Oxidation-Reduction Reactions Chemistry Worksheet Page 7
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6.2 Oxidation Numbers
6.2
Oxidation Numbers
Phosphates, like ammonium phosphate, are important components of fertilizers used
to stimulate the growth of agricultural crops and to make our gardens green. Their
commercial synthesis requires elemental phosphorus, which can be acquired by heating
phosphate rock (containing calcium phosphate) with sand (containing silicon dioxide)
and coke (a carbon‑rich mixture produced by heating coal). This method for isolating
phosphorus, called the furnace process, is summarized in the first equation below. The
other equations show how phosphorus can be converted into ammonium phosphate.
+ 6SiO
+ 10C → P
+ 10CO + 6CaSiO
2Ca
(PO
)
3
4
2
2
4
3
+ 5O
+ 6H
O → 4H
P
PO
4
2
2
3
4
+ NH
→ (NH
H
PO
)
PO
3
4
3
4
3
4
Are these reactions oxidation‑reduction reactions? Are electrons transferred? Simply
reading a chemical equation does not always tell us whether oxidation and reduction
have occurred, so chemists have developed a numerical system to help identify a reaction
as redox. For redox reactions, this system also shows us which element is oxidized,
which is reduced, what the oxidizing agent is, and what the reducing agent is.
The first step in this system is to assign an oxidation number to each atom in the
reaction equation. As you become better acquainted with the procedure, you will gain
a better understanding of what the numbers signify, but for now, just think of them as
tools for keeping track of the flow of electrons in redox reactions. Oxidation numbers
are also called oxidation states.
If any element undergoes a change of oxidation number in the course of a reaction,
O
4
bjeCtive
the reaction is a redox reaction. If an element’s oxidation number increases in a reaction,
O
5
bjeCtive
that element is oxidized. If an element’s oxidation number decreases in a reaction, that
O
6
bjeCtive
element is reduced. The reactant containing the element that is oxidized is the reducing
agent. The reactant containing the element that is reduced is the oxidizing agent (Table
6.1).
Table 6.1
O
4
O
5
O
6
Questions Answered by the Determination of Oxidation Numbers
bjeCtive
bjeCtive
bjeCtive
Question
Answer
Is the reaction redox?
If any atoms change their oxidation number, the reaction is
redox.
Which element is oxidized?
The element that increases its oxidation number is oxidized.
Which element is reduced?
The element that decreases its oxidation number is reduced.
What’s the reducing agent?
The reactant that contains the element that is oxidized is the
reducing agent.
What’s the oxidizing agent?
The reactant that contains the element that is reduced is the
oxidizing agent.
Study Sheet 6.1 on the next page describes how you can assign oxidation numbers
to individual atoms.
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