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1/2 reactions AP style

1/2 reactions AP style. What are 1/2 reactions. Reactions in aqueous solutions can be complicated and hard to balance. Our solution… break the one reaction into 2 smaller reactions (1/2 reactions). Balancing acidic 1/2 reactions. Write 1/2 reactions for oxidation and reduction

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1/2 reactions AP style

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  1. 1/2 reactions AP style

  2. What are 1/2 reactions • Reactions in aqueous solutions can be complicated and hard to balance. • Our solution… break the one reaction into 2 smaller reactions (1/2 reactions)

  3. Balancing acidic 1/2 reactions • Write 1/2 reactions for oxidation and reduction 2. For each 1/2 reaction - balance all elements except O &H - balance O using H2O - balance H using H+ - balance the charge using electrons - May need to multiply by a common factor 3. Add the 1/2 reactions together and cancel spectators. 4. Check that charges are balanced

  4. Example Start with a net ionic equation MnO4- (aq) + Fe2+ (aq) --> Mn2+ (aq)+ Fe3+ (aq) Isolate 1/2 reactions MnO4- (aq) --> Mn2+ (aq) +7 -2 +2 Fe2+ (aq) --> Fe3+ (aq) +2 +3

  5. Balance each 1/2 reactions MnO4- (aq) --> Mn2+ (aq)

  6. 5e- + 8H+(aq)+ MnO4-(aq) --> Mn2+ (aq) + 4H2O (l)

  7. Balance each 1/2 reactions Fe2+ (aq) --> Fe3+ (aq)

  8. Fe2+ (aq) --> Fe3+ (aq)+ e-

  9. 5e- + 8H+(aq)+ MnO4-(aq) --> Mn2+ (aq) + 4H2O (l) Balance the charges 5Fe2+ (aq) -->5 Fe3+ (aq)+5 e-

  10. Add 1/2 reactions together 5Fe2+ (aq) + 5e- + 8H+(aq)+ MnO4-(aq) --> Mn2+ (aq) + 4H2O (l) + 5 Fe3+ (aq)+5 e- Cancel spectators 5Fe2+ (aq) + 8H+ (aq)+ MnO4-(aq) --> Mn2+ (aq) + 4H2O (l) + 5 Fe3+ (aq)

  11. Balancing Basic 1/2 reactions • Balance 1/2 reactions as in acidic as if H+ ions were present • Add OH- to balance H+ • Combine OH- and H+ to form H2O • Cancel out spectator H2O’s

  12. Example of basic 1/2 rxn balancing Ag(s) + CN- (aq) + O2 (g) --> Ag(CN)2- (aq)

  13. Ag(s) + CN- (aq) --> Ag(CN)2- (aq) Balance C and N Ag(s) + 2CN- (aq) --> Ag(CN)2- (aq) Balance charge Ag(s) + 2CN- (aq) --> Ag(CN)2- (aq) + e-

  14. Balance red. 1/2 reaction O2 (g) --> Balance O O2 (g) --> 2H2O (l) Balance H O2 (g) + 4H+--> 2H2O (l) Balance e- 4e- +O2 (g) + 4H+--> 2H2O (l)

  15. Red 1/2 rxn 4e- +O2 (g) + 4H+--> 2H2O (l) Ox 1/2 rxn Ag(s) + 2CN- (aq) --> Ag(CN)2- (aq) + e- Balance charge 4(Ag(s) + 2CN- (aq) --> Ag(CN)2- (aq) + e-) Add ‘em together 4Ag(s) + 8CN- (aq) --> 4Ag(CN)2- (aq) + 4e- 4e- +O2 (g) + 4H+--> 2H2O (l) 4Ag(s)+ 8CN- (aq)+ O2 (g) + 4H+ --> 4Ag(CN)2- (aq) +2H2O (l)

  16. Add OH- to eliminate H+ and H2O 4Ag(s)+ 8CN- (aq)+ O2 (g) + 4H+ +4OH ---> 4Ag(CN)2- (aq) +2H2O (l) + 4OH - FYI: 4H+ +4OH -= 4H2O Cancel 4Ag(s)+ 8CN- (aq)+ O2 (g) + 2H2O (l) --> 4Ag(CN)2- (aq) + 4OH -

  17. Homework • Pg 183 • #’s 63-66

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