Acids

1
Chapter 16

• Acids Bases

2
Chapter 16 Test Review Section

• Reaction Rate (Chapter 14)

3
Acid-Base Theory

• How would you describe an acid or a base?
• Similar questions about acids and bases have been
  asked ever since their discovery in the 1200s
• Because of these curiosities, several theories
  were generated in attempts to clarify what makes
  an acid an acid (or a base a base)

4
Acid-Base Theory - Arrhenius

• Arrhenius defined the terms acid and base as
  follows
• An acid is a substance that produces H ions when
  dissolved in water (now described as hydronium
  rather than H).
• A base is a substance that produces OH- ions when
  dissolved in water

5
Hydronium Ion (H3O) or (H(aq))
6
Acid-Base Theory - Arrhenius

• We can demonstrate these definitions as follows

H2O
HNO3(l) H(aq) NO3-(aq)
H2O
Ca(OH)2(s) Ca2(aq) 2OH-(aq)
7
Acid-Base Theory - Arrhenius

• Are there any errors, or oversights, in the
  Arrhenius Theory?
• Consider ammonia, NH3, which is a well known
  base
• What happens when ammonia is placed in water?

H2O
NH3(g) ????
8
Br?nsted-Lowry Theory

• Bronsted and Lowry came up with an alternative
  definition for acids and bases

9
Bronsted Acids Bronsted Bases

• A bronsted acid is a substance from which a
  proton (H) can be removed (proton donor).
• A bronsted base is a substance that can remove a
  proton from an acid (proton acceptor).

10
Conjugate Acids Bases

• A conjugate base is formed when an acid releases
  a proton (H).
• A conjugate acid is formed when a base accepts a
  proton (H).

11
Acids and Conjugate Bases (Page 491)
12
Bases and Conjugate Acids (Page 491)
13
Bronsted Acid Base Reactions

• An acid and a base that are related by the gain
  and loss of a proton are called a conjugate
  acid-base pair.
• Every acid has associated with it a conjugate
  base.
• Likewise, every base has associated with it a
  conjugate acid.

14
Conjugate Acid Base Pairs
15
Conjugate Acid Base Pairs
16
Water is amphoteric
17
Amphoteric

• A substance that may either gain or lose a proton
  is amphoteric.
• In other words it can act like either a Bronsted
  acid or a Bronsted base.
• Amphoteric substances include hydrogen containing
  anions, hydroxides of some metals (especially
  those near the metal/nonmetal dividing line).

18
What ion do you think is present in the solution
below?

• For every ion in a solid or solution there must
  be an ion of opposite charge, a counter ion, in
  order for the solid or solution to be
  electrically neutral.

Cu2
19
Self ionization (autoionization) of water
Water can react with itself in a process known as
self ionization (autoionization).

• Hydronium ion is the conjugate acid of water and
  hydroxide ion is the conjugate base of water.

20
Self ionization of water

• Occurs to a very slight extent (2 in every
  billion water molecules self ionize at 25C).

Write the equilibrium expression for this
reaction.
21
Ion Product of water

• Kw H3OOH-
• Kw H3OOH- 1.0 x 10-14 (at 25C)

Kw H3OOH- 1.0 x 10-12 (at 100C)
Is this reaction exothermic or endothermic?
Support your answer.
22

• How does adding and acid (i.e. HCl) to water
  affect H3O, OH- and the Kw?
• How does adding and base (i.e. NaOH) to water
  affect H3O, OH- and the Kw?

23
(No Transcript)
24
Weak Acids

• Weak acids are much less than 100 ionized in
  water.
• One of the best known is acetic acid (CH3CO2H)

25
A
26
Strong Bases

• Strong Base 100 dissociated in water.
• NaOH (aq) ? Na (aq) OH- (aq)

27
Weak Bases

• Weak base less than 100 ionized in water
• One of the best known weak bases is ammonia
• NH3 (aq) H2O (l) ? NH4 (aq) OH- (aq)

28
CH3CO2H is added to H2O

• Write the reaction that occurs.
• Write the equilibrium expression for the reaction.

29
Equilibrium Constants for Weak Acids

• A Weak acid has a Ka
• The higher the Ka the stronger the acid

30
NH3 is added to H2O

• Write the reaction that occurs.
• Write the equilibrium expression for the reaction.

31
Equilibrium Constants for Weak Bases

• A Weak base has a Kb
• The higher the Kb the stronger the base

32
Appendix F and G in the front of your notebook
contain many Ka and Kb values.

• This is not a complete listing of weak acids and
  weak bases however it does seem that there are
  more weak acids.

33
We can use Ka and Kb values to rank the strength
of weak acids and weak bases.
Weak Acids Weak Acids Weak Acids
Name Chemical Formula KA
Acetic acid HC2H3O2 1.80 x 10-5
Acetylsalicylic acid HC9H7O4 3.2 x 10-4
Benzoic acid HC7H5O2 6.0 x 10-5
Chloroacetic acid ClCH2COOH 1.40 x 10-3
Formic acid HCOOH 1.77 x 10-4
Hydrocyanic acid HCN 4.00 x 10-10
Hypochlorous acid HOCl 4.00 x 10-8
Hydrofluoric acid HF 7.20 x 10-4
Hypoiodous acid HOI 2.30 x 10-11
Lactic Acid HC3H5O3 1.5 x 10-4
Nicotinic acid HC6H4NO2 1.40 x 10-5
Nitrous Acid HNO2 4.5 x 10-4
34
Which weak acid below is the strongest?
Weak Acids Weak Acids Weak Acids
Name Chemical Formula KA
Acetic acid HC2H3O2 1.80 x 10-5
Acetylsalicylic acid HC9H7O4 3.2 x 10-4
Benzoic acid HC7H5O2 6.0 x 10-5
Chloroacetic acid ClCH2COOH 1.40 x 10-3
Formic acid HCOOH 1.77 x 10-4
Hydrocyanic acid HCN 4.00 x 10-10
Hypochlorous acid HOCl 4.00 x 10-8
Hydrofluoric acid HF 7.20 x 10-4
Hypoiodous acid HOI 2.30 x 10-11
Lactic Acid HC3H5O3 1.5 x 10-4
Nicotinic acid HC6H4NO2 1.40 x 10-5
Nitrous Acid HNO2 4.5 x 10-4
35
The higher the Ka the stronger the acid.
36
Why does a higher Ka value indicate that an acid
is stronger?
For two acids of the same molarity the one with
the higher Ka is stronger because it dissociates
more. This increases the concentration of the
products (more specifically hydronium).
37
Which weak base below is the strongest?
Weak Bases Weak Bases Weak Bases
Name Chemical Formula KB
Ammonia NH3 1.80 x 10-5
Diethylamine (CH3CH2)2NH 8.6 x 10-4
Methylamine CH3NH2 4.2 x 10-4
Pyridine C5H5N 1.5 x 10-9
Quinoline C9H7N 6.3 x 10-8
Trimethylamine (CH3)3N 7.4 x 10-5
38
Why does a higher Kb value indicate that an base
is stronger?
For two bases of the same molarity the one with
the higher Kb is stronger because it dissociates
more. This increases the concentration of the
products (more specifically hydroxide).
39
Relative Strengths of Conjugate Pairs
40
Ka x Kb Kw 1 x 10-14

• This equation is used to determine the Ka or Kb
  value for an acid or base within a conjugate
  pair.
• Teacher Example Determine the Kb of acetate
  ion.
• (See Appendix F) Ka CH3CO2H 1.8 x 10-5
• 1.8 x 10-5 x Kb 1 x 10-14
• Kb CH3CO2- 5.6 x 10-10

41
What is the Kb of PO43-?

• Ka HPO42- 3.6 x 10-13
• For any conjugate pair
• Ka x Kb Kw 1 x 10-14
• Kb for PO43- 2.8 x 10-2

42
For the conjugate pair HPO42- PO43-

• See Appendix F and G in the front of your
  notebook
• Ka HPO42- 3.6 x 10-13
• Ka x Kb Kw 1 x 10-14
• Kb for PO43- 2.8 x 10-2

43
Conjugate Pair Acid Base Strength Comparison

• CH3CO2H and CH3CO2-
• Ka CH3CO2H 1.8 x 10-5
• Kb CH3CO2- 5.6 x 10-10
• HPO42- and PO43-
• Ka HPO42- 3.6 x 10-13
• Kb for PO43- 2.8 x 10-2

44
Is NH4 a stronger acid than HCN? Support your
answer.
45
Acid Base Neutralization

• Acid Base ? Salt Water
• These reactions go to completion. (?)

46
Acid Base Neutralization

• Acid Base ? Salt Water
• HCl NaOH ? NaCl H2O
• 2HNO3 Ba(OH)2 ? Ba(NO3)2 2H2O
• salt (an ionic compound containing the cation
  from a base and the anion from an acid)

47
(No Transcript)
48
Acid Base Neutralization

• Assume stoichiometric amounts of acid and base.
• A strong acid and a strong base form a neutral
  solution.
• A strong acid and a weak base form an acidic
  solution.
• A weak acid and a strong base form a basic
  solution.
• A weak acid and a weak base can form an acidic,
  basic, or neutral solution.
• The salt formed in the neutralization reaction is
  responsible for the observations listed above.

49
Neutralization Reaction

• A neutralization reaction occurs when an acid and
  a base react to form water and a salt (an ionic
  compound containing the cation from a base and
  the anion from an acid)
• NaOH HCl ? NaCl H2O
• KOH CH3CO2H ? KCH3CO2 H2O
• Is the solution that results from each reaction
  acidic, basic or neutral?

50
Neutralization Reaction

• NaOH HCl ? NaCl H2O (neutral)
• KOH CH3CO2H ? KCH3CO2 H2O (basic)
• Why are the solutions that result from
  neutralization reactions acidic, basic or
  neutral?
• NaCl is a neutral salt and
• KCH3CO2 is a basic salt

51
Acidic, Basic, and Neutral Salts

Cations from groups I and II



All anions other than those from strong acids or
HSO4- and H2PO4-

All cations other than those from strong bases
52
A basic ion reacts as a base when added to water.
Na2CO3 Na CO32-
53
An acidic ion reacts as an acid when added to
water.
KHSO4 K HSO4-
54
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• NH4I

55
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• KClO4

56
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• Na3PO4

57
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• Zn(NO3)2

58
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• AlCl3

59
Is the salt acidic, basic, or neutral?Write an
equation to support your answer if the salt is
acidic or basic.

• NH4NO2

60
Acid Base Strength

• We have already discussed some methods for
  comparing strengths of acids and bases
• Percent ionization
• Ka or Kb comparison
• Example HCl is a stronger acid than CH3CO2H
• (Even though it may sound odd. This is the same
  as saying that CH3CO2H is a stronger base than
  HCl).

61
Strengths of Binary Acids (HX)

• Binary Acid Examples HCl, H2O, NH3, CH4
• Binary acid strength is based on
• Bond Polarity (Electronegativity)
• Bond Strength

62
Strengths of Binary Acids (HX)

• Rule 1
• When moving from left to right in a period the
  acid strength increases with the increasing
  electronegativity of X. This is because the H
  X bond becomes more polarized.
• Example HCl is a stronger acid than H2S.

63
Strengths of Binary Acids (HX)
64
Strengths of Binary Acids (HX)

• Rule 2
• When moving down a group the acid strength
  increases because the H X bond is weaker and
  easier to break.

65

• The H F bond is very strong and hard to break
  and therefore HF is weaker than the other acids.
  
• The H I bond is the weakest and easiest to
  break and therefore HI is the strongest acid.

66
Confused?

• These rules seem to contradict each other but if
  we look at the bond energies it may make more
  sense.
• Bond energy is a greater factor in a group than
  in a period.

Bond Bond Energy (kJ/mol)
H C 415
H F 569
H I 295
67

• Even though HI is the strongest acid we consider
  HCl, HBr, and HI all to be strong acids.
• This is because of the leveling effect.

68
The Leveling Effect

• Some solvents such as water, can make these acids
  appear to be the same strength as they all ionize
  100.
• This is known as the leveling effect.
• This leveling effect can also be seen with the
  strong bases.

69
Strengths of Oxyacids (HOX)

• Oxyacid examples HClO, H2SO4, NaOH
• Oxyacid strength is based on
• Electronegativity of X
• The number of oxygen atoms

70
Strengths of Oxyacids (HOX)

• Rule 1
• As the electronegativity of X increases the
  strength of the oxyacid increases.
• This can be explained by the electron withdrawing
  power of oxygen compared to X.
• Oxygen is an electron bully

71
Strengths of Oxyacids (HOX)

Na O H

72
Strengths of Oxyacids (HOX)
73
Strengths of Oxyacids (HOX)
74
Strengths of Oxyacids (HOX)

• Rule 2
• For oxyacids with the same element X. The acid
  strength increases as the number of oxygen atoms
  increases.
• This can be explained by the electron withdrawing
  power of oxygen.
• Oxygen is an electron bully

75
Strengths of Oxyacids (HOX)
76
Mono-, Di-, and Triprotic Acids

• Acids can be classified as to the number of
  hydrogen atoms that they can give up (labile
  protons).
• Monoprotic acids contain 1 ionizable H.
• HCl, HNO2, CH3CO2H
• Diprotic acids contain 2 ionizable Hs.
• H2SO4, H2S, H2CO3
• Triprotic acids contain 3 ionizable Hs.
• H3PO4, H3AsO4

77
The Stepwise Dissociation of Phosphoric Acid
H3PO4 (aq) H2O(l)
H2PO4-(aq) H3O(aq)
H2PO4-(aq) H2O(l)
HPO42-(aq) H3O(aq)
HPO42-(aq) H2O(l) PO43-(aq)
H3O(aq)
78
For polyprotic acids each successive ionization
is weaker
a
79
Practice
80
Practice
81
Practice
82
Practice
83
Practice
84
Practice
85
Practice

• Which is the stronger acid H2C2O4 or HC2O4 ?

86
Practice

• Which is the stronger acid H2C2O4 or HC2O4 ?

87
This is in your notebook
88
Volumetric Analysis (Titration)
(Standard solution)
89
Titration of a 40.00mL sample of H3PO4 solution
requires 35.00mL of 0.1500M KOH to reach the end
point. Determine the molar concentration of the
H3PO4.

• 2KOH H3PO4 ? K2HPO4 2H2O

0.06563M H3PO4
90
10.0 grams of vinegar (containing acetic acid,
CH3CO2H) is titrated with 65.40mL of 0.150 M NaOH
to reach an endpoint. What is the percent by mass
of acetic acid in the vinegar?
91
10.0 grams of vinegar (containing acetic acid,
CH3CO2H) is titrated with 65.40mL of 0.150 M NaOH
to reach an endpoint. What is the percent by mass
of acetic acid in the vinegar?
5.89