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OXIDATION-REDUCTION REACTIONS (REDOX)

OXIDATION-REDUCTION REACTIONS (REDOX). A Change in Charge Clic k here to start the podcast. REDOX BASICS. transfer of electrons between two atoms. Na. F. Oxidation Numbers. the ox. # of free elements is ZERO (including diatomics like O 2 and polyatomics like P 4 or S 8 )

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OXIDATION-REDUCTION REACTIONS (REDOX)

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  1. OXIDATION-REDUCTION REACTIONS (REDOX) A Change in Charge Click here to start the podcast

  2. REDOX BASICS • transfer of electrons between two atoms Na F

  3. Oxidation Numbers • the ox. # of free elements is ZERO (including diatomics like O2 and polyatomics like P4 or S8) • the ox. # of a monatomic ion = its charge (like Na+1) • Other elements: • O = -2 (except in peroxide, each O = -1) • H = +1 (except in metal hydrides, then H = -1) • Halogens = -1 (except when central atom of polyatomic ions, like ClO3-1)

  4. Identify the oxidation number for the underlined element • H2O • H2O2 • FeSO4 • FeSO4 • HClO • HClO2 • HClO3 • HClO4 Click here for answers

  5. Is it Oxidation or Reduction or neither? • Silver ions change to raw (elemental) silver • Hydrogen gas changes to hydrogen ions • Blue CrCl2 changes to green CrCl3 • Yellow K2CrO4 changes to orange K2Cr2O7 Click here for answers

  6. Oxidation-Reduction Reactions Single Replacement: Zn(s) + HCl(aq) ZnCl2(aq) + H2(g) Synthesis: Mg(s) + O2(g)  MgO(s) Decomposition: Fe2O3(s)  Fe(s) + O2(g) Combustion: CH4(g) + O2(g)  CO2(g) + H2O(g) These types of reactions can be balanced by inspection

  7. Redox reactions in acid/base solutions MnO4- + S2O3-2 + H+ Mn+2 + SO4-2 + H2O Br2 + OH-  Br- + BrO3- + H2O S8 + O2 + H2O  SO4-2 + H+ These will be balanced using the method of half-reactions

  8. Balancing with Half-Reactions • Split the reaction into half reactions involving similar species (ignore H+, OH-, and H2O) • Balance atoms of all elements EXCEPT O and H • Balance O by adding H2O to the side that has fewer O atoms • Balance H by adding H+ to the side that has fewer H atoms • If acidic solution: continue on to step 5 • If basic solution: add enough OH- to the same side that you added H+ to. Change H+ and OH- to H2O. Add same amount of OH- to the other side. • Balance by CHARGE • Determine total charge on each side of each half-reaction • Add electrons to the more positive side • Use a multiplier to get electrons in each reaction to be equal • Add half reactions together and cancel any terms that are the same on opposite sides.

  9. MnO4- + S2O3-2 + H+ Mn+2 + SO4-2 + H2O

  10. Br2 + OH-  Br- + BrO3- + H2O

  11. Fe+2 + Cr2O7-2 Fe+3 + Cr+3 (in acidic solution) Click here for answers

  12. S8  S2O3-2 + S-2(in basic solution) Click here for answers

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