11

ELECTROCHEMISTRYELECTROCHEMISTRYChapter 20Chapter 20

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TRANSFER REACTIONSTRANSFER REACTIONS

Atom/Group transferAtom/Group transfer

HCl + HHCl + H22O ---> ClO ---> Cl-- + H + H33OO++

Electron transferElectron transfer

Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> Cu(aq) ---> Cu2+2+(aq) + 2 Ag(s) (aq) + 2 Ag(s)

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Electron Transfer ReactionsElectron Transfer ReactionsElectron Transfer ReactionsElectron Transfer Reactions

• Electron transfer reactions are Electron transfer reactions are oxidation-reductionoxidation-reduction or or

redoxredox reactions. reactions.

• Redox reactions can result in the generation of an electric Redox reactions can result in the generation of an electric

current or be caused by imposing an electric current. current or be caused by imposing an electric current.

• Therefore, this field of chemistry is often called Therefore, this field of chemistry is often called

ELECTROCHEMISTRY.ELECTROCHEMISTRY.

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Why Study Electrochemistry?Why Study Electrochemistry?Why Study Electrochemistry?Why Study Electrochemistry?

• BatteriesBatteries

• CorrosionCorrosion

• Industrial Industrial production of production of chemicalschemicals such as such as Cl Cl22, NaOH, F, NaOH, F22 and Al and Al

• Biological redox Biological redox reactionsreactions

The heme groupThe heme group

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Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions

Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions

• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.

• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.

• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.

• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.

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Review of Cheesy Ways to Review of Cheesy Ways to remember these termsremember these terms

Review of Cheesy Ways to Review of Cheesy Ways to remember these termsremember these terms

• OIL RIGOIL RIG

• (Oxidation Is Losing, Reduction Is (Oxidation Is Losing, Reduction Is Gaining)Gaining)

• LEO (the lion) goes GERLEO (the lion) goes GER

• Losing Electrons Oxidation, Gaining Losing Electrons Oxidation, Gaining Electrons ReductionElectrons Reduction

• OLEOLE--

• Oxidation Losing ElectronsOxidation Losing Electrons

• OIL RIGOIL RIG

• (Oxidation Is Losing, Reduction Is (Oxidation Is Losing, Reduction Is Gaining)Gaining)

• LEO (the lion) goes GERLEO (the lion) goes GER

• Losing Electrons Oxidation, Gaining Losing Electrons Oxidation, Gaining Electrons ReductionElectrons Reduction

• OLEOLE--

• Oxidation Losing ElectronsOxidation Losing Electrons

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Oxidation Haiku!Oxidation Haiku!

• Lost an electron

•But now feeling positive

•Oxidized is cool!

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OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Direct Redox Direct Redox ReactionReaction

Oxidizing and Oxidizing and reducing reducing agents in agents in

direct direct contact.contact.

Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

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Copper + Silver IonCopper + Silver Ion

1010

OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Indirect Redox ReactionIndirect Redox Reaction

A battery functions by transferring electrons A battery functions by transferring electrons through an external wire from the reducing through an external wire from the reducing

agent to the oxidizing agent.agent to the oxidizing agent.

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Electrochemical CellsElectrochemical CellsElectrochemical CellsElectrochemical Cells• An apparatus that allows a redox An apparatus that allows a redox

reaction to occur by transferring reaction to occur by transferring electrons through an external electrons through an external connector.connector.

• Product favored reaction ---> Product favored reaction ---> voltaic voltaic or galvanic cellor galvanic cell ----> electric current ----> electric current

• Reactant favored reaction ---> Reactant favored reaction ---> electrolytic cellelectrolytic cell ---> electric current ---> electric current used to cause chemical change.used to cause chemical change.

• This is important to distinguish these This is important to distinguish these types due to subtle differences!types due to subtle differences!

Batteries are voltaic Batteries are voltaic cellscells

1212Balancing Equations Balancing Equations for Redox Reactionsfor Redox Reactions

Some redox reactions have equations that must be balanced Some redox reactions have equations that must be balanced by special techniques.by special techniques.

MnOMnO44-- + 5 Fe + 5 Fe2+2+ + 8 H + 8 H++ ---> Mn---> Mn2+ 2+ + 5 Fe+ 5 Fe3+3+ + 4 H + 4 H22OO

• This is all review from Chapter 5, so you don’t need any This is all review from Chapter 5, so you don’t need any review…. Right?review…. Right?

Mn = +7Mn = +7 Fe = +2Fe = +2Fe = +3Fe = +3Mn = +2Mn = +2

1313Fine, lets look at Fine, lets look at some examples!some examples!

Consider the Consider the reduction of reduction of AgAg++ ions with ions with copper metal.copper metal.

Cu + AgCu + Ag++ --give--> Cu --give--> Cu2+2+ + Ag + Ag

1414Balancing Balancing EquationsEquations

Step 1:Step 1: Divide the reaction into half-reactions, one Divide the reaction into half-reactions, one for oxidation and the other for reduction.for oxidation and the other for reduction.

OxOx Cu ---> CuCu ---> Cu2+2+

RedRed Ag Ag++ ---> Ag ---> Ag

Step 2:Step 2: Balance each for mass. Already done in this Balance each for mass. Already done in this case.case.

Step 3:Step 3: Balance each half-reaction for charge by Balance each half-reaction for charge by adding electrons.adding electrons.

OxOx Cu ---> Cu Cu ---> Cu2+2+ + + 2e-2e-

RedRed Ag Ag++ + + e- e- ---> Ag---> Ag

1515Balancing Balancing EquationsEquations

Step 4:Step 4: Multiply each half-reaction by a factor so that Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the reducing agent supplies as many electrons as the oxidizing agent requires.the oxidizing agent requires.

Reducing agentReducing agent Cu ---> Cu Cu ---> Cu2+2+ + 2e- + 2e-

Oxidizing agentOxidizing agent 22 Ag Ag++ + + 22 e- ---> e- ---> 22 Ag Ag

Step 5:Step 5: Add half-reactions to give the overall equation.Add half-reactions to give the overall equation.

Cu + 2 AgCu + 2 Ag++ ---> Cu ---> Cu2+2+ + 2Ag + 2Ag

The equation is now balanced for both The equation is now balanced for both charge and mass.charge and mass.

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ReductionReduction of VO of VO22++ with Zn with Zn

1717Balancing Balancing EquationsEquations

Balance the following in acid solution—Balance the following in acid solution—

VOVO22++ + Zn ---> VO + Zn ---> VO2+ 2+ + Zn+ Zn2+2+

Step 1:Step 1: Write the half-reactionsWrite the half-reactions

OxOx Zn ---> ZnZn ---> Zn2+2+

RedRed VOVO22++ ---> VO ---> VO2+2+

Step 2:Step 2: Balance each half-reaction for mass.Balance each half-reaction for mass.

OxOx Zn ---> ZnZn ---> Zn2+2+

RedRedVOVO22

++ ---> VO ---> VO2+2+ + + HH22OO2 H2 H++ ++

Add HAdd H22O on O-deficient side and add HO on O-deficient side and add H++

on other side for H-balance.on other side for H-balance.

1818Balancing Balancing EquationsEquations

Step 3:Step 3: Balance half-reactions for charge.Balance half-reactions for charge.

OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e- 2e-

RedRed e- e- + 2 H+ 2 H++ + VO + VO22++ ---> VO ---> VO2+2+ + H + H22OO

Step 4:Step 4: Multiply by an appropriate factor.Multiply by an appropriate factor.

OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e-2e-

RedRed 22e-e- + + 44 H H++ + + 22 VO VO22++ ---> ---> 22 VO VO2+2+ + + 22

HH22OO

Step 5:Step 5: Add Add balancedbalanced half-reactions half-reactions

Zn + 4 HZn + 4 H++ + 2 VO + 2 VO22++ ---> Zn ---> Zn2+2+ + 2 VO + 2 VO2+2+ + 2 H + 2 H22OO

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Tips on Balancing Tips on Balancing EquationsEquations

• Never add ONever add O22, O atoms, or , O atoms, or OO2-2- to balance oxygen. to balance oxygen.

• Never add HNever add H22 or H atoms to or H atoms to balance hydrogen.balance hydrogen.

• Be sure to write the correct Be sure to write the correct charges on all the ions.charges on all the ions.

• Check your work at the end Check your work at the end to make sure mass and to make sure mass and charge are balanced.charge are balanced.

• PRACTICE!PRACTICE!

2020

Lets Look at A Demo!

• The Hydrolysis of Water!!!