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.

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Presentation transcript:

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 H 2 O - 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 MnO 4 - (aq) + Fe 2+ (aq) --> Mn 2+ (aq) + Fe 3+ (aq) Isolate 1/2 reactions MnO 4 - (aq) --> Mn 2+ (aq) Fe 2+ (aq) --> Fe 3+ (aq) +2 +3

Balance each 1/2 reactions MnO 4 - (aq) --> Mn 2+ (aq)

5e- + 8H + (aq) + MnO 4 - (aq) --> Mn 2+ (aq) + 4H 2 O (l)

Balance each 1/2 reactions Fe 2+ (aq) --> Fe 3+ (aq)

Fe 2+ (aq) --> Fe 3+ (aq)+ e -

5e- + 8H + (aq) + MnO 4 - (aq) --> Mn 2+ (aq) + 4H 2 O (l) Balance the charges 5Fe 2+ (aq) -->5 Fe 3+ (aq)+5 e -

Add 1/2 reactions together 5Fe 2+ (aq) + 5e- + 8H + (aq) + MnO 4 - (aq) --> Mn 2+ (aq) + 4H 2 O (l) + 5 Fe 3+ (aq)+5 e - Cancel spectators 5Fe 2+ (aq) + 8H + (aq) + MnO 4 - (aq) --> Mn 2+ (aq) + 4H 2 O (l) + 5 Fe 3+ (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) + O 2 (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 O 2 (g) --> Balance O O 2 (g) --> 2H 2 O (l) Balance H O 2 (g) + 4H +-- > 2H 2 O (l) Balance e- 4e- +O 2 (g) + 4H +-- > 2H 2 O (l)

Red 1/2 rxn 4e- +O 2 (g) + 4H +-- > 2H 2 O (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- +O 2 (g) + 4H +-- > 2H 2 O (l) 4Ag(s)+ 8CN - (aq)+ O 2 (g) + 4H + --> 4Ag(CN) 2 - (aq) +2H 2 O (l)

Add OH- to eliminate H+ and H2O 4Ag(s)+ 8CN - (aq)+ O 2 (g) + 4H + +4OH - --> 4Ag(CN) 2 - (aq) +2H 2 O (l) + 4OH - FYI: 4H + +4OH - = 4H 2 O Cancel 4Ag(s)+ 8CN - (aq)+ O 2 (g) + 2H 2 O (l) --> 4Ag(CN) 2 - (aq) + 4OH -

Homework Pg 183 #’s 63-66