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 → Zn 2+ ions (oxidized) Cu 2+ ions → Cu atoms (reduced)

Agents of redox Oxidizing agent: causes oxidation of another substance –Cu 2+ ions oxidize the Zn atoms Reducing agent: causes reduction of another substance Reducing agent: causes reduction of another substance –Zn atoms reduce the Cu 2+ 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 Cu 2+ by Zn to produce a voltage –In left beaker is Cu and CuSO 4 –In right beaker is Zn and ZnSO 4 –Adding a “salt bridge” completes the circuit and the reaction occurs Zn + Cu 2+ = Zn 2+ + 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 →Zn e Cathode: Cu e →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 = H 2 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 + Cl 2 = ZnCI 2 proceed? Zn + Cl 2 = Zn Cl - Positive cell voltage means reaction happens What are the reduction potentials? –Zn e = Zn V –CI 2 + 2e = 2CI V But…in the reaction Zn is oxidized (reverse sign) –Zn = Zn e V Overall voltage: 0.76 V 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 Fe 2+ –Cathode: O 2 is reduced to H 2 O 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 + H 2 SO 4 = PbSO 4 + 2H + + 2e Reduction: PbO 2 + H 2 SO 4 + 2H + + 2e = PbSO 4 + 2H 2 O Overall: Pb + PbO 2 + 2H 2 SO 4 = 2PbSO 4 + 2H 2 O Discharge Recharge

Dry cell batteries Acid dry cell –Zn anode Zn → Zn e –MnO 2 cathode 2MnO 2 + 2NH e → Mn 2 O 3 + 2NH 3 + H 2 O Alkali cell –Zn anode Zn + 2OH - → ZnO + H 2 O + 2e –MnO 2 cathode 2MnO 2 + H 2 O + 2e → Mn 2 O 3 + 2OH -

Fuel cells and the hydrogen economy A battery with the “electrolyte” supplied from without –Cathode: 2H 2 → 4H + + 4e –Anode: O 2 + 4H + +4e → 2H 2 O –Overall 2H 2 + O 2 → 2H 2 O

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 + O 2 → Fe 2 O 3 Combustion CH 4 + O 2 → CO 2 + H 2 O Bleaching Cl + e - = Cl -

Biological systems Respiration Cytochrome c (Fe 3+ ) + e - = cytochrome c (Fe 2+ ) Followed by: O 2 + 4e - + 4H + → 2H 2 O Metabolism Ethanol → acetaldehyde → acetic acid → CO 2 + H 2 O

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?