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Redox Half Reactions

Redox Half Reactions. What is the purpose of creating a half reaction? How to balance a half reaction?. What is a half reaction?. does not occur by itself Two reactions are coupled so that electron released by one is accepted by another

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Redox Half Reactions

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  1. Redox Half Reactions What is the purpose of creating a half reaction? How to balance a half reaction?

  2. What is a half reaction? • does not occur by itself • Two reactions are coupled so that electron released by one is accepted by another • oxidation and reduction take place simultaneously in a system Zn + Cu2+ Zn2+ + Cu Zn being oxidized, reducing agent Cu2+ being reduced, oxidizing agent Redox reactions take place in battery operations.

  3. How does a voltaic cell work? • REDOX reaction takes place when the reactants are separated into two cells • REDOX requires the flow of electrons • Electrical connections (salt bridge and wire) facilitate the flow • WATCH THIS! Redox Reactions provide a continuous stream of flowing electrons from the anode to the cathode.

  4. Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq) • may be written as two half-reactions Zn  Zn2+(aq) + 2 e- GER Cu2+(aq) + 2 e- Cu(s) LEO Zn(s) + Cu2+(aq) Cu(s) + Zn2+(aq) Half Reaction is the reaction of those species undergoing oxidation + reduction.

  5. How to write + balance half reactions? +1 +2 0 0 Ag+ + Cu ---> Ag + Cu2+ 1. Assign Oxidation Numbers 2. Separate out LEO and GER GER: Ag+ Ag LEO: Cu  Cu+2 3. Add electrons to the more positive side to balance the reaction GER: Ag++ 1 e- Ag LEO: Cu  Cu+2 + 2 e-

  6. Balanced Redox Reaction 4. Recombine reactants + products GER: Ag++ 1 e- Ag LEO: Cu  Cu+2 + 2 e- Ag+ + e¯ ---> Ag Cu ---> Cu2+ + 2e¯ Do the electrons cancel? Electrons on both sides must cancel. 5. Multiply eqn(s) by electron factor 2 Ag+ + 2 e¯ ---> 2 Ag 6. Recombine both half reactions for final rxn ⁄ ⁄ 2 Ag+ + 2e¯ + Cu ---> 2 Ag + Cu2+ + 2e¯

  7. REDOX and Batteries • REDOX reactions generate electric current in voltaic cells • What’s all this got to do with batteries? In redox reactions, electrons are transferred from one species to another. If the reaction is spontaneous, energy is released, which can then be used to do useful work. To harness this energy, the reaction must be split into two separate half reactions: the oxidation and reduction reactions. The reactions are put into two different containers and a wire is used to drive the electrons from one side to the other

  8. e- flow into cell • Copper gets reduced • e- produced • Cations/Cathode • Zincis oxidized • Zn+2 + 2 e- in sln • e- flow to wire • Anions/Anode e- e- e- e- e- Zn+2 Cu+2

  9. Simple Version ANODE ANIONS CATHODE CATIONS

  10. The Salt Bridge • Necessary to balance + and – charges • Without it, cell would cease to function • WATCH THIS!

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