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Oxidation-Reduction Reactions “ Redox ”

Oxidation-Reduction Reactions “ Redox ”. LEO SAYS GER. Oxidation and Reduction ( Redox ). Electrons are transferred. Spontaneous redox rxns can transfer energy Electrons (electricity) Heat. Non-spontaneous redox rxns can be made to happen with electricity.

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Oxidation-Reduction Reactions “ Redox ”

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  1. Oxidation-Reduction Reactions“Redox” LEO SAYS GER

  2. Oxidation and Reduction (Redox) • Electrons are transferred • Spontaneous redoxrxns can transfer energy • Electrons (electricity) • Heat • Non-spontaneous redoxrxns can be made to happen with electricity

  3. Reactions in which one or more electrons are transferred. Transfer may occur to form ions Oxidation – increase in oxidation state (loss of electrons); reducing agent Reduction – decrease in oxidation state (gain of electrons); oxidizing agent Note: Combustion reactions are redox! Double replacement reactions are NEVER redox! Redox Reactions

  4. Reaction of Sodium and Chlorine

  5. FIMHO 1. F—free elements and fluorine. Free elements always have an oxidation number of 0. Fluorine is always -1. Rules for Assigning Oxidation Numbers

  6. 2. I—ions. The oxidation number of ions is the charge of the ion. 3. M—metals. Use the Periodic Table to get the charge of a metal. If the metal has more than one charge, try to determine the charge from the second element. Ex—ZnCl2

  7. 4. H—hydrogen. H is always +1 EXCEPT when H is bonded to a Group 1 metal. When H is bonded to a Group 1 metal it is -1. Ex.—NaH vs. HCl 5. O—oxygen. O is always -2 EXCEPT when it is a peroxide (-1) or bonded to F (+2). Ex—Na2O2

  8. ALL OXIDATION NUMBERS MUST ADD TO THE TOTAL CHARGE ON THE COMPOUND OR ION!!

  9. Trends in Oxidation and Reduction • Active metals: • Lose electrons easily • Are easily oxidized • Active nonmetals: • Gain electrons easily • Are easily reduced

  10. Find the oxidation states for each of the elements in each of the following compounds: K2Cr2O7 CO32- MnO2 PCl5 SF4 Example K = +1; Cr = +6; O = –2 C = +4; O = –2 Mn = +4; O = –2 P = +5; Cl = –1 S = +4; F = –1

  11. Which of the following are oxidation-reduction reactions? Identify the oxidizing agent and the reducing agent. Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) Cr2O72-(aq) + 2OH-(aq)  2CrO42-(aq) + H2O(l) 2CuCl(aq)  CuCl2(aq) + Cu(s) Example

  12. Write the unbalanced equation. Determine the oxidation states of all atoms in the reactants and products. Show electrons gained and lost. Use coefficients to equalize the electrons gained and lost. Balance the rest of the equation by inspection. Add appropriate states. Balancing Oxidation–Reduction Reactions by Oxidation States

  13. Balance the reaction between solid zinc and aqueous hydrochloric acid to produce aqueous zinc chloride and hydrogen gas.

  14. Zn(s) + HCl(aq)  Zn2+(aq) + Cl–(aq) + H2(g) 1. What is the unbalanced equation?

  15. Zn(s) + HCl(aq)  Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0 2. What are the oxidation states for each atom?

  16. 1 e– gained (each atom) Zn(s) + HCl(aq)  Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0 2 e– lost The oxidation state of chlorine remains unchanged. 3. How are electrons gained and lost?

  17. 1 e– gained (each atom) × 2 Zn(s) + HCl(aq)  Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0 2 e– lost Zn(s) + 2HCl(aq)  Zn2+(aq) + Cl–(aq) + H2(g) 4. What coefficients are needed to equalize the electrons gained and lost?

  18. Zn(s) + 2HCl(aq)  Zn2+(aq) + 2Cl–(aq) + H2(g) 5. What coefficients are needed to balance the remaining elements?

  19. Balancing Practice Cl2+ OH-ClO- + Cl- + H2O Kent Chemistry Balancing Redox

  20. Balancing Redox Rxns in Acidic/Basic Sol’ns How will I know that the rxn is in an acidic sol’n? 1) It is explicitly said in the problem. 2) An acid (usually a strong acid) is included as one of the reactants. 3) An H+ is written just above the reaction arrow.

  21. Balancing Redox Rxns in Acidic Sol’ns 3 other species are available: • H2O – b/c the rxn takes place in sol’n • H+ - b/c it’s in an acidic sol’n • e¯ - b/c that’s what’s transferred during redox • All three will be used in getting the final answer.

  22. Balancing Redox Reactions in Acidic SolutionsAEIOU Method After writing unbalanced half reactions… A: Balance all Atoms EXCEPT for H & O. E: Add Electrons to appropriate side to balance oxidation state of element that is changing. I: Add H+Ions to appropriate side to balance overall charge in half reactions. O: Add H2O to appropriate side to balance Oxygens. U: Multiply half reaction(s) by a factor to cancel the electrons, Unite the half reactions, & consolidate.

  23. Balancing Redox Rxns in Acidic Sol’ns MnO4-+ Fe2+ Fe3+ + Mn2+(acidic)

  24. Balancing Redox Rxns in Acidic Sol’ns MnO4- (aq) + C2O42- (aq) → Mn2+ (aq) + CO2 (g)

  25. Balancing Redox Rxns in Acidic Sol’ns • Cr2O72¯ + Fe2+ ---> Cr3+ + Fe3+

  26. Balancing Redox Rxns in Basic Sol’ns How will I know that the rxn is in a basic sol’n? 1) It is explicitly said in the problem. 2) A base (usually a strong base) is included as one of the reactants. 3) An OH-is written just above the reaction arrow.

  27. Balancing Redox Rxns in Basic Sol’ns 4other species are available: • H2O – b/c the rxn takes place in sol’n • H+ - b/c some water ionizes • OH- - b/c it’s in a basic sol’n • e¯ - b/c that’s what’s transferred during redox • All fourwill be used in getting the final answer.

  28. Balancing Redox Rxns in Basic Sol’ns Rules—Basic solution • Write reactions as ½ reactions—One for oxidation & one for reduction • Balance all elements except H and O • Balance H with H+ • Balance O with H2O • Add equal #s of OH- to both sides to cancel the H+’s, & make them into H2O • Cancel any resulting water molecules with water molecules on the opposite side of the equation. • Balance charge in each half rxnwith e- • Multiply ½ reactions by factors such that the e- cancel • Add & consolidate both ½ reactions

  29. Balancing Redox Reactions in Acidic SolutionsAEIOU Method After writing unbalanced half reactions… A: Balance all Atoms EXCEPT for H & O. E: Add Electrons to appropriate side to balance oxidation state of element that is changing. I: Add H+Ions to appropriate side to balance overall charge in half reactions. O: Add H2O to appropriate side to balance Oxygens. **Add OH- on each side – a # that will cancel out the H+ you added – & form H2O.** U: Multiply half reaction(s) by a factor to cancel the electrons, Unite the half reactions, & consolidate.

  30. Balancing Redox Rxns in Basic Sol’ns NO2- (aq) + Al (s) → NH3 (aq) + Al(OH)4- (aq) (basic)

  31. Balancing Redox Rxns in Basic Sol’ns Br¯ + MnO4¯ ---> MnO2 + BrO3¯

  32. Redox Reaction Prediction #1

  33. Redox Reaction Prediction #2

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