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