Download 1 General Information: Rules for Determining Oxidation Numbers 1

115 PLTL Activity Sheet # 9
General Information: Rules for Determining Oxidation Numbers
1. Elements in molecules consisting of just that element are assigned an oxidation number of zero
Na, Fe, Cl2, N2, P4, S8
2. Monatomic ions and ionic compounds have an oxidation number equal to the individual ion
charge
Cl-, S-2, Al+3, MnCl2, PbBr2, TiCl4
3. In compounds:
Group IA metals are assigned an oxidation number of +1
Group IIA metals are assigned an oxidation number of +2
Group IIIA metals are assigned an oxidation number of +3
4. In a compound, fluorine is assigned an oxidation number of -1
5. In a compound, hydrogen is assigned an oxidation number of +1
Exception: Metal hydrides (Rule 3 takes priority)
6. In a compound, oxygen is assigned and oxidation number of -2
Exception: Peroxides- H2O2, Na2O2, BaO2 (O = -1)
Superoxides – KO2, RbO2, CsO2 (O = -1/2)
Others – OF2 (O = +2)
7. The sum of oxidation numbers in a neutral compound is zero and in a polyatomic ion the
sum is equal to its net charge.
8. Apply the oxidation number rules in the order given.
Activity 1: Determine the oxidation number of the underlined element in the following compounds.
Oxidation Number
Oxidation Number
Oxidation Number
HClO
NaClO
H2SO3
NH4+
SO42−
HNO3
CO32−
S6
Ca2+
CO
MnO4−
H3PO4
AlO2−
HClO
P4
PbO2
LiH
Fe(OH)3
UF6
K2Cr2O7
Na4P2O7
Ag
ZnO22−
UO22+
HNO2
Fe(OH)2
HAsO42−
Na2S2O3
O3
BaH2
N2
VO2+
S2O32−
Na2O
HBr
NO2
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115 PLTL Activity Sheet # 9
BeO
Al3+
Na+
P2O3
Cl2
Sb2O5
General Information: Balancing Redox Reactions via the Oxidation Number[Arrow] Method
1. Assign oxidation numbers for every species in the reaction.
2. Draw arrows connecting the species that have been oxidized and reduced.
3. Determine the electrons gained and lost.
4. Set the gained electrons equal to the lost electrons (you will have to multiply).
5. The multiplier becomes the new coefficient for everything associated with the arrow. See below
for Acidic or Basic conditions.
6. Re-write the reaction with the new coefficients.
7. Check the mass and charge.
Acidic and Basic Conditions:
a. Acidic Conditions: Add H2O to balance oxygen atoms then add H+ to balance hydrogen atoms.
b. Basic Conditions: Add OH− to balance charge then add H2O to balance H atoms (or O atoms)
Activity 2: Balance the following redox reactions using the oxidation state/ arrow method.
Fe3+
+ Tl+
→
Fe2+
+ Tl3+
Al
+ Cd+2
→
Al3+
+ Cd
MnO4−
+ Fe+2
→
Mn2+
+ Fe3+
[Acidic solution]
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115 PLTL Activity Sheet # 9
General Information: Balancing Simple, Acidic & Basic Redox Reactions via the Half−
−Reaction
Method
Step 1: Split the equation up into a reduction half reaction and an oxidation half reaction.
Ignoring hydrogen and oxygen, be sure to match pairs with a common element, in other
words, don't create elements out of thin air.
Step 2: Balance all elements that are not H or O.
Step 3: Balance O atoms by adding H2O's to one side of the equation.
Step 4: Balance H atoms by adding H+ ions to one side of the equation.
Step 5: Balance the electric charge by adding e– to the more positive side.
Note that you are not trying to get both sides equal to zero, just equal to each other.
At this point, the reduction reaction has e– as a reactant and the oxidation reaction has e– as a
product.
Step 6: Make sure the number of e– that were added to the total reduction reaction is the same as
the number of e– that were added to the total oxidation reaction. If not, multiply either one or both
total reactions by an integer that will make them equal.
Step 7: Combine the two half reactions and cancel species that appear on both sides and reduce the
coefficients to the smallest whole numbers.
No e– should appear in the final equation.
Activity 3
Fe3+
+ Tl+
→
Fe2+
+ Tl3+
Al
+ Cd+2
→
Al3+
+ Cd
MnO4−
+ Fe+2
→
Mn2+
+ Fe3+
[Acidic solution]
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115 PLTL Activity Sheet # 9
Activity 4: Identify the oxidizing and the reducing agents for the following reactions
Reaction
1. FeS2
2. Al
Oxidizing Agent
+ O2
Fe(OH)3
→
+ NO3−
AlO2−
→
+ SO4
Reducing Agent
2−
+ NH3
3. Cr2O72−
+ Fe2+
→
Cr3+ + Fe3+
4. MnO4−
+ Fe+2
→
Mn2+
+ Fe3+
5. Na + H2O → Na+ + H2
6. PbO2
7. Fe3+
8. Al
+
Cr3+
→ Pb2+
+ CrO42−
+ Tl+
→
Fe2+
+ Tl3+
+ Cd+2
→
Al3+
+ Cd
Activity 5
Balance the following reaction via the half-reaction method and present your results to the whole
group.
Cr2O72−
+ Fe2+
→
Cr3+ + Fe3+
[acidic]
Now balance the same reaction via the oxidation state/ arrow method and present your results to the
whole group.
Cr2O72−
+ Fe2+
→
Cr3+ + Fe3+
[acidic]
As a group discuss the advantages and disadvantages of each balancing method as you see them. What
strategy will you employ on the exam when asked to balance redox reactions?
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115 PLTL Activity Sheet # 9
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