chapter 20 oxidation-reduction reactions_2

8/8/2019 Chapter 20 Oxidation-Reduction Reactions_2

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Oxidation and Reduction (Redox)Oxidation and Reduction (Redox)

11

2

00

22

p ClNaClNa

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

p eNaNa

10

10

p CleCl


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LEO says GER :LEO says GER :

p eNaNa

10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10

CleCl Chlorine is reduced


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LEO says GER :LEO says GER :

- Losing electrons is oxidation, and thesubstance that loses the electrons is

called the reducing agent.

- Gaining electrons is reduction, and

the substance that gains the electrons is

called the oxidizing agent.

Mg(s) + S(s) MgS(s)

Mg is oxidized: loses e-, becomes a Mg2+ ion

S is reduced: gains e- = S2- ion

Mg is thereducing

agent

S is the oxidizing agent


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Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

1)The oxidation number of any

uncombined element is zero.

2)The oxidation number of a

monatomic ion equals its charge.

11

2

00

22

p ClNaClNa


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3)The oxidation number ofoxygen incompounds is -2, except in

peroxides, such as H2O2 where it is -1.

4)The oxidation number ofhydrogen in

compounds is +1, except in metal

hydrides, like NaH, where it is -1.

2

2

1

OH


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5) The sum of the oxidation numbers of theatoms in the compound must equal 0.

2

2

1

OH2(+1) + (-2) = 0

H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0Ca O H


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6) The sum of the oxidation numbers in

the formula of a polyatomic ion is equal

to its ionic charge.

3

2?

ON

X + 3(-2) = -1N O

2

4

2?

OS

thus X= +5

thus X = +6

X + 4(-2) = -2S O


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Reducing Agents and Oxidizing AgentsReducing Agents and Oxidizing Agents

An increase in oxidation number = oxidation

A decrease in oxidation number = reduction

p eaa

10

10

p CleCl

Sodium is oxidized it is the reducing agent

Chlorine is reduced it is the oxidizing agent


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Trends in Oxidation and ReductionTrends in Oxidation and Reduction

Active metals:

Lose electrons easily

Are easily oxidized

Are strong reducing agents

Active nonmetals:

Gain electrons easily

Are easily reduced

Are strong oxidizing agents


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BalancingBalancing RedoxRedox EquationsEquations

OBJECTIVES

Describe how oxidation

numbers are used to identify

redox reactions.


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OBJECTIVES

Balance a redox equation

using the oxidation-number-

change method.


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OBJECTIVES

Balance a redox equation

by breaking the equation into

oxidation and reduction half-

reactions, and then using thehalf-reaction method.


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Identifying Redox EquationsIdentifying Redox Equations

In general, all chemical reactions can

be assigned to one of two classes:

1)oxidation-reduction, in which

electrons are transferred:

Single-replacement, combination,

decomposition, and combustion

2) this second class has no electrontransfer, and includes all others:

Double-replacement and acid-

base reactions


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Identifying Redox EquationsIdentifying Redox EquationsIn an electrical storm, nitrogen and

oxygen react to form nitrogen monoxide:

N2(g) + O2(g) 2NO(g)

Is this a redox reaction?If the oxidation number of an element

in a reacting species changes, then

that element has undergone eitheroxidation or reduction; therefore, the

reaction as a whole must be a redox.

Conceptual Problem 20.4, page 647

YES!


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Balancing Redox EquationsBalancing Redox EquationsIt is essential to write a correctly

balanced equation that represents

what happens in a chemical reaction

Fortunately, two systematic methodsare available, and are based on the

fact that the total electrons gained in

reduction equals the total lost in

oxidation. The two methods:

1)Use oxidation-number changes

2)Use half-reactions


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Using OxidationUsing Oxidation--Number ChangesNumber ChangesSort of like chemical bookkeeping, you

compare the increases and decreases inoxidation numbers.

start with the skeleton equation

Step 1: assign oxidation numbers to all

atoms; write above their symbols

Step 2: identify which are oxidized/reduced

Step 3: use bracket lines to connect them

Step 4: use coefficients to equalize

Step 5: make sure they are balanced for

both atoms and charge Problem 20.5, 649


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Using halfUsing half--reactionsreactions

A half-reaction is an equation showing

just the oxidation or just the reduction thattakes place

they are then balanced separately, and

finally combinedStep 1: write unbalanced equation in ionic

form

Step 2: write separate half-reactionequations for oxidation and reduction

Step 3: balance the atoms in the half-

reactions (More steps on the next screen.)


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ChoosingChoosing a Balancing Methoda Balancing Method

1) The oxidation number changemethod works well if the oxidized

and reduced species appearonly

once on each side ofthe equation,andthere are no acids orbases.

2) The half-reaction method worksbest for reactions takingplace in

acidicor alkaline solution.

chapter 20 oxidation-reduction reactions_2

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