Guided Practice: Determine the OXIDATION...

Section 4.9

Oxidation–Reduction Reactions

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Guided Practice: Determine the OXIDATION NUMBERS!

NH3 N =

ClO- Cl =

H3PO4 P =

MnO4- Mn =

Oxidation

number of F in

HF?

Section 4.9


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Independent Practice:

Find the oxidation states for each of the

elements in each of the following

compounds:

• K2Cr2O7

• CO32-

• MnO2

• PCl5

• SF4

K = +1; Cr = +6; O = –2

C = +4; O = –2

Mn = +4; O = –2

P = +5; Cl = –1

S = +4; F = –1

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Section 4.9


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• Reactions that are concerned with the transfer

of electrons.

• Also called ‘Redox’ reactions because oxidation

and reduction occur at the same time.

Oxidation-Reduction Reactions

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Section 4.9


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• The atom that undergoes oxidation is called the

‘reducing agent’

• The atom that undergoes reduction is called the

‘oxidizing agent’


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http://chemwiki.ucdavis.edu/@api/deki/files/140/reducingoxidizingdiagram.png

Section 4.9


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Why Study Redox Reactions

Manufacturing metals

Energy from

food!

Batteries

Corrosion

Rusting

Green color

on the copper

due to

Redox!

Section 4.9


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• Oxidation is the loss of electrons.

• Reduction is the gain of electrons.


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General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc.

9

In light-sensitive sunglasses, UV light initiates an

oxidation-reduction reaction.

uv light

2Ag+ + 2Cl− 2Ag + Cl2

A. Which reactant is oxidized?

Cl − 2Cl− Cl + 2e−

B. Which reactant is reduced?

Ag+ 2Ag+ + 2e− 2Ag

Problem

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Not All Reactions are Redox Reactions

- Reactions in which there has been no

change in oxidation number are NOT redox reactions.

Example: silver nitrate and sodium

chloride solutions combine

)()()()( 3

2511111

3

251

aqONNasClAgaqClNaaqONAg

Section 4.9


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Example:

Zinc foil is added to a copper(II)nitrate solution.

Zn+ Cu(NO3)2 Cu + Zn(NO3)2

The net ionic for this reaction is:

Zn + Cu2+ Cu + Zn2+

The copper ion is being reduced: Cu2+ + 2e- Cu

The zinc atom is being oxidized: Zn Zn2+ + 2e-

The number of electrons gain / lost must be equal!



12

Zn replaces Cu2+

zinc undergoes oxidation

Zn(s) Zn2+(aq) + 2e-

Cu2+ undergoes reduction

Cu2+(aq) + 2e- Cu(s)


Zn Transfers Electrons to Cu2+

OIL Oxidation is losing electrons.

RIG Reduction is gaining electrons.

13


14

Identify each of the following as (O) Oxidation or

(R) Reduction

__A. Sn(s) Sn4+(aq) + 4e−

__B. Fe3+(aq) + 1e− Fe2+(aq)

__C. Cl2(g) + 2e− 2Cl−(aq)

Learning Check


15

Identify each of the following as (O) Oxidation or

(R) Reduction

O A. Sn(s) Sn4+(aq) + 4e−

R B. Fe3+(aq) + 1e− Fe2+(aq)

R C. Cl2(g) + 2e− 2Cl−(aq)

Solution

Section 4.9


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Example:

1. Write the molecular formula when Iron(III)oxide

and carbon monoxide react to form iron metal

and carbon dioxide.

2. Use oxidation numbers to determine which

atoms are becoming oxidized and reduced


Section 4.10

Balancing Oxidation–Reduction Equations

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Problem:

1. Write the balanced equation for the reaction that

occurs when zinc metal is placed in hydrochloric acid.

2. Write the oxidation states for each atom below.

Zn(s) + 2HCl(aq) ZnCl2 (aq) + H2(g)

0 +1 –1 +2 –1 0

Section 4.10


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1 e– gained (each atom)(Reduced)

• Zn(s) + HCl(aq) ZnCl2 (aq) + H2(g)

0 +1 –1 +2 –1 0

2 e– lost (Oxidized)

• The oxidation state of chlorine remains unchanged.

3. Connect the atoms that are being oxidized or reduced.

How many electrons are gained and lost? What compound is

being oxidized? Reduced?

Section 4.10


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Balancing Oxidation reactions using half reactions will

be covered in a later chapter.


Guided Practice: Determine the OXIDATION...

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