Chapter 18 Electrochemistry. Chapter 18 Table of Contents Copyright © Cengage Learning. All rights...

Chapter 18

Electrochemistry

Chapter 18

Table of Contents

Copyright © Cengage Learning. All rights reserved 2

18.1Balancing Oxidation–Reduction Equations

18.2 Galvanic Cells

18.3 Standard Reduction Potentials

18.4Cell Potential, Electrical Work, and Free Energy

18.5 Dependence of Cell Potential on Concentration

18.6 Batteries

18.7 Corrosion

18.8 Electrolysis

18.9 Commercial Eletrolytic Processes

Chapter 18

Table of Contents

Introduction

• car, calculator, digital watch, radio...

• corrosion, industrial preparation (Al, Cl2...)analytical chemistry:

pH meter, pollutants, diseases…• electrochemistry:

the study of the interchange of chemical & electrical energy.

Section 18.1

Balancing Oxidation–Reduction Equations

Return to TOC


Review of Terms

• Oxidation–reduction (redox) reaction – involves a transfer of electrons from the reducing agent to the oxidizing agent

• Oxidation – loss of electrons• Reduction – gain of electrons• Reducing agent – electron donor• Oxidizing agent – electron acceptor

Section 18.1


Return to TOC


Half–Reactions

• The overall reaction is split into two half–reactions, one involving oxidation and one reduction.

8H+ + MnO4– + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O

Reduction: 8H+ + MnO4– + 5e– → Mn2+ + 4H2O

Oxidation: 5Fe2+ → 5Fe3+ + 5e–

Section 18.1


Return to TOC


The Half–Reaction Method for Balancing Equations for Oxidation–Reduction Reactions Occurring in Acidic Solution

Section 18.1


Return to TOC


The Half–Reaction Method for Balancing Equations for Oxidation–Reduction Reactions Occurring in Basic Solution

Section 18.2

Atomic MassesGalvanic Cells

Return to TOC


Galvanic Cell

• Device in which chemical energy is changed to electrical energy.

• Uses a spontaneous redox reaction to produce a current that can be used to do work.

Section 18.2


Return to TOC

1) oxidation-reduction (redox) reactionrxn:

half-reactions:

eFeFe 5 :ox

O4HMn5eMnO8H :re

O4H)5FeMn

)5FeMnO8H

32

22-

4

)(2(aq3

(aq)2

(aq2

(aq)4(aq)

Section 18.2


Return to TOC

2) When the oxidizing agent & reducing agent are physically separated, e transfer through an external wire.

generates electricity.

3) salt bridge: connect two solns

4) electrodes; where the redox rxn occur

anode: oxidation occur

cathode: reduction occur

cathodeanode e

Section 18.2


Return to TOC

5 ) Cell potential ( )

a) The voltage difference between the electrodes.

electromotive force (emf)

b) can be measured by voltmeter.

c) emf of a cell depends on

The nature of the electrodes

[ions]

Temp.

cellE

Section 18.3

The Mole Standard Reduction Potentials

Return to TOC

1) It is impossible to measure εcell of ahalf-rxn directly,need a reference rxn SHE

standard hydrogen electrode:

1atmP 1M,H

0ε H2e2H

2H

0red cell2

Section 18.3


Return to TOC

2) Standard reduction potential

Table 18.1

Zn2eZn

H2e2H

ε

2

2

0red

Section 18.3


Return to TOC

Section 18.3


Return to TOC

3)

0.34V

0

CuCu

0.76V

0

ZnZn

1.10V

0cell

(s)(aq2

(aq)2

(s)

0.76V

0

ZnZn

0

0

HH

0.76V

0cell

2(g)(aq)2

(s)(aq)

0red

0ox

0cell

22

22

εεε

Cu)ZnCu Zn:rxn

εεε

HZnZn2H :rxn

εεε

Section 18.3


Return to TOC

Section 18.3


Return to TOC

4).

rxn

0.34V0.77Vεεεεε

0.77Vε 2Fe2e2Fe

0.34Vεε 2eCuCu

0.77Vε FeeFe:red

FeCuCuFe

(anode)0re(cathode)

0re

0ox

0re

0cell

0re

23

0re

0ox

2

0re

23

(aq)2

(aq)2

(s)(aq)3

2

Section 18.3


Return to TOC


Concept Check

Order the following from strongest to weakest oxidizing agent and justify. Of those you cannot order, explain why.

Fe Na F- Na+ Cl2

Section 18.3


Return to TOC


Line Notation

• Used to describe electrochemical cells.• Anode components are listed on the left.• Cathode components are listed on the right.• Separated by double vertical lines.• The concentration of aqueous solutions should

be specified in the notation when known.• Example: Mg(s)|Mg2+(aq)||Al3+(aq)|Al(s)

Mg → Mg2+ + 2e– (anode) Al3+ + 3e– → Al (cathode)

Section 18.3


Return to TOC


Concept Check

Sketch a cell using the following solutions and electrodes. Include:

The potential of the cell The direction of electron flow Labels on the anode and the cathode

a) Ag electrode in 1.0 M Ag+(aq) and Cu electrode in 1.0 M Cu2+(aq)

Section 18.3


Return to TOC


Concept Check

Sketch a cell using the following solutions and electrodes. Include:

The potential of the cell The direction of electron flow Labels on the anode and the cathode

b) Zn electrode in 1.0 M Zn2+(aq) and Cu electrode in 1.0 M Cu2+(aq)

Section 18.3


Return to TOC


Concept Check

Consider the cell from part b.

What would happen to the potential if you increase the [Cu2+]?

Explain.

The cell potential should increase.

Section 18.4

Cell Potential, Electrical Work, and Free Energy

Return to TOC


Work

• Work is never the maximum possible if any current is flowing.

• In any real, spontaneous process some energy is always wasted – the actual work realized is always less than the calculated maximum.

Section 18.4


Return to TOC

-nFεΔG

:condition standard

nFεΔG

ΔG w& nFq & qε w

q

w-ε

(C) charge

(J)work (V) emf

maxe mol

c96485

Section 18.4


Return to TOC

(ex) 18.5

P812

nFεΔG

εεε

FeCuFeCu

ΔG calculate

0ox

0re

0rxn

(aq)2

(s)(s)(aq)2

Section 18.4


Return to TOC

(ex) 18.6

Predict whether 1M HNO3 will dissolve gold metal to form 1M Au3+?

0.54Vεεε

-1.50Vε 3eAuAu

0.96Vε

O2HNO3e4HNO

0ox

0re

0cell

0ox

3

0re

23

Section 18.5

Dependence of Cell Potential on Concentration

Return to TOC

1) [C] & εcell

standard conditions: [C]=1M

what if [C]≠1M?

(ex) 18.7

a) [Al3+]=2.0M, [Mn2+]=1.0M εcell<0.48V

b) [Al3+]=1.0M, [Mn2+]=3.0M εcell>0.48V

0.48Vε

3Mn2Al3Mn2Al

:ε

0cell

(s)(aq)3

(aq)2

(s)

0cell

Section 18.5


Return to TOC

2) The Nernst eqn

logQn

0.0592εε

lnQnF

RTεε

lnQ RTnFεnFε

lnQ RTΔGΔG

Section 18.5


Return to TOC

3) Concentration Cells

Determine

a) e flow direction?

b) anode? cathode?

c) ε=? at 25℃

Section 18.5


Return to TOC

4) Ion-Selective Electrodes: pH meter

Section 18.5


Return to TOC

5) at equilibrium: ΔG=0, εcell=0, Q=K

ex:18.10 at p818

logKn

0.0592lnK

nF

RTε

lnK RTΔG

Section 18.5


Return to TOC

Ag Ag

1MAgNO 3(aq)

1M NaCl(s)

& AgCl(s)

(ex) Calculate Ksp for AgCl at 25℃

ε=0.58V soln:

1.0Msp

0

ClAgK

1.0

Aglog

1

0.05920

0.58Vε

0?ε

Section 18.5


Return to TOC


Concept Check

Explain the difference between E and E °.

When is E equal to zero?

When the cell is in equilibrium ("dead" battery).

When is E ° equal to zero?

E is equal to zero for a concentration cell.

Section 18.6

Batteries

Return to TOC


One of the Six Cells in a 12–V Lead Storage Battery

Section 18.6

Batteries

Return to TOC


A Common Dry Cell Battery

Section 18.6

Batteries

Return to TOC


A Mercury Battery

Section 18.6

Batteries

Return to TOC


Schematic of the Hydrogen-Oxygen Fuel Cell

Section 18.7

Corrosion

Return to TOC


• Process of returning metals to their natural state – the ores from which they were originally obtained.

• Involves oxidation of the metal.

Section 18.7

Corrosion

Return to TOC

1) (ex)

2) Table 18.1 : : metals & O2

3) Al & Al2O3 [Al2(OH)6] protect layer

4) Compare of Au, O2, Cu, Ag.

0reε

reddish)~(blackrust iron

OnHOFeFe 232OH,O

(s)22(g)

0.6Vε0re

0reε

0reε

Section 18.7

Corrosion

Return to TOC

5) Corrosion

cathodeat rust

8HOnHO2Fe

O2n)H(4O4Fe &

4OH4eO2HO :Cathode

2eFeFe :Anode

(aq)(s)232

)(22(g)(aq)2

)(22(g)

(aq)2

(s)

Section 18.7

Corrosion

Return to TOC

6) Moisture (H2O) act as a “salt bridge”

Salt (NaCl) corroision

∵① [ ] & conductivity

② complex ion

Section 18.7

Corrosion

Return to TOC

7) Prevention Corrosion

① paint or metal plating

Sn: cans oxide

Cr: bumpers for automobiles coating

Zn: sacrificial coating

② alloying:

Stainless steel: (Cr, Ni, Fe)

form oxide coatings & change

0reε

0reε

Section 18.7

Corrosion

Return to TOC

③ cathodic protection

fuel tanks & pipelines (Mg… need to be replaced)

any metal that is more easily oxidized than Fe ( is more )

force metal become a cathode. (reduction)

0reε

0reε

Section 18.8

Electrolysis

Return to TOC


• Forcing a current through a cell to produce a chemical change for which the cell potential is negative.

Section 18.8

Electrolysis

Return to TOC

1) galvanic cell electrolytic cell

① chemical E electrical E

electrical E chemical E

② Fig 18.19(a) Fig 18.19(b)

③ redox rxn: spontaneous nonspontaneous

1.10Vε

CuZnCuZn0

22

1.10Vpower external

CuZnCuZn 22

Section 18.8

Electrolysis

Return to TOC

2) Stoichiometry

(ex) determine the mass of Cu that is plated out? I = 10 amp (c/s) for t = 30 mins

① charge (c) = amp × sec = 10 × 30 × 60 (c)

② c mol of e = 96485 : 1

③ mol of e mol of Cu = 2 : 1

④ . Cu of gCu of mol MW

Section 18.8

Electrolysis

Return to TOC

3) Electrolysis of H2O

① .

2.06Vε

O2HO2H

)OH4(HO2HO6H :rxnNet

0.83Vε

4OH2H4eO4H :rxn Cathod

1.23Vε

4e4HOO2H :rxn Anode

0

222

-222

0re

-2

-2

0ox

-22

Section 18.8

Electrolysis

Return to TOC

② in pure water: [H+] = [OH-] = 10-7M

from

for electrolysis of H2O

1.23Vε

logQn

0.0592εε 0

Section 18.8

Electrolysis

Return to TOC

red

uce

dfir

st

oxi

diz

ing

ab

ility

0reε

4) Electrolysis of mixtures of ions

① .

0.76Vε

Zn2eZn

0.34Vε

Cu2eCu

0.80Vε

AgeAg

0

2

0

2

0

Section 18.8

Electrolysis

Return to TOC

② Electrolysis of NaCl(aq): Na+, Cl-, H2O

1.23Vε

4e4HOO2H*

1.36Vε 2eCl2Cl

:ox

0.83Vε

2OHH2eO2H*

2.17Vε NaeNa

:re

0ox

22

0ox2

0re

22

0re(aq)

Section 18.8

Electrolysis

Return to TOC


Concept Check

Consider a solution containing 0.10 M of each of the following: Pb2+, Cu2+, Sn2+, Ni2+, and Zn2+.

Predict the order in which the metals plate out as the voltage is turned up from zero.

Cu2+, Pb2+, Sn2+, Ni2+, Zn2+

Do the metals form on the cathode or the anode? Explain.

Section 18.9

Commercial Electrolytic Processes

Return to TOC


• Production of aluminum• Purification of metals• Metal plating• Electrolysis of sodium chloride• Production of chlorine and sodium hydroxide

Section 18.9


Return to TOC


Producing Aluminum by the Hall-Heroult Process

Section 18.9


Return to TOC


Electroplating a Spoon

Section 18.9


Return to TOC


The Downs Cell for the Electrolysis of Molten Sodium Chloride

Section 18.9


Return to TOC


The Mercury Cell for Production of Chlorine and Sodium Hydroxide

Chapter 18 Electrochemistry. Chapter 18 Table of Contents Copyright © Cengage Learning. All rights...

Documents

Transcript of Chapter 18 Electrochemistry. Chapter 18 Table of Contents Copyright © Cengage Learning. All rights...