Redox Reaction Lecture Notes, Lecture notes of Chemistry

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Electrochemistry

Chapter 8

Electrochemistry

• Electrochemistry is the branch of chemistry

that deals with the inter-conversion of

electrical energy and chemical energy.

• Electrochemical - redox reactions (oxidation-

reduction) in which the energy released by a

spontaneous reaction is converted to electricity.

• Electrolysis - electrical energy is used to cause

a nonspontaneous reaction to occur.

• In redox reactions, electrons are transferred

from one substance to another.

Balancing Redox Equations

1. Write the unbalanced equation for the reaction ion ionic form.

The oxidation of Fe

2+ to Fe3+ by Cr 2O

2- in acid solution?

Fe

2+ + Cr 2O

2- Fe3+ + Cr3+

2. Separate the equation into two half-reactions.

Oxidation:

Cr

2O

2- Cr3+

Reduction:

Fe

2+ Fe3+

3. Balance the atoms other than O and H in each half-reaction.

Cr2O

2- 2Cr3+

Balancing Redox Equations

4. For reactions in acid, add H

2O to balance O atoms and H

• to balance H atoms.

Cr

2O

2- 2Cr3+ + 7H 2O

14H

• • Cr 2O

2- 2Cr3+ + 7H 2O

5. Add electrons to one side of each half-reaction to balance the charges

on the half-reaction.

Fe

2+ Fe3+ + 1e-

6e

• • 14H+ + Cr 2O

2- 2Cr3+ + 7H 2O

6. If necessary, equalize the number of electrons in the two half-reactions

by multiplying the half-reactions by appropriate coefficients.

6Fe

2+ 6Fe3+ + 6e-

6e

• • 14H+ + Cr 2O

2- 2Cr3+ + 7H 2O

Galvanic Cells

• (^) Salt bridge: To complete the electrical

circuit is an inverted U tube containing an

inert electrolyte solution, such as KCl or

NH4NO3, whose ions will not react with other ions in solution or with

the electrodes.

Galvanic Cells

The difference in electrical potential

between the anode and cathode is

called:

• (^) cell voltage
• (^) electromotive force (emf)
• (^) cell potential

Cell Diagram

Zn ( s ) + Cu

2+ ( aq )^ Cu ( s )^ + Zn2+ ( aq )

[Cu

2+] = 1 M & [Zn2+] = 1 M

Zn ( s ) | Zn

2+ (1 M ) || Cu2+ (1 M ) | Cu ( s )

anode cathode

What is the standard emf of an electrochemical cell made of a Cd

electrode in a 1.0 M Cd(NO

3)2 solution and a Cr electrode in a 1.0 M Cr(NO3)3 solution?

Cd

2+ ( aq ) + 2e- Cd ( s ) E0 = -0.40 V

Cr

3+ ( aq )^ + 3e- Cr ( s )^ E0 = -0.74 V

Cd is the stronger

oxidizer

Cd will oxidize Cr

2e

• • Cd2+ (1 M ) Cd (s)

Cr (s) Cr

3+ (1 M ) + 3e-

Anode (oxidation):

Cathode (reduction):

2Cr ( s ) + 3Cd

2+ (1 M ) 3Cd ( s ) + 2Cr3+ (1 M )

x 2

x 3

E

0 = E cathode - Eanode

cell

E

0 = -0.40 – (-0.74)

cell

E

0 = 0.34 V

cell

Batteries

A fuel cell is an

electrochemical cell

that requires a

continuous supply of

reactants to keep

functioning

Anode:

Cathode: O

2 ( g ) + 2H2O ( l ) + 4e

• 4OH- (^) ( aq )

2H

2 ( g ) + 4OH

• ( aq ) 4H 2O ( l ) + 4e -

2H

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

Cathodic Protection of an Iron Storage Tank

Electrolysis

• Electrolysis is the process in which

electrical energy is used to cause a

nonspontaneous chemical reaction to

occur.

Electrolysis molten sodium chloride

• In its molten state, sodium chloride, an

ionic compound, can be electrolyzed to

form sodium metal and chlorine.

• This process is a major source of pure

sodium metal and chlorine gas

Electrolysis of Water

Electrolysis and Mass Changes

charge (C) = current (A) x time (s)

1 mole e

• = 96,500 C