The Finish Line is in site… Electrochemistry. “Oxidation-Reduction Reactions” LEO SAYS GER.

The Finish Line is in site…

Electrochemistry

““Oxidation-Reduction Reactions”Oxidation-Reduction Reactions”

LEO SAYS GER

The Meaning of Oxidation and The Meaning of Oxidation and Reduction (called “redox”)Reduction (called “redox”)

OBJECTIVES

Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons.

OBJECTIVES

State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.

OBJECTIVES

Describe what happens to iron when it corrodes.

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

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

• For example, when methane burns in air, it oxidizes and forms oxides of carbon and hydrogen.

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

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 fabricsHydrogen peroxide also releases oxygen when it decomposes

Oxidation and Reduction (Redox)Oxidation and Reduction (Redox) 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 (Redox)Oxidation and Reduction (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 (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg2+

•The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S2-

(MgS)

Oxidation and Reduction (Redox)Oxidation and Reduction (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 :

eNaNa10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10 CleCl Chlorine is reduced

LEO says GER :LEO says GER : Losing electrons is oxidation, and the

substance 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 the reducing

agent

S is the oxidizing agent

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

Not All Reactions are Redox ReactionsNot All Reactions are Redox Reactions

Reactions in which there has been no change in oxidation number are

NOT redox reactions.

Examples:

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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

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 secondMagnesium, chromium, or even zinc (called

galvanized) coatings can be applied