Part I: Solubility equilibria and factors affecting solubility

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Part I Solubility equilibria and factors
affecting solubility

• Monday, July 23rd
• CHM 102

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Solubility Equilibria

• Weve covered general equilibrium problems and
  acid-base equilibria.
• Now were moving into solubility equilibria.
• This deals with the dissolution and precipitation
  of ionic compounds in solution.
• Some natural examples of thisKidney stones
  (precipitation of salts)Coke dissolves tooth
  enamel!

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Solubility Equilibria and Ksp

• When a salt dissolves, it completely ionizes,
  resulting in an equation of the general form
• AB(s) A(aq) B-(aq)
• Because the solid is not included in the
  equilibrium expression (just as pure liquids,
  solvents, etc arent included), then the
  equilibrium expression becomes the product of the
  two dissociated ions
• Ksp AB-

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Solubility Equilibria

• While in CHM 101 we covered general qualitative
  rules for solubility, now were covering the way
  to figure things out quantitatively!
• Whereas before we would say nitrate salts are all
  soluble, now we could potentially put numbers to
  each salt to compare their solubilities more
  accurately!

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Solubility Equilibria

• Lets go back to the general form of a
  dissociation of a salt in water
• AB(s) A(aq) B-(aq)
• We can use the same conventions we used before
  for equilibrium constants and Kas/Kbs to figure
  out if the dissolution of a particular salt is
  really product-favored (soluble), or
  reactant-favored (insoluble).Kgtgt1 product
  favored (soluble)Kltlt1 reactant favored
  (relatively insoluble)

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Practice Solubility Equilibria

• Q1 What is the solubility equilibrium
  expression for the dissolution of barium
  phosphate in water?
• Q2 If the Ksp for the following salts at 25oC
  are AgCl 1.810-10
• AgBr 5.010-13
• AgI 8.310-17
• Given saturated solutions of each salt at 298 K,
  which solution would contain the highest
  concentration of silver ion?

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Practice Solubility Equilibria

• An aqueous solution of Be(OH)2 is made up with
  excess Be(OH)2 at the bottom of the flask, and
  was found to have a pOH of 8.9. What is the Ksp
  for beryllium hydroxide?

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Practice Solubility Equilibria

• Whats the concentration of silver ions in a
  saturated solution of AgBr at 25oC, if the Ksp of
  AgBr is 5.010-13 at 25oC?

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Factors that affect solubility

• Think back, whats a major factor that affects
  solubility that weve gone over?
• There are three more factors affecting solubility
  that were going to cover
• Presence of a common ion
• pH of the solution
• Presence of a complexing agent

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Common Ion

• The solubility of a salt decreases when another
  solute sharing a common ion is present.
• Weve already done similar calculations, but to
  refresh Consider a trying to dissolve CaF2 in a
  solution already containing NaF.
• Because the solution already contains F, less
  CaF2 will dissolve than if the solution had been
  made in just DI water!

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pH and solubility

• pH affects the solubility of some salts more than
  others!
• Consider the dissolution of M(OH) in water
• M(OH)(aq) M(aq) OH-(aq)
• Now heres where thinking back to Le Chatliers
  principle will help things along!
• According to LCs principle, what would happen if
  at equilibrium, we removed some of the OH- from
  solution?
• How could we remove some OH-?

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pH and solubility

• Consider the dissolution of Ba(OH)2 in water
• Ba(OH)2(aq) Ba2(aq)
  2OH-(aq)
• Lets think of a saturated solution of barium
  hydroxide (weve got some solid left
  undissolved).
• What will happen if we add a few drops of
  concentrated HCl to the solution?

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Practice pH and solubility

• Predict the effect of lowering the pH on the
  dissolution of the following solid salts
• Ni(OH)2
• Ca(CO3)
• NaCl
• BaF2

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Complexing and solubility

• Presence of a complexing agent can increase the
  solubility of a salt.
• Think of the reactions
• AgCl(s) Ag(s) Cl-(aq)
• NH3(aq) Ag(aq) Ag(NH3)2(aq)
• The ammonia can be thought of as a complexing
  agent. It complexes the silver ions that
  dissolve!
• When the silver ions become complexed, were
  removing product (think LCs principle) and the
  silver chloride will dissolve more (shift right).

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Part II Oxidation-ReductionReactions

• Monday, July 23rd
• CHM 102

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Electrochemistry

• This is the study between electricity and
  chemical reactions!
• When you sleep, youre constantly doing redox
  chemistry and never realized it! (cell
  respiration and NADH)
• When you drive here in the morning, youre using
  a redox reaction!
• Anyone wearing plated jewelry? Youre wearing
  the product of electrochemistry!

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Oxidation-Reduction Reactions

• When we look at electrochemistry reactions, well
  be looking at redox reactions.
• In redox reactions, well see changes in
  oxidation state and a transfer of electrons!
• Zn(s) 2H(aq) ? Zn2(aq) H2(g)

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Summing up what we just said

• Oxidizing agents cause another substance to be
  oxidized, are reduced themselves, and gain
  electrons in a redox reaction.
• Reducing agents cause another substance to be
  oxidized, are oxidized themselves, and lose
  electrons in an oxidation reduction reaction.
• Zn(s) 2H(aq) ? Zn2(aq) H2(g)

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Red without the ox reactions?

• Wherever you have one substance oxidized, one has
  to be reduced.
• You will always see one with the other!
• 2H2(g) O2(g) ? 2H2O(g)

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Practice finding oxidation states

• FeCl3
• FeCO3
• AlPO4
• Al(s)
• H2(g)
• Cr2O72-(aq)
• CrO42-(aq)

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Redox Reactions

• Cd(s) NiO2(s) 2H2O(l) ? Cd(OH)2(s)
  Ni(OH)2(aq)
• MnO4-(aq) C2O42-(aq) ? Mn2 2CO2 (unbal.)
• thermite