Redox Reactions: HALF REACTIONS To play the movies and simulations included, view the presentation in Slide Show Mode.

Electrochemical Reactions REDOX REACTIONS REACTIONS Day 1: Using Oxidation numbers

Terminology; Redox Reactions OXIDATION—loss of electron(s) increase in oxidation number. Or called REDUCING AGENT. REDUCTION—gain of electron(s); decrease in oxidation number. Or called OXIDIZING AGENT

Lose electrons: OXIDATION Gain electrons: REDUCTION LEO says GER Lose electrons: OXIDATION Gain electrons: REDUCTION

REDOX REACTIONS Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor. Transfer leads to— 1. increase in oxidation number of some element = OXIDATION 2. decrease in oxidation number of some element = REDUCTION

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Assigning oxidation numbers 7

Copper + Silver Ion

Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Oxidized? HALF- REACTIONS Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Oxidized? Cu(s) ---> Cu2+(aq) Reduced? 2 Ag+(aq) ---> 2 Ag(s)

Balancing Equations Balance BOTH mass and CHARGE Cu + Ag+ ----> Cu2+ + Ag Balance BOTH mass and CHARGE Step 1: Divide into half-reactions: oxidation and reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag

Balancing Equations Ox Cu ---> Cu2+ + 2e- Red Ag+ + e- ---> Ag 1. Balance each for mass (coefficients). Already done in this case. 2. Balance each half-reaction for charge Ox Cu ---> Cu2+ + 2e- (Oxidation electrons OUT: Product) Red Ag+ + e- ---> Ag (Reduction electrons IN: Reactant)

Balancing Equations 2 Ag+ + 2 e- ---> 2 Ag 3 Multiply each half-reaction so the electrons lost equal the number gained Cu ---> Cu2+ + 2e- 2 Ag+ + 2 e- ---> 2 Ag Add the two half reactions to the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag balanced for BOTH charge and mass.

Space Fight Identify two half reactions ID which element reduced and which is oxidized Why are liquid fuel preferred over gaseous? Why use hydrogen and oxygen as fuels if solid fuels provide better propulsion? D8 C10 13

2 Al(s) + 3 Cu2+(aq) --> 2 Al3+(aq) + 3 Cu(s) Redox Equations 2 Al(s) + 3 Cu2+(aq) --> 2 Al3+(aq) + 3 Cu(s) 14

Recognizing a Redox Reaction 2 Al(s) + 3 Cu2+(aq) --> 2 Al3+(aq) + 3 Cu(s) Al is OXIDIZED Al(s) --> Al3+(aq) + 3 e- Al Ox. # increases (more positive): loses e- : Oxidation

Recognizing a Redox Reaction 2Al(s) + 3Cu2+(aq) ’ 2Al3+(aq) + 3Cu(s) Cu is REDUCED Cu2+(aq) + 2 e- ’ Cu(s) Ox. # of Cu decreases (more neg.) gaining electrons Reduction

Recognizing a Redox Reaction 2 half-reactions add up to give the overall reaction 2 Al(s) --> 2 Al3+(aq) + 6 e- 3 Cu2+(aq) + 6 e- --> 3 Cu(s) -------------------------------------------------- 2 Al(s) + 3 Cu2+(aq) ---> 2 Al3+(aq) + 3 Cu(s) balanced for mass and charge.

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“REDOX on Wheels”: Automobiles D8 C13 20

Ready for more complex reactions? No!