Summary Chapter 13

1
Summary (Chapter 13)

• Equilibrium is dynamic forward, reverse
  reactions balanced
• Law of mass action (K and KP)
• equilibrium depends on overall reaction,not on
  the mechanism
• Equilibrium Constant vs. Reaction Quotient
• Le Chateliers principle

2
Acids and Bases

• Zumdahl
• Chapter 14

3
Topics we will cover

• Definition of acids and bases
• Definition of acid strength
• Acid-base equilibrium
• Polyprotic acids
• Definition of base strength

4
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.
• H2O is both an acid and a base.
• HCl H2O Cl- H3O
• acid base

5
Examples

• Arrhenius Concept
• HCl (aq) H (aq) Cl- (aq)
• H2O H (aq) OH- (aq)
• NaOH Na (aq) OH- (aq)
• NH4OH NH4 (aq) OH- (aq)

acids
bases
6
Examples

• Bronsted-Lowry Concept
• HCl (aq) H (aq) Cl- (aq)
• H2O H (aq) OH- (aq)
• NaOH Na (aq) OH- (aq)
• NH3 H2O NH4 (aq) OH- (aq)

acids
bases
7
Arrhenius vs. Bronsted-Lowry

• All Arrhenius acids are Bronsted-Lowry acids
• and vice versa.
• All Arrhenius bases are Bronsted-Lowry bases, but
  not vice versa

8
Conjugate Acid/Base Pairs

• HA(aq) H2O(l) A-(aq) H3O(aq)
• 
  conj conj
• acid (1) base (2) base (1)
  acid (2)
• conjugate base everything that remains of the
  acid molecule after a proton is lost.
• conjugate acid species formed when the proton
  is accepted by the base.

9
Acid Dissociation Constant (Ka)

• HA(aq) H2O(l) H3O(aq) A-(aq)

10
Acid Strength
Strong Acid Equilibrium lies far to the right.
(HNO3) Yields a weak conjugate base. (NO3-) Weak
Acid Equilibrium lies far to the left. (small Ka)
(CH3COOH) Yields a strong conjugate base
(CH3COO-)
11
Strong Acids

• Hydrochloric (HCl)
• Nitric (HNO3)
• Perchloric (HClO4)
• Sulfuric (H2SO4) diprotic
• (only 1st proton strongly acidic)

12
Which proton gets donated?

• Most acids are oxyacids the acidic H is attached
  to an O atom (exception HCl, HBr, HF, etc.)
• Organic acids most commonly have a carboxyl group

O
C
H
O
13
Water as an Acid and a Base

• Water is amphoteric (it can behave either as an
  acid or a base).
• Self-dissociation of water
• H2O H2O H3O OH-
• 
  conj conj
• acid 1 base 2 acid 2 base 1
• Kw 1 10-14 at 25C

14
Amphoteric Substances

• NH3 NH3 NH2- NH4

Acid 1
Acid 2
ConjugateBase 1
ConjugateBase 2
15
The pH Scale

• pH -logH
• Kw 1.00 10-14 H OH-
• pKw 14.00 pH pOH
• As pH rises, pOH falls (sum 14.00).
• Typical pH and pOH values in water lie between 0
  and 14

16
Acid Base Reactions

• Most reactions in water can be thought of in
  terms of a competition between bases for a
  proton.
• The equilibrium position favors the association
  of the proton with the stronger base
• HCl (aq) H (aq) Cl- (aq)
• H2O H (aq) OH- (aq)
• HCl (aq) H2O H3O (aq) Cl- (aq)

17
Classification of Acids
Acid Strength
----- Large Ka
Strong acids
----- Small Ka
Weak Acids
Chemical Structure
Oxyacids
---- acidic proton attached to O atom
Organic Acids
---- acidic proton part of CARBOXYL group
of acidic protons
Monoprotic
---- single acidic proton
---- more than one acidic proton
Polyprotic
18
pH of Strong Acid Solutions
Consider the different sources of protons.
If one source is the dominant one, use its
concentration
to determine the pH
Calculate the pH of 0.10 M nitric acid
Nitric acid is a strong acid, so it will
dissociate
0.10 M nitric acid gt 0.10 M protons
Water is another source. By itself, it will
produce
-
7
1.0 x 10 M protons,
negligible
pH - log ( 0.10) 1.00
Calculate the pH of 1.0 x 10-10 M nitric acid
pH
7.00
19
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.

20
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. lt Small x approx
• Verify assumptions using 5 rule.
• Calculate H and pH.

21
Percent Dissociation (Ionization)
22
Example 1 Equilibrium of a weak acid
What is the pH of a 0.1 M solution of
hydrofluoric acid ?
-
4
Ka of HF (hydrofluoric acid) 6.6 x 10 M
-

HF
H

F
-

H O
H

OH
2
Ka of HF is larger than that of water, so HF
is the dominant
producer of protons
Ka (x)(x)/(0.1 -x) 0.00066 M
Small x
x 0.00812
M
Exact x 0.00780 M
pH - log(x) 2.11
23
Example 2 Equilibrium of a weak acid
What is the pH of a 1.0 mM solution of
hydrocyanic acid ?
Ka of HCN (hydrocyanic acid) 6.2 x 10 -10
M
-

HCN
H

CN
-

H O
H

OH
2
Ka of HCN is larger than that of water, so
HCN is the
dominant producer of protons (??)
-
10
Ka (x)(x)/(0.001 -x) 6.2 x 10
M
-
7
Small x
x 7.9 x 10
M
pH - log(x) 6.10
24
Percent Dissociation of a weak acid
What is the dissociation of a 1.0 mM solution
of HCN?
x 7.9 10-7 M
Dissociation 100 (x)/(0.001) 0.079
What is the dissociation of a 0.1 M solution of
HF ?
x 0.0078 M
Dissociation 100 (x)/(0.1) 7.8
25
100
1
0.1
80
0.01
60
0.001
Concentration
Percent Dissociation
0.0001
40
1E-05
20
1E-06
0
1E-07
1
2
3
4
5
6
7
pH of Aqueous solution of HF
26
Polyprotic Acids

• . . . can furnish more than one proton (H) to
  the solution.

27
Polyprotic Acids

• Deprotonation (loss of H) occurs stepwise
• Equilibrium constants are successively smaller
• pH of 0.10 M sulfuric acid
• H2SO4 H HSO4- strong acid
• SO4- H SO42- Ka 1.210-2 M
• First step produces 0.10 M protons
• Second step produces 9.8 10-3 M protons
• pH -log10(0.10 0.0098) 0.96

28
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)

29
pH of Basic Solutions

• What is the pH of a 0.10 M aqueous solution of
  KOH ?
• KOH is a strong base gt complete dissociation
• OH- 0.10 M
• pH 14.00 - pOH 14.00 - (-log10(0.10))
• 14.00 - (-(-1.00)) 13.00
• The comments about low concentrations of strong
  acids apply equally well here

30
Bases(continued)

• weak very little dissociation (or reaction with
  water)
• H3CNH2(aq) H2O(l) H3CNH3(aq) OH-(aq)

31
Bases

• Strong Bases
• All Alkali Metal hydroxides (Group 1A)
• Most Alkaline Earth hydroxides (Group 2A)
• Weak Bases
• Other metal hydroxides (Al, Fe, Ag,)
• Compounds containing N atoms
• NH3 H2O NH4 OH-

32
Amines

• Prevasive as neuro transmitters/regulators
• Norepinephrine
• Adrenaline
• Novocain

33
Amino Acids

• Building blocks of proteins
• Acid and basic functions, but not amphoteric

H
C
CH2CH2CH2NH2
HOOC
acid
base
CH2NH2
Lysine
34
Basicity Constant

• B H2O ltgt BH OH-
• Kb BHOH-/B
• This definition is most readily applied to weak
  bases.

35
What is the pH of a 1.00 M solution of methyl
amine?
-
Kb of methylamine 4.38 x 10-4 M

• CH3NH2 CH3NH3 OH-
• Methyl amine will produce hydroxyl ions in water
• Kb (x)(x)/(1.00 -x) 0.000438 M
• x OH- 0.0209 M (small x)
• pH 14.00 - pOH 12.316

36
Relation between Ka and Kb

• In any acid base reaction, we can identify
  acid/base conjugate pairs.
• Kb Kw / Ka
• Ka is the acidity constant of the acid and Kb is
  the basicity constant of the conjugate pair
• HCOOH H2O ltgt HCOO- H3O
• Kb (HCOO-) Kw / Ka (HCOOH)

37
Acid-Base Properties of Salts
SALTS are ionic compounds
formed from reactions between acids and bases
When they are dissolved in water, they can
dissociate to regenerate the parent acid and base
The pH of the resulting solution will be
determined
by the relative strengths of the parent acid/base
Neutral pH 7.0
STRONG ACID STRONG BASE
NaCl, KI, Ba(NO3)2
Acidic pH lt 7.0
STRONG ACID WEAK BASE
NH4Cl, AlCl3
Basic pH gt 7.0
WEAK ACID STRONG BASE
NaF, KI, Na2(C3H2O2)
38
Acid-Base Properties of Salts
39
Structure and Acid-Base Properties

• Two factors for acidity in binary compounds
• Bond Polarity (high is good)
• Bond Strength (low is good)

40
Why are some acids stronger than others ?
Acid strengths are related to bond energies
stronger bonds lead to weaker acids
Binary acids
(H - X , X is a halogen)
Bond strengths are proportional to
electronegativity
Bond strength
HI gt HBr gt HCl gt HF
Acid strength
Oxo-acids
(H - O - E, E is a nonmetal)
Electronegativity of E weakens the O - H bond
Bond strength
HOCl gt HOBr gt HOI
Acid strength
41
Lewis Acids and Bases

• Lewis Acid electron pair acceptor
• Lewis Base electron pair donor

42
Chapter 14

• Sections 14.1 - 14.7
• Parts of 14.8 --- qualitative aspects
• very direct problems
• Section 14.12
• No need to do assigned problems 117 - 129

43
Summary of Chapter 14

• Definition of Acids and Bases
• Acid and Base Strength
• The pH scale
• Relation between Ka and Kb
• Acid-Base Equilibrium Calculations
• Polyprotic Acids
• Acid-base properties of salt solutions