11Electron Transfer Reactions: CH Electron Transfer Reactions: CH 19: 19: Oxidation-reduction or redox Oxidation-reduction or redox

reactionsreactions..

Electron Transfer Reactions: CH Electron Transfer Reactions: CH 19: 19: Oxidation-reduction or redox Oxidation-reduction or redox

reactionsreactions..

• Results in generation of an electric Results in generation of an electric

current (electricity) or caused by current (electricity) or caused by

imposing an electric current. imposing an electric current.

• In previous chapter energy was in form of In previous chapter energy was in form of

heat, here it is electricity.heat, here it is electricity.

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

called called ELECTROCHEMISTRY.ELECTROCHEMISTRY.

22

Terminology for Redox Terminology for Redox ReactionsReactions

Terminology for Redox Terminology for Redox ReactionsReactions

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

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

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

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

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

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

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

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

33You can’t have one… without the other!

• Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

• You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation

LEOLEO the lion says the lion says GERGER! Another way to ! Another way to

remember. remember.

GER!GER!

OIL RIGOIL RIG

44

Activity Series• For metals, the higher up the chart

the element is, the more likely it is to be oxidized. This is because metals like to lose electrons, and the more active a metallic element is, the more easily it can lose them.

• For nonmetals, the higher up the chart the element is, the more likely it is to be reduced. This is because nonmetals like to gain electrons, and the more active a nonmetallic element is, the more easily it can gain them.

55

Metal Activity

• Metallic elements start out with a charge of ZERO, so they can only be oxidized to form (+) ions.

• The higher of two metals MUST undergo oxidation in the reaction, or no reaction will happen.

• The reaction 3 K + FeCl3 3 KCl + Fe WILL happen, because K is being oxidized, and that is what the activity series says should happen.

• The reaction Fe + 3 KCl FeCl3 + 3 K will NOT happen.

3 K3 K00 + Fe + Fe+3+3ClCl-1-133

REACTIONREACTION

FeFe00 + 3 K + 3 K+1+1ClCl-1-1

NO REACTIONNO REACTION

66

Electrochemical CellsElectrochemical CellsElectrochemical CellsElectrochemical Cells• Allows a redox reaction to Allows a redox reaction to

occur by transferring e- occur by transferring e- through an external connector.through an external connector.

• Product favored reaction --> Product favored reaction --> voltaic or galvanic cellvoltaic or galvanic cell

Converts chemical energy from a Converts chemical energy from a spontaneous redox rxtn into spontaneous redox rxtn into electrical energy.electrical energy.

• Reactant favored reaction --> Reactant favored reaction --> electrolytic cellelectrolytic cell. .

Converts chem. energy to electrical Converts chem. energy to electrical energy using a nonspontaneous energy using a nonspontaneous redox rxtn & an electrical energy redox rxtn & an electrical energy source.source.

Batteries are voltaic Batteries are voltaic cellscells

77

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

•Zn is oxidized Zn is oxidized and is the reducing agent and is the reducing agent Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-•CuCu2+2+ is reduced is reduced and is the oxidizing agentand is the oxidizing agentCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)

88

•To obtain a useful To obtain a useful current, we separate the current, we separate the oxidizing and reducing oxidizing and reducing agents so that electron agents so that electron transfer occurs thru an transfer occurs thru an external wire. external wire.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

99

Anode: Anode: half cell where half cell where oxidation occursoxidation occurs

Cathode: Cathode: half cell half cell where reduction occurswhere reduction occurs

Basic Concepts of Electrochemical Basic Concepts of Electrochemical CellsCells

Electrons travel thru external wire.Electrons travel thru external wire.Salt bridge Salt bridge allows anions and cations to move between electrode allows anions and cations to move between electrode compartments, without mixing of metal atoms.compartments, without mixing of metal atoms.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

1010

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Notation: Notation: Anode I Anode product I Salt I Cathode ICathode productAnode I Anode product I Salt I Cathode ICathode productZn (s) I ZnZn (s) I Zn2+ 2+ (aq, 1 M) I NaNO(aq, 1 M) I NaNO33I CuI Cu2+2+ (aq, 1 M) I Cu (s) (aq, 1 M) I Cu (s)

Notation: Notation: Anode I Anode product I Salt I Cathode ICathode productAnode I Anode product I Salt I Cathode ICathode productZn (s) I ZnZn (s) I Zn2+ 2+ (aq, 1 M) I NaNO(aq, 1 M) I NaNO33I CuI Cu2+2+ (aq, 1 M) I Cu (s) (aq, 1 M) I Cu (s)

Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

<--Anions<--AnionsCations-->Cations-->

OxOxidationidationAnAnodeodeNegativeNegative

OxOxidationidationAnAnodeodeNegativeNegative

RedReductionuctionCatCathodehodePositivePositive

RedReductionuctionCatCathodehodePositivePositive

RED CATRED CATAn OXAn OX

1111

Terms Used for Voltaic Terms Used for Voltaic CellsCells

Electrode: Electrode: metal metal connected by connected by an external an external circuitcircuit

1212

CELL POTENTIAL,CELL POTENTIAL,EEcellcell EMF:electromotive EMF:electromotive

forceforce

• For Zn/Cu cell, For Zn/Cu cell, potentialpotential is is +1.10 V+1.10 V at 25 ˚C at 25 ˚C and when [Znand when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.

• This is the This is the STANDARD CELL STANDARD CELL POTENTIAL, EPOTENTIAL, Eoo

• ——a quantitative measure of the tendency of a quantitative measure of the tendency of reactants to proceed to products when all reactants to proceed to products when all are in their standard states at 25 ˚C. are in their standard states at 25 ˚C.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

1313

Calculating Cell VoltageCalculating Cell Voltage

• Balanced half-reactions can be added Balanced half-reactions can be added together to get overall, balanced together to get overall, balanced equation. equation.

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

If we know EIf we know Eoo for each half-reaction, we for each half-reaction, we could get Ecould get Eoo for net reaction. for net reaction.

1414TABLE OF STANDARD REDUCTION TABLE OF STANDARD REDUCTION POTENTIALSPOTENTIALS

the standard for measuring is SHE: Standard the standard for measuring is SHE: Standard Hydrogen ElectrodeHydrogen Electrode

TABLE OF STANDARD REDUCTION TABLE OF STANDARD REDUCTION POTENTIALSPOTENTIALS

the standard for measuring is SHE: Standard the standard for measuring is SHE: Standard Hydrogen ElectrodeHydrogen Electrode

2

Eo (V)

Cu2+ + 2e- Cu +0.34

2 H+ + 2e- H 0.00

Zn 2+ + 2e- Zn -0.76

oxidizingability of ion

reducing abilityof element

To determine an oxidation To determine an oxidation from a reduction table, just from a reduction table, just take the opposite sign of the take the opposite sign of the reduction!reduction!

1515Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell

Ox: Zn(s) ---> ZnOx: Zn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VRed:CuRed:Cu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 V------------------------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s) (aq) + Cu(s)

EEoo = +1.10 V = +1.10 V

Cathode, Cathode, positive, positive, sink for sink for electronselectrons

Anode, Anode, negative, negative, source of source of electronselectrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons ++

1616

1717

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Cd --> CdCd --> Cd2+2+ + 2e- + 2e-oror

CdCd2+2+ + 2e- --> Cd + 2e- --> Cd

Fe --> FeFe --> Fe2+2+ + 2e- + 2e-oror

FeFe2+2+ + 2e- --> Fe + 2e- --> Fe

EEoo for a Voltaic Cell for a Voltaic Cell

All ingredients are present. Which way does All ingredients are present. Which way does reaction proceed?reaction proceed?

1818

From the table, you see From the table, you see •• Fe is a better reducing Fe is a better reducing

agent than Cdagent than Cd•• CdCd2+2+ is a better is a better

oxidizing agent than oxidizing agent than FeFe2+2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

EEoo for a Voltaic Cell for a Voltaic Cell

Eo = Eo = Eo Eo reductionreduction (reduction process-cathode )– (reduction process-cathode )– Eo Eo reduction reduction (oxidation process- anode)(oxidation process- anode)Positive value indicates spontaneous process. Negative value indicates nonspontaneous.Positive value indicates spontaneous process. Negative value indicates nonspontaneous.

Steps for prediction spontaneity:Steps for prediction spontaneity:1.1.Write the reaction as oxidation and reduction half reactions.Write the reaction as oxidation and reduction half reactions.2.2.Plug standard reduction potentials in the equation given above.Plug standard reduction potentials in the equation given above.3.3.Check for the spontaneity by a positive or negative value.Check for the spontaneity by a positive or negative value.

1919More About More About Calculating Cell VoltageCalculating Cell Voltage

Assume IAssume I-- ion can reduce water. ion can reduce water.

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

Assuming reaction occurs as written, Assuming reaction occurs as written,

E˚ = E˚E˚ = E˚catcat+ E˚+ E˚anan= (-0.828 V) + (- +0.535 V) = = (-0.828 V) + (- +0.535 V) = -1.363 V-1.363 V

Minus E˚ means rxn. occurs in opposite Minus E˚ means rxn. occurs in opposite

directiondirection

(the connection is backwards or you are (the connection is backwards or you are

recharging the battery)recharging the battery)

2020

Charging a BatteryCharging a BatteryWhen you charge a battery, you are When you charge a battery, you are forcing the electrons backwards (from forcing the electrons backwards (from the + to the -). To do this, you will the + to the -). To do this, you will need a higher voltage backwards than need a higher voltage backwards than forwards. This is why the ammeter in forwards. This is why the ammeter in your car often goes slightly higher your car often goes slightly higher while your battery is charging, and then while your battery is charging, and then returns to normal.returns to normal.

In your car, the battery charger is In your car, the battery charger is called an alternator. If you have a called an alternator. If you have a dead battery, it could be the dead battery, it could be the battery needs to be replaced OR battery needs to be replaced OR the alternator is not charging the the alternator is not charging the battery properly.battery properly.

2121

Dry Cell BatteryDry Cell Battery

Anode (-)Anode (-)

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

Cathode (+)Cathode (+)

2 NH2 NH44++ + 2e- ---> + 2e- --->

2 2 NHNH33 + H + H22

2222

Alkaline BatteryAlkaline Battery

Nearly same reactions as Nearly same reactions as in common dry cell, but in common dry cell, but under basic conditions.under basic conditions.

Anode (-): Anode (-): Zn + 2 OHZn + 2 OH-- ---> ZnO + H ---> ZnO + H22O + 2e-O + 2e-

Cathode (+): Cathode (+): 2 MnO2 MnO22 + H + H22O + 2e- ---> O + 2e- --->

MnMn22OO33 + 2 OH + 2 OH--

2323

HH22 as a Fuel as a Fuel

Cars can use electricity generated by HCars can use electricity generated by H22/O/O22

fuel cells.fuel cells.HH22 carried in tanks or generated from carried in tanks or generated from

hydrocarbonshydrocarbons

2424Balance 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+

RedRed

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 H H22OO

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

VOVO22++ ---> VO ---> VO2+2+ + + HH22OO2 H2 H++ ++

Add HAdd H22O on O-O on O-

deficient side and add deficient side and add HH++ on other side for on other side for

H-balance.H-balance.