Chapter 16: Aqueous Ionic Equilibria

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Chapter 16 Aqueous Ionic Equilibria

• Common Ion Effect
• Buffer Solutions
• Titrations
• Solubility
• Precipitation
• Complex Ion Equilibria

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Solubility Equilibria

• All ionic compounds dissolve in water to some
  degree
• We can apply the concepts of equilibrium to salts
  that dissolve to a small extent (insoluble)
• We can use the equilibrium constant for the
  process to measure relative solubilities in water

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Solubility Product

• Equilibrium constant for the dissociation of a
  solid salt into its aqueous ions solubility
  product, Ksp
• For an ionic solid MnXm, the dissociation
  reaction is
• MnXm(s) ? nMm(aq) mXn-(aq)
• The solubility product would be
• Ksp MmnXn-m

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Molar Solubility

• Solubility is the amount of solute that will
  dissolve in a given amount of solution at a
  particular temperature (in grams or moles)
• The molar solubility is the number of moles of
  solute that will dissolve in a liter of solution.
• The molarity of the dissolved solute in a
  saturated solution.

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Calculating Ksp from Solubility 1. Copper (I)
bromide has a measured solubility of 2.0 x 10-4 M
at 25C. Calculate Ksp. 2. Calculate Ksp for
bismuth sulfide (Bi2S3) which has a solubility of
1.0 x 10-15
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Calculating Solubility from Ksp The Ksp for
copper (II) hydroxide is 1.6 x 10-19 at 25C.
Calculate the solubility, Cu2, OH-.
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Ksp and Relative Solubility

• Molar solubility is related to Ksp
• However, you cannot always compare solubilities
  of compounds by comparing their Ksps
• In order to compare Ksps, the compounds must
  have the same dissociation stoichiometry

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The Effect of Common Ion on Solubility

• Addition of a soluble salt that contains one of
  the ions of the insoluble salt, decreases the
  solubility of the insoluble salt
• For example, addition of NaCl to the solubility
  equilibrium of solid PbCl2 decreases the
  solubility of PbCl2
• PbCl2(s) ? Pb2(aq) 2 Cl-(aq)

addition of Cl- shifts the equilibrium to the left
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1. Calculate the solubility of Ag2CrO4 in pure water
   (Ksp 9.0 x 10-12)
2. Calculate the solubility of Ag2CrO4 in a 0.1M
   solution of AgNO3

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The Effect of pH on Solubility

• For insoluble ionic hydroxides, the higher the pH
  (and the greater OH-, the lower the solubility
  of the ionic hydroxide
• OH- acts as a common ion,
• M(OH)n(s) ? Mn(aq) nOH-(aq)
• For insoluble ionic compounds that contain anions
  of weak acids, the lower the pH, the higher the
  solubility
• M2(CO3)n(s) ? 2 Mn(aq) nCO32-(aq)
• H3O(aq) CO32- (aq) ? HCO3- (aq) H2O(l)

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Precipitation

• Precipitation will occur when the concentrations
  of the ions exceed the solubility of the ionic
  compound
• If we compare the reaction quotient, Q, for the
  current solution concentrations to the value of
  Ksp, we can determine if precipitation will occur
• Q Ksp, the solution is saturated, no
  precipitation
• Q lt Ksp, the solution is unsaturated, no
  precipitation
• Q gt Ksp, the solution would be above saturation,
  the salt above saturation will precipitate
• Some solutions with Q gt Ksp will not precipitate
  unless disturbed these are called
  supersaturated solutions

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a supersaturated solution will precipitate if a
seed crystal is added
precipitation occurs if Q gt Ksp
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Selective Precipitation

• A solution containing several different cations
  can often be separated by addition of a solution
  that will form an insoluble salt with one of the
  ions, but not the others

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If 10.0 mL of 0.50 M Pb(NO3)2 and 20.0 mL 1.0 M
NaI are mixed, will a precipitate form?
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Chapter 18Electrochemistry
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Redox Reaction

• Elements change oxidation number
• e.g., single displacement, and combustion, some
  synthesis and decomposition
• Oxidation--oxidation number increases
• Reduction--oxidation number decreases
• Both must occur in a reaction--two half reactions
• oxidizing agent is reactant molecule that causes
  oxidation
• contains element reduced
• reducing agent is reactant molecule that causes
  reduction
• contains the element oxidized

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Rules for Assigning Oxidation States

• Free elements have an oxidation state 0
• Monatomic ions have an oxidation state equal to
  their charge.
• The sum of the oxidation states of all the atoms
  in a compound is 0.
• The sum of the oxidation states of all the atoms
  in a polyatomic ion equals the charge on the ion.
• The oxidation number of fluorine is always -1 in
  compounds with other elements.

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Rules for Assigning Oxidation States

• 6. Chlorine, bromine and iodine always have
  oxidation numbers of -1 except when bonded to O
  or F.
• 7. The oxidation number of oxygen is almost
  always -2 the oxidation number of hydrogen is
  almost always 1.
• Exceptions
• --When oxygen is in the form of a peroxide
  (O22-), the oxidation number is -1.
• --When hydrogen forms a binary compound with a
  metal, the oxidation number is -1 and the
  compound is called a hydride.

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Oxidation and Reduction

• Oxidation occurs when an atoms oxidation state
  increases during a reaction
• Reduction occurs when an atoms oxidation state
  decreases during a reaction

Reducing agent
Oxidizing agent
CH4 2 O2 ? CO2 2 H2O -4 1
0 4 2 1 -2
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Identify the element that is oxidized and the
element that is reduced in each of the following
reactions. What is the oxidizing and the
reducing agent in each reaction?

• 3 H2S 2 NO3 2 H 3 S 2 NO 4 H2O
• MnO2 4 HBr MnBr2 Br2 2 H2O

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Common Oxidizing Agents
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Common Reducing Agents
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Balancing Redox Reactions

• 1. Assign oxidation numbers
• --determine element oxidized and element reduced
• 2. Separate the reaction into oxidation and
  reduction half- reactions.
• 3. Balance half-reactions by mass
• a. First balance elements other than H and O
• b. Balance O using H2O
• Balance H using H
• 4. Balance each half-reaction by charge by
  adding electrons to the reactants side of the
  reduction and the product side of the oxidation.
• 5. Multiply half-reactions by integers to make
  electrons the same in both half-reactions
• Add half-reactions and cancel the electrons to
  produce a balanced equation.
• For reactions that occur in acidic solutions,
  skip to step 9.
• For reactions that occur in basic solutions, add
  the same of OH- as H to both sides of the
  equation.
• 9. Check that reaction is balanced for mass and
  charge.

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