leo says ger textbook ch 4.4 oxidation-reduction reactions “redox” unit 1: chemistry basics 1.42...

LEO SAYS GERLEO SAYS GERLEO SAYS GERLEO SAYS GER

Textbook ch 4.4

Unit 1: Chemistry Basics

1.42 Oxidation-Reduction Oxidation-Reduction ReactionsReactions“Redox”“Redox”

Oxidation and Reduction (Redox)

Electrons are transferred

Spontaneous redox rxns can transfer energy

Electrons (electricity) Heat

Non-spontaneous redox rxns can be made to happen with electricity

Oxidation Reduction Reactions(Redox)

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

11

2

00

22

ClNaClNa

eNaNa10

10 CleCl

LEO says GER :Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

Chlorine is reduced

eNaNa10

10 CleCl

One cannot occur without the other

Rules for Assigning Oxidation Numbers

Rules 1 & 21. The oxidation number elements in their

elemental form is zero.

2. The oxidation number of a monatomic ion equals its charge

11

2

00

22

ClNaClNa

Rules for Assigning Oxidation Numbers

Rules 3 & 4

3. The oxidation number of oxygen in compounds is -2

4. The oxidation number of hydrogen in compounds is +1

2

2

1

OH

Rules for Assigning Oxidation Number Rule 5

5. The sum of the oxidation numbers in the formula of a compound is 0

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

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

2

2

1

OH 2

122

)(

HOCa

Rules for Assigning Oxidation Numbers

Rule 66. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge

3

2?

ON

24

2?

OS

Not All Reactions are Redox Reactions

Reactions in which there has been no change in oxidation number are not redox rxns.

Examples:

1 5 2 1 1 1 1 1 5 2

3 3( ) ( ) ( ) ( )Ag N O aq NaCl aq AgCl s Na N O aq

1 2 1 1 6 2 1 6 2 1 2

2 24 2 42 ( ) ( ) ( ) ( )NaOH aq H S O aq Na S O aq H O l

Trends in Oxidation and Reduction

Active metals: Lose electrons easily Are easily oxidized

Active nonmetals: Gain electrons easily Are easily reduced

Calculate the oxidation #s

Weirdo Weirdo exceptionsexceptions: : Rules 3 & 43. The oxidation number of oxygen in compounds is -2 (Note: Except in the peroxide ion in which it has an oxidation number of −1.)

4. The oxidation number of hydrogen in compounds is +1 (Note: Except when bonded to a metal, in which it has an oxidation number of -1)

2

1

2

1

OH

2

12

HaC

Oxidation of a metal in Single Replacement reactions

• Hydrochloric acid reacts with Zinc metal.

• Write oxidation #s 2HCl(aq) + Zn(s) ZnCl2 + H2(g)

Note: As written Zinc replaces the hydrogen ion in the reaction

WAIT! - How do you know the reaction WILL occur???

• Will hydrochloric acid oxidize zinc metal?HCl(aq) + Zn(s) ??????

Sometimes chemicals don’t react.

For single replacement reactions we consult activity series to predict if reaction will occur.

We want to “see” if single element can oxidized and “replace” a like ion in the compound.

Look at single element…is it higher in the chart than a like ion

(metal replace metals)• I will give you a copy of

the activity series to use on test

Activity series

•metals–greater activity = easier to lose electrons– i.e. easier to become a cation

•nonmetals–greater activity = easier to gain

electrons– i.e. easier to become an anion

• Will an aqueous solution hydrochloric acid oxidize zinc metal?

• If so, write the net ionic equation

• Because Zn is higher on activity series than H-

the reaction will occur. Zn will replace H in the compound.

HCl(aq) + Zn(s)

• calcium and lead (II) nitrate

• copper and lithium sulfate