Oxidation-Reduction Reactions Chemistry Worksheet Page 4
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41210
Chapter 6
Oxidation-Reduction Reactions
to lose electrons and be oxidized, another element or compound must be there to gain
the electrons and be reduced. In other words, oxidation (loss of electrons) must be
accompanied by reduction (gain of electrons). In the reaction that forms ZnO from
Zn and O
, the uncharged zinc atoms cannot easily lose electrons and be oxidized
2
unless something such as oxygen is there to gain the electrons and be reduced. In the
reaction that converts NaCl to Na and Cl
, the chloride ions can lose electrons and be
2
oxidized because the sodium ions are available to gain the electrons and be reduced.
By similar reasoning, we can say that reduction requires oxidation. Because electrons
are not likely to be found separated from an element or compound, a substance cannot
gain electrons and be reduced unless there is another substance that is able to transfer
the electrons and be oxidized. Oxidation and reduction take place together.
Reactions in which electrons are transferred, resulting in oxidation and reduction,
are called oxidation-reduction reactions. Because the term oxidation-reduction is a bit
cumbersome, we usually call these reactions redox reactions.
Even though the oxidation and reduction of a redox reaction take place simultaneously,
each making the other possible, chemists often have reason to describe the reactions
separately. The separate oxidation and reduction equations are called half-reactions.
For example, in the reaction:
2Zn(s) + O
(g) → 2ZnO(s)
2
the oxidation half‑reaction is
2Zn → 2Zn
2+
+ 4e
‒
and the reduction half‑reaction is
+ 4e
‒
→ 2O
2‒
O
2
Because the zinc atoms lose the electrons that make it possible for the oxygen atoms to
gain electrons and be reduced, the zinc is called the reducing agent. A reducing agent
is a substance that loses electrons, making it possible for another substance to gain
electrons and be reduced. The oxidized substance is always the reducing agent.
Because the oxygen atoms gain electrons and make it possible for the zinc atoms to
lose electrons and be oxidized, the oxygen is called the oxidizing agent. An oxidizing
agent is a substance that gains electrons, making it possible for another substance to
lose electrons and be oxidized. The reduced substance is always the oxidizing agent.
In the reaction that forms sodium chloride from the elements sodium and chlorine,
sodium is oxidized, and chlorine is reduced. Because sodium makes it possible for
chlorine to be reduced, sodium is the reducing agent in this reaction. Because chlorine
makes it possible for sodium to be oxidized, chlorine is the oxidizing agent.
+
(g) → 2NaCl(s)
2Na(s)
Cl
2
reducing
oxidizing
agent
agent
+
‒
2Na → 2Na
+ 2e
Oxidation Half‑reaction:
‒
‒
+ 2e
→ 2Cl
Reduction Half‑reaction:
Cl
2
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