Chem 30 Redox 7
30–B1.7k write and balance equations for redox reactions in acidic and neutral solutions by
using half-reaction equations obtained from a standard reduction potential table
Breathalyzer units use a reaction that can be used to determine if a person has a BAC over
the legal limit.
Cr2O72- + C2H5OH  CH3COOH + Cr3+ (need complete balanced equation to convict)
http://en.wikipedia.org/wiki/Breathalyzer
(See myths about breathalyzers)
Demo:
- Decomposition of ammonium dichromate
(NH4)2Cr2O7  Cr2O3 + N2
(complete balanced equation?)
http://www.youtube.com/watch?v=SbXC3qAB6F8&feature=related
●Note that 5-step method can’t be used because entities not all on chart
Balancing reactions using Oxidation numbers method
*(If oxidation states don’t change, the rxn is not redox)!
1. Assign oxidation numbers to the individual atoms and write the e- transferred per
individual atom only
2. Write the e- transferred per reactant unit
3. Balance the e- lost and the e- gained. Do this by multiplying the e- transferred by the
simplest whole number coefficients. Write these coefficients for the reactant units.
4. Write coefficients for product units so that the OA and RA atoms on both sides are
equal.
5. Balance O atoms by adding H2O to one side. Balance H atoms by adding H+ to one side.
Basic solutions only…
6. Add OH- to both sides to equal the number of H+ present.
7. Combine the H+ and OH- on the same side to form H2O, and cancel H2O on both sides.
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- Worksheet
- p. 593 #12
- p. 595 #6 (identify the OA and RA unit only)
8a,c
12 (identify the OA and RA unit only)