chapter20_section03_edit.............ppt

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chemistry

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Balancing Redox Equations>
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Oxidation-Number Changes in
Chemical Reactions
Bell Question
How are oxidation and reduction defined
in terms of a change in oxidation
number?
20.2

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Balancing Redox Equations>
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Oxidation-Number Changes in
Chemical Reactions
Bell Answer:
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.
20.2

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Balancing Redox Equations>
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Oxidation-Number Changes in
Chemical Reactions
20.2

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Balancing Redox Equations>
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Balancing Redox Equations>
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Identifying Redox Reactions
Section 3
How are oxidation numbers used to
identify redox reactions?
20.3

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Balancing Redox Equations>Identifying Redox Reactions
If the oxidation number of an element in
a reacting species changes, then that
element has undergone either oxidation
or reduction. Therefore, the reaction as a
whole must be a redox reaction.
20.3

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Balancing Redox Equations>Identifying Redox Reactions
A redox reaction takes place between nitrogen
and oxygen when lightning bolts heat the air.
20.3

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Balancing Redox Equations>Identifying Redox Reactions
A color change can signal a redox reaction.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Two Ways to Balance Redox Equations
Using changes in oxidation numbers
Using half-reactions
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Using Oxidation-Number Changes
In the oxidation-number-change method, you
balance a redox equation by comparing the
increases and decreases in oxidation numbers.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
To use the oxidation-number-change method,
start with the skeleton equation for the redox
reaction.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 1Assign oxidation numbers to all the
atoms in the equation.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 2Identify which atoms are oxidized and
which are reduced.
Step 3Use one bracketing line to connect the
atoms that undergo oxidation and another such
line to connect those that undergo reduction.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
In a balanced redox equation, the total
increase in oxidation number of the
species oxidized must be balanced by
the total decrease in the oxidation
number of the species reduced.
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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 4Make the total increase in oxidation
number equal to the total decrease in oxidation
number by using appropriate coefficients.
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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 5 Finally, make sure that the equation is
balanced for both atoms and charge.
20.3

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Practice Problemsfor Conceptual Problem 20.5
Problem Solving 20.20Solve
Problem 20 with the help of an
interactive guided tutorial.

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Using Half-Reactions
•A half-reactionis an equation showing just
the oxidation or just the reduction that takes
place in a redox reaction.
•In the half-reaction method, you write and
balance the oxidation and reduction half-
reactions separately before combining them
into a balanced redox equation.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
To balance a redox reaction using half-
reactions, write separate half-reactions
for the oxidation and the reduction. After
you balance atoms in each half-reaction,
balance electrons gained in the
reduction with electrons lost in the
oxidation.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
The oxidation of sulfur by nitric acid in aqueous
solution is one example of a redox reaction that
can be balanced by the half-reaction method.
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Steps 1 and 2
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 3a
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Step 3b
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Steps 4 and 5
20.3

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Balancing Redox Equations>Two Ways to Balance Redox
Equations
Steps 6 and 7
20.3

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Practice Problemsfor Conceptual Problem 20.6
Problem Solving 20.21Solve
Problem 21 with the help of an
interactive guided tutorial.

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Balancing Redox Equations>Choosing a Balancing Method
Choosing a Balancing Method
•Balancing by oxidation number change
usually works well if the oxidized and reduced
species appear only once on each side of the
equation.
•Balancing by the half-reaction method works
best in redox reactions where the same
element is both oxidized and reduced and in
reactions that take place in acidic or alkaline
solution.
20.3

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Section Quiz
-or-
Continue to: Launch:
Assess students’ understanding
of the concepts in Section
20.3 Section Quiz.
20.3.

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1.Choose the correct words for the spaces. One
method used to balance redox reactions
compares changes in ______ numbers.
When an equation is balanced, the total
number of _________ lost must be equal to
the total number gained.
a.oxidation, electrons
b.oxidation, protons
c.atomic, electrons
d.mass, protons
20.3 Section Quiz

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2.Choose the correct numbers for the spaces. Balance
the equation below with whole-number coefficients
using the oxidation-number-change method.
NH
3+ O
2NO
2+ H
2O
The least common multiple of the changes in
oxidation numbers of atoms is _____, and the
coefficient of water in the balanced equation is _____.
a.14, 6
b.14, 3
c.28, 3
d.7, 6
20.3 Section Quiz

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3.Balance the equation below with the lowest whole-
number coefficients using the half-reaction method.
Ag
+
(aq) + SO
2(g) + H
2O(l) Ag(s) + SO
4
2–
(g) + H
+
(aq)
The reduction half-reaction must be multiplied by
a.1
b.2
c.3
d.4
20.3 Section Quiz

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Balancing Redox Equations>Concept Map 20
Concept Map 20Create
your concept map using
the computer.

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