Oxidation and Reduction Chapters 20 & 21

Oxidation vs Reduction

What’s really happening…

What???? Oxidation= increase in oxygen atoms, increase in oxno, loss in electrons Reduction= loss of oxygen atoms, decrease in oxno, gain in electrons Remember: “L.E.O. says G.E.R.” Loss of electrons oxidation Gain electrons reduction

Agents

Activity Series of Metals (p. 668)

When the the reaction happens, electrons move from Al to Cu

Types of Redox Reactions Direct Combination: S + O 2 → SO 2 Decomposition: HgO → 2Hg + O 2 Single Replacement: Cu (s) + 2AgNO 3(aq) → Cu(NO 3 ) 2(aq) + 2Ag (s) Cu (s) + 2Ag + (aq) → Cu + (aq) + 2Ag (s) (net ionic) But: Cu (s) + ZnCl 2(aq) → NR Cu (s) + Zn +2 (aq) → No reaction (due to relative reactivity rank)

Balancing Redox Equations Some equations are are difficult to balance by inspection or trial and error that worked up until now. The fundamental principle is that the number of electrons lost in the oxidation process must equal the number of electrons gained in the reduction process.

Electrochemical cells Use redox reactions to either produce or use electricity.

Voltaic Cells In late 1700’s Italian physician Luigi Galvani twitched frog legs by connecting two metals. Italian scientist Alessandro Volta concluded the two metals in the presence of water produce electricity.

Voltaic Cells Zn (s) + CuSO 4(aq) → ZnSO 4(aq) + Cu (s) Zn (s) → Zn +2 (aq) + 2e- oxidation Cu +2 (aq) + 2e- → Cu (s) reduction Half Cell- Zn (anode) Pushes e- to Cu (cathode)

Voltaic Cell Electrons move spontaneously from the anode (-) to the cathode (+) The salt bridge allows Electrons to move freely Without mixing solutions.

Cell Potential Ability to move e- through a wire from one electrode to another is the electrical or cell potential. It is measured in volts (v) For Example: A Zn-Cu cell with 1 M solutions produces 1.10 volts. Here is how it works:

Standard Reduction Potentials (p. 693)

Standard Electrode Potentials E cell = E oxidation + E reduction E cell 0 = sum of the oxidation potential (E ox 0 ) plus reduction potential (E red 0 ) The standard state conditions are noted with the 0. E 0 are determined by measuring half cell potential differences. Zn (s) → Zn +2 (aq) + 2e- E 0 ox = V Zn +2 (aq) + 2e- → Zn (s) E 0 red = V

Calculating Cell Potentials Zn (s) → Zn +2 (aq) + 2e- E 0 ox = V Cu +2 (aq) 2e- → Cu (s) E 0 red = V Total Voltage (E cell ) = Volts Practice Problems #1 and 2 on P. 696.

That’s it for this Electrifying lecture!