Lecture 15 Chemical Reactions

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Lecture 15 - Chemical Reactions
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Precipitation Reactions

• Will the reaction proceed?
• Product must be less soluble than the reagents
• see section 4.4 for solubility rules

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Things are soluble if

• The cation is Li, Na, K, Rb, Cs, NH4
• The anion is Cl-, Br-, I- (except Ag, Hg22 and
  Pb2 compounds)
• The anion is NO3-, ClO4-, CH3COO-

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Uses of precipitation reactions

• 1. determination of Ag in solution
• pitfall
• AgCl(s) Ag(aq) Cl-(aq)
• Ksp Ag Cl- 1.6 x 10-10

1.6 x 10-10
i.e. Ag
Cl-
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Uses of precipitation reactions

• 2. determination of Ra2 in solution

radioactive decay
238U
226Ra
this is also radioactive
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Cant measure Ra radiation directly!
Ra2
Ra2
Ra2
Ra2
Ra2
Ra2
Ra2
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but RaSO4 is only slightly soluble...
RaSO4(s) Ra2(aq) SO4-2(aq) Ksp
Ra2(aq) SO4-2(aq) 2.0 x 10-11 thus,
Ra2(aq) 2.0 x 10-11
SO4-2(aq)
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• suppose a water sample has
• Ra2 1 x 10-13 mol/L
• and suppose we add H2SO4 until
• SO4-2 1 mol/L
• then Ra2(aq) 2 x 10-11 / 1
• 2 x 10-11 mol/L
• i.e. the Ra stays in solution!

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234Th4(aq) (radioactive)
BaCl2(aq) H2SO4(aq)
Ra2
Ra2
Ra2
Ra2
Ra2
Ra2
Ra2
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• Ba2(aq) SO4-2(aq) BaSO4(s) (lots!)
• AND
• Ra2(aq) SO4-2(aq) RaSO4(s)
• AND
• Th4(aq) 2 SO4-2(aq) Th(SO4)2(s)

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Ba(Ra)(Th)SO4(s)
Th radiation
Ra radiation
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• 1. Measure Ra radiation
• 2. Measure Th radiation indication of how
  complete the precipitation was

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e.g. Uranium Processing

• U ore
• UO22(aq)
• (NH4)2U2O7(s)

acid

NH4OH

U concentrate for shipping
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e.g. production of Polyvinyl Chloride (PVC)

• ethylene
• vinyl chloride
• PVC

chlorine

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wheres the Cl2 from?

• brine (sea water)
• Cl2 NaOH

electrolysis
but brine is a solution of many salts!
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Mg2(aq) and Ca2(aq) interfere with electrolysis!

• anion Ksp of Mg salt
• Cl- 735 (v. soluble)
• OH- 1 x 10-11
• CO3- 1 x 10-5
• SO4-2 752

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Mg2(aq) and Ca2(aq) interfere with electrolysis!

• anion Ksp of Ca salt
• Cl- 1210
• OH- 6 x 10-6
• CO3- 5 x 10-9
• SO4-2 2 x 10-5

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thus, add OH- and CO3-2
NaOH(aq)
Na2CO3(aq)
brine
NaCl(aq)


Mg(OH)2(s)
CaCO3(s)
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2. Acid Base Reactions

• stay tuned!
• in general, acid base salt water

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3. Oxidation Reduction Reactions

• REDOX, for short
• a transfer of electrons takes place
• one substance gets REDuced, the other gets
  OXidized
• cant have one without the other!

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e.g. Na H2O

• overall,
• Na(s) H2O Na(aq) OH-(aq) ½ H2(g)

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e.g. Na H2O

• actually,
• Na(s) Na(aq) e- (Na gets oxidized)
• H2O e- OH-(aq) ½ H2(g) (H2O gets reduced)

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e.g. dissolving copper

• Cu(s) HNO3(aq) Cu2(aq) N2O4(g)
• Cu(s) Cu2 2 e- (copper gets oxidized)
• 2 NO3-(aq) 4 H(aq) 2 e- N2O4(g) 2 H2O
  (nitrate gets reduced)