Equilibria Involving Acids

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• Chapter 18
• Equilibria Involving Acids bases

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ARRHENIUS THEORY for ACIDS and BASES

• ACIDS produce hydrogen ions (protons), H, in
  solution
• BASES produce hydroxide ions, OH-,in solution,
  
• NEUTRALIZATION H OH- ?? H2O
• Problems with Arrhenius Theory
• H3O Hydronium ion rather than H
• OH(H2O)3- present in solution, not OH-
• Other substances also have acidic or basic
  properties

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H surrounded by four H-bonded H2O molecules
H9O4
4
OH- surrounded by three H-bonded H2O molecules
OH(H2O)3-
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Bronsted-Lowery Theory of Acids and Bases

• Acid any substance donating a proton, H
• Base any substance accepting a proton
• Conjugate Acid-Base Pairs
• e.g. HF NH3 ??? NH4 F-
• acid 1 base 2 acid 2
  base 1
• AMPHOTERIC substances have both acidic and basic
  properties.
• Mono-, di-, tri-,. to polyprotic acids.
• Acidic versus nonacidic H atoms in compounds.

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For each of the following reactions, identify the
acid, the base, the conjugate base, and the
conjugate acid

• H2O H2O ?? H3O OH-
• H2PO4- H2PO4- ?? H3PO4 HPO42-
• H2SO4 H2O ?? H3O HSO4-
• H2PO4- H2O ?? H3PO4 OH-
• CO2 2H2O ?? HCO3- H3O
• H2PO4- H2O ?? HPO42- H3O
• Fe(H2O)63 H2O ?? Fe(H2O)5(OH) 2 H3O
• HCN CO32-?? CN- HCO3-

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Graphic representations of strong and weak acid
equilibria
Strong Acid 100 Dissociation into ions

• HA(aq) H2O(l) ?? H3O(aq) A- (aq)
• Equilibrium Favors undissociated acid

Weak Acid Very little Dissociation into ions
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Acid strength versus conjugate base strength
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Bronsted-Lowery Theory Acid and Base Strengths

• Proton transfers occur from a
• Strong acid to a strong base
• e.g. HCl NaOH ? H2O NaCl
• Weak acid to strong base
• e.g. CH3COOH NaOH ? ??
• Weak base to a stronger base
• e.g. HSO41- HSO31- ?
  ???
• Will a Reaction occur between..
• a. HS1- and F 1- ?? b. HCl
  and ClO2 1- ??
• c. HCl and ClO4 1- ?? d. HCl and
  HNO3??

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Relative strengths of some Bronsted-Lowry acids
and their conjugate bases

• Acid Base
• Strongest HClO4 ClO4- Weakest
• Acids H2SO4 HSO4- bases
• HI I-
• HBr Br-
• HCl Cl-
• HNO3 NO3-
• H3O H2O
• HSO4- SO42-
• H2SO3 HSO3-
• H3PO4 H2PO4-
• HNO2 NO2-
• HF F-
• CH3CO2H CH3CO2-
• H2CO3 HCO3-
• H2S HS-
• NH4 NH3
• HCN CN-
• HCO3- CO32-

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Bronsted-Lowery Theory Acid and Base Strengths

• LEVELING EFFECT of SOLVENTS
• The strongest acid in a solvent is the conjugate
  acid of the solvent. The strongest base is the
  conjugate base.
• Acid H3O in water
• Base OH- in water

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Autoionization of Water and the pH Scale


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Autoionization of Water

• H2O(l) H2O(l) ??? H3O(aq) OH-(aq)
• Kw H3O OH- 1.0 x 10-14 at 25oC
• At equilibrium
• H3O OH- 1.0 x 10-7
• Kw changes with temperature
• but H3O OH-

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pH and pOH SCALES

• pH - log H3O
• pOH - log OH-

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pH, pOH CALCULATIONS

• pH - log H3O
• pOH - log OH-
• Kw H3O OH- 1.0 x 10-14
• so pH3O pOH- 14.00
• or pH 14.00 pOH
• CALULATE SOME pH and pOH VALUES

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• H pH
• 10-14 14 1 M NaOH
• 10-13 13
• Basic 10-12 12 Ammonia 10-11 11
• 10-10 10
• 10-9 9
• 10-8 8
• Neutral 10-7 7 Pure Water
• 10-6 6
• 10-5 5
• 10-4 4
• 10-3 3
• Acidic 10-2 2
• 10-1 1
• 1 0

(Household Cleaner)
Blood
Milk
Vinegar
Lemon juice
Stomach acid
1 M HCl
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• Calculate the pH of each solution
• a. H 1.4 x 10-3 M e. OH- 8 x 10-11 M
• b. H 2.5 x 10-10 M f. OH- 5.0 M
• c,. H 6.1 M g. pOH 10.5
• d. OH- 3.5 x 10-2 M h. pOH 2.3
• Calculate H and OH- for each solution
• a. pH 7.41 (the normal pH of blood)
• b. pH 15.3
• c. pH -1.0 e. pOH 5.0
• d. pH 3.2 f. pOH 9.6
• How many significant figures are there in the
  numbers 10.78, 6.78, 0.78? If these were pH
  values, to how many significant figures can you
  express the H? Explain any discrepancies
  between your answers to the two questions.

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• Values of Kw as a function of temperature are as
  follows
• Temp (oC) Kw
• 0 1.14 x 10-15
• 25 1.00 x 10-14
• 35 2.09 X 10-14
• 40 2.92 x 10-14
• 50 5.47 x 10-14
• a. Is the autoionization of water exothermic or
  endothermic?
• b. What is the pH of pure water at 50oC?

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• Values of Kw as a function of temperature are as
  follows
• Temp (oC) Kw
• 0 1.14 x 10-15
• 25 1.00 x 10-14
• 35 2.09 X 10-14
• 40 2.92 x 10-14
• 50 5.47 x 10-14
• a. Is the autoionization of water exothermic or
  endothermic?
• b. What is the pH of pure water at 50oC?
• c. Restate your answers to water at 50oC. Which
  of the three criteria for neutrality is most
  general?
• d. From a plot of ln(Kw) versus 1/T (using the
  Kelvin scale), estimate Kw at 37oC, normal
  physiological temperature.
• e. What is the pH of a neutral solution at 37oC?

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-28







Y -9.2338 6870.6x R2 0.999
-29
-30
ln Kw
-31
-32
-33
-34
-35
0.0028
0.0030
0.0032
0.0034
0.0036
1/T
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pH MEASUREMENT

• Indicators colored weak acids and bases
• pH Meters Glass membrane with a voltage
  (potential) difference across the glass.
• pH and BODY CHEMISTRY
• Normal pH 7.3 to 7.5
• Acidosis pH lt 7.3
• Alkalosis pH gt 7.45
• Body Chemistry is buffered with bicarbonates
  (HCO3-) dihydrogenphosphates (H2PO4-) and
  proteins which help to maintain a constant pH

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WEAK ACIDSIONIZATION CONSTANTS

• HA(aq) H2O(l) ??? H3O(aq) A- (aq)
• Ka values at 25oC are known and tabulated for a
  large number of weak acids.

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Graphic representations of strong and weak acid
equilibria
Strong Acid 100 Dissociation into ions

• HA(aq) H2O(l) ?? H3O(aq) A- (aq)
• Equilibrium Favors undissociated acid

Weak Acid Very little Dissociation into ions
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Values of Ka for Some Common Monoprotic Acids

• Formula Name Value of Ka
• HSO4- Hydrogen sulfate ion 1.2 x 10-2
• HClO2 Chlorous acid 1.2 x 10-2
• HC2H2ClO2 Monochloroacetic acid 1.35 x 10-3
• HF Hydrofluoric acid 7.2 x 10-4
• HNO2 Nitrous acid 4.0 x 10-4
• HC2H3O2 Acetic acid 1.8 x 10-5
• Al(H2O)63 Hydrated aluminum (III) ion 1.4
  x10-5
• HOCl Hypochlorous acid 3.5 x 10-8
• HCN Hydrocyanic acid 6.2 x 10-10
• NH 4 Ammonium ion 5.6 x 10-10
• HOC6H5 Phenol 1.6 x 10-10
• The units of Ka are mol/L, but are customarily
  omitted.

Increasing acid strength
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Write the dissociation reaction and the
corresponding equilibrium expression for each of
the following acids in water.

• a. H3PO4
• b. H2PO41-
• c. HCO31-
• d. HCN
• e. Glycine, H2NCH2COOH
• f. Acetic acid, CH3COOH (HC2H3O2)
• g. Phenol, C6H5OH
• h. Benzoic acid, C6H5COOH

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• Write the reaction and the corresponding Kb
  equilibrium expression for each of the following
  substances acting as bases in water.
• a. PO43- g. Glycine, NH2CH2COOH
• b. HPO42- h. Ethylamine, CH3CH2NH2
• c. H2PO4- I. Aniline, C6H5NH2
• d. NH3 j. Dimethylamine, (CH3)2NH
• e. CN-
• f. Pyridine, C5H5N

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WEAK ACID CALCULATIONS

• HA(aq) H2O(l) ??? H3O(aq) A-(aq)
• 2H2O(l) ??? H3O(aq) OH-(aq)
• To simplify calculations, if ionization is lt
  5, then CHA ? HA OR HA CHA H3O,
• Set up pH equilibria calculations in tables as in
  previous equilibria problems.

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Solving Weak Acid Equilibrium Problems

1. List the major species in the solution
2. Choose the species that can produce H, and write
   balanced equations for the reactions producing H
3. Using the values of the equilibrium constants for
   the reactions you have written, decide which
   equilibrium will dominate in producing H
4. Write the equilibrium expression for the dominant
   equilibrium.
5. List the initial concentrations of the species
   participating in the dominant equilibrium.
6. Define the change needed to achieve equilibrium
   that is, define x..
7. Write the equilibrium concentrations in terms of
   x.
8. Substitute the equilibrium concentrations into
   the equilibrium expression.
9. Solve for x the easy way that is , by assuming
   that HA0-x ? HA0.
10. Use the 5 rule to verify whether the
    approximation is valid.

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• Ka Problems
• For trichlorophenol (HC6H2Cl3O),
  Ka 1 x 10-6, Calculate the concentrations
  of all species and the pH of a 0.05 M solution of
  trichlorophenol in water.
• A solution is prepared by dissolving 0.56 g
  benzoic acid (C6H5CO2H), Ka 6.4 x 10-5) in
  enough water to make 1.0 L of solution.
  Calculate C6H5CO2H. C6H5CO2-, H, OH-,
  and the pH in this solution.
• Calculate the pH of a solution containing a
  mixture of 0.050 M HNO3 and 0.50M HC2H3O2.

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WEAK BASES IONIZATION CONSTANTS

• B(aq) H2O(l) ??? BH (aq) OH- (aq)
• Kb values at 25oC are tabulated or may be
  calculated from Kw and Ka
• Kw (Ka)(Kb) so Kb Kw/Ka
• Where Ka is the conjugate acid constant

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• Kb Problems
• Thallium (Tl) hydroxide is a strong base used in
  the synthesis of some organic compounds.
  Calculate the pH of a solution containing 2.48 g
  TlOH per liter.
• For the reaction of hydrazine (N2H4) in water.
• H2NNH2 H2O ??? H2NNH3 OH-
• Kb is 3.0 x 10-6. Calculate the concentrations
  of all species and the pH of a 2.0 M solution of
  hydrazine in water.

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Two Weak Acids in Solution

• A solution of 0.100 M HClO, Ka 3.5 x 10-8 and
  0.100 M Formic acid, HCO2H, Ka 1.8 x 10-4 are
  mixed in equal proportions. What is the
  resulting solution pH?

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Polyprotic Acid Equilibria

• What is the pH of a solution 0.100 M sulfurous
  acid, H2SO3, Ka1 1.5 x 10-2 and Ka2
  1.0 x 10-7?

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ACID-BASE PROPERTIES OF THE OXIDES (PART I)
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ACID BASE PROPERTIES OF THE OXIDES (PART II)
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ACID-BASE PROPERTIES OF THE OXIDES (PART III)
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ACID BASE PROPERTIES OF THE OXIDES (PART IV)
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ACID-BASE PROPERTIES OF THE OXIDES (PART V)
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ACID BASE PROPERTIES OF THE OXIDES PART (VI)
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ACID BASE PROPERTIES OF THE OXIDES PART (VII)
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ACID-BASE PROPERTIES OF THE OXIDES PART (VIII)
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BRONSTED-LOWRY THEORY

• OXIDES, HYDROXIDES, ANHYDRIDES
• Acid, base reactions
• Dehydrations (formation of anhydrides)
• Hydration of oxides

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ACID STRENGTHSof WEAK ACIDS

• Oxoacids Ka ? up as the central atom
• oxidation state ? up.
• Ka ? up as central atom of same oxidation state
  moves left to right in the periodic table.
• Ka ? up as the central atom of the same
  oxidation state moves UP in the same Group or
  family.
• Polyprotic acids
• Ka decreases by approximately 105 for each
  successive H ionized.
• Binary acids (only H and another element)
• Within a period, Ka ? up as electronegativity
  of the other element.
• Within a group, Ka ? up going down the group
  to higher mass and larger size.

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• SnO2 ? H2O ?? ?
• ?HCrO4- ?? ?Cr2O72- ?
• ?HMnO4- ?? ? Mn (VI) compound ?

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HYDROLOYSIS OF IONIC SALTS

• The pH of each type salt in solution depends on
  the Ka or Kb of the hydrolyzing ion(s).

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• Salt Derived Ions Undergoing
• From Hydrolysis pH Examples
• Strong base Neither Neutral NaCl, KNO3,
• strong acid pH 7 BaCl2, CaBr2
• Strong base Anion Basic LiCN, KNO2, CaF2
• weak acid pH gt 7 NaCH3CO2
• Weak base Cation Acidic, NH4Cl, Al(NO3)3,
• strong acid pH lt 7 (CH3)3NHBr
• Weak base, Both Acidic NH4NO2
• weak acid if Kblt Ka
• Neutral NH4CH3CO2
• if Kb Ka
• basic NH4CN

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• Arrange the following 0.10 M solutions in order
  from most acidic to most basic
• KOH, KBr, KCN, NH4Br, NH4CN, HCN
• Given that the Ka value for acetic acid is 1.8 x
  10-5 and the Ka value for hypochlorous acid is 3
  x 10-8, which is the stronger base, OCl- or
  C2H3O2-

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• Acid-Base Equilibria
• What is the pH of a solution of 0.150 M sodium
  nitrite, NaNO2?
• HNO2, Ka 4.0 x 10-4.
• What is the pH of a solution of 0.150 M
  hydrazinnium chloride, H2NNH3?
• H2NNH3, Kb 3.0 x 10-6

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• Calculate the pH of each of the following
  solutions.
• a. 0.10 M CH3NH3Cl
• b. 0.050 M NaCN
• c. 0.20 M Na2CO3 (consider only the reaction )
• CO32- H2O ?? HCO3- OH-
• Sodium azid (NaN3) is sometimes added to water to
  kill bacteria. Calculate the concentration of
  all species in a 0.010 M solution of NaN3. The
  Ka value for hydrazoic acid (HN3) is 1.9 x 10-5.

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LEWIS THEORYACIDS and BASES

• Lewis base - an electron pair donor
• Lewis acid - an electron pair acceptor
• Lewis acid Lewis base ??? Adduct
• (coordination compound)
• e.g. Cu2(aq) 4CN-(aq) ?? Cu(CN)42-(aq)
• Look at Lewis Dot structures for lone pairs.