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Oxidation-Reduction Reactions

Oxidation-Reduction Reactions. *What kind of chemical reaction is responsible for burning, rusting, bleaching clothes, powering your cell phone as well as your body, and giving fireflies their glow?. Oxidation-Reduction Reactions , also called REDOX reactions. I. What are Redox Reactions?.

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Oxidation-Reduction Reactions

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  1. Oxidation-Reduction Reactions

  2. *What kind of chemical reaction is responsible for burning, rusting, bleaching clothes, powering your cell phone as well as your body, and giving fireflies their glow? • Oxidation-Reduction Reactions, also called • REDOX reactions

  3. I. What are Redox Reactions? • Redox reactions involve the transfer of ELECTRONS (e–) between different atoms Na + Cl → Na + Cl

  4. Oxidation Reduction

  5. * CONSERVATION OF CHARGE: oxidation and reduction happen simultaneously (at the same time) – you can’t have one without the other

  6. Remember: LEO the lion says GER (or LEO the liger says GER) • LOSING ELECTRONS is OXIDATION • GAINING ELECTRONS is REDUCTION

  7. II. Determining if a reaction is a redox reaction • If the oxidation number/state of an atom changes from reactant to product, a redox reaction has occurred *Note: Double replacement reactions are NOT redox reactions Single replacement reactions are ALWAYS redox reactions

  8. A. Assigning Oxidation Numbers(listed on the Periodic Table) 1. If a species is all by itself (not combined with a different element), its oxidation number is ZERO Ex: Mg O2 S2 Cu H2

  9. 2. In a polyatomic ion, the elements’ oxidation numbers add up to equal the CHARGE on the ion * SOS: you must multiply the Subscript by the Oxidation number to get the Sum NH4+ ClO–

  10. 2. In a polyatomic ion, the elements’ oxidation numbers add up to equal the CHARGE on the ion * SOS: you must multiply the Subscript by the Oxidation number to get the Sum SO32– SO42–

  11. 3. In a compound, the oxidation numbers of the elements add up to ZERO (compounds are neutral) * SOS: you must multiply the Subscript by the Oxidation number to get the Sum NaCl

  12. 3. In a compound, the oxidation numbers of the elements add up to ZERO (compounds are neutral) • SOS: you must multiply the Subscript by the Oxidation number to get the Sum NH4Cl

  13. 3. In a compound, the oxidation numbers of the elements add up to ZERO (compounds are neutral) • SOS: you must multiply the Subscript by the Oxidation number to get the Sum H2SO3

  14. More Examples a.)  What is the oxidation number of chlorine in iron (III) chloride?

  15. More Examples b.) What is the oxidation state of chromium in potassium chromate?

  16. B. Checking for Changes in Oxidation Numbers 2 Na (s) + Cl2 (g) → 2NaCl (s) • The oxidation # on Na changes from 0 to +1 • Na loses electrons • Na is oxidized • Na is the reducing agent

  17. B. Checking for Changes in Oxidation Numbers 2Mg (s) + O2 (g) → 2MgO (s) • The oxidation # on Mg changes from 0 to +2 • Mg loses electrons • Mg is oxidized • Mg is the reducing agent

  18. a.) Zn + 2HCl → ZnCl2 + H2

  19. b.) 4Al + 3O2 → 2Al2O3

  20. Orange Packet #s 2, 3, 19, 20, 22, 24, 26-30, 32-35

  21. In the presence of oxygen, iron forms iron (III) oxide, more commonly known as rust. On a half-sheet of paper with your name on it, write the oxidation and reduction half-reactions for the reaction described above.

  22. What causes fruit to turn brown? • Oxidation of compounds in fruit after exposure to oxygen in the air causes a change of color (browning) of fruit

  23. Oxidation (losing electrons) is a process that can be very damaging to living things. • results in the creation of free radicals, which are chemical species that have unpaired electrons (they are unhappy – they don’t have a completed valence shell) • free radicals will rip electrons away from other compounds in living things, such as proteins or molecules of DNA, causing damage which can lead to cancer • Antioxidants get rid of free radicals • Vitamins A, C, and E are antioxidants

  24. V. Electrochemical Cells A. Use REDOX reactions to convert chemical energy into electrical energy OR convert electrical energy into chemical energy

  25. V. Electrochemical Cells A. Use REDOX reactions to convert chemical energy into electrical energy OR convert electrical energy into chemical energy

  26. 1. Voltaic (Galvanic) Cells =batteries a.) SPONTANEOUSredox reactions – NO energy put in (energy is released) Use Table J to determine what is oxidized and what is reduced Higher on Table J is OXIDIZED Lower on Table J is REDUCED

  27. *Parts to Know* • Electrodes: the places where oxidation or reduction happens • Anode: the site of oxidation(An Ox) • Cathode: the site of reduction(Red Cat) • Wire: electrons flow through the wire from the anode (–) to the cathode (+) • Salt bridge: allows IONS to flow between the electrodes to keep the charges balanced

  28. The electrodes must be separated in order to produce an electric current (flow of electrons). The energy present in the flowing electrons (ELECTRICITY) is captured and used to power other processes.

  29. b.) Diagram of a voltaic cell using Zn and Zn(NO3)2 with Cu and Cu(NO3)2

  30. c.) Voltaic cell problems: • Look on Table J and find which element is higher – this element is OXIDIZED (On Table J – electrons flow DOWNHILL, spontaneously) • Under each beaker write “oxidation” or “reduction” • Label the oxidation electrode ANODE (An Ox) • Label the reduction electrode CATHODE (Red Cat)

  31. 5. Place a (–) charge on the anode 6. Place a (+) charge on the cathode 7. Draw in the direction of electron flow (from ANODE (–) to CATHODE (+)) • Write the half reactions under the correct beakers

  32. 9. The salt bridge allows for ions to flow between the electrodes Positive (+) ions flow toward the CATHODE (to balance the negative electrons) Negative (–) ions flow toward the ANODE (to replace the electrons that are leaving)

  33. How many of you have had cavities filled? …if you do, you have the potential to make a tiny battery IN YOUR MOUTH! • Cavities are filled with a mixture of metals including zinc (Zn), tin (Sn), copper (Cu), and silver (Ag) • If you bite down on a piece of aluminum foil, the saliva in your mouth, the aluminum foil, and the filling make a little voltaic cell that produces a tiny current that travels through your tooth to the nerve below the filling…it’s a bit UNPLEASANT!

  34. Voltaic cell animation • http://www.blackgold.ab.ca/ICT/Division4/Science/Div.%204/Voltaic%20Cells/demo.htm

  35. 2. Electrolytic Cells a.) NONSPONTANEOUS redox reactions – need to put in energy b.) The species more likely to lose electrons is forced to gain electrons (On Table J – electrons are moving UPHILL, nonspontaneously)

  36. 2. Electrolytic Cells c.) Electrons still flow from the anode to the cathode, but now the signs are reversed • The anode is (+) and the cathode is (–) (electrons are being forced to travel to the negative electrode)

  37. 2. Electrolytic Cells d.) Used for 1. Electrolysis: using electricity to break apart (lyse) compounds into their elements Ex:Obtaining active elements from compounds (a) Complete and balance the following electrolysis reactions. (b) Assign oxidation numbers to each element. ___NaCl (l)

  38. ____H2O (l)

  39. 2. Electrolytic Cells d.) Used for 2. Electrolytic cells can also be used for electroplating – putting a metal coating on something

  40. Anode:

  41. Cathode:

  42. Comparing & Contrasting Voltaic and Electrolytic Cells Voltaic Cells (Batteries) Electrolytic Cells) • SPONTANEOUS • redoxrxn • energy is produced • (electricity) • anode is (-) • cathode is (+) • nonspontaneous • redoxrxn • energy is needed • (needs electricity) • anode is (+) • cathode is (-)

  43. Comparing & Contrasting Voltaic and Electrolytic Cells Voltaic Cells (Batteries) Electrolytic Cells) • redox reactions • anode is where oxidation happens • (An Ox) • anode loses mass • cathode is where reduction happens • (Red Cat) • cathode gains mass • electrons flow through wire • from anode to cathode

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