Oxidation numbers answers
1.
Determine the oxidation number of the specified element in each of the following.
a.
N in NF3
NF3 does not contain H or O, ox. # of F (highest EN) = -1 Because NF3 is a
compound, ox.# = 0.
Let ox.# of N be x:
x + 3(-1)  0
x - 3  0, x  3
The ox.# of nitrogen is +3.
b.
S in S8
Because S8 is the formula of a pure element, the ox.# of S in S8 is 0.
c.
Cr in CrO42O has its usual ox.# of -2. ox.# = -2. Let ox.# of Cr be x.
x + 4(-2)  -2
x - 8 2, x  6
d.
The ox.# of chromium is +6.
P in P2O5
O has its usual ox.# of -2. ox.# = 0.
Let ox.# of P be x.
2x + 5(-2)  0
2x - 10  0, 2x  10, x  5
e.
The ox.# of phosphorus is +5.
C in C12H22O11
ox.# of H is +1. ox.# of O is -2.
ox.# = 0. Let ox.# of C be x.
12x + 22(+1) + 11(-2)  0
12x + 22 - 22  0, 12x  0,
f.
x0
The ox.# of carbon is 0.
C in CHCl3
ox.# of H is +1. ox. # of Cl (highest EN) = -1
x.
ox.# = 0
Let ox.# of C be
x + 1(+1) + 3(-1)  0
x + 1 - 3  0, x - 2  0, x  2
2.
The ox.# of carbon is +2.
Determine the oxidation number of each element in each of the following.
a.
H2SO3
ox.# of H is +1. ox.# of O is -2. ox.# = 0. Let ox.# of S be x.
2(+1) + x + 3(-2)  0
2 + x - 6  0, x - 4  0, x  4
b.
The ox.# of sulfur is +4
OHox.# of H is +1. ox.# of O is -2. ox.# = -1. To check if our assignment is right:
1(-2) + 1(+1)  -1,
c.
-2 + 1  -1 Yeah!
HPO42ox.# of H is +1. ox.# of O is -2.
ox.# = -2.
Let ox.# of P be x.
1(+1) + x + 4(-2)  -2
1 + x - 8  -2,
x - 7  -2,
x5
The ox.# of phosphorus is +5
3.
As stated in rule 4, oxygen does not always have its usual oxidation number of -2.
Determine the oxidation number of oxygen in each of the following.
a.
the compound oxygen difluoride, OF2
Oxygen has an electronegativity of 3.44. Fluorine has an electronegativity
of 3.98. Therefore, ox.# of F is -1.
ox.# = 0. Let ox.# of O be x.
x + 2(-1)  0, x - 2  0, x  2
b.
The ox.# of oxygen is +2.
the peroxide ion, O22ox.# = -2.
Let ox.# of O be x.
2x  -2, x  -1,
4.
The ox.# of oxygen is -1.
Determine the oxidation number of each element in each of the following ionic
compounds by considering the ions separately.
a.
Al(HCO3)3
The compound Al(HCO3)3 contains Al3+ ions and HCO3- ions. ox.# of
Al is +3, ox.# of H is +1, ox.# of O is -2.
ox.# of
HCO3- = -1. Let ox.# of C be x.
1(+1) + x + 3(-2)  -1
1 + x - 6  -1, x - 5  -1, x  4.
b.
The ox.# of carbon is +4
(NH4)3PO4
The compound(NH4)3PO4 contains NH4+ ions and PO43- ions. ox.# of H
is +1, ox.# of NH4+ = +1. Let ox.# of N be x.
x + 4(+1)  +1
x + 4  +1, x  +1 - 4, x  -3
ox.# of O is -2,
ox.# of
The ox.# of nitrogen is -3.
PO43-
= -3. Let ox.# of P be y.
y + 4(-2)  -3
y - 8  -3, y  -3 + 8, y  5
c.
The ox.# of phosphorus is +5
K2H3IO6
The compound K2H3IO6 contains K+ ions and H3IO62- ions. ox.# of K is +1.
H3IO62- : ox.# of H is +1, O is -2.
Let ox.# of I be x.
ox.# of H3IO62- = -2
3(+1) + x + 6(-2)  -2
3 + x - 12  -2,
x - 9  -2, x  7
The ox.# of iodine is +7.
5.Use oxidation numbers to determine whether each reaction is a redox reaction.
a.
H2O2 +
2Fe(OH)2 
2Fe(OH)3
+1 -1
+2 -2 +1
+3 -2 +1
The ox.# of O that originates in Fe(OH)2 is -2 on both sides. The ox.# of O
that originates in H2O2  from -1 to -2. The ox.# of Fe from +2 to +3 the
reaction is a redox reaction.
b.
PCl3 +
3 H2O 
H3PO3
+
3 HCl
+3-1
+1 -2
+1+3-2
+1-1
No elements undergo changes in ox.#, so the reaction is not a redox
reaction.
6.
Identify the oxidizing agent and the reducing agent for the redox reaction(s) in the
previous question.
a.
7.
The ox.# of O in H2O2  from -1 to -2, H2O2 is reduced; ox. agent. The
ox.# of Fe in Fe(OH)2.  from +2 to +3 Fe(OH)2 is oxidized; red.
agent.
For the following balanced net ionic equation, use oxidation numbers to identify the
reactant that undergoes oxidation and the reactant that undergoes reduction.
Br2
+
2 ClO2-  2 Br- +
2 ClO2
0
+3 -2
-1
+4 -2
The ox.# of O is the same on both sides. The ox.# of Cl in ClO2-  from +3 to +4 
ClO2- is oxidized. The ox.# of Br in Br2 from 0 to -1.  Br2 is reduced.
8.
Nickel and copper are two metals that have played a role in the economy of
Newfoundland and Labrador. Nickel and copper ores usually contain the metals as
sulfides, such as NiS and Cu2S. Do the extractions of these pure elemental metals from
their ores involve redox reactions? Explain your reasoning.
In NiS, Ni has an ox.# of +2. Metallic nickel, Ni, is an element with an ox.# of 0.
In Cu2S, Cu has an ox.# of +1. Metallic copper, Cu, is an element with an ox.# of
0.Because each of these metals undergoes a  in its ox.# during extraction, the
extraction process must involve reduction. Oxidation and reduction always occur
together, so the extraction processes must be redox reactions. We do not need to
know the other reactants to know that this conclusion is true.