Download Oxidation Numbers

Chemistry 20.2
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20.2
Oxidation Numbers
When aluminum or magnesium
powder in fireworks are heated
to high temperatures in the
explosion, they burn with an
intense white light. As elements
burn, their oxidation numbers
change. You will learn about how
oxidation and reduction are
defined in terms of a change in
oxidation number.
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
Assigning Oxidation Numbers
What is the general rule for assigning
oxidation numbers?
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
An oxidation number is a positive or negative
number assigned to an atom to indicate its
degree of oxidation or reduction.
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
As a general rule, a bonded atom’s
oxidation number is the charge that it
would have if the electrons in the bond
were assigned to the atom of the more
electronegative element.
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
The oxidation number of any element in the free
or uncombined state is zero.
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
Oxidation numbers are often written above the
chemical symbols in a formula.
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20.2
Oxidation Numbers
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Assigning Oxidation Numbers
K2CrO4
Cr oxidation
number = +6
Cr2O3
Cr oxidation
number = +3
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Practice Problems for Conceptual Problem 20.2
Problem Solving 20.9 Solve
Problem 9 with the help of an
interactive guided tutorial.
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20.2
Oxidation Numbers
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Oxidation-Number Changes in
Chemical Reactions
Oxidation-Number Changes in Chemical Reactions
How are oxidation and reduction defined
in terms of a change in oxidation
number?
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20.2
Oxidation Numbers
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Oxidation-Number Changes in
Chemical Reactions
An increase in the oxidation number of
an atom or ion indicates oxidation. A
decrease in the oxidation number of an
atom or ion indicates reduction.
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20.2
Oxidation Numbers
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Oxidation-Number Changes in
Chemical Reactions
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20.2
Oxidation Numbers
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Oxidation-Number Changes in
Chemical Reactions
When an iron nail is placed in a copper(II)
sulfate solution, the iron reduces Cu2+ ions in
solution and is simultaneously oxidized to Fe2+
The iron becomes coated with metallic copper.
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Practice Problems for Conceptual Problem 20.3
Problem Solving 20.11 Solve
Problem 11 with the help of an
interactive guided tutorial.
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20.2 Section Quiz.
Assess students’ understanding
of the concepts in Section 20.2.
Continue to:
-or-
Launch:
Section Quiz
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20.2 Section Quiz.
1. Which of the following oxidation numbers is
INCORRECT?
a. ClO3– ; Cl +5, O –2
b. SO32– ; S +4, O –2
c. H2O2; H +1, O –1
d. Ca(ClO)2; Ca +2, Cl +2, O –2
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20.2 Section Quiz.
2. What is the oxidation number of Co in CoCl63–
?
a. –3
b. –6
c. +9
d. +3
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20.2 Section Quiz.
3. In the reaction PbO2(aq) + 4HBr(aq) 
PbBr2(aq) + Br2(l) + 2H2O(l) which element
has the greatest change in oxidation number?
a. hydrogen
b. oxygen
c. bromine
d. lead
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END OF SHOW