20.1 the meaning of oxidation and reduction 1 > copyright © pearson education, inc., or its...

20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction

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Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Chapter 20Oxidation-Reduction Reactions

20.1 The Meaning of Oxidation and Reduction

20.2 Oxidation Numbers20.3 Describing Redox Equations


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Why do you need to wash the bottom of your car?

CHEMISTRY & YOUCHEMISTRY & YOU

Remember: During winter in cold climates, salt is often spread on roads to lower the freezing point of water and thereby prevent the buildup of slippery ice.


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What Are Oxidation and What Are Oxidation and Reduction?Reduction?

What Are Oxidation and Reduction?

What happens to a substance that undergoes oxidation?

What happens to a substance that undergoes reduction?


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• The combustion of gasoline in an automobile engine and the burning of wood in a fireplace are reactions that require oxygen as they release energy.

• The reactions that break down food in your body and release energy use oxygen from the air you breathe.


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Oxygen and Redox

When methane (CH4), the main component of natural gas, burns in air, it oxidizes and forms oxides of carbon and hydrogen.• One oxide of carbon is carbon dioxide, CO2.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)


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Oxygen and Redox

Not all oxidation processes involve burning.• When elemental iron turns to rust, it slowly

oxidizes to compounds such as iron(III) oxide (Fe2O3).

4Fe(s) + 3O2(g) → 2Fe2O3(s)


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Oxygen and Redox

Not all oxidation processes involve burning.• Common liquid household bleach contains

sodium hypochlorite (NaClO), a substance that releases oxygen, which oxidizes stains to a colorless form.

• Hydrogen peroxide (H2O2) also releases oxygen when it decomposes. It is both a bleach and a mild antiseptic that kills bacteria by oxidizing them.


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Oxygen and Redox

A process called reduction is the opposite of oxidation.• Originally, reduction meant the loss of

oxygen from a compound.


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Oxygen and RedoxThe reduction of iron ore to metallic iron involves the removal of oxygen from iron(III) oxide.• The reduction is accomplished by heating the

ore with carbon, usually in the form of coke.

2Fe2O3(s) +3C(s) → 4Fe(s) + 3CO2(g)Iron(III) oxide

Carbon Iron Carbon dioxide


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Oxygen and Redox

The reduction of iron also includes an oxidation process.

• As iron(III) oxide is reduced to iron by losing oxygen, carbon oxidizes to carbon dioxide by gaining oxygen.




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Oxygen and Redox

The reduction of iron also includes an oxidation process.

• As iron(III) oxide is reduced to iron by losing oxygen, carbon oxidizes to carbon dioxide by gaining oxygen.

• Oxidation and reduction always occur simultaneously.




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Oxygen and Redox

A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen.• No oxidation occurs without reduction,

and no reduction occurs without oxidation.


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Oxygen and Redox

A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen.• Reactions that involve the processes of

oxidation and reduction are called oxidation-reduction reactions.

• Oxidation-reduction reactions are also known as redox reactions.


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Electron Shift in Redox Reactions

The modern concepts of oxidation and reduction have been extended to include many reactions that do not even involve oxygen.

• Redox reactions are currently understood to involve any shift of electrons between reactants.


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Electron Shift in Redox Reactions

• Oxidation is now defined to mean complete or partial loss of electrons or gain of oxygen.

• Reduction is now defined to mean complete or partial gain of electrons or loss of oxygen.

OxidationLoss of electronsGain of oxygen

ReductionGain of electronsLoss of oxygen


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Redox Reactions That Form IonsDuring a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal.

• When magnesium metal is heated with the non-metal sulfur, the ionic compound magnesium sulfide is produced.

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


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Redox Reactions That Form Ions

• Two electrons are transferred from a magnesium atom to a sulfur atom.

• The magnesium atoms are made more stable by the loss of electrons.

• The sulfur atoms become more stable due to the gain of electrons.

Mg + S Mg2+ + S 2–

Magnesium atom

Sulfur atom

Magnesium ion

Sulfide ion

→


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• Because it loses electrons, the magnesium atom is said to be oxidized to a magnesium ion.

• The sulfur atom gains two electrons and is reduced to a sulfide ion.

Mg + S Mg2+ + S 2–

Magnesium atom

Sulfur atom

Magnesium ion

Sulfide ion

→


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• The overall process is represented as the two component processes below.

Oxidation: Mg → Mg2+ + 2e– (loss of electrons)

Reduction: S + 2e– → S 2– (gain of electrons)

Mg + S Mg2+ + S 2–

Magnesium atom

Sulfur atom

Magnesium ion

Sulfide ion

→


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A substance that undergoes oxidation loses electrons.

A substance that undergoes reduction gains electrons.


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• The substance that loses electrons is the reducing agent.

• The substance that accepts electrons is the oxidizing agent.


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Redox Reactions That Form IonsAnother way to identify oxidizing and reducing agents is to remember that the species that is reduced is the oxidizing agent, and the species oxidized is the reducing agent.

Mg(s) + S(s) → MgS(s)

Magnesium (reducing

agent)

Sulfur (oxidizing

agent)

Magnesium sulfide

oxidized

reduced


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Identifying Oxidized and Reduced ReactantsSilver nitrate reacts with copper to form copper nitrate and silver. From the equation below, determine what is oxidized and what is reduced. Identify the oxidizing agent and the reducing agent.

2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)

Sample Problem 20.1Sample Problem 20.1


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• Identify the ions in the reaction.

• Then trace how the electrons were transferred.

Analyze Identify the relevant concepts.1



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Rewrite the equation in ionic form so it will be easier to analyze the reaction.

Solve Apply concepts to this situation. 2


2Ag+ + 2NO3– + Cu → Cu2+ + 2NO3

– + 2Ag

In this reaction, two electrons are lost from a copper atom (Cu) when it becomes a Cu2+ ion. These electrons are gained by two silver ions (Ag+), which become neutral silver atoms.


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• The species that loses electrons is oxidized and is the reducing agent.

• The species that gains electrons is reduced and is the oxidizing agent.

Solve Apply concepts to this situation. 2


The Cu is the reducing agent. The Ag+ is the oxidizing agent.

Oxidation: Cu → Cu2+ + 2e– (loss of electrons)

Reduction: 2Ag+ + 2e– → 2Ag (gain of electrons)


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Redox With Covalent CompoundsSome reactions involve covalent compounds, that is, compounds in which complete electron transfer does not occur.

2H2(g) + O2(g) → H2O(l)


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Redox With Covalent Compounds

• In each reactant hydrogen molecule, the bonding electrons are shared equally between the hydrogen atoms.

• In water, however, the bonding electrons are pulled toward oxygen because it is much more electronegative than hydrogen.

• Hydrogen is oxidized because it undergoes a partial loss of electrons.

H—H O—O H—O

H electrons

shared equally

electrons shared equally

shift of bonding electrons away from hydrogen and toward oxygen


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• In oxygen, the bonding electrons are shared equally between oxygen atoms in the reactant oxygen molecule.

• However, when oxygen bonds to hydrogen in the water molecule, there is a shift of electrons toward oxygen.

• Oxygen is thus reduced because it undergoes a partial gain of electrons.

H—H O—O H—O

H electrons shared equally

electrons shared equally

shift of bonding electrons away from hydrogen and toward oxygen


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• Hydrogen is the reducing agent because it is oxidized.

• Oxygen is the oxidizing agent because it is reduced.

2H2(g) + O2(g) → H2O(l)

This redox reaction is highly exothermic—that is, it releases a great deal of energy.


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In some reactions involving covalent reactants or products, the partial electron shifts are less obvious. Processes Leading to Oxidation and Reduction

Oxidation Reduction

Complete loss of electrons (ionic reactions)

Complete gain of electrons (ionic reactions)

Shift of electrons away from an atom in a covalent bond

Shift of electrons toward an atom in a covalent bond

Gain of oxygen Loss of oxygen

Loss of hydrogen by a covalent compound

Gain of hydrogen by a covalent compound

Increase in oxidation numberDecrease in oxidation number



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The element that donates electrons in a redox reaction is called the reducing agent. Which of the following is always true of the reducing agent?

A. It is oxidized.

B. It is reduced.

C. It is ionic.

D. It is covalent.


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The element that donates electrons in a redox reaction is called the reducing agent. Which of the following is always true of the reducing agent?

A. It is oxidized.

B. It is reduced.

C. It is ionic.

D. It is covalent.


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CorrosionCorrosion

Corrosion

How does the presence of salts and acids accelerate the corrosion of metals?


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CorrosionCorrosion

Iron corrodes by being oxidized by oxygen to ions of iron.

• Water in the environment accelerates the rate of corrosion.


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CorrosionCorrosion

Iron corrodes by being oxidized by oxygen to ions of iron.

• Oxygen, the oxidizing agent, is reduced to oxide ions (in compounds such as Fe2O3) or to hydroxide ions.

2Fe(s) + O2(g) + 2H2O(l) → 2Fe(OH)2(s)

4Fe(OH)2(s) + O2(g) + 2H2O(l) → 4Fe(OH)3(s)• Corrosion occurs more rapidly in the

presence of salts and acids.


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CorrosionCorrosion

The presence of salts and acids accelerates corrosion by producing conductive solutions that make electron transfer easier.


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If your car is exposed to salt on the roads in the winter, why is it important to wash the salt off your car?



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If your car is exposed to salt on the roads in the winter, why is it important to wash the salt off your car?


Many parts of your car’s body are made of steel, an iron alloy. The salt can cause the metal to corrode, or rust, faster than it would otherwise because when mixed with water, the salt creates a conductive solution that allows electrons to transfer more easily.


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CorrosionCorrosion

The corrosion of some metals can be a desirable feature.

• The copper on this building reacted with water vapor, carbon dioxide, and other substances in the air to form a patina. This patina consists of a pale-green film of basic copper(II) carbonate.


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CorrosionCorrosion

• Gold and platinum are called noble metals because they are very resistant to losing their electrons by corrosion.

• Other metals lose electrons easily but are protected from extensive corrosion by the oxide coating formed on their surface.

Resistance to Corrosion

Not all metals corrode easily.


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CorrosionCorrosion

Resistance to CorrosionIron(III) oxide

Aluminum oxide

Water Water

Oxygen Oxygen

Iron forms a coating when it corrodes, but the coating ofiron oxide that forms is not tightly packed. Water and air can penetrate the coating and attack the iron metal beneath it.

Aluminum oxidizes quickly in air to form a coating of very

tightly packed aluminum oxide particles. This coating protects the aluminum object from further corrosion.


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CorrosionCorrosion

Controlling Corrosion

To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal.• These coatings exclude air and water from

the surface, thus preventing corrosion.

• If the coating is scratched or worn away, however, the exposed metal will begin to corrode.


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CorrosionCorrosion


To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal.• Painting a surface,

like the Golden Gate Bridge, protects it from the effects of the environment.


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CorrosionCorrosion


To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal.• Chromium metal also

serves as a protective coating.

• Like aluminum, chromium forms a corrosion-resistant oxide film on its surface.


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CorrosionCorrosion


• To protect an iron object, a piece of magnesium may be placed in electrical contact with the iron.

• When oxygen and water attack the iron object, the iron atoms lose electrons as the iron begins to be oxidized.

• However, because magnesium is a better reducing agent than iron, the magnesium immediately transfers electrons to the iron atoms, preventing their oxidation.

In another method of corrosion control, one metal is “sacrificed,” or allowed to corrode, to save a second metal.


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CorrosionCorrosion


• Zinc blocks are attached to the steel (iron) hull of this ship. The zinc blocks oxidize (corrode) instead of the iron, preventing the hull from corroding.

In another method of corrosion control, one metal is “sacrificed,” or allowed to corrode, to save a second metal.

zinc block


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Explain why metal trash cans are frequently made from zinc-coated steel.


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Explain why metal trash cans are frequently made from zinc-coated steel.

A trash can made from uncoated steel would rust if left outside in the rain. The zinc coating corrodes first, protecting the steel underneath.


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Key Concepts Key Concepts

A substance that undergoes oxidation gains oxygen or loses electrons, while a substance that undergoes reduction loses oxygen or gains electrons.

The presence of salts and acids accelerates corrosion by producing conductive solutions that make electron transfer easier.


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Glossary TermsGlossary Terms

• oxidation-reduction reaction: a reaction that involves the transfer of electrons between reactants

• oxidation: a process that involves complete or partial loss of electrons or a gain of oxygen; it results in an increase in the oxidation number of an atom


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Glossary TermsGlossary Terms

• reduction: a process that involves a complete or partial gain of electrons or the loss of oxygen; it results in a decrease in the oxidation number of an atom

• reducing agent: the substance in a redox reaction that donates electrons; in the reaction, the reducing agent is oxidized

• oxidizing agent: the substance in a redox reaction that accepts electrons; in the reaction, the oxidizing agent is reduced


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• Oxidation-reduction reactions always occur simultaneously in redox reactions.

• Losing electrons is oxidation.

• Gaining electrons is reduction.

• If oxygen is involved in the reaction, then the substance gaining oxygen is oxidized, while the substance losing oxygen is reduced.

• The species that is reduced is the oxidizing agent, while the oxidized species is the reducing agent.

BIG IDEABIG IDEA

Reactions


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END OF 20.1END OF 20.1

20.1 the meaning of oxidation and reduction 1 > copyright © pearson education, inc., or its...

Documents

meaning of oxidation

oxidation processes

oxidation numbers

opposite of oxidation

copyright pearson education

reduction of iron ore

removal of oxygen

loss of oxygen