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 1. 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
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
Balance each 1/2 reactions
MnO4- (aq) --> Mn2+ (aq)
5e- + 8H+(aq) + MnO4-(aq) --> Mn2+ (aq) + 4H2O (l)
Balance each 1/2 reactions
Fe2+ (aq) --> Fe3+ (aq)
Fe2+ (aq) --> Fe3+ (aq)+ e-
5e- + 8H+(aq) + MnO4-(aq) --> Mn2+ (aq) + 4H2O (l)
Balance the charges
5Fe2+ (aq) -->5 Fe3+ (aq)+5 e-
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)
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
Example of basic 1/2 rxn balancing
Ag(s) + CN- (aq) + O2 (g) --> Ag(CN)2- (aq)
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-
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)
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)
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 -
Homework
• Pg 183
• #’s 63-66
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