Redox Reactions and Electrochemistry

By Aktr

Loss of e

Gain of e

Reducing Agent

Oxidising agent

Oxidation

Reduction

Electrochemical cell occur both

SnCl2(aq) + 2FeCl3(aq) → SnCl4(aq) + 2FeCl2(aq)

CuO(s) + H2(g) → Cu(s) + H2O(l)

Example

+2 +3 +4 +2

+100+2

Conductance in electrolytic solution

Also act as a conductor

Conductance in electrolytic solutions

Metallic conductor depend upon

• Nature and structure of metal.• No. of valence electron per atom.• Temperature of the sample.

conductivity aqueous solution depend upon

• Nature of electrolyte.• Size of ion.• Solvation of ion.• Concentration of electrolytic.• Temperature.

One cell const. and resistivityKnown then we can find value Easily.

Molar conductivity

Electrochemical cell

Chemical to electrical Electrical to chemical

2. Voltaic or Galvanic cells- An electrochemical cell in which a spontaneous reaction produces electricity.Eg. Dry cell, lead storage cell etc.

1. Electrolytic cell-An electrochemical cell in which a non spontaneous

reaction is forced to occur by passing a direct current from an external source into the solution.

Eg. Refining metal(purify), electroplating & production of many chemical substance.

Gets Smaller -> <- Gets Larger

Cell Notation

1. Anode2. Salt Bridge3. Cathode

Anode | Salt Bridge | Cathode

| : symbol is used whenever there is a different phase

19.2

Cell Notation

Zn (s) + Cu2+ (aq) Cu (s) + Zn2+ (aq)

[Cu2+] = 1 M & [Zn2+] = 1 M

Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)anode cathode

Zn (s)| Zn+2 (aq, 1M)| K(NO3) (saturated)|Cu+2(aq, 1M)|Cu(s)anode cathodeSalt bridge

More detail..

K(NO3)

Zn (s) + 2 H+(aq) -> H2 (g) + Zn+2 (aq)

Zn(s)| Zn+2|KNO3|H+(aq)|H2(g)|Pt

Electrochemical Cells

The difference in electrical potential between the anode and cathode is called:

• cell voltage

• electromotive force (emf)

• cell potential

000reductionoxidationCell EEE

UNITS: Volts Volt (V) = Joule (J) Coulomb, C

Standard Electrode Potentials

Standard reduction potential (E0) is the voltage associated with a reduction reaction at an electrode when all solutes are 1 M and all gases are at 1 atm.

V

Standard hydrogen electrode (SHE)

eatm

Reduction Reaction

Determining if Redox Reaction is Spontaneous

• + E°CELL ; spontaneous reaction

• E°CELL = 0; equilibrium• - E°CELL; nonspontaneous

reaction

More positive E°CELL ; stronger oxidizing agent ormore likely to be reduced

Relating E0Cell to G0

echworkECell arg

Unitswork, Joulecharge, CoulombEcell; Volts

charge = nFFaraday, F; charge on 1 mole e-F = 96485 C/mole

work = (charge)Ecell = -nFEcell

G = work (maximum)

G = -nFEcell

Relating CELL to the

Equilibrium Constant, KG0 = -RT ln K

G0 = -nFE0cell

-RT ln K = -nFE0cell

K

nFRTECell ln0

0257.0

96485

29831.8

moleC

KmolKJ

FRT

Kn

Kn

ECell log0592.0ln0257.00

Effect of Concentration on Cell Potential

G =G0 + RTlnQ

G0 = -nFE0cell

-nFEcell= -nFE0cell + RTln Q

Ecell= E0cell - RTln Q

nF

Ecell= E0cell - 0.0257ln Q

nEcell= E0

cell – 0.0592log Q n

Corrosion – Deterioration of Metals by Electrochemical Process

Corrosion – Deterioration of Metals by Electrochemical Process

Corrosion – Deterioration of Metals by Electrochemical Process

Cathodic Protection

Abbreviated Standard Reduction Potential Table

Batteries

19.6

Leclanché cell

Dry cell

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

Cathode: 2NH4+ (aq) + 2MnO2 (s) + 2e- Mn2O3 (s) + 2NH3 (aq) + H2O (l)

Zn (s) + 2NH4 (aq) + 2MnO2 (s) Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)

Batteries

Zn(Hg) + 2OH- (aq) ZnO (s) + H2O (l) + 2e-Anode:

Cathode: HgO (s) + H2O (l) + 2e- Hg (l) + 2OH- (aq)

Zn(Hg) + HgO (s) ZnO (s) + Hg (l)

Mercury Battery

19.6

Batteries

19.6

Anode:

Cathode:

Lead storagebattery

PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- PbSO4 (s) + 2H2O (l)

Pb (s) + SO42- (aq) PbSO4 (s) + 2e-

Pb (s) + PbO2 (s) + 4H+ (aq) + 2SO42- (aq) 2PbSO4 (s) + 2H2O (l)

Fuel Cell vs. Battery

• Battery; Energy storage device– Reactant chemicals already in device– Once Chemicals used up; discard (unless rechargeable)

• Fuel Cell; Energy conversion device– Won’t work unless reactants supplied– Reactants continuously supplied; products continuously

removed

Fuel Cell

A fuel cell is an electrochemical cell that requires a continuous supply of reactants to keep functioning

Anode:

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

2H2 (g) + 4OH- (aq) 4H2O (l) + 4e-

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

Types of Electrochemical Cells

• Voltaic/Galvanic Cell; Energy released from spontaneous redox reaction can be transformed into electrical energy.

• Electrolytic Cell; Electrical energy is used to drive a nonspontaneous redox reaction.

Faraday’s Constant Redox Eqn

Molar Mass

Charge =(Current)(Time)