AcidBase Chemistry

1
Acid-Base Chemistry

• Assignment 7

2
Acids and Bases - 3 Definitions

• Arrhenius
• Bronsted-Lowry
• Lewis

3
Arrhenius Definition

• Acid proton donorHA H A-
• Base hydroxide donorBOH B OH-
• Dilemma NH3

4
Bronsted-Lowry Definition

• Acid proton donor
• Base proton acceptorNH3 H NH4
• Dilemna ferrocene (organometallics)

5
Lewis Definition

• Acid electron pair acceptor (electrophile)
• Base electron pair donor (nucleophile)

6
Acid and Base Strength

• Strong acids and Bases completely dissociate
  (ionize) in aqueous solution
• EXHClaq -gt Haq Cl-aq
• Weak acids and Bases incompletely dissociate in
  aqueous solution
• EXHC2H3O2aq Haq C2H3O2-aq

7
Not All Mineral Acids are Strong!

• HCNHCN H CN- Ka 2.1 x 10-9
• HFHF H F- Ka 6 x 10-4

8
Polyprotic Acids

• Protons are always lost one at a time!
• acids produced by proton loss from polyprotic
  acids are weak acids, characterized by a unique
  Ka value
• H2SO4 HSO4- H Ka1 not measurableHSO4-
  SO42- H Ka2 1.1 x 10-2

9
Strong Acids

• Most mineral acids
• HCl
• H2SO4
• HNO3
• HClO4
• Not HF, H3PO4

10
Weak Acids

• Organic acids (need C and usually have
  COOH)EXAMPLESformic acidacetic acidpropionic
  acid

11
Strong Bases

• Inorganic hydroxides containing metals from
  families IA or IIA
• Note names of these families Alkali metals,
  alkaline earth metals

12
Weak Bases

• NH3, organic amines, and hydroxides other than
  group Ia or IIa hydroxides
• organic amines contain amino groupR-NH2 H
  R-NH3

13
Scheme for Identification of Acids and Bases
14
Practice

• Identify the acid/base nature of the following
  compounds
• Hydroxyl amine
• Calcium hydroxide
• Carbon dioxide
• Pthalic acid
• Hydrogen sulfide
• pyridine

15
Conjugates

• Acids and Bases exist in a conjugate
  relationshipHA H A-acid
  baseBOH B OH-base acid

16
Example

• NH4OH NH4 OH-
• base acid
• HC2H3O2 H C2H3O2-
• acid base

17
Problem

• Identify the conjugate acid-base pairs for each
  of the following compounds
• Ammonium hydroxide
• Diethylamine
• Iodic acid
• Formic acid
• HPO4-

18
Amphoterism

• Some compounds can function as both acids or
  bases depending on the situatione.g., H2O
• HC2H3O2 H2O H 3O C2H3O2-acid base
  acid base
• NH3 H2O NH4 OH-base acid acid
  base

19
Dissociation Constants for Weak Acids and Bases

• Recall for HA
• Ka HA- HA
• The bigger Ka, the _____ the H and the _____
  the HA

20
Question

• What is the comparatively strongest weak acid on
  Table A?
• ANS iodic acid, Ka 0.18
• What is the comparatively strongest weak base on
  Table B?
• ANS diethylamine and piperidine are equally
  strong, Kb 0.0013

21
Conjugate Acid-Base Strength

• For HA H A-
• Recall,Ka HA- HA
• HA is the conjugate _____ and A- is its conjugate
  ____
• HA is a ____ ____
• A- is a ____ ____
• If HA is a relatively strong weak acid, then A-
  is a comparatively ____ ____ base

22
Problem

• HSO4- isa) the conjugate acid of SO4-2b) a
  strong acidc) the conjugate base of H2SO4d) a
  strong basee) the conjugate acid of H2SO4

23
Problem

• Which of the following is/are amphoterica)
  H2PO4-b) C2H3O2-c) CH3CH2NH2d) HCO3-e)
  CH3CH(NH2)COOH

24
Conjugate Acid-Base Strength

• The stronger the conjugate acid is, the weaker
  the conjugate base is and vice versa

HA
A-
25
Salts

• There are four kinds of salts
• salts of weak acidsexample sodium acetate
• salts of weak basesexample ammonium chloride
• salts of strong acids and strong basesexample
  sodium chloride
• salts of weak acids and weak basesexample
  ammonium acetate

26
Identification of Salts

• Salts hydrolyze in water
• Salt water ? acid base
• EX
• NaCl HOH ? Na OH- H Cl-

27
Identification of Salts

• Salts are obtained by reacting acids and bases
• Acid Base Salt water
• Note this is just the reverse of the hydolysis
  reaction
• Salt water Acid Base

28
Identification of Salts

• So, salts are classified based on their parent
  acids
• Their parents are the acids and bases used to
  form them
• EX NaCl
• NaOH HCl ? Na Cl- H2O
• So, parents are strong acid and strong base and
• NaCl is the salt of a strong acid and strong base

29
Examples

• NaC2H3O2 salt of ________________
• NH4Cl salt of ___________________
• NH4COOH salt of _______________
• LiF salt of _____________________

30
Homework Problem 1

• Classify each of the following as a strong or
  weak acid, base, or salt (identify
  parents)CsOH AgOH sodium
  salicylate H2PO4-HClO4 H2CO3ferric
  hydroxide oxalic acidNH4C2H3O2 trimethylamine

31
The Autoionization of Water

• HOH(l) HOH(l) H3O OH-
  hydronium ion
• This is an equilibrium process and is
  characterized by an equilibrium constant, KwKw
  H3OOH- 10-14 at 250C

32
Kw DOES vary with Temperature
lower
higher
33
The Relationship between H and OH-

• Kw HOH- 10-14Useful Equivalent forms
• H 10-14/OH-
• OH- 10-14/H

34
The pH Scale

• pH -log H no units
• H antilog-pH
• pH of pure water 7

35
A Brief Review of LOG Math

• Taking a logarithm corresponds to answering the
  questionTo what power do I raise 10 in order to
  represent the number of interest?log 100 gt 10?
  100
• NOTE LOG is not same thing as LNnatural log is
  based on e? number

36
A Brief Review of LOG Math

• log (ab) log a log b
• log (a/b) log a - log b
• log ab b log a

37
The pOH scale

• pOH -log OH- no units
• OH- antilog-pOH
• What is the pOH in pure water?
• ANS pOH -log (10-7) 7

38
How is pH related to pOH?

• recall Kw H3OOH- 10-14 at 250C
• Derivation
• call pKw -logKw 14 at 250C
• thenpKw pH pOH 14

39
The pH of Some Common Substances
40
Aspirin and Ibuprofen

• Clean and Clear Sensitive Skin Deep Cleaning
  Astringent
• active ingredient 0.5 salicylic acid
• Advil
• active ingredient ibuprofen

41
Orange Juice

• Tropicana Pure Premium with Calcium
• active ingredients calcium hydroxide, malic
  acid and citric acid

42
Toothpaste

• Aquafresh Whitening Toothpaste
• active ingredient sodium fluoride
• Colgate Maximum Cavity Protection Fluoride
  Toothpaste
• active ingredient 0.76 sodium
  monofluorophosphate

43
The Relationship between pH and pOH
acidic
neutral
basic
44
Acidic Solution

• high H concentration
• low pH valuevalue below 7
• low OH- concentration
• high pOH valuevalue greater than 7

45
Basic Solution

• high OH- concentration
• low pOH valuevalue less than 7
• low H concentration
• high pH valuevalue greater than 7

46
pKa and pKb

• pKa - log Ka
• pKb - log Kb
• pKa pKb pKw

47
Conjugate Acid-Base Strength

• The stronger the conjugate acid is, the weaker
  the conjugate base is and vice versa
• Ka Kb 10-14
• pKa pKb 14

HA
A-
48
pKa , pKb, and Weak Acid/ Base Strength
HA

• The lower the pKa the ______ the weak acid
• The higher the pKa, the ______ the weak acid
• The lower the pKa of a weak acid, the ______ the
  pKb of its conjugate weak base and the _____ its
  conjugate base

A-
pKa pKb 14
49
Question

• Which is the comparatively stronger weak acid
  ammonium or pyridinium?

50
Strong Acids and Strong Bases

• HA ? H A-
• BOH ? B OH-
• Strong acids and strong bases completely
  dissociate in water so their concentration gives
  us the H in solution directly

sea slugs secrete H2SO4
sea squirts squirt HNO3
Photographs from Atkins, P.W. Molecules W.H.
Freeman New York, 1987.
51
Problem

• Determine the pH of the following solutions. Are
  the solutions acidic or basic?a) 0.001 M HCl
  solutionb) a solution whose OH- is 10-3 Mc)
  0.0001 M NaOH solution
• ANSa) pH 3, acidic b) pH 11, basic c) pH 10,
  basic

52
Homework Problem 1

• Calculate the hydrogen ion concentration and pH
  of a solution prepared by placing 11.5 g of HClO4
  (perchloric acid FW 100) in a 500 mL volumetric
  flask subsequently filled to the mark with
  water.(Hint What kind of acid is HClO4???)

53
Problem

• What are the pH and pOH of a solution prepared by
  mixing 25 mL of 0.20 M NaOH with 60 mL of 0.10 M
  HCl?(Suggestion Draw a picture)
• ANS pH 1.93 pOH 12.07

54
Weak Acids and Bases

• HA H A-
• BOH B OH-
• Weak acids and weak bases incompletely dissociate
  so their concentration does not provide
  meaningful insight into the aqueous pH of their
  solutions

Fire ants venom contains formic acid
Photograph from Atkins, P.W. Molecules W.H.
Freeman New York, 1987.
55
Weak Acids (Table A)
56
Weak Acids

• The stronger a weak acid the greater the H in
  solution
• The weaker a weak acid, the greater the HA in
  solution
• Recall pKa -log Ka

57
Acid-Base Strength
A-
HA
58
Reminder Weak Acids
The stronger the weak acid, the larger the Ka and
the lower the pKa
59
Calculating the pH of Weak Acids

• the dissociation of weak acids determines the
  equilibrium concentration of H and therefore the
  pH
• the dissociation of weak acids is characterized
  by the equilibrium constant Ka
• Derivation

60
Problem

• What is the pH and pOH of a solution of 0.05 M
  butyric acid (CH3CH2CH2COOH) given the pKa for
  butyric acid 4.81?
• Q Where found in nature?

61
Weak Bases

• The stronger a weak base the greater the OH- in
  solution
• The weaker a weak base, the greater the BOH in
  solution
• Define pKb -log Kb
• Derivation

62
Problem

• What is the pH of an 0.026 M solution of
  hexamethylenetetramine (Kb 10-9)?

63
Salts

• There are four kinds of salts
• salts of weak acidsexample sodium acetate
• salts of weak basesexample ammonium chloride
• salts of strong acids and strong basesexample
  sodium chloride
• salts of weak acids and weak basesexample
  ammonium acetate

64
Salts of Strong Acids and Bases

• Example NaCl
• NaCl H2O -gt Naaq Cl-aq H2O
• pH ? 7
• These salts do not directly perturb the water
  equilibrium

65
Salts of Weak Acids

• Example NaC2H3O2
• NaC2H3O2 H2O Na HC2H3O2 OH-
• pH 0.5(14 pKa logsalt)
• solutions are weakly basic

66
Salts of Weak Acids

• Derivation
• pH 0.5(14 pKa logsalt)
• the weaker the weak acid, the more basic the
  solution (the more tightly the weak acid holds
  onto the H and the higher the OH- in solution

67
Question

• Solutions of which salt would be more basic -
  sodium acetate or sodium cyanide?

68
Salts of Weak Bases

• Example NH4Cl
• NH4Cl H2O NH4OH H Cl-
• Derivation
• pH 0.5(14 - pKb - logsalt)
• solutions are weakly acidic

69
Salts of Weak Bases

• pH 0.5(14 - pKb - logsalt)
• The weaker the weak base, the lower the solution
  pH (the more tightly the weak base holds onto the
  OH- and the higher the H in solution)

70
Salts of Weak Acids and Weak Bases

• Example NH4C2H3O2
• NH4C2H3O2 H2O NH4OH HC2H3O2
• Derivation
• pH 0.5(14 pKa - pKb)
• solution pH depends on the relative strength of
  the acid vs. the base

71
Homework Problem 2

• Calculate the pH of an 0.1 M solution of each of
  the following compounds
• A) sodium acetate
• B) sodium nitrate
• C) sodium hydroxide
• D) hydrogen fluoride
• E) pyridine

72
Problem

• Identify the following compounds in terms of
  their acid/base properties and predict whether
  aqueous solutions of these compounds will be
  acidic, basic, or neutrala) sodium cyanideb)
  ammonium nitratec) potassium nitrate

73
Buffers

• DEFINITIONa solution containing both a weak
  acid/base and its salt which resists change in pH
  due to
• temperature
• dilution
• and addition of SMALL amounts of strong acid or
  base

74
Examples

• HC2H3O2 and NaC2H3O2
• HCOOH and HCOONa
• NaH2PO4 and Na2HPO4
• pyridine and pyridinium chloride
• ammonia and ammonium chloride

75
Problem

• Solutions are made by combining equal volumes of
  the following. Which is/are a buffer(s)?a) 0.1
  M NH4Cl 0.1 M NH4b) 0.1 M HF 0.05 M NaOHc)
  0.05 M HF 0.1 M NaOHd) 0.1 M NaF 0.05 M
  HCle) 0.1 M NaF 0.05 M Na

76
Henderson Hasselbalch Equation

• pH pKa log conj. base/conj. acidorpH
  pKa log salt/acidfor a weak acid and its
  salt
• Recall pKa -log Ka
• derivation

77
Problem

• Calculate the pH of a solution that is 0.25 M
  sodium acetate and 0.30 M acetic acid given Ka
  1.8 x 10-5 for acetic acid.(hint what is the
  pKa?)
• ANSpH pKa log salt/acidpH 4.74
  log(0.25/0.30) 4.74 - 0.08pH 4.66

78
Question

• If we have a base buffer containing ammonia and
  ammonium chloride, what is the correct form of
  the Henderson-Hasselbach equation based on these
  species?
• pH pKa log ammonia/ammonium chloride

79
Problem

• Calculate the pH of the solution that results
  when 200 mL of 0.300 M ammonium hydroxide are
  mixed with 250 mL of 0.150 M ammonium chloride.
• ANSpH pKa log (conj. base/conj. acid)
  9.25 log ammonia/ammonium 9.25
  log (0.06/0.0375) 9.25 0.20 9.45

80
Buffers - Effect of Dilution

• Consider the pH of the solution that results from
  mixing 100 mL 0.1 M NaH2PO4 and 100 mL 0.1 M
  Na2HPO4? What is the pH if the solution is
  diluted by a factor of 2? 5? Generalize your
  findings.

81
Buffers - Effect of Temperature
Buffers Calbiochem Corp., Doc. No. CB0052-591
Perrin Dempsey Buffers for pH and Metal Ion
Control Chapman Hall London, 1979.
82
Comparison - Effect of Addition of SMALL Amount
of Strong Acid or Base

• Buffer itself (100 mL 0.1 M NaH2PO4 and 100 mL
  0.1 M Na2HPO4) pH 7.20vs. 200 mL water pH 7.00
• Upon addition 0.005 moles strong acid, buffer pH
  6.72water pH 1.60
• Upon addition 0.005 moles strong base, buffer pH
  7.68water pH 10.40

83
Preparing Buffers - Useful References

• Perrin Dempsey Buffers for pH and Metal Ion
  Control New York Wiley, 1974.
• Chemical Company booklets.Example Calbiochem
  Doc. No. CB0052-591

84
Practical Preparation

• Practical
• Identify reagent based on pKa
• Prepare appropriate molarity and add NaOH/HCl to
  adjust pH

85
Special Types of Buffers

• GOOD Buffers - temperature resistant
• Volatile Buffers - can be removed by freeze
  drying
• Universal Buffers - wide effective pH range

86
GOOD Buffers

• zwitterionic - have both amino and sulfonyl
  groups
• EXAMPLESMES pK 6.15HEPES pK 7.55

87
GOOD Buffers

• pK typically 6-8 (physiological pH)
• No complexation with metal ions (no inhibition of
  enzymes)
• High aqueous solubility
• Minimal salt effects
• No UV-vis absorption (240-280 nm)

88
Desirable Characteristics of GOOD Buffers

• pH independent of temperature
• Compare with TRIS

89
Volatile Buffers

• Can be removed by simple evaporation or
  lyophilization
• good for electrophoresis or preparative ion
  exchange chromatography

90
Volatile Buffers

• EXAMPLES
• ammonium acetate pH 4-6
• pyridinium formate pH 3-6
• ammonium carbonate pH 8-10

91
Universal Buffers

• Mixture of two or more buffers
• Effects of buffers are additive
• greater buffering capacity
• wider effective pH range
• EXAMPLEcitric acid 3.13, 4.76, 6.40phosphoric
  acid 2.15, 7.20, 12.15boric acid 9.24, 12.74,
  13.80

92
Amino Acids

• Given
• pKa (COOH) 2.3
• pKa (NH3) 9.6
• What form do amino acids assume at pH 7?

93
Titrimetry

• Purpose
• Determine concentration of an acid or base of
  unknown concentration (MAVA MBVB)
• Identification of unknown acid or base based on
  pKa (pH pKa at ½ volume at equivalence point)
• Method volumetrically using biuret
• At endpoint moles acid moles baseMa Va Mb
  Vb

94
Terminology

• Titrant standardized strong acid or base
  delivered from a biuret
• Standardized concentration made known both in
  terms of accuracy and precision
• EndpointpH at which visual indicator changes
  color
• Equivalence pointpH at which moles of acid equal
  moles of base

95
Typical Experimental Methodology - Weak Acid

• Standardize titrant (NaOH)titrate NaOH with KHP
  of known concentration
• Titrate unknown (weak acid)titrate unknown with
  standardized titrant

96
Indicators

• Organic weak acids that have different colors in
  their acid and conjugate base forms
• EXphenolphthaleinHA H
  A-colorless pink

97
Indicators

• pH pKa logA-/HA
• Your eye can detect color for 10-fold excess of
  A-/HA
• At equivalence point pH changes rapidly
• Bottom line endpoint may not be equivalence
  point if indicator pKa not near equivalence point

98
Titration of a Weak Acid

• Lets calculate the pH of the solution produced
  by adding 0, 10, 20, 25, 50, and 70 mL of 0.1 M
  sodium hydroxide to 50 mL of 0.1 M formic acid

99
Titration of Weak Acid

• 1 weak acidpH 0.5 (pKa logacid
• 2 bufferpH pKa logbase/acid
• 3 equivalence point salt of weak acidpH 0.5
  (14 pKa logsalt)
• 4 strong basepH 14logOH-

4
3
pH
X
2
1
X
Vol. Titrant, mL
100
Identification of Weak Acid

• At equivalence pointMacid Vacid Mbase Vbase
• At ½ volume corresponding to equivalence
  pointpH pKa

101
Titration of a Weak Base

• Lets calculate the pH of the solution produced
  by adding 0, 10, 20, 25, 50, and 70 mL of 0.1 M
  hydrochloric acid to 50 mL of 0.1 M ammonium
  hydroxide

102
Titration of Weak Base

• 1 weak basepH 14 - 0.5 (pKb logbase
• 2 bufferpH pKa logbase/acid
• 3 equivalence point salt of weak basepH 0.5
  (14 - pKb - logsalt)
• 4 strong acidpH -logH

1
X
2
3
pH
X
4
Vol. Titrant, mL