calculations Continued
What is the pH of 0.10 M acetic acid?
What is the % dissociation?
Percent dissociation =
!
H+[ ]
HA+[ ]x100%
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Capable of donating more than one
proton.
What happens to Ka with each successive
dissociation?
H2CO3(aq) H+ (aq) + HCO3!(aq)
Ka1 = 4.3 x 10!7
HCO3!(aq) H+ (aq) + CO3
2!(aq)
Ka2 = 5.6 x 10!11
H3PO4 (aq) H+(aq) + H2 PO4!(aq)
Ka1 = 7.5 x 10!3
H2PO4!(aq) H+ (aq) + HPO4
2!(aq)
Ka2 = 6.2 x 10!8
HPO42!(aq) H+(aq) + PO4
3!(aq)
Ka3 = 4.2 x 10!13
POLYPROTIC ACIDS
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What are the concentrations of H+,
HCO3!, and CO3
2! in 1 x 10!3 M H2CO3?
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In H2CO3(aq) solution, what are the
conjugate acids and bases present?
Rank them in order of:a) increasing acid strength.
b) increasing base strength.
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Distribution curve for phosphoric
acid (H3PO4)
NaOH is added to a solution of H3PO4
until the pH reaches 12. Which species
are present in the solution?
1.00
0.50
0.00
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STRONG BASES
Examples:
Strong Bases DISSOCIATE
COMPLETELY to form:
Strong bases are strong electrolytes.
KNOW:
! Group I and II hydroxides are
strong bases (EXCEPT Mg and Be).
! Arrhenius bases donate OH!.
! BrØnsted bases accept H+
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WEAK BASES
PARTIALLY DISSOCIATE WATER to form :
Weak bases are weak electrolytes
[OH!]final < [B]initial
Base Hydrolysis:Bases react REVERSIBLY with water to form OH!
ions
What is the equilibrium constant for basehydrolysis?
Keq = Kb =
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WEAK BASESBase Hydrolysis
" Weak bases can be neutral Example: NH3, amines (NR3)
NH3(aq) + H2O(l) NH4+(aq) + OH!(aq)
" Weak bases can be anions (anyion derived from a weak acid)
Example: F!, NO2!, CH3COO!
F!(aq) + H2O(l) HF(aq) + OH!(aq)
32CH112 LRSVDS ACIDS BASES Part 2
What is the pH of 0.1 M NH3?
Kb = 1.8 x 10-5
Base Hydrolysis
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Things to KNOW
Acid Dissociation
HA + H2O(l) H3O+(aq) + A!(aq)
Equilibrium constant = Ka
!
Ka
=[H
+][A
"]
[HA]
!
Kb
=[HB
+][OH
"]
[B]
Base Hydrolysis
B + H2O(l) HB+(aq) + OH!(aq)
Equilibrium constant = Kb
34CH112 LRSVDS ACIDS BASES Part 2
Acid: CH3COOH H+ + CH3COO!
Base: CH3COO! + H2O CH3COOH + OH-
-------------------------------------------------------
H2O H+ + OH-
Ka = Kb =
How is Ka related to Kb?
What is the Conjugate Acid/ Base pair for acetic acid?
Ka x Kb =
Ka x Kb =
35CH112 LRSVDS ACIDS BASES Part 2
When we add two reactions together, we
multiply their equilibrium constants.
For conjugate acid-base pairs:
Ka x Kb = Kw = 1 x 10!14
Larger Ka means smaller Kb
Stronger the acid, weaker the base
36CH112 LRSVDS ACIDS BASES Part 2
Conjugate acid/base
Strength
H-F + OH! F! + H2O
Stronger acid
6.9 x 10!4
Weaker acid
Ka = 10!14
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).
• The conjugate acid of OH! (strong base) is water.
37CH112 LRSVDS ACIDS BASES Part 2
What are the Spectator Ions?
Strong Acids Conjugate Base
Conjugate Acids of Strong Bases (Arrhenius):
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Salts dissociate completely in H2O
CH3COONa "
NH4Cl "
NaCl "
What is the pH of a solution formedby dissolving these salts in water?
Acid-Base Properties of AqueousSalt Solutions
39CH112 LRSVDS ACIDS BASES Part 2
Salt Solutions
When a salt is added to water will thesolution 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?
Hydrolysis: a cation or anion reactswith H2O to form H+(aq) or OH!(aq)
40CH112 LRSVDS ACIDS BASES Part 2
What is the pH of 0.12 M CH3COONa?
Ka (CH3COOH) = 1.8 x 10!5
Finding the pH of an aqueoussalt solution
41CH112 LRSVDS ACIDS BASES Part 2
Formation of Salt Solutions
Acid + Base " Salt + water
HCl + NaOH " NaCl + H2O
Na+ is the conj ACID of NaOH
Cl! is the conj BASE of HCl
CH3COOH +NaOH " CH3COONa + H2O
CH3COO! is the conj. BASE of CH3COOH
Na+ is the conj ACID of NaOH
HCl + NH3 " NH4Cl
NH4+ is the conj ACID of NH3
Cl! is the conj BASE of HCl
42CH112 LRSVDS ACIDS BASES Part 2
Will a salt be acidic or basic?
1. Salt derived from a strong acid and a
strong base
Neutral solution (pH = 7)
Example: NaCl (from NaOH and HCl)
2. Salt derived from a weak acid and a
strong base
Basic solution (pH > 7)
Examples:
NaClO (NaOH and HClO)
ClO! (aq) + H2O HClO (aq) + OH!(aq)
(CH3COO)2Ba (Ba(OH)2 and CH3COOH)
CH3COO!(aq) + H2O CH3COOH(aq) +OH!(aq)
Salt Solutions
43CH112 LRSVDS ACIDS BASES Part 2
3. Salt derived from a strong acid and a
weak base
Acidic solution (pH <7)
Example: NH4Cl (NH3 and HCl)
NH4+ + H2O NH3 + H3O
+
4. Salt derived from a weak acid and a
weak base
pH depends on acid/base involved
Example: NH4CN (NH4+ and CN!)
Salt Solutions
44CH112 LRSVDS ACIDS BASES Part 2
1. Is it a salt? Dissociates
completely in H2O
2. Does the cation act as an acid?
(donate a proton)? If so use Ka
3. Does the anion act as a base?
(hydrolyze water?) If so use Kb
# What is the pH of 0.02 M KN3?
Ka (HN3) = 1.9 x 10!5
Finding the pH of a salt solution
45CH112 LRSVDS ACIDS BASES Part 2
Summary: Salt Solutions
XY(s) + H2O(l) " X+(aq) + Y!(aq)
X+ yields a neutral pH solution if it is the
conjugate acid of a strong base.
Examples: Na+, K+
X+ yields an acid (low pH) if it is the
conjugate acid of a weak base.
Examples: NH4+
Y- yields a neutral pH solution if it is the
conjugate base of a strong acid.
Examples: Cl!, Br!, NO3!
Y- yields a basic (high pH) solution if it is
the conjugate base of a weak acid.
Examples: CH3COO!, OCl!, F!, CN!
46CH112 LRSVDS ACIDS BASES Part 2
Acid Strength
Larger Ka = Stronger Acid
What if you don’t know Ka?
H-XX = Cl-, CH3COO-, NO3
-, etc.
How easily does H+ break away from X-?
Weaker H-X bond = Stronger Acid
What does HX bond strength depend on?
H X
H X
47CH112 LRSVDS ACIDS BASES Part 2
• Remember the Order for
Electronegativity
For Our New Clambakes
Bring Internationally-Shipped
Canned Hams, Please
48CH112 LRSVDS ACIDS BASES Part 2
Correlation of Acid Strength withStructure: Polarization of H#X
Acid strength increases withincreasing polarization ofH#X bond
49CH112 LRSVDS ACIDS BASES Part 2
Draw the Lewis Structure
OxyacidsMany acids consist of a central atom withseveral attached oxygen atoms. These arecalled oxyacids.
Examples:
HOClO3 HOClO2
HOClO HOCl
50CH112 LRSVDS ACIDS BASES Part 2
Acid strength increases with:1) Increasing electronegativity of thecentral atom
HOCl > HOBr > HOI
2) Increasing oxidation state of thecentral atom
HOClO3 > HOClO2 > HOClO > HOCl
Acid Strength
General rule for uncharged oxyacids HxEOy:
If y-x > 2 then strong (H2SO4, HNO3,…)
If < 2 then weak (H2CO3, HBrO, HNO2,…)
51CH112 LRSVDS ACIDS BASES Part 2
Sample Problem
Of the following, __________
is the strongest acid.
A. F3C-COOH
B. Cl3C-COOH
C. Br3C-COOH
D. Br2ClC-COOH
E. H3C-COOH
52CH112 LRSVDS ACIDS BASES Part 2
Inductive effect stabilizes
the conjugate base
Ka
CH3COOH 1.8 x 10!5
CH2ClCOOH 1.4 x 10!3
CHCl2COOH 3.3 x 10!2
CCl3COOH 2.0 x 10!1
53CH112 LRSVDS ACIDS BASES Part 2
LEWIS ACIDSAny substance that can accept
_________________________
•Have positive charge
•Need more electron density
Examples of Lewis Acids:
Highly charged transition metal cations:
Group III cations:
and compounds:
Smaller group II cations:
Compounds with Incomplete Octet:
LEWIS BASES
Any substance that can donate
_______________________
•Have lone pair electrons
•May be neutral or anionic
Examples of Lewis Bases:
Broader Definition of Acids and Bases:
54CH112 LRSVDS ACIDS BASES Part 2
Ni
H3N
H3N NH3
NH3
NH3
NH3
2+
Ni2+(aq) + 6NH3(aq) " Ni(NH3)62+(aq)
Lewis Bases Coordinate to
Metal Cations
55CH112 LRSVDS ACIDS BASES Part 2
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 radiusNa+ 1.0
Li+ 1.5
Ca2+ 2.1
Mg2+ 3.1
Zn2+ 2.7
Cu2+ 2.8
Al3+ 6.7
Cr3+ 4.8
Fe3+ 4.7
56CH112 LRSVDS ACIDS BASES Part 2
57CH112 LRSVDS ACIDS BASES Part 2
Hydration
Metal ions attract the lone pairs on the oxygen in
water molecules. This is a Lewis acid – Lewisbase reaction – the metal ion is the Lewis acid.
Hydration increases with increasing charge/sizeratio of the metal ions.
Hydrated metal ions have acidic character whichincreases with increasing charge/size ratio ofthe metal ions.
Mz+ OH
H
:
:$-
$+
58CH112 LRSVDS ACIDS BASES Part 2
HYDRATION
WHY do metal ions behave like acids?
Hydrolysis is a reaction that dissociates water.
M(H2O)nz+ 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+ Ka=1.2x 10-5
Fe(H2O)63+ Ka=6.7x 10-3
Zn(H2O)42+ Ka=3.3x 10-10
Ag(H2O)2+ Ka=1.2x 10-12
59CH112 LRSVDS ACIDS BASES Part 2
Sample problem
What is the concentration ofphosphate ions in a 2.5 Msolution of phosphoric acid? (ThepH of the solution is 0.87.)
The acid-dissociation constants ofphosphoric acid (H3PO4) at 25oCareKa1 = 7.5 x 10!3
Ka2 = 6.2 x 10!8
Ka3 = 4.2 x 10!13
A. 1.9 x 10!19 M
B. 1.0 x 10!6 M
C. 0.13 M
D. 6.2 x 10!8 M
E. 4.2 x 10!13 M
60CH112 LRSVDS ACIDS BASES Part 2
Sample Problem
Which of the following salts will
give the most basic solution
when dissolved in water?
1. KBrO4
2. KBrO3
3. KBrO2
4. KBrO
5. KClO4
61CH112 LRSVDS ACIDS BASES Part 2
Acid/Base SALTS Review
1) Which one of the following saltswould have a basic aqueoussolution?
1. KF 3. NaI
2. Al(NO3)3 4. NH4Br
2) Arrange the following in the orderof increasing base strength:
NO2! NO3
! PO43! HSO4
!
3) Which of the following cannot actas a Lewis base?
1. Cl!
2. OH!
3. CN!
4. NH3
5. H+
62CH112 LRSVDS ACIDS BASES Part 2
ACIDS
React with water to form H3O+ ions.
2 types
Molecule containing ionizable protons
CH3COOH, HF, HNO3
Cations/Lewis Acids.
Non-metal cations ex. NH4+
Metal cations (Lewis acids)
BASESReact with water to form OH!
Molecules that contain OH!
NaOH, Ba(OH)2
Anions/Lewis Bases
Hydrolyze water to form OH!
Example: CH3COO!, F!
Molecules with extra lone pairs(LewisBases)
Examples: H2O, NH3
63CH112 LRSVDS ACIDS BASES Part 2
SALTS
MA 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
64CH112 LRSVDS ACIDS BASES Part 2
YOU SHOULD KNOW
GIVEN FIND
pH [H+], [OH-],pOH[H+] or [OH-] pHList of acids Weaker /StrongerList of pKa’s or Ka’s Weaker /StrongerKa or pKa and [HX] pH, [H+], [OH-]pH and [HX] Ka
Recall that a small Ka " high pKa, and
both mean weak acid and not much
dissociation.