oxidation & reduction reactions chapter 6 chemistry: the molecular nature of matter, 6 th ...

Oxidation & Reduction Reactions

CHAPTER 6

Chemistry: The Molecular Nature of Matter, 6th editionBy Jesperson, Brady, & Hyslop

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CHAPTER 6: Oxidation & Reduction

Learning Objectives

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

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

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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 metalsMg(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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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

Oxidizing agent

Reducing agent

Cu(s) + 2NO3–(aq) + 4H+(aq) Cu2+(aq) + 2NO2(g) + 2H2O

0 +5 +2 +4


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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 acid3Cu(s) + 8HNO3(dil, aq) 3Cu(NO3)2(aq) + 2NO(g) + 4H2O

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Focus on Sulfuric Acid


Hot, Concentrated H2SO4

– Becomes potent oxidizer– SO2 is product– Partial reduction of S (+6 to +4)– SO4

2– + 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)– SO4

2– + 10H+ + 8e– H2S(g) + 4H2O Ex: 4Zn + 5H2SO4(hot, conc.) 4ZnSO4 + H2S + 4H2O


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Acids reacting with metal – Special case of more general phenomena:

Single Replacement Reaction• Reaction where one element replaces another• A + BC → AC + B1. 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

Reduction Oxidation Single Replacement Reaction


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• 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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• 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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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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• 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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• 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 substancesCombustion

– 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 productse.g. CH4(g) + 2O2(g) CO2(g) + 2H2O2C8H18(g) + 25O2(g) 16CO2(g) + 18H2O


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Reduction Oxidation

2. Incomplete Combustion – Not enough O2 a. Limited O2 supply

• CO(g) is the carbon product2CH4(g) + 3O2(g) 2CO(g) + 4H2O

b. Very limited O2

• C(s) is the carbon productCH4(g) + O2(g) C(s) + 2H2O• Gives tiny black particles• Soot—lamp black• Component of air pollution

Oxygen as an Oxidizing Agent: Combustion


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Reduction Oxidation

3. Combustion of Organics containing O– Still produce CO2 and H2O– Need less added O2

C12H22O11(s) + 12O2(g) 12CO2(g) + 11H2O4. Combustion of Organics containing S

– Produce SO2 as product2C4H9SH + 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

Oxygen as an Oxidizing Agent: Combustion


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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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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 equation8H+(aq) + Cr2O7

2–(aq) + 3SO32–(aq)

3SO42–(aq) + 2Cr3+(aq) + 4H2O

• Then do calculations1. g K2Cr2O7 moles K2Cr2O7 moles Cr2O7

2–(aq) 2. moles Cr2O7

2–(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)

moles Cr2O72–(aq) moles 3SO3

2–(aq)

moles SO32–(aq) moles Na2SO3 g Na2SO3

722

272

722722

722 OCrK mol 1OCr 1mol

OCrK g 294.2OCrK mol 1OCrK g 12.4

272OCr mol 0.0421

2

72

232

72 OCr mol 1SO mol 3OCr mol 0.0421 2

3SO mol 0.126

3232

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3223 SONa mol 1

SONa g 126.1SO mol 1

SONa mol 1SO mol 0.126

32SONa g 9.15

oxidation & reduction reactions chapter 6 chemistry: the molecular nature of matter, 6 th ...

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