Oxidation-Reduction Reactions Chemistry Worksheet Page 2
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41208
Chapter 6
Oxidation-Reduction Reactions
6.1
An Introduction to Oxidation-Reduction Reactions
Zinc oxide is a white substance used as a pigment in rubber, sun‑blocking ointments,
and paint. It is added to plastics to make them less likely to be damaged by ultraviolet
radiation and is also used as a dietary supplement. It can be made from the reaction of
pure zinc and oxygen:
2Zn(s) + O
(g) → 2ZnO(s)
2
In a similar reaction that occurs every time you drive your car around the block,
nitrogen monoxide is formed from some of the nitrogen and oxygen that are drawn
into your car’s engine:
(g) + O
(g) → 2NO(g)
N
2
2
This nitrogen monoxide in turn produces other substances that lead to acid rain and
help create the brown haze above our cities.
When an element, such as zinc or nitrogen, combines with oxygen, chemists say
it is oxidized (or undergoes oxidation). They also use the term oxidation to describe
many similar reactions that do not have oxygen as a reactant. This section explains
the meaning of oxidation and shows why oxidation is coupled with a corresponding
chemical change called reduction.
Oxidation, Reduction, and the Formation of Binary Ionic Compounds
2+
Zinc oxide is an ionic compound made up of zinc cations, Zn
, and oxide anions,
2‒
O
. When uncharged zinc and oxygen atoms react to form zinc oxide, electrons are
transferred from the zinc atoms to the oxygen atoms to form these ions. Each zinc
atom loses two electrons, and each oxygen atom gains two electrons.
2Zn(s) + O
(g) → 2ZnO(s)
Overall reaction:
2
2+
‒
2+
‒
O
2
Zn → Zn
+ 2e
2Zn → 2Zn
+ 4e
What happens to Zn:
or
bjeCtive
O + 2e
‒
→ O
2‒
+ 4e
‒
→ 2O
2‒
What happens to O:
or
O
2
As we saw in Chapter 3, this transfer of electrons from metal atoms to nonmetal
atoms is the general process for the formation of any binary ionic compound from its
elements. For example, when sodium chloride is formed from the reaction of metallic
sodium with gaseous chlorine, each sodium atom loses an electron, and each chlorine
atom gains one.
2Na(s) + Cl
(g) → 2NaCl(s)
Overall reaction:
2
+
+
Na → Na
+ e
‒
2Na → 2Na
+ 2e
‒
or
Cl + e
‒
→ Cl
‒
+ 2e
‒
→ 2Cl
‒
or
Cl
2
The reactions that form sodium chloride and zinc oxide from their elements are so
similar that chemists find it useful to describe them using the same terms. Zinc atoms
O
2
bjeCtive
that lose electrons in the reaction with oxygen are said to be oxidized; therefore, when
sodium atoms undergo a similar change in their reaction with chlorine, chemists say
they too are oxidized, even though no oxygen is present. According to the modern
convention, any chemical change in which an element loses electrons is called an
oxidation (Figure 6.1).
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