Acid-Base chemistry
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Acidity of blood (pH range of
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Heartburn (acid-reflux) – Tums, Rolaids, Milk of Magnesia; The
Purple Pill, Nexium
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Acidity regulation (tropical fish / goldfish tanks)
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Pepsi (most sodas); just how acidic?
– Loosens rusty bolts; cleans windshields
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Battery acid (H2SO4)
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Acid Rain (SO2, NO2, CO2)
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Drain cleaners (Drano, Liquid Plumber)
Nature of acids and bases
Acids – sour, tart taste (vinegar, lemon juice)
Bases – bitter taste, slippery feel between fingers (drano,
detergent)
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Arrhenius definitions (1884):
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Acids: produce H+ ions (protons) in solution
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Bases: produce OH- (hydroxide) ions in solution
Lowry-Brønsted definitions
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Danish & British chemists
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More general definitions than Arrhenius definition:
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Acids: Proton donors
Bases: proton acceptors
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H 2O
+
HCl  H3O+
+
Cl-
Lewis Acids / Lewis Bases
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LA: electron pair acceptor
LB: electron pair donor

NH3(aq) + H+(aq)  NH4+(aq)
LB
LA

OH-(aq) + H+(aq)  H2O(l)
General reaction for HA + H2O
HA(aq) + H2O(l)  H3O+(aq) + A-(aq)
Acid
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Base
Conjugate
Acid
Conjugate
Base
Position of equilibrium dictated between bases in equation
(competition for H+)
H2O stronger base:
A- stronger base:

Keq = [H3O+][A-]
[HA][H2O]
Ka = Acid dissociation constant
[H3O+] = [H+] in H2O

Ka = [H3O+][A-]
[HA]
Ka = Acid dissociation constant
Acid dissociation reactions
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Dissociation of acid (HA) most important here
HA (aq)  H+(aq) + A-(aq)
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H2O still important (required for aqueous conditions)
Ka for this equilibrium process?
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Can predict dissociation reaction for any acid (no matter how
complex-looking)
HCl (aq)
HC2H3O2 (aq)
NH4+ (aq)
C6H5NH3+ (aq)
Acid Strength

Defined by equilibrium position of dissociation reaction:
HA(aq) + H2O(l)  H3O+(aq) + A-(aq)
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Strong acid – lies far to right side (a)
Weak acid – lies far to left side (b)
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Describing acid strength
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Insert Figure 14.6
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Insert table 14.1
strong acid
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Property
Ka value
Equilibrium Posn.
[H+]e vs [HA]o
Strength of conj.
base compared
to water
weak acid
Strong Acid
Weak Acid
Common strong acids
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HCl, HClO4, HNO3, HI, HBr (monoprotic acids)
H2SO4 (diprotic acid)
Ka values – very large
Relative base strengths
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Arrange F-, Cl-, NO2-, CN- in order of increasing base strength:
Water: Amphoteric substance
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Can exist as both an acid and a base
Autoionization of water
H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
Kw = [H3O+][OH-]; Kw = Dissociation constant for H2O
Kw = [H+][OH-]
[H+]=[OH-] = 1.0 x 10-7 M (at 25 °C, in pure water)
Kw = [H+][OH-] = 1.0 x 10-14
In any aqueous solution, the product of [H+] and [OH-] must
always equal 1.0 x 10-14
Kw
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Kw = [H+][OH-] = 1.0 x 10-14
3 possible situations:
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Neutral solutions; [H+] = [OH-]
Acidic solutions; [H+] > [OH-]
Basic solutions; [H+] < [OH-]
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In all cases: Kw = [H+][OH-] = 1.0 x 10-14
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Varies with T (as do all K values)
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Calculating [H+],[OH-]
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[H+] with [OH-] = 1.0 x 10-5 M
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[OH-] with [H+] = 10.0 M
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[OH-] and [H+] in neutral solution at 60 °C (Kw = 1x10-13 at 60 °C)
pH scale
(acidity measurement)
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pH = -log [H+]
[H+] = 1.0 x 10-7 M; pH =
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pOH = -log[OH-]; pK = -log Ka
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Log scale; pH changes by 1 for
every power of 10 change in [H+]
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pH decreases as [H+] increases
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Calculating pH / pOH
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1.0 x 10-3 M OH-
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1.0 M H+
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pH + pOH = 14.00
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pH of sample of human blood = 7.41 @ 25 °C. Calculate pOH,
[H+], [OH-]
pH of strong acid solutions
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What species are present? 1M HCl
What are the major species that can furnish H+ ions?
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pH of 0.10 M HNO3
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pH of 1.0 x 10-10 M HCl
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pH of weak acid solutions
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pH of 1.00 M HF (weak acid); Ka = 7.2 x 10-4
Major species in solution? HF, H2O
Major species which furnish H+?
H2O(l)
H+(aq) + OH-(aq)
Kw = 1.0 x 10-14
HF(aq)
H+(aq) + F-(aq)
Ka = 7.2 x 10-4
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Which of these two is the stronger acid?
Ka = [H+][F-] / [HF] = 7.2 x 10-4
In order to calculate pH, need equilibrium value of [H+]
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[HF]o = 1.00 M
[H+]o = 0 M (approximation, as H+ from H20 not included here)
[F-]o = 0 M
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Let x be the change required to reach equilibrium…..
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pH of weak acid solutions
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Equilibrium concentration of [HF] =
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Equilibrium concentration of [H+] =
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Equilibrium concentration of [F-] =
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Substitute these into Ka = [H+][F-] / [HF] = 7.2
x 10-4 eqn, and solve for x….
Bases
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Cleaning solutions (ammonia, bleach)
Antacids (Tums, Rolaids, Milk of Magnesia)
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Arrhenius: produces OH- ions
Lowry – Brønsted: H+ acceptor
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Strong Bases - NaOH
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pH of strongly basic solution
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5.0 x 10-2 M NaOH solution (same procedure as for acidic pH
calculations); Expected pH range?
Major species:
Weak bases
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Many types of bases don’t contain OH-, but do increase [OH-]
when dissolved in water (through reaction with water).
NH3(aq) + H2O(l)
Base
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NH4+(aq) + OH-(aq)
Acid
Lone pair of electrons on N picks up H+ from H2O
Weak bases
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General reaction with H2O:
B(aq) + H2O(l)
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BH+(aq) + OH-(aq)
Kb =
Kb always refers to reaction of a base with H2O to produce a
conjugate acid and OH-
pH of weak bases - calculations
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Very similar to those of weak acids
pH of 15.0 M solution of NH3 (Kb = 1.8 x 10-5)
Polyprotic acids
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More than 1 acidic H; H2SO4, H3PO4; H2CO3
Consider H3PO4
H3PO4(aq)
H+(aq) + H2PO4-(aq)
Ka1 = [H+][H2PO4-] / [H3PO4]= 7.5 x 10-3
H2PO4-(aq)
H+(aq) + HPO42-(aq)
Ka2 = [H+][HPO42-] / [H2PO4-] = 6.2 x 10-8
HPO42-(aq)
H+(aq) + PO43-(aq)
Ka3 = [H+][PO43-] / [HPO42-] = 4.8 x 10-13
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Only 1st dissociation step usually important for [H+]
determination
Ka1 >>> Ka2 >>> Ka3
Polyprotic acids
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pH of 5.0 M H3PO4 solution; eq. concs. of H3PO4, H2PO42-,
HPO43- and PO43-.
Sulfuric acid (H2SO4)
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Unique acid
Strong acid in 1st dissociation step
Weak acid in 2nd dissociation step
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pH of 1.0 M H2SO4 solution
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Does HSO4- make significant contribution to [H+]? No.
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Acid-base properties of salts
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Salts producing neutral solutions
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Salts producing basic solutions
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Salts producing acidic solutions
Acid-base properties of salts
Structure effects on acid/base
properties
 Oxyacids
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Hydrogen halides