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Title: Chemistry 100


1
Chemistry 100
  • Acids and Bases

2
The Brønsted Definitions
  • Brønsted Acid proton donor
  • Brønsted Base proton acceptor
  • Conjugate acid - base pair an acid and its
    conjugate base or a base and its conjugate acid

3
Example Acid-Base Reactions
  • Look at acetic acid dissociating
  • CH3COOH(aq) ?CH3COO-(aq) H(aq)
  • ?
  • Brønsted acid Conjugate base
  • Look at NH3(aq) in water
  • NH3(aq) H2O(l) ?NH4(aq) OH-(aq)
  • ? ?
  • Brønsted base conjugate acid

4
Representing Protons in Aqueous Solution
  • CH3COOH(aq) ? CH3COO-(aq) H(aq)
  • CH3COOH(aq) H2O(l) ? CH3COO-(aq) H3O(aq)
  • HCl (aq) ? Cl-(aq) H(aq)
  • HCl(aq) H2O(l) ? Cl-(aq) H3O(aq)

5
Representing Protons
  • Both representations of the proton are equivalent
  • H5O2 (aq), H7O3 (aq), H9O4 (aq) have been
    observed
  • We will use either H(aq) or H3O(aq)

6
What is H (aq)?

H3O
H
H5O2
H9O4
7
The Hydroxide Bases
  • KOH, RbOH, NaOH, are not strictly Brønsted Bases
    since none of these substances accepts a proton
  • KOH(aq) ? K(aq) OH-(aq)
  • NaOH(aq) ? Na(aq) OH-(aq)
  • OH-(aq) H3O(aq) ?2 H2O(l)

8
The Autoionization of Water
  • Water autoionizes (self-dissociates) to a small
    extent
  • 2H2O(l) ? H3O(aq) OH-(aq)
  • H2O(l) ? H(aq) OH-(aq)
  • These are both equivalent definitions of the
    autoionization reaction. Water is acting as a
    base and an acid in the above reaction ? water is
    amphoteric.

9
The Autoionization Equilibrium
  • from the preceding chapter
  • but we know H2O is constant

10
The Defination of Kw
  • Keq H2O Kw HOH-
  • Ion product constant for water, Kw, is the
    product of the molar concentrations of H and OH-
    ions in pure water at a temperature of 298.15 K
  • Kw HOH- 1.0x10-14 at 298.2 K

11
The Definition of an Acidic Solution
  • We define an acidic solution as one where the
    H in the solution is greater than the the H
    in pure water
  • acidic solution ? H gt 1.0 x 10-7 mole/L at
    298.2 K

12
The Definition of a Basic Solution
  • Basic solutions are those where the H in the
    solution is less than its concentration in pure
    water at 298.2 K.
  • Basic solution ? H lt 1.0 x 10-7 mole/L
  • An alternative definition of a a basic solution
    is as follows
  • Basic solution ? OH- gt 1.0 x 10-7 mole/L

13
The Definition of a Neutral Solution
  • A neutral solution is defined as one where the
    H in the solution is equal to the hydrogen ion
    concentration in pure water
  • Neutral solution ? H OH- 1.0 x 10-7
    mole/L at 298.2 K!

14
The Dependence of Kw on Temperature
  • In our definitions of an acidic, basic, and a
    neutral solutions, we had explicitly stated the
    temperature as 298.2 K. Why?

Kw is temperature dependent
  • How will that affect our definition of an acidic,
    basic, or a neutral solution?

15
Neutrality at Body Temperature
  • At T 310.15 K (physiological temperature) ? Kw
    2.4 x 10-14
  • A neutral solution has H OH- (Kw)½
  • At 310.15 K, a neutral solution is one where H
    OH- 1.5 x 10-7 M

(UNLESS OTHERWISE INDICATED, ALL CALCULATIONS
WILL BE AT 298.15 K)
16
The pH scale
Sørenson - 1909 pH -log H
Solution Type H / M pH range
neutral solutions H OH- 1.0x10-7 pH 7.00
basic solutions H lt1.0x10-7 pH gt 7.00
acid solutions H gt1.0x10-7 pH lt 7.00
17
The Relationship between pH and pOH
  • pH -log H
  • pOH -log OH-
  • From the Kw expression
  • Kw HOH- 1.0x10-14 at 298.2 K
  • -log (1 x 10-14) -log H -log OH-

14.00 pH pOH
18
Acid Strength and Dissociation
  • CH3COOH(aq) ? CH3COO-(aq) H(aq)
  • HCOOH(aq) ? HCOO-(aq) H(aq)
  • both weak acids lt 5 ionized
  • Other examples of weak acids ? HF, HNO2, HCN

19
Acid Strength
  • The strength of an acid is directly dependent on
    its dissociation (? value)
  • For an acid
  • n the number of groups that donate a proton

20
Base Strength and Dissociation
  • Strong Bases also 100 ionized in water
  • NaOH(aq) Na(aq) OH-(aq)
  • Ba(OH)2(aq) Ba2(aq) 2OH-(aq)
  • Some bases are weak bases they dont ionize
    completely.
  • NH3(aq) H2O(l) ? NH4 (aq) OH-(aq)
  • lt 5 ionized in aqueous solution

21
Base Strength
  • The strength of a base is also directly dependent
    on its dissociation (? value)
  • For a base
  • baseo ? the original concentration of base
  • m the number of basic groups in the molecule

22
Conjugate Acid-Base Strengths
  • CH3COOH (aq) ? CH3COO-(aq) H(aq)
  • Note that the conjugate base of acetic acid is a
    reasonable proton acceptor
  • CH3COO-(aq) H2O (l) ? CH3COOH (aq) OH-(aq)

23
Other Examples
  • HNO3 (aq) H(aq) NO3-(aq)
  • conjugate base (very weak)
  • HCOOH (aq) ? HCOO-(aq) H(aq)
  • conjugate base is relatively strong
  • NH3(aq) H2O(l) ? NH4 (aq) OH-(aq)
  • relatively strong
    conjugate acid

24
  • HCl (aq) ? Cl-(aq) H(aq)
  • The conjugate base, Cl- ion, is extremely weak
  • Cl-(aq) H2O (l) ? HCl (aq) OH-(aq)

25
  • S2-(aq) H2O (l) ? HS-(aq) OH- (aq)
  • The conjugate acid, the HS- ion, is extremely
    weak
  • HS-(aq) ? H (aq) S2- (aq)
  • The equilibrium lies very far to the left for
    this reaction

26
  • The greater the acid strength (large Ka), the
    weaker the conjugate base of that acid
  • The weaker the acid (smaller Ka), the stronger
    its conjugate base
  • If the base strength is high (Kb is large), its
    conjugate acid is very weak
  • The weaker the base (small Kb value), the
    stronger the conjugate acid of the base

27
Calculating the pH of Solution of Strong Acids
  • For the dissolution of HCl, HI, or any of the
    other seven strong acids in water
  • HCl (aq) ? H (aq) Cl- (aq)
  • HI (aq) ? H (aq) I- (aq)
  • The pH of these solutions is obtained from the
    molarity of the dissolved acid
  • pH -log H -logHCl

28
Calculating the pH of Solution of Strong Bases
  • For the dissolution of NaOH, Ba(OH)2, or any of
    the other strong bases in water
  • NaOH (aq) ? Na (aq) OH- (aq)
  • Ba(OH)2 (aq) ? Ba2 (aq) 2OH- (aq)

29
  • The pH of these solutions is obtained by first
    calculating the pOH from the molarity of the
    dissolved base
  • pOH -log OH- -logNaOH
  • pOH -log OH- -log2 Ba(OH)2
  • pH 14.00 - pOH

30
The Seven Strong Acids
  • chloric acid HClO3
  • hydrobromic acid HBr
  • hydrochloric acid HCl
  • hydroiodic acid HI
  • nitric acid HNO3
  • perchloric acid HClO4
  • sulphuric acid H2SO4
  • What about the relative strength of the strong
    acids?

31
The Leveling Effect
  • H (aq) (or H3O(aq)) is the strongest acid that
    can exist in aqueous solution. Any acid stronger
    than H(aq) reacts with water completely to
    produce H(aq) and the weak conjugate base.

32
  • HNO3 (aq) is a stronger acid than H(aq) (H3O) \
    reacts with water completely to form H(aq)
  • HNO3 (aq) ? H (aq) NO3- (aq)
  • Acids weaker than H(aq) have the equilibrium
    lying primarily to the left.
  • HNO2(aq) ? H(aq) NO2- (aq)

33
  • The OH- ion is the strongest base that can exist
    in aqueous solution. Bases stronger than
    OH-(aq) react with water to produce the hydroxide
    ion (OH-).

34
  • NH2- (the amide ion) is an extremely strong base
    (much stronger than OH-). Therefore,
  • NaNH2 (aq) H2O (l) NH3 (aq) NaOH(aq)
  • NH2- cannot exist in aqueous solution.
  • NH3 is a much weaker base than OH-. Therefore,
    when it reacts with water, the equilibrium
    favours the reactants
  • NH3 (aq) H2O (l) ?NH4 (aq) OH-(aq)

35
The Leveling Effect Defined
  • Any acid that is stronger than H(aq) means that
    we have 100 ionisation of the acid.
  • For acids like HCl(aq), HClO4(aq), HNO3(aq), the
    appearance is one of equal acid strength.
  • Water is said to have a levelling effect on the
    acid strength

36
Equilibria in Aqueous Solutions of Weak Acids/
Weak Bases
  • By definition, a weak acid or a weak base does
    not ionize completely in water (? ltlt100).
  • How would we calculate the pH of a solution of a
    weak acid or a weak base in water?

37
The Ka Value
  • To obtain the pH of a weak acid solution, we must
    apply the principles of chemical equilibrium
  • Define the acid dissociation constant Ka
  • For a general weak acid reaction
  • HA (aq) ? H (aq) A- (aq)

38
Weak Acid/Bases and pH
  • For a solution of hydrofluoric acid in water
  • HF (aq) ? H (aq) F- (aq)

39
Equilibria of Weak Bases in Water
  • To calculate the percentage dissociation of a
    weak base in water (and the pH of the solutions)
  • CH3NH2 (aq) H2O ? CH3NH3(aq) OH- (aq)
  • We approach the problem as in the case of the
    weak acid above, i.e., from the chemical
    equilibrium viewpoint.

40
The Kb Value
  • Define the base dissociation constant Kb
  • For a general weak base reaction with water
  • B (aq) H2O (l) ? B (aq) OH- (aq)
  • For the above system

41
Diprotic/Polyprotic Acids
  • Look at the following system.
  • H2C2O4 (aq) ?HC2O4- (aq) H (aq) Ka1
  • HC2O4- (aq) ?C2O42- (aq) H (aq) Ka2
  • For the dissociation of diprotic and polyprotic
    acids, the magnitudes of the dissociation
    constants decrease in the direction
  • Ka1 gt Ka2 gt Ka3 etc.

42
Example
  • For oxalic acid in water,
  • Ka1 6.5 x 10-2
  • Ka2 6.1 x 10-5
  • Since Ka1gtgt Ka2, the H (and the pH) in the
    solution is due primarily to the first
    dissociation ONLY.

43
Obtaining the Relationship between Ka and Kb
  • We have already seen that there is a relationship
    between the strength of an acid and the ability
    of its conjugate base to hydrolyse.
  • HCOOH (aq) ?HCOO- (aq) H (aq)
  • Ka (HCOOH) 1.8 x 10-4
  • Examine the reverse reaction, the hydrolysis
    (reaction of the substance with water) of HCOO-
  • HCOO-(aq) H2O (l) ? HCOOH (aq) OH-(aq)

44
Obtaining the Kb of the Conjugate Base
  • HCOOH (aq) ? HCOO- (aq) H(aq)
  • HCOO- (aq) H2O (l) ?HCOOH (aq) OH- (aq)
  • Keq(1) Ka (HCOOH)
  • Keq(2) Kb (HCOO-)
  • Add the two reactions together
  • HCOOH (aq) ? HCOO- (aq) H(aq)
  • HCOO- (aq) H2O (l) ?HCOOH (aq) OH- (aq)

45
  • We are left with the overall reaction
  • H2O (l) ?H (aq) OH- (aq)
  • Kw H OH-
  • From our rules for the equilibria of multiple
    reactions.
  • Kw K (1) K (2)

Kw Kb Ka
46
Variation of Conjugate base Strength with Ka
  • HCOOH (aq) ?HCOO- (aq) H (aq)
  • Ka (HCOOH) 1.8 x 10-4
  • Kb (HCOO-) 5.6 x 10-11
  • CH3COOH (aq) ?CH3COO- (aq) H (aq)
  • Ka (CH3COOH) 1.8 x 10-5
  • Kb (CH3COO-) 5.6 x 10-10

47
Salts of Conjugate Bases
  • Look at the dissolution of CH3COONa in water.
  • CH3COONa (aq) ? Na (aq) CH3COO- (aq)
  • But we know that the acetate ion, CH3COO- (aq),
    will hydrolyze in aqueous solution according to
    the following reaction.
  • CH3COO- (aq) H2O (l) ?CH3COOH (aq) OH-(aq)

48
Hydrolysis reaction produces OH-
Dissolving a salt of a strong base/ weak acid in
water ? basic solution.
49
Salts of Conjugate Acids
  • Look at the dissolution of NH4Cl in water
  • NH4Cl (aq) ? NH4 (aq) Cl- (aq)
  • But we know that the ammonium ion, NH4 (aq),
    will donate a proton aqueous solution
  • NH4 (aq) ?NH3 (aq) H(aq)

50
Hydrolysis reaction produces H(aq)
Dissolving a salt of a strong acid/ weak base in
water ? acidic solution.
51
Both the cation and anion Hydrolyse
  • What about salts in which both the cation and
    anion hydrolyze?
  • The pH of the solution will depend on the
    magnitude of the Ka and the Kb values of the
    respective acidic and basic ions.

52
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53
Salts of Weak Acids/Strong Bases
  • How do we prepare a solution of HCOONa?
  • Titration of HCOOH with NaOH according to the
    following reaction

HCOOH (aq) NaOH (aq) ? HCOONa (aq) H2O (l)
? ?
? Weak Acid Strong Base
Basic Salt
Dissolution of the salt of a weak acid/strong
base produces a basic solution (pH gt 7.00).
54
The Strong Acid/Weak Base Case
  • How do we prepare a solution of NH4Cl?
  • Titration of HCl with NH3 according to the
    following reaction

HCl (aq) NH3 (aq) ? NH4Cl (aq) ?
? ? Strong Weak
Acidic Acid Base Salt
Dissolution of the salt of a strong acid/weak
base produces a acidic solution (pH lt 7.00).
55
The Weak Acid/Weak Base Case
  • What would be the pH of a solution of CH3COONH4?
  • Look at the following reactions
  • NH4 (aq) ? NH3 (aq) H (aq)
  • K Ka (NH4)
  • CH3COO- (aq) H2O (l) ? CH3COOH (aq) OH- (aq)
  • K Kb (CH3COO-)

56

The pH of a solution the salt of a weak acid/weak
base depends on the magnitudes of the equilibrium
constants.
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