Transcript Slide 1

Balancing Redox Reactions
3 I2 (s) + 2 Al (s)  2 AlI3
+7
+3
+2
+4
MnO4- + C2O42-  Mn2+ + CO2
MnO4- + C2O42-  Mn2+ + 2 CO2 2 O2
HELP
4 H2O
4 OH-
MnO4- + C2O42-  Mn2+ + CO2
1. Break it into two half-reactions
MnO4-  Mn2+
C2O42-  CO2
2. Balance redox active species
MnO4-  Mn2+
3. Balance oxygen using H2O
MnO4-  Mn2+ + 4 H2O
+7
+2
MnO4-  Mn2+
4. Balance hydrogen using H+
8 H+ + MnO4-  Mn2+ + 4 H2O
5. Balance charge using e5 e- + 8 H+ + MnO4-  Mn2+ + 4 H2O
This is a balanced half-reaction
electrons are a ________
reactant
MnO4- is _______
gaining electrons
MnO4- is being _______
reduced
Where do the e- come from?
C2O42-  CO2
2. Balance redox active species
C2O42-  2 CO2
3. Balance oxygen using H2O
C2O42-  2 CO2
4. Balance hydrogen using H+
C2O42-  2 CO2
5. Balance charge using eC2O42-  2 CO2 + 2e-
+3
+4
C2O42-  2 CO2 + 2eThis is a balanced half-reaction
electrons are a ________
product
C2O42- is _______
losing electrons
C2O42- is being _______
oxidized
Now combine the two half-reactions
2 ( 5 e- + 8 H+ + MnO4-  Mn2+ + 4 H2O )
5 ( C2O42-  2 CO2 + 2e- )
16H+ + 2MnO4- + 5C2O42- 2Mn2+ + 8H2O + 10CO2
stoichiometry
additional product, H2O
additional reactant, acid
Ag+ (aq) + C6H12O6 (aq) Ag (s) + C6H12O7 (aq)
Ag+  Ag (s)
C6H12O6

C6H12O7
Ag+ + C6H12O6  Ag (s) + C6H12O7
balance mass
Ag+ + e-  Ag (s)
balance oxygen H2O
balance hydrogen H+
balance charge e-
H2O + C6H12O6
balance mass
balance oxygen H2O
balance hydrogen H+
balance charge e-
 C6H12O7 + 2 H+ + 2 e-
H2O + C6H12O6
 C6H12O7 + 2 H+ + 2 e-
2 ( Ag+ + e-  Ag (s) )
H2O + C6H12O6+ 2Ag+  C6H12O7+ 2 H+ + 2Ag (s)
Ag+ reduced
Ag+ oxidizing agent
C6H12O6 oxidized
C6H12O6 reducing agent
C+
C 3+
H2O + C6H12O6
 C6H12O7 + 2 H+ + 2 e-
2 ( Ag+ + e-  Ag (s) )
H2O + C6H12O6+ 2Ag+  C6H12O7 +2 H+ + 2Ag (s)
2 OH2 OH2 H2O
This reaction takes place in a basic solution OH1
+
2OH +H2O+C6H12O6+2Ag  C6H12O7 +2Ag (s)+2 H2O
2OH- + C6H12O6 +2Ag+  C6H12O7 +2Ag (s) + H2O
CN-(aq) + MnO4- (aq)  CNO- (aq) + MnO2 (s)
separate half-reactions
CN-  CNO-
MnO4-  MnO2
balance mass
CN-  CNO-
MnO4-  MnO2
balance oxygen
H2O+ CN-  CNO-
MnO4-  MnO2+ 2 H2O
H2O+ CN-  CNO-
MnO4-  MnO2+ 2 H2O
balance hydrogen
H2O+ CN- CNO-+2 H+ MnO4-+4H+ MnO2+ 2H2O
balance charge
3 ( H2O+ CN-CNO- + 2 H+ +2 e- )
2 ( 3 e-+ MnO4-+4H+ MnO2+ 2H2O )
3 CN- +2 MnO4- + 2 H+  3 CNO-+ 2 MnO2 + H2O
3 CN- +2 MnO4- + 2 H+  3 CNO-+ 2 MnO2 + H2O
2 OH2 OHmake the solutions basic
add OH-
3 CN- + 2 MnO4- + H2O  3 CNO- + 2 MnO2+ 2 OH-