Ch 22sec1

CHAPTER 22CHAPTER 22““Oxidation-Reduction Reactions”Oxidation-Reduction Reactions”

LEO SAYS GER

Ch 22.1 Oxidationand

Reduction

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

Early chemists saw “oxidation” reactions only as the combination of a material with oxygen to produce an oxide.

• For example, when coal burns in air, it oxidizes and forms oxides of carbon and hydrogen, as shown in Fig. 22.1, p. 645


But, not all oxidation processes that use oxygen involve burning:

•Elemental iron slowly oxidizes to compounds such as iron (III) oxide, commonly called “rust”•Bleaching stains in fabrics•Hydrogen peroxide also releases oxygen when it decomposes


A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur

simultaneously The substance gaining oxygen (or

losing electrons) is oxidized, while the substance losing oxygen (or gaining electrons) is reduced.

Oxidation and Reduction Oxidation and Reduction (Redox)(Redox) Today, many of these reactions may

not even involve oxygen Redox currently says that electrons

are transferred between reactants Mg + S → Mg2+ + S2-

•The magnesium atom changes to a magnesium ion by losing 2 electrons, and is thus oxidized

•The sulfur atom is changed to a sulfide ion by gaining 2 electrons, and is thus reduced.

LEO says GER :LEO says GER :

eNaNa10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10 CleCl Chlorine is reduced

Oxidation and Reduction Oxidation and Reduction (Redox)(Redox) 11

2

00

22

ClNaClNa

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

eNaNa10

10 CleCl

LEO says GER :LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. ( gets oxided) - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent.( gets reduced)

Mg(s) + S(s) → MgS(s)

Mg is oxidized – loses e-

S is reduced – gains e-

Mg is the reducing agent

S is the oxidizing agent

REDOX PROBLEM

What is oxidized and reduced in this reaction?

2AgNO3(aq) + Cu(s) ----Cu(NO3)(aq)

+2Ag(s)

… and what are the oxidizing agents and reducing agents?

REDOX PROBLEM

2AgNO3(aq) + Cu(s) ----Cu(NO3)2(aq)

+2Ag(s)

+1 -1 0 +2 -1 0

Ag+ went from +1 to 0, gained electrons so it got reduced, and is the oxidizing agent

Cu went from 0 to +2 , lost electrons so it got oxidized, and is the reducing agent


It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds?

In water, we learned that oxygen is highly electronegative, so:

the oxygen gains electrons (is reduced and is the oxidizing agent), and the hydrogen loses electrons (is oxidized and is the reducing agent)

• Look for the more electronegative element- it will “gain” electrons and so be reduced

Not All Reactions are Redox Not All Reactions are Redox ReactionsReactions- Reactions in which there has been no change in oxidation number are not redox reactions.

Examples:

)()()()( 3

2511111

3

251

aqONNasClAgaqClNaaqONAg

)()()()(22

2

1

4

26

2

1

4

26

2

1121

lOHaqOSNaaqOSHaqHONa

CorrosionCorrosion•Damage done to metal is costly to prevent and repair•Iron, a common construction metal often used in forming steel alloys, corrodes by being oxidized to ions of iron by oxygen.

•This corrosion is even faster in the presence of salts and acids, because these materials make electrically conductive solutions that make electron transfer easy

CorrosionCorrosion•Luckily, not all metals corrode easily

•Gold and platinum are called noble metals because they are resistant to losing their electrons by corrosion•Other metals may lose their electrons easily, but are protected from corrosion by the oxide coating on their surface, such as aluminum – Figure 22.8, page 652 Iron has an oxide coating, but it is not tightly packed, so water and air can penetrate it easily

CorrosionCorrosion•Serious problems can result if bridges, storage tanks, or hulls of ships corrode

•Can be prevented by a coating of oil, paint, plastic, or another metal•If this surface is scratched or worn away, the protection is lost

•Other methods of prevention involve the “sacrifice” of one metal to save the second

•Magnesium, chromium, or even zinc (called galvanized) coatings can be applied

Education

Ch 22sec1