Reactions in Aqueous Solution

1
Chapter 4

• Reactions in Aqueous Solution

2
Part I

• Solutions and Concentration

3
Solutions

• Solute
• Substance that gets dissolved
• Ex. the sugar in a sugar-water solution
• Solvent
• Substance in which something is dissolved
• Ex. the water in a sugar-water solution
• Aqueous solution a solution in which water is
  the solvent

4
Concentration Terms

• Dilute small amount of solute dissolved in
  solvent
• Concentrated large amount of solute dissolved
  in solvent
• Unsaturated more solute could be dissolved in
  solvent
• Saturated maximum amount of solute has been
  dissolved in solvent
• Supersaturated more than the maximum amount
  of solute has been dissolved in solvent
• Usually achieved by heating solution, adding more
  solute then letting solution cool

5
Calculating Concentration

• Molarity (mol solute/L solution)
• Brackets are used to indicate molar
  concentrations.
• NaCl means concentration (molarity) of sodium
  chloride

6
Dilution

• When a solution is diluted, the number of moles
  of solute does not change. Only more solvent is
  added.
• M mol/L
• Rearranges to mol M x L
• Diluting changes M and L, but their product is
  the same
• McLc MdLd
• c concentrated, d dilute
• How many liters of a 12 M solution should be
  diluted to produce 850 mL of a 1.5 M solution?
• (12 M)Lc(1.5 M)(0.85 L)
• Lc 0.11 L or 110 mL

7
Electrolytes and Nonelectrolytes

• Nonelectrolyte does not conduct electricity
  when dissolved
• Electrolyte does conduct electricity when
  dissolved
• Strong electrolyte dissociates almost
  completely into ions when dissolved, conducts
  electricity very well
• Weak electrolyte dissociates much less, weakly
  conducts electricity
• Salts, acids, bases are electrolytes

8
Concentration of Ions in Soln

• Ionic compounds contain two or more ions.
• Consider the number of the ion of interest.
• For example
• Al2(SO4)3
• There are 2 Al3 ions in this compound and 3
  SO42- ions.
• 1.5 M Al2(SO4)3
• Al3 3.0 M
• SO42- 4.5 M
• You can think of this as a dissolution reaction
  (which it is)
• Al2(SO4)3 (s) ? 2 Al3 (aq) 3 SO42- (aq)
• Use the coefficient ratio to determine
  concentration of ions

9
Part II

• Precipitation Reactions

10
Precipitation Reactions

• Precipitate
• Insoluble solid resulting from mixing two aqueous
  solutions
• Qualitative analysis
• Used to determine whether specific ions are
  present in solution

11
Solubility Rules

• Soluble
• Group 1, NH4 cmpds
• NO3-, C2H3O2-,ClO3- , ClO4- cmpds
• Cl-, Br-, I- cmpds
• except w/ Ag, Hg22, Pb2
• SO4 cmpds
• except w/ Ba2, Sr2, Pb2, Ca2, Ag, Hg22

• Insoluble
• CO32-, PO43- cmpds
• except w/ Group 1, NH4
• S2- cmpds
• except w/ Group 1, NH4, Ba2
• OH-, O2- cmpds
• except w/ Group 1, Ba2

12
Net Ionic Equations

• Write complete formula for all compounds
• Dissociate (separate into ions) all soluble cmpds
• Eliminate spectator ions
• Example
• Sodium carbonate is mixed with calcium chloride
  to form calcium carbonate and sodium chloride
• Na2CO3 CaCl2 ? CaCO3 2 NaCl
• 2 Na CO32- Ca2 2 Cl- ? CaCO3 2 Na 2
  Cl-
• Ca2 CO32- ? CaCO3

13
Gravimetric Analysis

• Precipitate filtered, dried, weighed
• Mass of component (ion) determined

14
Example

• It is suspected that the paint from an old house
  contains lead. A 3.110 g sample of dried paint
  is dissolved in acid, and a solution of sodium
  chloride is added. The precipitate is filtered,
  dried, and weighed. The mass of the precipitate
  is 0.0565 g. What is the percent of Pb in the
  paint?

15
Solution

• Write equation for precipitation
• Pb2 2 Cl- ? PbCl2
• Determine the mass of lead in the precipitate
• (0.0565 g PbCl2)(1 mol PbCl2/278.2 g PbCl2)(1 mol
  Pb/1 mol PbCl2)(207.2 g Pb/1 mol Pb) 0.0421 g
  Pb
• Determine the Pb in original sample
• (0.0421 g Pb/3.110 g paint) x 100 1.35

16
Part III

• Acids and Bases

17
Arrhenius Acids and Bases

• Arrhenius acid produces hydrogen ions (protons)
  in solution
• Hydrogen ions
• Bond to water to become hydronium ions H3O
• Arrhenius base produces hydroxide ions in
  solution

18
Strong Acids and Bases

• Dissociate 100
• Strong acids
• HNO3
• H2SO4
• HClO4
• HCl
• HBr
• HI

• Strong Bases
• NaOH
• KOH
• Ca(OH)2
• Ba(OH)2

19
Weak Acids and Bases

• Partially dissociate
• Strengths vary
• Examples
• Weak acid
• HC2H3O2
• Weak base
• NH3

20
Neutralization Reactions

• Reactions btwn acid base
• Total equation shows complete formula for each
  reactant and product
• HNO2 KOH ? KNO2 H2O
• Ionic equation strong acids/bases and soluble
  salts written as ions
• HNO2 K OH- ? K NO2- H2O
• Net ionic equation spectator ions (ions not
  changed in reaction) cancel and are not shown
• HNO2 OH- ? NO2- H2O

21
Volumetric Analysis

• Base added to acid gradually via buret
• Standard soln
• Known concentration
• Used to determine unknown concentration of other
  soln
• Primary standard concentration doesnt change
• Indicator used to visualize end point
• Different colors in acidic and basic soln

22
Equivalence Point vs. End Point

• Equivalence point point at which just enough
  base has been added to neutralize all of the acid
• End point point at which indicator changes
  color
• Usually this is about a drop beyond the
  equivalence point

23
Titration Calculations

• If 24.7 mL of 0.157 M NaOH is required to
  neutralize a 15.0 mL solution HCl, what is the
  HCl concentration?
• NaOH HCl ? H2O NaCl
• M mol/L ? 0.157 M NaOH x mol/0.0247L
• x 3.87 x 10-3 mol NaOH
• (3.87 x 10-3 mol NaOH)(1 mol HCl/1 mol NaOH)
  3.87 x 10-3 mol HCl
• M mol/L ? x M HCl 3.87 x 10-3 mol/0.015 L
  0.259 M HCl

24
Part IV

• Oxidation-Reduction Reactions

25
Oxidation-Reduction Reactions

• Also known as redox rxns
• Electrons transferred from one reactant to
  another (change in oxidation state)
• Types of redox reactions
• Combination/synthesis rxns
• Some decomposition rxns
• Single displacement rxns

26
Oxidation States

• Element in standard state 0
• Monatomic ion charge
• O -2 (except peroxides, O -1)
• H 1 (except hydrides H -1)
• Sum of ox. states in neutral cmpd 0
• Sum of ox. states in polyatomic ion charge
• In binary covalent cmpds, element closest to top
  right of periodic table has negative charge

27
Calculating Oxidation States

• H2SO3
• O -2
• H 1
• Sum for cmpd must 0
• 2(1) S 3(-2) 0
• S 4
• K2Cr2O7
• Monatomic ion charge
• K 1
• O -2
• Sum for polyatomic ion must charge
• 2Cr 7(-2) -2
• Cr 6

28
Oxidation vs. Reduction

• Oxidation loss of electrons, increase in
  oxidation number
• Reduction gain of electrons, decrease in
  oxidation number
• Oxidizing agent substance that causes something
  else to be oxidized
• Reducing agent substance that causes something
  else to be reduced

29
Example

• Cu 2 AgNO3 ? Cu(NO3)2 2 Ag
• Cu is oxidized.
• Ox. state changes from 0 to 2
• Cu is the reducing agent.
• It causes the Ag to be reduced.
• Ag is reduced.
• Ox. state changes from 1 to 0
• AgNO3 is the oxidizing agent.
• It causes the Cu to be oxidized.
• (Usually, an element is oxidized or reduced. The
  entire substance is the oxidizing or reducing
  agent.)

0
1
5
-2
-2
5
2
0
Ox. States
30
Balancing Redox Rxns

• Separate into oxidation and reduction half rxns.
• Balance all elements except O and H.
• Balance O with H2O in acid or with OH- in base.
• Balance H with H in acid or with H2O in base.
• Balance charge with electrons.
• Multiply half rxns by whole numbers so that the
  number of electrons in both half rxns is equal.
• Add half rxns and simplify if necessary.

31
Redox Rxn in Acid

• Pb2 NO3- ? PbO2 HNO2
• 2 5
  -2 4 2 1 3 -2

• Oxidation
• Pb2 ? PbO2
• Pb2 ? PbO2
• Pb2 2 H2O? PbO2
• Pb2 2 H2O ? PbO2 4 H
• Pb2 2 H2O ? PbO2 4 H 2 e-
• Pb2 2 H2O ? PbO2 4 H 2 e-

• Reduction
• NO3- ? HNO2
• NO3- ? HNO2
• NO3- ? HNO2 H2O
• NO3- 3 H ? HNO2 H2O
• NO3- 3 H 2 e- ? HNO2 H2O
• NO3- 3 H 2 e- ? HNO2 H2O

• Pb2 H2O NO3- ? PbO2 H HNO2

32
Redox Rxn in Base

• Mn(OH)2 SiO32- ? MnO2 Si
• 2 -2 1
  4 -2 4 2 0

• Oxidation
• Mn(OH)2 ? MnO2
• Mn(OH)2 ? MnO2
• Mn(OH)2 ? MnO2
• Mn(OH)2 2 OH- ? MnO2 2 H2O
• Mn(OH)2 2 OH- ? MnO2 2 H2O 2 e-
• 2 Mn(OH)2 4 OH- ? 2 MnO2 4 H2O 4 e-

• Reduction
• SiO32- ? Si
• SiO32- ? Si
• SiO32- ? Si 3 OH-
• SiO32- 3 H2O ? Si 6 OH-
• SiO32- 3 H2O 4 e- ? Si 6 OH-
• SiO32- 3 H2O 4 e- ? Si 6 OH-

• 2 Mn(OH)2 SiO32- ? 2 MnO2 H2O Si 2 OH-