Oxidation and Reduction
! Reactions that involve electron transfer ! Batteries and chemistry
What’s in a battery
! A battery does work with electricity ! A circuit is required ! An electric current flows through the circuit ! A chemical reaction provides the electricity
Energy and electricity
! All chemical reactions involve energy change
! Reactions where energy is given out can be made to provide the energy in the form of electricity
! Volta made the first battery (Voltaic cell) ! All batteries involve electron transfer ! Electron transfer involves oxidation/
reduction
Oxidation-Reduction (Redox)
! Oxidation is loss of electrons Na → Na+ + e-
! Reduction is gain of electrons Cl + e- = Cl-
! Formation of NaCl from elements is redox
Single displacement is redox
! Zn atoms → Zn2+ ions (oxidized) ! Cu2+ ions → Cu atoms (reduced)
Agents of redox
! Oxidizing agent: causes oxidation of another substance – Cu2+ ions oxidize the Zn atoms
! Reducing agent: causes reduction of another substance – Zn atoms reduce the Cu2+ ions
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
! With elements forming ionic compounds identifying oxidation and reduction is usually straightforward – Follow path of electrons from reactant to
product ! What about covalent molecules and
reactions involving only compounds? ! System of oxidation numbers is used ! Oxidation numbers keep track of electrons
A numbers game
Daniell cell
! An electrolytic cell which uses the reduction of Cu2+ by Zn to produce a voltage – In left beaker is Cu and CuSO4 – In right beaker is Zn and ZnSO4 – Adding a “salt bridge” completes the circuit
and the reaction occurs Zn + Cu2+ = Zn2+ + Cu
Galvanic cell: long distance chemistry
! Each metal in touch with a solution of its own ions ! External circuit carries electrons transferred during the
redox process ! A “salt bridge” containing neutral ions completes the
internal circuit. ! With no current flowing, a potential develops – the
potential for work ! Unlike the reaction in the beaker, the energy released by
the reaction in the cell can perform useful work – like lighting a bulb
Odes to a galvanic cell ! Cathode
– Where reduction occurs
– Where electrons are consumed
– Where positive ions migrate to
– Has positive sign
! Anode – Where oxidation
occurs – Where electrons are
generated – Where negative ions
migrate to – Has negative sign
Cell notation ! Anode on left, cathode on right ! Electrons flow from left to right ! Oxidation on left, reduction on right ! Single vertical = electrode/electrolyte boundary ! Double vertical = salt bridge
Anode: Zn →Zn2+
+ 2e
Cathode: Cu2+ + 2e →Cu
Volts and amps ! Volt is the measure of potential – the driving
force to move electrons. Voltage depends on the type of chemical process and not on the size of the battery
! Other forms of potential: – Pressure moves air or liquids – Temperature moves heat – Chemical potential moves reactions
! Amp is the flow of current. The size of the current flowing can be increased by making the electrodes larger (more reaction per second)
Measuring tendency for reduction
! Reduction potential measures the tendency for a substance to reduce another substance
! The tendency is measured relative to some standard – taken to be hydrogen
2H+ + 2e = H2 ! Standard reduction potentials are all measured
under the same conditions – Negative value means that process is unfavourable – Positive value means that process is favourable
Making predictions ! Will the reaction
Zn + Cl2 = ZnCI2 proceed? Zn + Cl2 = Zn2+ + 2Cl-
! Positive cell voltage means reaction happens
! What are the reduction potentials? – Zn2+ + 2e = Zn -0.76 V – CI2 + 2e = 2CI- +1.36 V
! But…in the reaction Zn is oxidized Overall voltage:
+1.36 V - -0.76 V = 2.12 V ! Verdict: reaction proceeds
A rusty nail: corrosion and electrochemistry
! The rusting of a nail is an electrochemical process – Anode: Fe is oxidized to Fe2+ – Cathode: O2 is reduced to H2O
! Why do things rust quicker in salt water?
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
Lead-acid batteries – a unique system
! Lead battery technology is 100 years old – Provides high current – Rechargeable – Inexpensive – Rugged
! Oxidation: Pb + H2SO4 = PbSO4 + 2H+ + 2e ! Reduction: PbO2 + H2SO4 + 2H+ + 2e = PbSO4 + 2H2O ! Overall: Pb + PbO2 + 2H2SO4 = 2PbSO4 + 2H2O
Discharge
Recharge
Dry cell batteries
! Acid dry cell – Zn anode Zn → Zn2+ + 2e – MnO2 cathode 2MnO2 + 2NH4
+ + 2e → Mn2O3 + 2NH3 + H2O
! Alkali cell – Zn anode Zn + 2OH- → ZnO + H2O + 2e – MnO2 cathode 2MnO2 + H2O + 2e → Mn2O3 + 2OH-
Fuel cells and the hydrogen economy
! A battery with the “electrolyte” supplied from without – Cathode: 2H2 → 4H+ + 4e – Anode: O2 + 4H+ +4e → 2H2O – Overall 2H2 + O2 → 2H2O
Electrolysis – driving against the stream
! In the spontaneous process (left): electrons flow from left to right (battery discharge powers cell phone)
! Nonspontaneous process (right): apply voltage to electrodes: electrons flow from right to left, reversing the chemical reaction, restores potential energy to the bonds (plugging phone into charger restores the battery)
Predicting spontaneity and the activity series
! A more active metal will reduce a less active metal ion
! A less active metal ion will oxidize a more active metal
Redox in life
! Corrosion Fe + O2 → Fe2O3
! Combustion CH4 + O2 → CO2 + H2O
! Bleaching Cl + e- = Cl-
Biological systems
! Respiration Cytochrome c (Fe3+) + e- = cytochrome c (Fe2+)
Followed by: O2 + 4e- + 4H+ → 2H2O
! Metabolism Ethanol → acetaldehyde → acetic acid → CO2 + H2O
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 ! Question: should we take antioxidant
supplements?