Balancing Redox Equations

Analyze: Mg + S MgS

• assign Oxidation Numbers

• figure out change in oxidation numbers• what is oxidized?

• what is reduced?

00 00 +2+2 -2-2

S: 0 to -2 = ReductionS: 0 to -2 = Reduction

Mg: 0 to +2 = OxidationMg: 0 to +2 = Oxidation

Mg + S MgS

half-reactions

Mg + S MgS

2 electrons2 electrons

2 electrons2 electrons

00 00 +2 -2+2 -2

Mg is oxidized:Mg is oxidized:

Mg Mg Mg Mg+2+2 + 2 + 2ee--

S is reduced:S is reduced:

S + 2S + 2ee-- S S-2-2

electrons lost = electrons gained

Mg Mg Mg Mg+2+2 + 2 + 2ee--

S + 2S + 2ee-- S S-2-2________++____________________________________________________________

Mg + S + 2Mg + S + 2ee-- Mg Mg+2+2 +2 +2ee-- + S + S-2-2

add half-reactions:

Zn + 2HCl H2 + ZnCl2

• Zn goes from 0 to +2 = oxidation

• H goes from +1 to 0 = reduction

• Cl goes from -1 to -1; no change

00 +1+1 -1-1 00 +2+2 -1-1

Zn +2HCl H2 + ZnCl2

Zn Zn+2 + 2e-

2H+1 + 2e- H2

2 electrons2 electrons

1 electron per H1 electron per H

______________________________________

Zn + 2H+1 +2e- Zn+2 +2e- + H2

Balancing Redox Equations

1.1. assignassign oxidation numbers to all atoms in equation oxidation numbers to all atoms in equation

2.2. determinedetermine elements that elements that changedchanged oxidation number oxidation number

3.3. identifyidentify element oxidized & element reduced element oxidized & element reduced

4.4. writewrite half-reactions (diatomics must stay as is) half-reactions (diatomics must stay as is)

5.5. # electrons# electrons lost & gained must be lost & gained must be equalequal; multiply ; multiply

half-reactions if necessaryhalf-reactions if necessary

6.6. addadd half-reactions; half-reactions;

transfertransfer coefficients to skeleton equation coefficients to skeleton equation

7.7. balancebalance restrest of equation by counting atoms of equation by counting atoms

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

00 +1+1 +5+5 -2-2 +2+2 +5+5 -2-2 00

Half-Reactions

Cu Cu+2 + 2e-

Ag+1 + 1e- AgMultiply Multiply by 2by 2

++____________________________________________

Cu + 2AgCu + 2Ag+1+1 + 2 + 2ee-- 2Ag + Cu 2Ag + Cu+2+2 + 2 + 2ee--

Cu Cu+2 + 2e-

2Ag+1 + 2e- 2Ag

Transfer Coefficients• Compare skeleton equation & sum of ½ rxns:

Cu + AgNO3 Ag + Cu(NO3)2

vs.Cu + 2AgCu + 2Ag+1+1 + 2 + 2ee- - 2Ag + Cu 2Ag + Cu+2+2 + 2 + 2ee--

• Transfer coefficients!

Cu + 2AgNO3 2Ag + Cu(NO3)2

Cu + HNOCu + HNO33 Cu(NO Cu(NO33))22 + NO + NO22 + H + H22OO

half-reactions

Cu Cu+2 + 2e-

N+5 + 1e- N+4

00 +1+1 +5+5 -2-2 +2+2 +5+5 -2-2 +4+4 -2-2 +1+1 -2-2

Cu from 0 to +2 = oxidizedCu from 0 to +2 = oxidizedH from +1 to +1; no changeH from +1 to +1; no changeO from -2 to -2; no changeO from -2 to -2; no changeN starts as +5; N starts as +5; ends as +5 [no change – Cu(NOends as +5 [no change – Cu(NO33))22]]

and ends as +4 = reduction [NOand ends as +4 = reduction [NO22]]

WhatWhat’’s oxidized? Whats oxidized? What’’s reduced?s reduced?

• What is oxidized?

• What is reduced?

• What is the oxidizing agent?

• What is the reducing agent?

CuCu

CanCan’’tt just say N! just say N!ItIt’’s N in the HNOs N in the HNO33 or N or N+5+5

NN+5+5

CuCu

half-reactions

Cu Cu+2 + 2e-

N+5 + 1e- N+4

multiply half-reactions as necessary:

# of electrons lost = # gained# of electrons lost = # gained

Cu Cu+2 + 2e-

2 (N+5 + e- N+4)

balance remaining atoms by inspection

balanced half-reaction coefficients might not completely balance the equationCu + 4HNOCu + 4HNO33 Cu(NO Cu(NO33))22 + 2NO + 2NO22 + 2H + 2H22OO