Oxidation-Reduction Reactions

REDOX Reactions

Oxidation State• Oxidation numbers are very

similar to charge. • There are some different rules

for assigning them.

Rules for Oxidation State• 1. The oxidation number for

any uncombined element is zero

• Ex. The oxidation state for Na is zero.

Rule 2• The oxidation number for a

monatomic ion equals the charge on the ion.

• Ex. Cl-1 has an oxidation of -1.

Rule 3• The more electronegative

element in a binary compound is assigned the number equal to the charge it would have if it were an ion.

• Ex. The oxidation number of O in NO is -2 because oxygen is more electronegative than nitrogen.

Rule 4• The oxidation number of

fluorine in a compound is always -1.

• Ex. The oxidation number of F in LiF is -1.

Rule 5• Oxygen has an oxidation

number of -2 unless it is combined with fluorine, when it is +2, or it is in a peroxide such as H2O2, when it is -1.

• Ex. The oxidation of O in NO2 is -2.

Rule 6• The oxidation state of

hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1.

• Ex. The oxidation of H in LiH is -1. The oxidation of H in HCl is +1.

Rule 7• In compounds, Group 1 and 2

elements and aluminum have oxidation numbers of +1, +2, and +3, respectively.

• Ex. The oxidation number of Ca in CaCO3 is +2.

Rule 8• The sum of the oxidation

numbers of all atoms in a neutral compound is 0.

• Ex. The oxidation number of each atom in CaCO3 can be found by knowing the rules above and knowing the compound is neutral

• Ca = +2 (rule 7)• O = -2 x 3 atoms = -6 (rule 5)• C = +4 (rule 8—balances out

the charge)

Rule 9• The sum of the oxidation

numbers of all atoms in a polyatomic ion equals the charge of the ion.

• Ex. The oxidation of each element in H2PO4

2- can be determined by knowing the overall charge is -2

• H = +1 x 2 atoms = +2 (rule 6)• O = -2 x 4 atoms = -8 (rule 5)• P = +4 (rule 9)

Types of Reactions5 types plus a new one

Type of Reactions

Chemical reactions are classified into five general types

Combination

Decomposition

Single Replacement

Double Replacement

Combustion

Combination (Synthesis)

Two or more elements or simple compounds combine to form (synthesize) one product

• A + B AB

• 2Mg + O2 2MgO

• 2Na + Cl2 2NaCl

• SO3 + H2O H2SO4

Decomposition

One substance is broken down (split) into two or more simpler substances.

• AB A + B

• 2HgO 2Hg +

O2

• 2KClO32KCl + 3

O2

Learning Check R1

Classify the following reactions as 1) combination or

2) decomposition:

___A. H2 + Br2 2HBr

___B. Al2(CO3)3 Al2O3 +

3CO2

___C. 4 Al + 3C Al4C3

Solution R1

Classify the following reactions as 1) combination or 2) decomposition:

_1_A. H2 + Br2 2HBr

_2_B. Al2(CO3)3 Al2O3 +

3CO2

_1_C. 4 Al + 3C Al4C3

Single Replacement

One element takes the place of an element in a reacting compound.

A + BC AB + C

Zn + 2HCl ZnCl2 + H2

Fe +CuSO4 FeSO4 +

Cu

Learning Check R2

Classify the following reactions as 1) single replacement 2) double replacement

__A. 2Al + 3H2SO4 Al2(SO4)3 + 3H2

__B. Na2SO4 + 2AgNO Ag2SO4 +

2NaNO3

__C. 3C + Fe2O3 2Fe + 3CO

Solution R2

Classify the following reactions as 1) single replacement 2) double replacement

1_A. 2Al + 3H2SO4 Al2(SO4)3

+ 3H2

2_B. Na2SO4 + 2AgNO3 Ag2SO4

+ 2NaNO3

1_C. 3C + Fe 2Fe + 3CO

Combustion

A reaction in which a compound (often carbon) reacts with oxygen

C + O2 CO2

CH4 + 2O2 CO2 + 2H2O

C3H8 + 5O2 3CO2 + 4H2O

C6H12O6 + 6O2 6CO2 + 6H2O

Learning Check R3

Balance the combustion equation

___C5H12 + ___O2 ___CO2 + ___H2O

Solution R3

Balance the combustion equation

1 C5H12 + 8 O2 5 CO2 + 6 H2O

Oxidation-Reduction Reactions

REDOX Rxns

Oxidation and Reduction

Reactions that involve a loss or gain of electrons

Occurs in many of the 4 types of reactions and combustion

Important in food metabolism, batteries, rusting of metals

Requirements for Oxidization-Reduction

Electrons are transferred

Two processes occur

Oxidation = Loss of electrons (LEO)

Zn Zn2+ + 2e-

Reduction = Gain of electrons (GER)

Cu2+ + 2e- Cu

Balanced Red-Ox Equations

Combine the oxidation and reduction reactions to make

Loss of electrons = Gain of electrons

Zn + Cu2+ + 2e- Zn2+ + 2e- + Cu

Zn + Cu2+ Zn2+ + Cu

Gain/Loss of Hydrogen

In organic and biological reactionsoxidation = Loss of Hreduction = Gain of H

Half-Reaction• The part of a reaction involving

oxidation or reduction alone. • The overall reaction is the sum

of the two half reactions• Ex. • 3Cu 3Cu2++ 6 e-

• 2NO3-1 + 6 e- + 8H+ 2NO + 4

H2O

• 3Cu + 2NO3-1 + 8 H+ 3Cu2+ + 2NO +

4H2O

Learning Check R3

Identify the following as an 1) oxidation or a reduction process:

__A. Sn Sn4+ + 4e-

__B. Fe3+ + 1e- Fe2+

__C. Cl2 + 2e- 2Cl-

Solution R3

Identify the following as an 1) oxidation or a reduction process:

1_ A. Sn Sn4+ + 4e-

2_ B. Fe3+ + 1e- Fe2+

2_ C. Cl2 + 2e- 2Cl-

Learning Check R4

In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction

Ag+ + Cl- Ag + ClA. Which reactant is oxidized

1) Ag+ 2) Cl- 3) AgB. Which reactant is reduced?

1) Ag+ 2) Cl- 3) Cl

Solution R4

In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction

Ag+ + Cl- Ag + ClA. Which reactant is oxidized

2) Cl- Cl- Cl + e-

B. Which reactant is reduced?1) Ag+ Ag+ + e-

Ag

Oxidizing and Reducing Agents

Reducing Agents• A substance that has the

potential to cause another substance to be reduced.

• Lose electrons (gain higher charge)

Oxidizing Agent• Substance that has the

potential to cause another substance to be oxidized

• Gains electrons (become more negative)

Summary Chart of TerminologyTerm Change in

oxidation number

Change in electron population

Oxidation Increases Loss of electrons

Reduction Decreases Gain of electrons

Oxidizing agent

Decreases Gains electrons

Reducing agent

Increases Loses electrons

Autooxidation• A process in which a substance

acts as both an oxidizing agent and a reducing agent

ElectrochemistryElectrodes, half cells, anodes, cathodes,

voltaic and electrolytic cells

Electrochemistry• The branch of chemistry that

deals with electricity-related applications of oxidation-reduction

A bright idea!• As redox rxns occur, electrons

are transferred which also produces heat.

• But if the substances are separated by a porous barrier then they are able to transfer electricity instead of heat

Electrode•Conductor used to establish electrical contact with a nonmetallic part of a circuit such as an electrolyte

Half-cell• A single electrode immersed in

a solution of its ions.

Anode• One half cell is the anode which

is the electrode where oxidation takes place.

Cathode• Other half cell where reduction

takes place.

Electrochemical cell• A system of electrodes and

electrolytes in which either chemical reactions produce electrical energy of an electric current produces chemical change.

Voltaic Cells•If the redox rxn in an electrochemical cell occurs spontaneously and produces electrical energy then it is a voltaic cell.

Types of Dry Cell BatteriesZinc-Carbon Dry Cells

Alkaline BatteriesMercury batteries

Zinc-Carbon Dry Cells•Ex. Flashlight batteries•Have Zn container which is the anode

•Filled with moist paste of MnO2, graphite, and NH4Cl

Alkaline Batteries•Ex. Batteries found in small, portable devices such as CD players•Very similar to the Zn-C but it does not contain the C which allows them to be smaller•Uses paste of Zn and KOH instead of a solid metal anode.

Mercury Batteries•Ex. Calculator batteries•The anode half rxn is the same as the alkaline but the cathode involves HgO

Rechargeable cells•Combination of voltaic and electrolytic cells•Acts as a voltaic cell during use and as an electrolytic cell during charging.•Ex. Car battery