Acid and Base Equilibria

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Chapter 18

• Acid and Base Equilibria

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AcidBase Concepts
Arrhenius Acid A substance which dissociates in
water to form hydrogen ions (H) in solution.
HA(aq) ? H(aq) A(aq) Arrhenius Base
A substance that dissociates in, or reacts with
water to form hydroxide ions (OH).
MOH(aq) ? M(aq) OH(aq)
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AcidBase Concepts

• BrønstedLowry Acid Substance that can donate H
• BrønstedLowry Base Substance that can accept H
• Chemical species whose formulas differ only by
  one proton are said to be conjugate acidbase
  pairs.

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For the following reaction in aqueous solution,
identify the BrønstedLowry acids, bases, and
conjugate acidbase pairs.
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AcidBase Concepts
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AcidBase Concepts
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AcidBase Concepts
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AcidBase Concepts

• A Lewis Acid is an electron-pair acceptor. These
  are generally cations and neutral molecules with
  vacant valence orbitals, such as Al3, Cu2, H,
  BF3.
• A Lewis Base is an electron-pair donor. These are
  generally anions and neutral molecules with
  available pairs of electrons, such as H2O, NH3,
  O2
• The bond formed is called a coordinate bond.

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AcidBase Concepts
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AcidBase Concepts

• Write balanced equations for the dissociation of
  each of the following BrønstedLowry acids.
• (a) H2SO4 (b) HSO4 (c) H3O
• Identify the Lewis acid and Lewis base in each of
  the following reactions
• (a) SnCl4(s) 2 Cl(aq) ? SnCl62(aq)
• (b) Hg2(aq) 4 CN(aq) ? Hg(CN)42(aq)
• (c) Co3(aq) 6 NH3(aq) ? Co(NH3)63(aq)

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Dissociation of Water

• Water can act as an acid or as a base. H2O(l) ?
  H(aq) OH(aq)
• This is called the autoionization of
  water.H2O(l) H2O(l) ? H3O(aq) OH(aq)

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Dissociation of Water

• This equilibrium gives us the ion product
  constant for water.
• Kw Kc HOH 1.0 x 1014
• If we know either H or OH then we can
  determine the other quantity.

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Dissociation of Water

• The concentration of OH ions in a certain
  household ammonia cleaning solution is 0.0025 M.
  Calculate the concentration of H ions.
• Calculate the concentration of OH ions in a HCl
  solution whose hydrogen ion concentration is 1.3
  M.

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pH A Measure of Acidity

• The pH of a solution is the negative logarithm of
  the hydrogen ion concentration (in mol/L).
• pH log H
• pH pOH 14
• Acidic solutions H gt 1.0 x 107 M, pH lt
  7.00Basic solutions H lt 1.0 x 107 M, pH
  gt 7.00Neutral solutions H 1.0 x 107 M,
  pH 7.00

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pH A Measure of Acidity

• Nitric acid (HNO3) is used in the production of
  fertilizer, dyes, drugs, and explosives.
  Calculate the pH of a HNO3 solution having a
  hydrogen ion concentration of 0.76 M.
• The pH of a certain orange juice is 3.33.
  Calculate the H ion concentration.
• The OH ion concentration of a blood sample is
  2.5 x 107 M. What is the pH of the blood?

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pH A Measure of Acidity
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pH A Measure of Acidity
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Strength of Acids and Bases

• Strong acids and bases are strong electrolytes
  that are assumed to ionize completely in water.
• Weak acids and bases are weak electrolytes that
  ionize only to a limited extent in water.
• Solutions of weak acids and bases contain ionized
  and non-ionized species.

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Strength of Acids and Bases

• If an acid is strong, its conjugate base has no
  measurable strength.
• H3O is the strongest acid that can exist in
  aqueous solution.
• OH ion is the strongest base that can exist in
  aqueous solution.

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Strength of Acids and Bases
ACID CONJ. BASE
ACID CONJ. BASE

• HClO4
• HI
• HBr
• HCl
• H2SO4
• HNO3
• H3O
• HSO4

HSO4 HF HNO2 HCOOH NH4 HCN H2O NH3
ClO4 I Br Cl HSO4 NO3 H2O SO42
SO42 F NO2 HCOO NH3 CN OH NH2
Increasing Acid Strength
Increasing Acid Strength
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Strength of Acids and Bases

• Stronger acid stronger base ?
• weaker acid weaker base
• Predict the direction of the following
• HNO2(aq) CN(aq) ? HCN(aq) NO2(aq)
• HF(aq) NH3(aq) ? F(aq) NH4(aq)

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Acid Ionization Constants

• Acid Ionization Constant the equilibrium
  constant for the ionization of an acid. HA(aq)
  H2O(l) ? H3O(aq) A(aq)
• Or simply HA(aq) ? H(aq) A(aq)

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Strength of Acids and Bases

• (a) Arrange the three acids in order of
  increasing value of Ka.
• (b) Which acid, if any, is a strong acid?
• (c) Which solution has the highest pH, and which
  has the lowest?

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Acid Ionization Constants

• pH of a Weak Acid
• Write a balanced equation and equilibrium
  expression.
• Identify all species present that may affect pH.
• Major species will be HA, H, A. We are not
  concerned with H2O or OH at this point.
• Summarize the changes in concentration of HA, H,
  A using a table.

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Acid Ionization Constants

• pH of a Weak Acid (Contd)
• 5. Substitute new values into equilibrium
  expression.
• 6. If Ka is significantly (gt1000 x) smaller than
  HA the expression (0.50 x) approximates to
  (0.50).
• 7. The equation can now be solved for x and pH.
• 8. If Ka is not significantly smaller than HA
  the quadratic equation must be used to solve for
  x and pH.

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Acid Ionization Constants

• The Quadratic Equation
• The expression must first be rearranged to
• The values are substituted into the quadratic and
  solved for a positive solution to x and pH.

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Acid Ionization Constants

• Initial Change Equilibrium Table Determine the
  pH of 0.50 M HA solution at 25C. Ka 7.1 x 104.

-


H

A
?
HA
(aq)
(aq)
(aq)
Initial

(
M
)

0.50
0.00
0.00
Change
(M)

x

x

x
Equilib
0.50

x
x
x
(M)
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Acid Ionization Constants

• Calculate the pH of a 0.036 M nitrous acid (HNO2)
  solution.
• What is the pH of a 0.122 M monoprotic acid whose
  Ka is 5.7 x 104?
• The pH of a 0.060 M weak monoprotic acid is 3.44.
  Calculate the Ka of the acid.

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Acid Ionization Constants

• Percent Dissociation A measure of the strength
  of an acid.
• Stronger acids have higher percent dissociation.
• Percent dissociation of a weak acid decreases as
  its concentration increases.

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Base Ionization Constants

• Base Ionization Constant The equilibrium
  constant for the ionization of a base.
• The ionization of weak bases is treated in the
  same way as the ionization of weak acids.
• B(aq) H2O(l) ? BH(aq) OH(aq)
• Calculations follow the same procedure as used
  for a weak acid but OH is calculated, not H.

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Base Ionization Constants
BASE Kb
CONJ. ACID Ka
5.6 x 10 4 4.4 x 10 4 4.1 x 10 4 1.8 x 10
5 1.7 x 10 9 3.8 x 10 10 1.5 x 10 14
C2H5NH2 (ethylamine) CH3NH2 (methylamine) C8H10N4O
2 (caffeine) NH3 (ammonia) C5H5N
(pyridine) C6H5NH2 (aniline) NH2CONH2 (urea)
C2H5NH3 CH3NH3 C8H11N4O2 NH4 C5H6N C6H5NH3 N
H2CONH3
1.8 x 10 11 2.3 x 10 11 2.4 x 10 11 5.6 x 10
10 5.9 x 10 6 2.6 x 10 5 0.67
Note that the positive charge sits on the
nitrogen.
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Base Ionization Constants

• Product of Ka and Kb multiplying out the
  expressions for Ka and Kb equals Kw.
• Ka ? Kb Kw
• What is the pH of a 0.40 M ammonia solution?
• Calculate the pH of a 0.26 M methylamine solution.

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Diprotic Polyprotic Acids

• Diprotic and polyprotic acids yield more than one
  hydrogen ion per molecule.
• One proton is lost at a time. Conjugate base of
  first step is acid of second step.
• Ionization constants decrease as protons are
  removed.

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Diprotic Polyprotic Acids
ACID Ka
CONJ. BASE Kb
Very Large 1.3 x 10 2 6.5 x 10 2 6.1 x 10
5 1.3 x 10 2 6.3 x 10 8 4.2 x 10 7 4.8 x 10
11 9.5 x 10 8 1 x 10 19 7.5 x 10 3 6.2 x 10
8 4.8 x 10 13
H2SO4 HSO4 C2H2O4 C2HO4 H2SO3 HSO3 H2CO3 HCO3
H2S HS H3PO4 H2PO4 HPO42
HSO4 SO4 2 C2HO4 C2O42 HSO3 SO3
2 HCO3 CO3 2 HS S 2 H2PO4 HPO42 PO43
Very Small 7.7 x 10 13 1.5 x 10 13 1.6 x 10
10 7.7 x 10 13 1.6 x 10 7 2.4 x 10 8 2.1 x 10
4 1.1 x 10 7 1 x 10 5 1.3 x 10 12 1.6 x 10
7 2.1 x 10 2
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Diprotic Polyprotic Acids

• Calculate the concentration of all species
  present in a 0.10 M solution of oxalic acid
  (C2H2O4). Determine the pH of the solution.
• Calculate the concentration of all species
  present in a 0.20 M solution of phosphoric acid
  (H3PO4). Determine the pH of the solution.

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Molecular Structure and Acid Strength

• The strength of an acid depends on its tendency
  to ionize.
• For general acids of the type HX
• The stronger the bond, the weaker the acid.
• The more polar the bond, the stronger the acid.
• For the hydrohalic acids, bond strength plays the
  key role giving HF lt HCl lt HBr lt HI

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Molecular Structure and Acid Strength

• The electrostatic potential maps show all the
  hydrohalic acids are polar. The variation in
  polarity is less significant than the bond
  strength which decreases from 567 kJ/mol for HF
  to 299 kJ/mol for HI.

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• For binary acids in the same group, HA bond
  strength decreases with increasing size of A, so
  acidity increases.
• For binary acids in the same row, HA polarity
  increases with increasing electronegativity of A,
  so acidity increases.

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• For oxoacids bond polarity is more important. If
  we consider the main element (Y) YOH
• If Y is an electronegative element, or in a high
  oxidation state, the YO bond will be more
  covalent and the OH bond more polar and the acid
  stronger.

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• For oxoacids with different central atoms that
  are from the same group of the periodic table and
  that have the same oxidation number, acid
  strength increases with increasing
  electronegativity.

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• For oxoacids having the same central atom but
  different numbers of attached groups, acid
  strength increases with increasing central atom
  oxidation number.
• As shown on the next slide, the number of oxygen
  atoms increases the positive charge on the
  chlorine which weakens the OH bond and increases
  its polarity.

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• Oxoacids of Chlorine

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• Predict the relative strengths of the following
  groups of oxoacids
• a) HClO, HBrO, and HIO.
• b) HNO3 and HNO2.
• c) H3PO3 and H3PO4.

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AcidBase Properties of Salts

• Salts that produce neutral solutions are those
  formed from strong acids and strong bases.
• Salts that produce basic solutions are those
  formed from weak acids and strong bases.
• Salts that produce acidic solutions are those
  formed from strong acids and weak bases.

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AcidBase Properties of Salts

• Calculate the pH of a 0.15 M solution of sodium
  acetate (CH3COONa). What is the percent
  hydrolysis?
• Calculate the pH of a 0.24 M sodium formate
  solution (HCOONa).

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AcidBase Properties of Salts

• Metal Ion Hydrolysis

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• Calculate the pH of a 0.020 M Al(NO3)3 solution.
• What is the pH of a 0.050 M AlCl3 solution?
• Predict whether the following solutions will be
  acidic, basic, or nearly neutral
• (a) NH4I (b) CaCl2 (c) KCN (d) Fe(NO3)3