1 ELECTROCHEMICAL CELLS Chapter 20 : D8 C20. 21.2 Half-Cells and Cell Potentials > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights

11ELECTROCHEMICAL ELECTROCHEMICAL CELLSCELLS

Chapter 20 : D8 C20Chapter 20 : D8 C20

21.2 Half-Cells and Cell Potentials >21.2 Half-Cells and Cell Potentials >

2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Electrical PotentialElectrical Potential

Electrical Potential

What causes the electrical potential of an

electrochemical cell?

• The electrical potential of a voltaic cell is a measure of the cell’s ability

to produce an electric current.

• Electrical potential is usually measured in volts (V).

33

Electrons are transferred from Al to Cu2+, but there is no useful electric current. Energy

released as HEAT.

Electrochemical ReactionsElectrochemical Reactions

44If Al and Cu are separated work is done by the electrons. Voltmeter is

used to measure the energy.

What determines

the measured

value?

55

Terms Used for Voltaic Terms Used for Voltaic CellsCells

Figure 20.3Figure 20.3

66The Cu|CuThe Cu|Cu2+2+ and Ag|Ag and Ag|Ag++ Cell Cell




The potential of an isolated half-cell cannot be measured.

• You cannot measure the electrical potential of a zinc half-cell or of a copper half-cell separately.

• When these two half-cells are connected to form a voltaic cell,

however, the difference in potential can be measured.

88

CELL POTENTIAL, ECELL POTENTIAL, E

• For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when

[Zn2+] and [Cu2+] = 1.0 M.

Zn and Zn2+,anode

Cu and Cu2+,cathode

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons1.10 V1.10 V

1.0 M1.0 M 1.0 M1.0 M

99CELL POTENTIAL, ECELL POTENTIAL, E

STANDARD CELL POTENTIAL, Eo

is when [Zn2+] and [Cu2+] = 1.0 M.

For Zn/Cu cell, potential is +1.10 V at 25 ˚C

a quantitative measure of the tendency of reactants to proceed to products when all are

in their standard states at 25 ˚C.




The standard hydrogen electrode is used with other electrodes so the

reduction potentials of the other cells can be measured.

• The standard reduction potential of the hydrogen electrode has been assigned a value of 0.00 V.

1111

CELL POTENTIALS, ECELL POTENTIALS, Eoo

Can’t measure 1/2 reaction Eo directly. Therefore, measure it

relative to a STANDARD HYDROGEN CELL..

2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm)2 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm)

Eo = 0.0 VSHE

1212




The difference between the reduction potentials of the two half-cells is called

the cell potential.

cell potential = – reduction potential of half-cell in which reduction occurs

reduction potential of half-cell in which

oxidation occurs

or Ecell = Ered – Eoxid

–

1414

Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V

Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V

Volts

ZnH2

Salt Bridge

Zn2+ H+

Zn Zn2+ + 2e- OXIDATION ANODE

2 H+ + 2e- H2REDUCTIONCATHODE

- +

Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Negative Negative electrodeelectrode

Supplier Supplier of of

electronselectrons

Acceptor Acceptor of of

electronselectrons

Positive Positive electrodeelectrode

2 H2 H++ + 2e- --> H + 2e- --> H22

ReductionReductionCathodeCathode

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

AnodeAnode

1515

Reduction of H+ by Zn

No currentNo current

1616Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Zn(s) + 2H+(aq) --> Zn2+ + H2(g) Eo =+0.76 V

Therefore, oxidation potential

Eo for Zn ---> Zn2+ (aq) + 2e- is +0.76 V

Overall: reduction of H+

by Zn metal.

1717

Volts

CuH2

Salt Bridge

Cu2+ H+

Cu2+ + 2e- Cu REDUCTION CATHODE

H2 2 H+ + 2e-OXIDATION ANODE

-+

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell

EEoo = +0.34 V = +0.34 V

Acceptor Acceptor of of

electronselectrons

Supplier Supplier of of

electronselectrons

CuCu2+2+ + 2e- --> Cu + 2e- --> CuReductionReductionCathodeCathode

HH22 --> 2 H --> 2 H++ + 2e- + 2e-

OxidationOxidationAnodeAnode

PositivePositive NegativeNegative

1818

Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell

Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H+(aq)

Measured Eo = +0.34 VTherefore, reduction potential

Eo for Cu2+ + 2e- ---> Cu is +0.34 V

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

Overall reaction is reduction of Cu2+ by H2 gas.

1919

Calculating Cell VoltageCalculating Cell Voltage

• Balanced half-reactions can be added together to get overall, balanced 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 Eo for each half-reaction, we could get Eo for net reaction..

2020

Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VCuCu2+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)

Eo (calc’d) = +1.10 V

Cathode, positive, sink for electrons

Anode, negative, source of electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons ++




The tendency of a given half-reaction to occur as a reduction is called the reduction potential.

• The half-cell in which reduction occurs has a greater reduction potential than the half-cell in which oxidation occurs.

2222

Using Standard Potentials, EUsing Standard Potentials, Eoo

Table 20.1Table 20.1

• Which is the best oxidizing agent (reduced):

O2, H2O2, or Cl2? _________________

• Which is the best reducing agent (oxidized):

Hg, Al, or Sn? ____________________




The standard cell potential (E0cell) is the

measured cell potential when the ion concentrations in the half-cells are 1M,

any gases are at a pressure of 101 kPa, and the temperature is 25°C.

E0cell = E0

red – E0oxid



Calculating Standard Calculating Standard Cell PotentialsCell Potentials

Calculating Standard Cell Potentials

How can you determine if a redox reaction is spontaneous?

Positive Cell Potentials



Calculating Standard Calculating Standard Cell PotentialsCell Potentials

If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as

written.

If the cell potential is negative, then the reaction is

non-spontaneous.

2626

Uses of EUses of Eoo Values ValuesUses of EUses of Eoo Values Values

• Organize half-reactions by relative ability to act as oxidizing agents (reduced)

• Table 20.1Table 20.1• Use this to predict

cell potentials and direction of redox reactions.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

2727

2828

REDUCTIONPOTENTIALS



Oxidation: Al(s) → Al3+(aq) + 3e– (at anode)

Reduction: Fe3+(aq) + 3e– → Fe(s) (at cathode)

Al(s) + Fe3+(aq) → Al3+(aq) + Fe(s)

Determine the cell reaction for a voltaic cell composed of the following half-cells.

Fe3+(aq) + 3e– → Fe(s) E0cell = –0.036 V

Al3+(aq) + 3e– → Al(s) E0cell = –1.66 V



Show that the following redox reaction between zinc metal and silver ions is

spontaneous.

Sample Problem 21.1Sample Problem 21.1

Determining Reaction Spontaneity

Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)

If E0cell is positive, the reaction is spontaneous.



• First identify the half-reactions.


Oxidation: Zn(s) → Zn2+(aq) + 2e–

Reduction: Ag+(aq) + e– → Ag(s)

• Write both half-cells as reductions with their standard reduction potentials.

Zn2+(aq) + 2e– → Zn(s) E0Zn

2+ = –0.76 V

Ag+(aq) + e– → Ag(s) E0Ag

+ = +0.80 V



• Calculate the standard cell potential.


E0cell = E0

red – E0oxid = E0

Ag+ – E0

Zn2+

= +0.80 V – (–0.76 V) = +1.56 V

E0cell > 0, so the reaction is spontaneous.

3333More About More About Calculating Cell VoltageCalculating Cell Voltage

Assume I- ion can reduce water.

2 H2O + 2e- ---> H2 + 2 OH- Cathode: Red 2 I- ---> I2 + 2e- Anode: Ox-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

2 H2O + 2e- ---> H2 + 2 OH- Cathode: Red 2 I- ---> I2 + 2e- Anode: Ox-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

Assuming reaction occurs as written,

E˚net = E˚cathode - E˚anode or = E˚red + E˚ox

= (-0.828 V) - (+0.535 V) = -1.363 V

Minus E˚ means rxn. occurs in opposite direction

3434

Using Standard Potentials, EUsing Standard Potentials, Eoo

Table 20.1Table 20.1

• In which direction do the following

reactions go?

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

What is Eonet for the overall reaction?

3535Standard Redox Standard Redox Potentials, EPotentials, Eoo

E˚net = “distance” from “top” half-reaction

(cathode) to “bottom” half-reaction (anode)

E˚net = E˚cathode - E˚anode

EEoonetnet for Cu/Ag+ reaction = +0.46 V for Cu/Ag+ reaction = +0.46 V

3636

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Cd --> Cd2+ + 2e-or

Cd2+ + 2e- --> Cd

Fe --> Fe2+ + 2e-or

Fe2+ + 2e- --> Fe

Eo for a Voltaic Cell

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



In a voltaic cell in which one half-reaction is Fe3+ + 3e– → Fe, which of the following

would occur as oxidation?

A.2H+ + 2e– → H2

B.Al3+ + 3e– → Al

C.Br2 + 2e– → 2Br–

D.Fe3+ + e– → Fe2+

B.



Key ConceptsKey Concepts

The electrical potential of a cell results from a competition for electrons between two half-cells.

You can determine the standard reduction potential of a half-cell by using a standard hydrogen electrode and the equation for standard cell potential.



Key Concepts & Key Concepts & Key Equation Key Equation

If the cell potential for a given redox reaction is positive, then the reaction is spontaneous as written. If the cell potential is negative, then the reaction is nonspontaneous.

E0cell = E0

red – E0oxid



Glossary TermsGlossary Terms

• electrical potential: the ability of a voltaic cell to produce an electric current

• reduction potential: a measure of the tendency of a given half-reaction to occur as a reduction (gain of electrons) in an electrochemical cell

• cell potential: the difference between the reduction potentials of two half-cells



Glossary TermsGlossary Terms

• standard cell potential (E0cell): the measured

cell potential when the ion concentrations in the half-cells are 1.00M at 1 atm of pressure and 25°C

• standard hydrogen electrode: an arbitrary reference electrode (half-cell) used with another electrode (half-cell) to measure the standard reduction potential of that cell; the standard reduction potential of the hydrogen electrode is assigned a value of 0.00 V

4242

4343

RED = REDUCED

BLACK = OXIDIZED

e-

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1 ELECTROCHEMICAL CELLS Chapter 20 : D8 C20. 21.2 Half-Cells and Cell Potentials > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights