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?