Oxidation & Reduction
Reactions
CHAPTER 6
Chemistry: The Molecular Nature of Matter, 6th edition
By Jesperson, Brady, & Hyslop
CHAPTER 6: Oxidation & Reduction
Learning Objectives
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Define oxidation and reduction
Identify redox reactions
Recognize oxidation numbers
Balancing redox reactions
 Acidic conditions
 Basic conditions
Acids as oxidizing agents
Oxygen as oxidizing agents
Single replacement reactions
Navigate the activity series
Practice Redox Stoichiometry
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Reduction
Oxidation
Acids as Oxidizing Agents
• Metals often react with acid
– Form metal ions and
– Molecular hydrogen gas
Molecular equation
Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)
Net ionic equation
Zn(s) + 2H+(aq) H2(g) + Zn2+(aq)
• Zn is oxidized
• H+ is reduced
• H+ is the oxidizing reagent
• Zn is the reducing reagent
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Reduction
Oxidation
Acids as Oxidizing Agents: Metals
• Ease of oxidation process depends on metal
– Metals that react with HCl or H2SO4
• Easily oxidized by H+
• More active than hydrogen (H2)
e.g. Mg, Zn, alkali metals
Mg(s) + 2H+(aq)  Mg2+(aq) + H2(g)
2Na(s) + 2H+(aq)  2Na+(aq) + H2(g)
– Metals that don’t react with HCl or H2SO4
• Not oxidized by H+
• Less active than H2
e.g. Cu, Pt
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Reduction
Oxidation
Acids as Oxidizing Agents:
Anions Determine Oxidizing Power
Acids are divided into two classes:
Nonoxidizing Acids
– Anion is weaker oxidizing agent than H3O+
– Only redox reaction is
• 2H+ + 2e–  H2 or
• 2H3O+ + 2e–  H2 + 2H2O
– HCl(aq), HBr(aq), HI(aq)
– H3PO4(aq)
– Cold, dilute H2SO4(aq)
– Most organic acids (e.g., HC2H3O2)
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Reduction
Oxidation
Acids as Oxidizing Agents
• Anion is stronger oxidizing agent than H3O+
– Used to react metals that are less active than H2
– No H2 gas formed
– HNO3(aq)
• Concentrated
• Dilute
• Very dilute, with strong reducing agent
– H2SO4(aq)
• Hot, conc’d, with strong reducing agent
• Hot, concentrated
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Reduction
Oxidation
Acids as Oxidizing Agents:
Focus on Nitrate Ion
Concentrated HNO3
– NO3– more powerful oxidizing agent than H+
– NO2 is product
– Partial reduction of N (+5 to +4)
– NO3–(aq) + 2H+(aq) + e–  NO2(g) + H2O
oxidation
reduction
0
+5
+2
+4
Cu(s) + 2NO3–(aq) + 4H+(aq)  Cu2+(aq) + 2NO2(g) + 2H2O
Reducing Oxidizing
agent
agent
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Reduction
Oxidation
Acids as Oxidizing Agents:
Focus on Nitrate Ion
Dilute HNO3
– NO3– is more powerful oxidizing agent than H+
– NO is product
– Partial reduction of N (+5 to +2)
– NO3–(aq) + 4H+(aq) + 3e –  NO(g) + 2H2O
– Used to react metals that are less active than H2
Reaction of copper with dilute nitric acid
3Cu(s) + 8HNO3(dil, aq)  3Cu(NO3)2(aq) + 2NO(g) + 4H2O
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Reduction
Oxidation
Acids as Oxidizing Agents:
Focus on Sulfuric Acid
Hot, Concentrated H2SO4
– Becomes potent oxidizer
– SO2 is product
– Partial reduction of S (+6 to +4)
– SO42– + 4H+ + 2e–  SO2(g) + 2H2O
Ex: Cu + 2H2SO4(hot, conc.)  CuSO4 + SO2 + 2H2O
Hot, concentrated with strong reducing agent
– H2S is product
– Complete reduction of S (+6 to –2)
– SO42– + 10H+ + 8e–  H2S(g) + 4H2O
Ex: 4Zn + 5H2SO4(hot, conc.)  4ZnSO4 + H2S + 4H2O
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Reduction
Oxidation
Single Replacement Reaction
Acids reacting with metal
– Special case of more general phenomena:
Single Replacement Reaction
• Reaction where one element replaces another
• A + BC → AC + B
1. Metal A can replace metal B
– If A is more active metal, or
2. Nonmetal A can replace nonmetal C
– If A is more active than C
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Reduction
Oxidation
Single Replacement Reaction
• Left = Zn(s) + CuSO4(aq)
• Center = Cu2+(aq) reduced to Cu(s)
Zn(s) oxidized to Zn2+(aq)
• Right = Cu(s) plated out on Zn bar
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
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Reduction
Oxidation
Single Replacement Reaction
• Zn2+ ions take place of Cu2+
ions in solution
• Cu atoms take place of Zn
atoms in solid
– Cu2+ oxidizes Zn0 to Zn2+
– Zn0 reduces Cu2+ to Cu0
• More active Zn0 replaces less
active Cu2+
• Zn0 is easier to oxidize!
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Reduction
Oxidation
Activity Series of Metals
• Cu less active, can't replace Zn2+
– Can't reduce Zn2+
– Cu(s) + Zn2+(aq)  No reaction
• General phenomenon
– Metal that is more easily oxidized will displace one that is
less easily oxidized from its compounds
Activity Series (Table 6.3)
– Metals at bottom more easily oxidized (more active) than
those at top
– This means that given metal ion will be displaced from its
compounds by any metal below it in table
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Reduction
Oxidation
Activity Series of Metals
2H+(aq) + Sr(s)  Sr2+(aq) + H2(g)
– H+ oxidizes Sr0 to Sr2+
– Sr0 reduces H+ to H2
– More active Sr0 replaces less active H+
– Sr0 is easier to oxidize!
H2(g) + Sr2+(aq)  NO REACTION!
• Why?
– H2 less active, can't replace Sr2+
– Can't reduce Sr2+
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Reduction
Oxidation
Activity Series of Metals
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Reduction
Oxidation
Activity Series of Metals
• Metals at very bottom of table
– Very strong reducing agents
– Very easily oxidized
– Na down to Cs
• Alkali and alkaline earth metals
– React with H2O as well as H+
2Na(s) + 2H2O  H2(g) + 2NaOH(aq)
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Reduction
Oxidation
Activity Series of Metals
• Ag – slight reaction (top of activity series)
– 2HCl(aq) + Ag(s)  2AgCl(aq) + H2(g)
• Zn – somewhat reactive (middle of activity series)
– 2HCl(aq) + Zn(s)  ZnCl2(aq) + H2(g)
• Mg – very reactive (bottom of activity series)
– 2HCl(aq) + Mg(s)  MgCl2(aq) + H2(g)
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Reduction
Oxidation
Activity Series of Metals:
Predicting Reactions
If M is below H
• Can displace H from solutions containing H+
2H+(aq)  H2(g)
If M is above H
• Doesn't react with nonoxidizing acids
– HCl, H3PO4, etc.
In general
• Metal below replaces ion above
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Reduction
Oxidation
Activity Series of Metals:
Predicting Reactions
• Predictive tool for determining outcome of single
replacement reactions
• Given a metal (M ) and the ion of a different metal
(M 'n+)
– Look at chart and draw arrow from M to M 'n+
– Arrow that points up from bottom left to
top right will occur
– Arrow that points down from top left to bottom
right will NOT occur
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Reduction
Oxidation
Oxygen as an Oxidizing Agent:
Combustion
Oxygen reacts with many substances
Combustion
– Rapid reaction of substance with oxygen that
gives off both heat and light
– Hydrocarbons are important fuels
– Products depend on how much O2 is available
1. Complete Combustion
– O2 plentiful
– CO2 and H2O products
e.g. CH4(g) + 2O2(g)  CO2(g) + 2H2O
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O
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Reduction
Oxidation
Oxygen as an Oxidizing Agent:
Combustion
2. Incomplete Combustion
– Not enough O2
a. Limited O2 supply
• CO(g) is the carbon product
2CH4(g) + 3O2(g)  2CO(g) + 4H2O
b. Very limited O2
• C(s) is the carbon product
CH4(g) + O2(g)  C(s) + 2H2O
• Gives tiny black particles
• Soot—lamp black
• Component of air pollution
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Reduction
Oxidation
Oxygen as an Oxidizing Agent:
Combustion
3. Combustion of Organics containing O
– Still produce CO2 and H2O
– Need less added O2
C12H22O11(s) + 12O2(g)  12CO2(g) + 11H2O
4. Combustion of Organics containing S
– Produce SO2 as product
2C4H9SH + 15O2(g)  8CO2(g) + 10H2O + 2SO2(g)
– SO2 turns into acid rain when mixed with water
• SO2 oxidized to SO3
• SO3 reacts with H2O to form H2SO4
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Reduction
Oxidation
Oxygen as an Oxidizing Agent:
Corrosion
Corrosion
– Direct reaction of metals with O2
– Many metals corrode or tarnish when exposed to O2
Ex.
2Mg(s)+ O2(g)  2MgO(s)
4Al(s) + 3O2(g)  2Al2O3(s)
4Fe(s) + 3O2(g)  2Fe2O3(s)
4Ag(s) + O2(g)  2Ag2O(s)
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Reduction
Oxidation
Oxygen as an Oxidizing Agent:
Reacting with Nonmetals
• Many nonmetals react directly with O2 to form
nonmetal oxides
• Sulfur reacts with O2
– Forms SO2
S(s) + O2(g)  2SO2(g)
• Nitrogen reacts with O2
– Forms various oxides
– NO, NO2, N2O, N2O3, N2O4, and N2O5
– Dinitrogen oxide, N2O
• Laughing gas used by dentists
• Propellant in canned whipped cream
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Reduction
Oxidation
Example of Redox Stoichiometry
Same as other stoichiometry problems, except
balancing the equation is more complicated.
Ex. How many grams of Na2SO3 (126.1 g/mol) are needed to
completely react with 12.4 g of K2Cr2O7 (294.2 g/mol)?
• First need balanced redox equation
8H+(aq) + Cr2O72–(aq) + 3SO32–(aq) 
3SO42–(aq) + 2Cr3+(aq) + 4H2O
• Then do calculations
1. g K2Cr2O7  moles K2Cr2O7  moles Cr2O72–(aq)
2. moles Cr2O72–(aq)  moles 3SO42–(aq)
3. moles SO32–(aq)  moles Na2SO3  g Na2SO3
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Reduction
Oxidation
Example of Redox Stoichiometry
grams K2Cr2O7  moles K2Cr2O7  moles Cr2O72–(aq)
1 mol K 2 Cr2 O 7
1mol Cr2 O 27 
12.4 g K 2 Cr2 O 7 

294.2 g K 2 Cr2 O 7 1 mol K 2 Cr2 O 7
 0.0421 mol Cr2 O27 
moles Cr2O72–(aq)  moles 3SO32–(aq)
0.0421 mol Cr2 O 27  
3 mol SO 23 
1 mol Cr2 O 27 
 0.126 mol SO 23 
moles SO32–(aq)  moles Na2SO3  g Na2SO3
0.126 mol SO 23 
Jesperson, Brady, Hyslop. Chemistry:
The Molecular Nature of Matter, 6E

1 mol Na2 SO 3
1 mol SO 23 
126.1 g Na2 SO 3

1 mol Na2 SO 3
 15.9 g Na2 SO 3
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