Oxidation and Reduction

Reactions that involve electron

transfer

Batteries and chemistry

What do rust and the hydrogen fuel

cell have in common?

Rusting is the

oxidation of iron

The fuel cell functions

by oxidation of

hydrogen gas

Both involve transfer

of electrons

Learning objectives

Define oxidation and reduction process

Identify oxidation/reduction and oxidizing/reducing agents

Describe basic principles of an electrochemical cell

Define units of electricity: volts and amps

Describe features of common battery types

Describe features of a fuel cell

Oxidation-Reduction (Redox)

Oxidation is loss of

electrons

Na → Na+ + e-

Reduction is gain of

electrons

Cl + e- = Cl-

Other ways to view oxidation and

reduction

The role of oxygen (oxidizing agent) – Oxidation is addition of O atoms

Fe + O2 → Fe2O3

– Reduction is loss of O atoms

Fe2O3 +H2 → Fe + H2O

The role of hydrogen (reducing agent) – Oxidation is loss of H atoms

C2H5OH → CO2 + H2O

– Reduction is gain of H atoms

C6H6 + H2 → C6H12

Redox with Zn and CuSO4

Zn metal reduces the

Cu2+ ions to Cu metal

– Zn loses electrons

– Cu2+ gains electrons

Zn + CuSO4 = Cu + ZnSO4

Zn + Cu2+ = Zn2+ + Cu

Zn is oxidized

Cu2+ is reduced

Nuggets of redox processes

Where there is oxidation there is always

reduction

Oxidizing agent Reducing agent

Is itself reduced Is itself oxidized

Gains electrons Loses electrons

Causes oxidation Causes reduction

Identifying oxidation and reduction:

Follow the electrons

Only reactants (things on left) are candidates

Metal elements:

– Generally form positive ions (lose electrons)

– Are reducing agents

– Are oxidized

Nonmetal elements:

– Form negative ions (gain electrons)

– Are oxidizing agents

– Are reduced

Worked example

Redox in life

Corrosion

Fe + O2 → Fe2O3

Combustion

CH4 + O2 → CO2 + H2O

Disinfectants

Antiseptics

Bleaching

Cl + e- = Cl-

Life-giving systems

Photosynthesis

Energy + 6CO2 +

6H2O = C6H12O6 +

6O2

Respiration

C6H12O6 + 6O2 =

6CO2 + 6H2O +

energy

Vitamin C and oxidation

Vitamins are organic compounds important for maintaining health

Vitamin C is also easily oxidized (it is a reducing agent)

Body produces free radicals which oxidize – aging, cancer, cardiovascular disease

Antioxidants (like vitamin C) defend against radicals

Questions remain as to benefits of antioxidants

Energy and electricity

All chemical reactions

involve energy change

Spontaneous reactions

can provide energy in the

form of electricity

Volta made the first

(maybe not) battery

(Voltaic cell)

Batteries involve electron

transfer

Electron transfer involves

oxidation/reduction

Daniell cell Electrochemical cell uses

reduction of Cu2+ by Zn to

produce electricity

– Left beaker Zn and Zn2+ ions

– Right beaker Cu and Cu2+ ions

– “Salt bridge” completes the

circuit carries current in ions

Zn + Cu2+ = Zn2+ + Cu

– Anode: Zn is oxidized (loses

electrons)

– Cathode: Cu2+ is reduced (gains

electrons)

– Current flows from anode to

cathode performing work

Volts and amps

Volt measures potential – driving force – to

move electrons. Voltage depends on type of

chemical process and not on size of battery

Other types of potential:

– Pressure moves air or liquids

– Temperature moves heat

– Chemical potential moves reactions

Amp is flow of charge (current). Size of current

flowing depends on size of electrodes

Your basic D cell

Oxidation

Zn = Zn2+ + 2e

Reduction 2MnO2 + H2O + 2e = Mn2O3 + 2OH-

Overall Zn + 2MnO2 + H2O = Zn(OH)2 + Mn2O3

Lithium batteries

Lithium has a very large

negative reduction

potential

Li = Li+ + e….E = 3.04 V

The basis for light-weight,

high energy density

batteries

– Low atomic mass of lithium

– High reduction potential

– Ability to make

rechargeable batteries

Lithium’s high energy high price

to pay Tesla’s battery pack

consists of 6,800

individual Li cells

Weighs 400 kg

Range 200 miles

Cost $36,000

One fuel tank 20 gal

Weighs 70 kg

Range 600 miles

Cost each fill $60

Lead-acid batteries – a unique system Lead battery technology is 100

years old

– Provide high current required to

crank engine instantly

– Rechargeable

– Inexpensive

– Rugged

Oxidation

Pb + SO42- = PbSO4 + 2e

Reduction

PbO2 + SO42- + 4H+ + 2e = PbSO4 + 2H2O

Overall

Pb + PbO2 + 2H2SO4 = 2PbSO4 + 2H2O

Discharge

Recharge

Fuel cells: There’s nothing new in

chemistry

Grove’s “Gas Voltaic Battery” was the first fuel

cell – in 1843

The modern gas voltaic battery

In a battery the

“fuel” is sealed

inside the cell

In a fuel cell it is

supplied constantly

from outside

The fuel cell advantage: tax-free

conversion

Fuel cells convert

chemical potential

energy into

electrical energy

directly

Intrinsically high

efficiency

Fuel cell applications

Portable

Mobile

Stationary

Types of fuel cells

All fuel cells are basically

the same but:

What’s the fuel

– Normally hydrogen

What’s the electrolyte

– Transports H+ (or)

– Transports O2-

Anode materials

Cathode materials

For an overview

SOFC

Solid Oxide Fuel Cell

All solid-state

High efficiency

Electrolyte conducts O2-

ions

Fuel can be varied

Requires high

temperature (800 -

1000ºC)