Acid-Base Equilibria and Solubility Equilibria

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Acid-Base Equilibria and Solubility Equilibria

• Chapter 16
• 16.1-16.9

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Acid-Base Equilibria and Solubility Equilibria

• We will continue to study acid-base reactions
• Buffers
• Titrations
• We will also look at properties of slightly
  soluble compunds and their ions in solution

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Homogeneous vs. Heterogeneous

• Homogeneous Solution Equilibria- a solution that
  has the same composition throughout after
  equilibrium has been reached.
• Heterogeneous Solution Equilibria- a solution
  that after equilibrium has been reached, results
  in components in more than one phase.

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The Common Ion Effect

• Acid-Base Solutions
• Common Ion
• The Common Ion Effect- the shift in equilibrium
  caused by the addition of a compound having an
  ion in common with the dissolved substance.
• pH

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The Common Ion Effect
Consider mixture of CH3COONa (strong electrolyte)
and CH3COOH (weak acid).
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Henderson-Hasselbach Equation

• Relationship between pKa and Ka.

pKa -log Ka
Henderson-Hasselbalch equation
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pH Calculations

• What is the pH of a solution containing 0.30 M
  HCOOH and 0.52 M HCOOK?

HCOOH pKa 3.77
Mixture of weak acid and conjugate base!
0.30
0.00
0.52
-x
x
x
0.30 - x
x
0.52 x
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pH Calculations
Common ion effect
0.30 x ? 0.30
0.52 x ? 0.52
4.01
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Buffer Solutions

• A buffer solution is a solution of
• A weak acid or a weak base and
• The salt of the weak acid or weak base
• Both must be present!

A buffer solution has the ability to resist
changes in pH upon the addition of small amounts
of either acid or base.
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Buffer Solutions

• IV solutions (7.4)
• Blood (7.4)
• Gastric Juice (1.5)

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Buffer Solutions
Add strong acid
Add strong base
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Buffer Solutions
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Effectiveness of Buffer Solutions
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Preparing a Buffer Solution with a Specific pH

• Work Backwards
• Choose a weak acid whose pKa is close to the
  desired pH
• Substitute pKa and pH values into the
  Henderson-Hasselbach Equation
• This will give a ratio of conjugate Base/Acid
• Convert ratio to molar quantities

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Acid-Base Titrations

• In a titration a solution of accurately known
  concentration is added gradually added to another
  solution of unknown concentration until the
  chemical reaction between the two solutions is
  complete.
• Types of Titrations
• Those involving a strong acid and a strong base
• Those involving a weak acid and a strong base
• Those involving a strong acid and a weak base

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Acid-Base Titrations

• Indicator substance that changes color at (or
  near) the equivalence point

Equivalence point the point at which the
reaction is complete
Slowly add base to unknown acid UNTIL
The indicator changes color (pink)
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Acid-Base Titrations
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Strong Acid-Strong Base Titrations

• Strong Acid-Strong Base Titrations

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Strong Acid-Base Titrations
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Strong Acid-Base Titrations

• Calculate the pH after the addition of 10.0mL of
  0.100M NaOH to 25.0mL of 0.100M HCl.
• moles NaOH
• 10.0mL x (0.100mol NaOH/ 1L NaOH) x (1L/ 1000mL)
• 1.00 x 10-3 mol
• moles HCl
• 25.0mL x (0.100mol HCl/ 1 L HCl) x (1L/ 1000mL)
• 2.50 x 10-3 mol
• Amount of HCl left after partial neutralization
• 2.50 x 10-3 mol - 1.00 x 10-3 mol 1.5 x x 10-3
  mol

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Strong Acid-Base Titrations

• Thus, H 1.5 x x 10-3 mol/ 0.035L
• H 0.0429 M
• pH -log H
• pH -log 0.0429
• pH 1.37

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Strong Acid-Base Titrations

• Calculate pH after the addition of 35.0mL of
  0.100M NaOH to 25.0mL of 0.100M HCl.
• moles NaOH
• 0.100 mol NaOH / 0.035 L NaOH
• 3.50 x 10-3 mol
• moles HCl
• 0.100 mol HCl / 0.025 L HCl
• 2.50 x 10-3 mol
• Amount of NaOH left after full HCl neutralization
• 3.50 x 10-3 mol 2.50 x 10-3 mol 1.0 x x 10-3
  mol

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Strong Acid-Base Titrations

• Thus, NaOH 1.0 x x 10-3 mol/ 0.06L
• NaOH 0.0167 M
• OH- 0.0167 M
• pOH -log H
• pOH -log 0.0167
• pOH 1.78
• pH 14.0-pOH
• pH 14.0-1.78
• pH 12.22

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Weak Acid-Strong Base Titrations

• Weak Acid-Strong Base Titrations

At equivalence point (pH gt 7)
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Weak Acid-Strong Base Titrations
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Strong Acid-Weak Base Titrations

• Strong Acid-Weak Base Titrations

At equivalence point (pH lt 7)
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Strong Acid-Weak Base Titrations
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Acid-Base Indicators

• Equivalence point occurs when OH- H originally
  present.
• Indicators
• End Point- Occurs when indicator changes color
• End point Equivalence point

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Acid-Base Indicators
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Acid-Base Indicators
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Acid-Base Indicators
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Solubility Equilibria

• Reactions that produce precipitates
• Importance
• Tooth Enamel Acid tooth decay
• Barium Sulfate used in x-rays
• Fudge!!!

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Solubility Equilibria

• Solubility Product Constant (Ksp)- the product of
  the molar concentrations of the constituent ions,
  each raised to the power of its stoichiometric
  coefficient in the equilibrium equation.

Ksp AgCl-
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Solubility Products

• What are the correct solubility products of the
  following equations?

Ksp Mg2F-2
Ksp Ag2CO32-
Ksp Ca23PO33-2
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Solubility Product Constants
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Solubility Constants

• What does a large Ksp mean?
• What does a small value mean?

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Molar Solubility and Solubility

• Molar solubility (mol/L)- is the number of moles
  of solute dissolved in 1 L of a saturated
  solution.

Solubility (g/L)- is the number of grams of
solute dissolved in 1 L of a saturated solution.
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Molar Solubility and Solubility
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Molar Solubility and Solubility

• The Ksp of silver bromide is 7.7 x 10-13.
  Calculate the molar solubility.
• AgBr(s) ? Ag(aq) Br-(aq)
• Initial (M) 0 0
• Change (M) -s s s
• Equilibrium (M) s s
• Ksp AgBr-
• 7.7 x 10-13 AgBr-
• 7.7 x 10-13 s2
• S 8.8 x 10-7 M

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Ksp vs. Q

• Dissolution of an ionic solid in aqueous
  solution

Q lt Ksp
Unsaturated solution
No precipitate
Q Ksp
Saturated solution
Q gt Ksp
Supersaturated solution
Precipitate will form
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Predicting Precipitation Reactions

• Calculate Q for the reaction
• Is Q larger, smaller or equal to Ksp?

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Separation of Ions by Fractional Precipitation

• Removal of ions from solution
• Useful in preparation of prescription medications
• Ions can be removed by filtration

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Fractional Precipitation

• Ions proper reagent
• Smallest ? Largest Ksp
• If AgNo3 is added to a solution containing Cl-,
  Br- and I- ions, which compound will precipitate
  out first?
• Compound Ksp
• AgCl 1.6 x 10 -10
• AgBr 1.7 x 10 -13
• AgI 8.3 x 10 -17

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The Common Ion Effect and Solubility
Ksp 7.7 x 10-13
s2 Ksp
s 8.8 x 10-7
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The Common Ion Effect and Solubility
Br- 0.0010 M
Ag s
Br- 0.0010 s ? 0.0010
Ksp 0.0010 x s
s 7.7 x 10-10
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pH and Solubility

• The solubility of many substances depends on the
  pH of the solution.
• ? pH
• ? pH
• Insoluble bases dissolve in acidic solutions
• Insoluble acids dissolve in basic solutions

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