Balancing Redox Equations

Analyze: Mg + S  MgS +2 -2 What is oxidized? What is reduced? -2 +2 Analyze: Mg + S  MgS What is oxidized? What is reduced? Assign Oxidation Numbers Figure out change in oxidation numbers Mg: 0 to +2 = Oxidation S: 0 to -2 = Reduction

Mg + S  MgS 2 electrons 2 electrons Identify what species is oxidized & what species is reduced Figure out the 2 half-reactions

Half-Reactions Mg + S  MgS +2 -2 Mg is oxidized: S is reduced: +2 -2 Mg + S  MgS Mg is oxidized: Mg  Mg+2 + 2e- S is reduced: S + 2e-  S-2

Adjust half-reactions so that electrons lost = electrons gained Add half-reactions: Mg  Mg+2 + 2e- S + 2e-  S-2 __________________________________ Mg + S + 2e-  Mg+2 +2e- + S-2 Balance elements by counting atoms

Zn + 2HCl  H2 + ZnCl2 +1 -1 +2 -1 Zn goes from 0 to +2 = oxidation +1 -1 +2 -1 Zn goes from 0 to +2 = oxidation H goes from +1 to 0 = reduction Cl goes from -1 to -1; No change

Zn +2HCl  H2 + ZnCl2 Zn  Zn+2 + 2e- 2H+1 + 2e-  H2 2 electrons Zn +2HCl  H2 + ZnCl2 1 electron per H Zn  Zn+2 + 2e- 2H+1 + 2e-  H2 ______________________________________ Zn + 2H+1 +2e-  Zn+2 +2e- + H2

Balancing Redox Equations Assign oxidation numbers to all atoms in equation Determine elements changed oxidation number Identify element oxidized & element reduced Write half-reactions (diatomics must stay as is) Number electrons lost & gained must be equal; multiply half-reactions if necessary Add half-reactions; Transfer coefficients to skeleton equation Balance rest of equation by counting atoms

Cu + AgNO3  Cu(NO3)2 + Ag +1 +5 -2 +2 +5 -2 +1 +5 -2 +2 +5 -2 Cu + AgNO3  Cu(NO3)2 + Ag Cu goes from 0 to +2 = oxidation Ag goes from +1 to 0 = reduction N goes from +5 to +5; No change O goes from -2 to -2; No change

Half-Reactions +______________________ Cu  Cu+2 + 2e- Ag+1 + 1e-  Ag Multiply by 2 +______________________ Cu + 2Ag+1 + 2e-  2Ag + Cu+2 + 2e-

Transfer Coefficients Compare skeleton equation & sum of ½ rxns: Cu + AgNO3  Ag + Cu(NO3)2 vs. Cu + 2Ag+1 + 2e-  2Ag + Cu+2 + 2e- Transfer coefficients! Cu + 2AgNO3  2Ag + Cu(NO3)2

Cu + HNO3  Cu(NO3)2 + NO2 + H2O Assign Oxidation Numbers +1 +5 -2 +2 +5 -2 +1 -2 +4 -2 Cu + HNO3  Cu(NO3)2 + NO2 + H2O Assign Oxidation Numbers Identify species oxidized & reduced Cu from 0 to +2 = oxidized H from +1 to +1; no change O from -2 to -2; no change N starts as +5; ends as +5 (no change) and ends as +4 = reduction

Cu + HNO3  Cu(NO3)2 + NO2 + H2O Find change in oxidation number Change of +2 Cu + HNO3  Cu(NO3)2 + NO2 + H2O Change of -1 Find change in oxidation number Write half-reactions Cu  Cu+2 + 2e- N+5 + 1e-  N+4

What’s oxidized? What’s reduced? What is oxidized? What is reduced? What is the oxidizing agent? What is the reducing agent? Cu Can’t just say N! It’s N in the HNO3 or N+5 N+5 Cu

Multiply half-reactions as necessary Cu  Cu+2 + 2e- 2N+5 + 2e-  2N+4 [# of electrons lost = # gained] Cu + 2HNO3  Cu(NO3)2 + 2NO2 + H2O Add half-reactions; Transfer coefficients Balance remaining atoms by inspection Cu + 4HNO3  Cu(NO3)2 + 2NO2 + 2H2O