Download Rules for Redox Rxns

Rules of Oxidation Numbers:
1. An element has an oxidation number of 0
2. A simple ion (Cl–, O2-, P3-, Pt4+, etc) has the same ox. no. as its charge.
3. Alkali metals are always +1;
Alkaline Earth Metals are +2
4. F is –1 {except F2 in rule 1}
5. H is +1 {except rule 3 NaH or CaH2}
6. O is –2 {except rule 3 & 5 Na2O2 and H2O2}
7. Defer to +1, +2, +3, ±4, –3, –2, –1 from valence e– (this is if you have 2 unknowns in
a compound and need to have an oxidation number other then #3 to 6)
8. Finally, unknown atoms have oxidation numbers chose to make compound neutral or
ion have correct charge
Rules for Balancing Redox Reactions
1. Determine the two 1/2 reactions. Find the elements that are changing their oxidation
number. Usually 2 elements, but sometimes only 1: Br2 --> Br– + BrO3–.
2. If a substance contains multiple atoms then use fractions:
1/2 Cl2 + 1e–➔Cl– ; 1/4P4 + 3e– ➔ P3–; 1/2Cr2O72– + 3e–➔ Cr3+
3. Balance the number of electrons in each half rxn; multiply each 1/2 rxn by a
coefficient; apply to all substances.
4. Add 1/2 reactions together (many times the 1/2 reactions are balanced with
5 and 6 before adding together!)
5. Balance O’s with H2O’s
6. Balance H’s with H+’s. Charges should be balanced.
7. For Basic Rxns add same number of OH– ‘s as H+’s, then cancel or combine H2O’s.
Ex 1. +4 (-2) (-1) (+1)
0
0 (+1) (-2)
SeO32- + I– + H+ ➔ Se + I2 + H2O
1•
4•
SeO32– + 4e– ➔ Se
I– ➔ 1e– + ½ I2
SeO32– + 4I– +
➔ Se + 2I2
➠
SeO32– + 4I– + 6H+ ➔ Se + 2I2 + 3H2O
Ex. 2. Br2 ➔ Br– + BrO3–
5•
1•
½Br2 + 1e– ➔ Br–
½Br2 ➔ 5e– + BrO3–
3 Br2 ➔ 5Br– + BrO3–
➠
3 Br2 + 3H2O ➔ 5Br– + BrO3– + 6H+