H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 20 Redox Equilibrium II: Electrochemical Cells 20.1Electrode Potentials 20.2Half.

H+

H+

H+

OH-

OH-OH

-

New Way Chemistry for Hong Kong A-Level Book 21

Chapter 20Chapter 20Redox Equilibrium II:

Electrochemical Cells

20.120.1 Electrode Potentials Electrode Potentials

20.220.2 Half cells Half cells

20.320.3 Electrochemical cells Electrochemical cells

20.420.4 Standard Electrode Potentials Standard Electrode Potentials

H+

H+

H+

OH-

OH-OH

-


20.520.5 Uses of Standard Electrode Potentials Uses of Standard Electrode Potentials

20.620.6 Primary and Secondary Cells Primary and Secondary Cells

20.720.7 Corrosion of Iron and its Prevention Corrosion of Iron and its Prevention

20.820.8 Socioeconomic Implications of Socioeconomic Implications of

Corrosion and Prevention Corrosion and Prevention

H+

H+

H+

OH-

OH-OH

-


Electrode Potentials

Different absolute electrode potentials (separation of charges) developed at the surface of the electrodes.

reduction

oxidationM(s)Mn+(aq) + ne-

Depend on:

20.1 Electrode Potentials (SB p.186)

Nature of electrodes & soln; temp and pressure ;

Conc. of ions in soln

H+

H+

H+

OH-

OH-OH

-


20.2 Half Cells (SB p.187)

Half Cell involving Metal–Metal Ion

Zn2+(aq) + 2e- Zn(s)

Cu2+(aq) + 2e- Cu(s)

H+

H+

H+

OH-

OH-OH

-


Half Cell involving Non-metal-Non-metal Ion


I2(aq) + 2e- 2I-(aq)

H+

H+

H+

OH-

OH-OH

-



Half Cell involving Ions in Different Oxidation States

Fe3+(aq) + e- Fe2+(aq)

H+

H+

H+

OH-

OH-OH

-


Electrochemical Cells20.3 Electrochemical Cells (SB p.189)

Reduction: Zn(s) Zn2+(aq) + 2e-

Oxidation: Cu2+(aq) + 2e- Cu(s)

Overall equation: Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

ΔH is -ve

H+

H+

H+

OH-

OH-OH

-





ΔH is -ve

20.3 Electrochemical Cells (SB p.189)

H+

H+

H+

OH-

OH-OH

-






ΔH is -ve

H+

H+

H+

OH-

OH-OH

-


Measuring the e.m.f. of a Cell ---- by high resistance voltmeterMeasuring the e.m.f. of a Cell ---- by high resistance voltmeter

Voltage difference across the two electrodes in a chemical cell


must be with high resistance for accurate measurement of e.m.f.(otherwise there will be internal voltage drop across the cell)

H+

H+

H+

OH-

OH-OH

-


Measuring the e.m.f. of a Cell ---- by potentiometerMeasuring the e.m.f. of a Cell ---- by potentiometer

PQ is a resistance wire, the potential difference across which is 20V.

PQ is a resistance wire, the potential difference across which is 20V.

Point R is located so there is no deflection in the Galvanometer reading.

Point R is located so there is no deflection in the Galvanometer reading.

(volts) 20x PQ

PR e.m.f. (volts) 20x PQ

PR e.m.f.


H+

H+

H+

OH-

OH-OH

-


Measuring the e.m.f. of a Cell ---- by digital multimeterMeasuring the e.m.f. of a Cell ---- by digital multimeter


Digitalmultimeter

1.10

H+

H+

H+

OH-

OH-OH

-


What factors determine the e.m.f. of a chemical cell?What factors determine the e.m.f. of a chemical cell?

Nature of electrodesNature of electrodes

Nature of ions in solutionNature of ions in solution

Conc. of ions in solutionConc. of ions in solution

Temp. at which the celloperates

Temp. at which the celloperates


H+

H+

H+

OH-

OH-OH

-


IUPAC cell convention


Cell Diagram

Zn(s) Zn2+(aq, 1M) Cu2+(aq, 1M) Cu(s) E = + 1.10V

ø

= =

=

anode Salt bridge cathode

Zn(s) | Zn2+(aq, 1M) Cu2+(aq, 1M) | Cu(s) E = +1.10V

ø

anode ---

porous pot cathode

H+

H+

H+

OH-

OH-OH

-


The absolute electrode potential of hydrogen is used as a reference standard.The absolute electrode potential of hydrogen is used as a reference standard.

20.4 Standard Electrode Potentials (SB p.194)

Standard Hydrogen Electrode

The standard conditions for a standard hydrogen electrode are:

1. Temperature of electrode: 298 K (25oC)

2. Pressure of hydrogen gas: 1 atm

3. Concentration of hydroxonium ion: 1M ( or 1 mol dm-3)

4. Platinum electrode with platinum black

H+

H+

H+

OH-

OH-OH

-



Metal-metal Ion System

Zn(s) + 2H+(aq) Zn2+(aq) + H2(g)

H+

H+

H+

OH-

OH-OH

-



Metal-metal Ion System

Cu2+(aq, 1M) Cu(s) Ered = +0.34V

ø

H+

H+

H+

OH-

OH-OH

-



Ion-ion System

Pt(s) H2(g, 1atm) 2H+(aq, 1M) Fe3+(aq, 1M) Pt(s)

= =

=

H+

H+

H+

OH-

OH-OH

-



Pt(s) H2(g, 1 atm) H+(aq, 1M) [MnO4-(aq, 1M) + 8H+(aq, 1M)],

[Mn2+(aq, 1M) + 4H2O(l)] Pt(s)

= =

=

H+

H+

H+

OH-

OH-OH

-


Very weak reducing

agent

Very strong

Oxidizing agent

Very weak oxidizing

agent

20.5 Uses of Standard Electrode Potentials (SB p.199)Standard reduction potentials of some common redox systems at 25oC

E (V)ø

Li+(aq) + e- Li(s)K+(aq) + e- K(s)Ca2+(aq) + 2e- Ca(s)Na+(aq) + e- Na(s)Mg2+(aq) + 2e- Mg(s)Al3+(aq) + 3e- Al(s)Zn2+(aq) + 2e- Zn(s)Fe2+(aq) + 2e- Fe(s)2H+(aq) + 2e- H2(g)

Cu2+(aq) + 2e- Cu(s)Fe3+(aq) + e- Fe2+(aq)Br2(g) + 2e- 2Br-(aq)

Cr2O72-(aq) + 6e- 2Cr3+(aq) + 7H2O(l)

MnO4-(aq) + 8H+(aq) + 5e- Mn2+(aq) + 4H2O(l)

F2(g) + 2e- 2F-(aq)

-3.04-2.92-2.87-2.71-2.38-1.66-0.76-0.440.00+0.34+0.77+1.07+1.33+1.52+2.87

Very strong

reducing agent

H+

H+

H+

OH-

OH-OH

-


To Calculate the e.m.f. of an Electrochemical Cell

20.5 Uses of Standard Electrode Potentials (SB p.200)

Ecell = Ecathode - Eanode

øø ø

H+

H+

H+

OH-

OH-OH

-



To Predict Feasibility of Redox ReactionsExample 20-4

Is the reaction Mg(s) + 2Ag+(aq) Mg2+(aq) + 2Ag(s) feasible?

Given: Mg2+(aq) + 2e- Mg(s) E = -2.38V

Ag+(aq) + e- Ag(s) E = +0.80Vø

øSolution

2 x (Ag+(aq) + e- Ag(s) E = +0.80V

-) Mg2+(aq) + 2e- Mg(s) E = -2.38V

2Ag+(aq) + Mg(s) Mg2+(aq) + 2Ag(s) E = +3.18V

As the E cell of the reaction has a positive value, the reaction is feasible and spontaneous.

øø

ø

ø

Answer

H+

H+

H+

OH-

OH-OH

-



To Predict Feasibility of Redox Reactions

Is the reaction Pb(s) + Ni2+(aq) Pb2+(aq) + Ni(s) feasible?

Given: Pb2+(aq) + 2e- Pb(s) E = -0.13V

Ni2+(aq) + 2e- Ni(s) E = -0.25V ø

øSolution

Ni2+(aq) + 2e- Ni(s) E = -0.25V

-) Pb2+(aq) + 2e- Pb(s) E = -0.13V

Ni2+(aq) + Pb(s) Ni(s) + Pb2+(aq) E = -0.12V

As the Ecell of the reaction has a negative value, the reaction is not feasible.

øø

ø

ø

Answer

H+

H+

H+

OH-

OH-OH

-


RemarksRemarks

1. The above predictions are based on standard reduction potential.

2. The predictions may not be applicable directly to other conditions.

3. Generally speaking, if the magnitude of the cell e.m.f. Calculated is greater than 0.4 V, it is quite safe to say that the prediction is valid.


H+

H+

H+

OH-

OH-OH

-


20.6 Primary and Secondary Cells (SB p.207)

Primary Cell

At anode: Zn(s) Zn2+(aq) + 2e-

At cathode: 2MnO2(s) + 2NH4+(aq) + 2e-

Mn2O3(s) + 2NH3(aq) + H2O(l)

The overall reaction is:

Zn(s) + 2MnO2(s) + 2NH4+(aq)

Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)

E = +1.50Vø

H+

H+

H+

OH-

OH-OH

-



Secondary Cell

At anode: Pb(s) + SO42-(aq) PbSO4(s) + 2e-

At cathode: PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-

PbSO4(s) + 2H2O(l)The overall reaction is:

Pb(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq)

2PbSO4(s) + 2H2O(l)

H+

H+

H+

OH-

OH-OH

-



Fuel Cell

At anode: H2(g) + 2OH-(aq) 2H2O(l) + 2e-

At cathode: O2(g) + 2H2O(l) + 4e- 4OH-(aq)

The overall reaction is:

2H2(g) + O2(g) 2H2O(l)

H+

H+

H+

OH-

OH-OH

-


20.7 Corrosion of Iron and its Prevention (SB p.210)

Electrochemical Process involved in Rusting

At anodic region: Fe(s) Fe2+(aq) + 2e-

At cathodic region: O2(g) + 2H2O(l) + 4e- 4OH-(aq)

Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)

2Fe(OH)2(s) + ½ O2(g) + (x-2) H2O(l) Fe2O3· xH2O(s)

rust

H+

H+

H+

OH-

OH-OH

-


1. Coating

2. Sacrificial protection

3. Alloying

4. Cathodic protection

20.7 Corrosion of Iron and its Prevention (SB p.211)

Prevention of Corrosion

H+

H+

H+

OH-

OH-OH

-


Socioeconomic Implication of Corrosion and Prevention

Factors concerned:

1. Direct cost of rusting

2. Indirect cost of rusting

3. Wastage of natural resource

4. Inconvenience to daily life

5. Safety

20.8 Socioeconomic Implications of Corrosion and Prevention (SB p.213)

H+

H+

H+

OH-

OH-OH

-


The END

H+H+ H+H+ H+H+ OH - New Way Chemistry for Hong Kong A-Level Book 2 1 Chapter 20 Redox Equilibrium II: Electrochemical Cells 20.1Electrode Potentials 20.2Half.

Documents

aq cus h

aq slide

hong kong

level book

cell slide

electrochemical cells

aq e fe

e cus slide