AqueousReactions

CHAPTER 20CHAPTER 20

““Oxidation-Reduction ReactionsOxidation-Reduction Reactions””

LEO SAYS GER

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Oxidation-Reduction Reactions

• An oxidation occurs when an atom or ion loses electrons.

• A reduction occurs when an atom or ion gains electrons.

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

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)

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Oxidation-Reduction Reactions

One cannot occur without the other.

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Problems

• 4.45 a,b

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Oxidation Numbers

To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

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Oxidation Numbers

• Elements in their elemental form have an oxidation number of 0. ( Al)

• The oxidation number of an ion is the same as its charge. B+2

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Oxidation Numbers

• Nonmetals have negative oxidation numbers based on the ions they form, although some are positive in certain compounds or ions.Oxygen has an oxidation number of −2,

except in the peroxide ion in which it has an oxidation number of −1.

Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

AqueousReactions

Oxidation Numbers

• Nonmetals tend to have negative oxidation numbers, Fluorine always has an oxidation number of −1.The other halogens have an oxidation

number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

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Oxidation Numbers

• The sum of the oxidation numbers in a neutral compound is 0.

• The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

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Problems

• 4.49

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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

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LEO says GER :LEO says GER :

eNaNa10

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 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

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Problems

• 4.51 a,b,c,d

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Displacement Reactions

In this reaction,

silver ions oxidize

copper metal.

Cu (s) + 2 Ag+ (aq) Cu2+ (aq) + 2 Ag (s)

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Displacement Reactions

The reverse reaction,

however, does not

occur.

Cu2+ (aq) + 2 Ag (s) Cu (s) + 2 Ag+ (aq) x

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Activity Series

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Using Molarities inStoichiometric Calculations

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TitrationThe analytical technique in which one can calculate the concentration of a solute in a solution.

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Titration