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
Discharge
Pb + PbO2 + 2H2SO4 = 2PbSO4 + 2H2O
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 O2ions
Fuel can be varied
Requires high
temperature (800 1000ºC)