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A very, very, very bad joke…

A very, very, very bad joke…. A cation and an anion were walking down the street…. …and the cation says to the anion , “Whoa, I think I lost an electron!”. The anion replies, “Are you sure?”. …to which the cation replies, “Yes…I’m POSITIVE !”. < FORCED LAUGHTER >.

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A very, very, very bad joke…

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  1. A very, very, very bad joke… A cation and an anion were walking down the street… …and the cation says to the anion, “Whoa, I think I lost an electron!” The anion replies, “Are you sure?” …to which the cation replies, “Yes…I’m POSITIVE!” <FORCED LAUGHTER> www.bitterfilms.com

  2. Al0 has been oxidized:Its oxidation state has increased from 0 3. Al3+ has been reduced:Its oxidation state has been reduced from 3 0. Cations and Anions and Electrons, Oh My! Al0 Al3+ + 3e- 13 ____ protons ____ protons ____ electrons ____ electrons 13 13 10 Al0 has lost 3 electrons. Al3+ Al0 + 3e- 13 ____ protons ____ protons ____ electrons ____ electrons 13 10 13 Al3+ has gained 3 electrons.

  3. NaBr (aq) + AgNO3 (aq) NaNO3 (aq) + AgBr (s) HBr (aq) + LiOH (aq) LiBr (aq) + H2O (l) Al0 Al3++ 3e- I20 + 2e- 2I- Redox Reactions: A New Type of Reaction All chemical reactions can be categorized, roughly into: • Acid-base reactions – depending on your definition • Displacement reactions – exchange of ions • Acid-base reactions – exchange of H+ or OH- • Oxidation-Reduction (redox) reactions – exchange of electrons

  4. Redox Rules! Calculating Oxidation States O 1. Oxidation states for elements are always zero. e.g. N2(g), Co(s), H2(g), C(gr), P4(s), Na(s), Ne(g) H H 2. Oxidation states for monatomic ions are the same as their charges. e.g. Al3+, Cd2+, Zn2+, Ag+,and Group IA(1) and IIA(2) ions 3. The oxidation state of oxygen is usually 2-, except for peroxides (-1). e.g. Na2O, Al2O3; compare with H2O2, Na2O2 – remember, O22- 4. The oxidation state of hydrogen is usually 1+, except for hydrides (-1). e.g. H2O, OH-; compare with NaH, LiAlH4 – look for H bonded to metal 5. The oxidation state of halides are 1-, except when bonded to oxygen. e.g. HF, NaCl, MgBr2 (F is always 1-); compare with ClO3- 6. The oxidation numbers MUST add up to the molecule’s total charge.

  5. Cr2O72- TiCl4 H2SO3 BaO2 P2O5 More Practice with Oxidation States Cr = ? O = -2 Cr = +6 dichromate 2(?) + 7(-2) = -2 4 Cl = 4(-1) titanium(IV) chloride +4 Ti = +4 S = sulfurous acid 2(-1) 2 O = barium peroxide 2(+5) 2 P = diphosphorus pentoxide

  6. Al: 0 +3 I: 0 -1 Back Again?! Aluminum and Iodine Al (s) + I2 (s) AlI3 (s) 3(-1) 2(0) +3 0 So, now that you know oxidation states…what is oxidized, and what is reduced?

  7. GER LEO Mnemonic Devices for Remembering Redox Rules LEO the lion goesGER When an atom/molecule Loses Electrons, it is Oxidized. -its oxidation state becomes more positive -it is the reducing agent, or reductant When an atom/molecule Gains Electrons, it is Reduced. -its oxidation state becomes more negative -it is the oxidizing agent, or oxidant Oxidation Is Loss O I L Reduction Is Gain R I G http://www.franklinhoward.com/

  8. Al: 0 +3 I: 0 -1 Back For Good!! Aluminum and Iodine Al (s) + I2 (s) AlI3 (s) 3(-1) 2(0) +3 0 decreases oxidation state increases oxidation state gains electrons loses electrons reduced I2 is oxidized Al is oxidizing agent I2 is the reducing agent Al is the 2 Al (s) 2 AlI3 (s) 3 I2 (s) +

  9. Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS! Oxidizing agents (oxidants) are reactants that are REDUCED. Reducing agents (reductants) are reactants that are OXIDIZED! Zn (s) + 2H+ (aq) H2 (g) + Zn2+ (aq) 2(+1) 2(0) +2 0 Zn loses/gains electrons H+ loses/gains electrons Zn is oxidized/reduced H+ is oxidized/reduced Zn is the reductant/oxidant H+ is the reductant/oxidant

  10. Identifying Oxidizing/Reducing Agents Oxidizing/reducing agents are ALWAYS REACTANTS! Oxidizing agents (oxidants) are reactants that are REDUCED. Reducing agents (reductants) are reactants that are OXIDIZED! 2 H2O (l) + Al (s) + MnO4- (aq) Al(OH)4- (aq) + MnO2 (s) +4 0 +3 +7 What is oxidized? What is reduced? Al (s) MnO4- (aq) What is the oxidizing agent? What is the reducing agent? MnO4- (aq) Al (s) How many electrons are transferred? 3 e-

  11. AgNO3 (s) Ag+ (aq) + NO3- (aq) H2O Ag+ (aq) + e- Ag (s) Cu+ (aq) + e- Cu (s) Practical Applications: Electroplating Electroplating deposits coats of metal onto objects Bronze baby shoes http://www.castinggifts.com/shoebronzing Recoating Tarnished objects http://www.collectorssecret.com/images/sprbeforeafter.jpg

  12. 2 H2O O2 + 4 H+ + 4 e- More Practical Applications: Photosynthesis Photosystem II 1. Chlorophyll in P680 molecule absorbs light. 2. P680 gives up electrons to a transport chain, becoming positive. 3. Electrons from 2 H2O molecules are given to P680 by a Mn4center. The Mn4 center works becauseMn2+, Mn3+, Mn4+, and Mn5+ areall accessible oxidation states. H2O is a reducing agent! http://www.lbl.gov/Science-Articles/Archive/PBD-water-split.html www.pdphoto.org

  13. Fe2O3 (s) + Al (s) Al2O3 (s) + Fe (l) Railroad trackrepairs More Practical Applications: Thermite Reaction UnderwaterWelding 2 Fe (l) + 3H2O (l) Fe2O3 (s) + 3H2 (g) www.wikipedia.org http://www.pagesbydave.com/divepics/pages/underwater%20welding.html

  14. 2(+3)iron(III) oxide More Practical Applications: Thermite Reaction D Fe2O3 (s) + Al (s) Al2O3 (s) + Fe (l) 0 2(+3) 0 Iron(III) oxide is the oxidizing agent. Aluminum oxide is the reducing agent.

  15. Limits to Redox Reactions What is the oxidation number of S in the following compounds? -2 H2S Which cannot be an oxidizing agent? (reduced) S8 0 H2S – cannot gain more electrons +2 SCl2 Which cannot be a reducing agent? (oxidized) Na2SO3 +4 SO42- – cannot lose more electrons SO42- +6

  16. H2O2 (aq) H2O (l) + O2 (g) 2(-1) -2 2(0) Cr1+ (aq) + Cr3+ (aq) 2 Cr2+ (aq) Disproportionation and Conproportionation These are special types of redox reactions: Disproportionation – One compound decomposes into two oxidation states catalyst H2O2 is BOTH oxidized and reduced! Conproportionation – Two oxidation states become one.

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