Oxidation and Reduction

……………………….

Objectives

• Oxidised, reduced

• Definition- oxidising/reducing agent, oxidant/reductant

• Rules of oxidation number

• Refer to oxidation number and decide-reduced or oxidised

Redox Reactions.

Oxidation

Reduction

= reactions involving REDuction and OXidation

Overview

• Oxidation and reduction reactions always occur simultaneously (redox reactions)

3 ways of looking at oxidation and reduction

• 1. oxidation is a gain of oxygen atoms, reduction is a loss of oxygen atoms

• 2. oxidation is a loss of hydrogen atoms, reduction is a gain of hydrogen atoms

• 3. oxidation is a loss of electrons, reduction is a gain of electrons– Most fundamental explanation, what we will

be dealing with the most

Oxidation Reduction

Gains oxygen Lose Oxygen

Lose Hydrogen Gains hydrogen

Lose Electrons Gains electrons

Oxygen

Hydrogen

e-

OXIDATIONOXIDATION REDUCTIONREDUCTION

= OXYGEN= OXYGEN

= HYDROGEN= HYDROGEN

= ELECTRON= ELECTRON

= OXIDATION STATE= OXIDATION STATE

REDOX REACTIONS

Definitions:

GAINGAIN LOSSLOSS

LOSSLOSS GAINGAIN

LOSSLOSS GAINGAIN

INCREASEINCREASE DECREASEDECREASE

Remember “OILRIG” : Oxidation Is Loss ; Reduction Is Gain (of electrons)

Oxidation states (also called oxidation numbers) are numbers assigned to EACH ATOM that takes part in a reaction.

Oxidation states are assigned using a set of International rules.

Redox-loss or gain of Oxygen

• Remember that reduction is a loss of oxygen from a compound

• We converted iron ore to metallic iron– We removed oxygen from the iron(III) oxide

– 2Fe2O3 + 3C 4Fe + 3CO2

– Carbon was oxidized because it gained an oxygen

Oxidizing and Reducing Agents

• Now the confusing part…

• CuO + H2 Cu + H2O

• Cu goes from +2 to 0– Cu is reduced, therefore it is called an oxidizing

agent (OXIDANT) because it causes some other substance to be oxidized

• H goes from 0 to +1– H is oxidized, therefore it is called a reducing agent

(REDUCTANT)because it causes some other substance to be reduced.

Identifying Agents in an Equation

• CuO + H2 Cu + H2O

Reduction: CuO is the oxidizing agent

Oxidation: H2 is the reducing agent

Examples

• Is the reactant oxidized or reduced?

• Pb PbO3

• SnO2 SnO

• KClO3 KCl

• C2H6O C2H4O

• C2H2 C2H6

Leo the Lion!

• LEO the lion says GER– Loss of electrons is oxidation, gain of

electrons is reduction

Pertaining to LEO…

• Mg + S MgS• Mg + S Mg2+ + S2-

• Magnesium is oxidized– Said to be the reducing agent– Substance in the reaction that loses electrons

• Sulfide sulfur atom is reduced – Said to be the oxidizing agent– Substance in the reaction that gains electrons

Oxidation Numbers

• A count of the electrons transferred or shared in the formation or breaking of chemical bonds

• To see if electron is lost or gained in the reaction an oxidation number is assigned to each element

• Follow a set of rules to assign oxidation number…

Rules for deciding Oxidation States (Numbers) :

5. In a BINARY (2 elements) COMPOUND

1. In all UNCOMBINED ELEMENTS, atom’s ox. no. = 0 .

2. In all COMPOUNDS, sum of ox. no.’s equals zero.

3. In all IONS, sum of ox. no.’s equals ion charge.

4. In all COMPOUNDS : Gp 1 elements + 1

+ 2

- 1

+ 3

the more electronegative atom given NEGATIVE ox. no. and the less electronegative atom given POSITIVE ox. no.

In most COMPOUNDS,

6. H = + 1 except when bonded to a metal- metal must have the positive ox. no.

7. O = - 2 except when bonded to F or in peroxides, e.g. Na2O2- F must have the negative ox. no. )

LEA

RN

and PR

AC

TIS

E

Gp 2 elements

Gp 3 elementsFluorine

Oxidation NumbersOxidation Numbers

• The oxidation numbers of atoms in a compound add up to zero.

F -1

O -2

H +1

Cl -1

Find the oxidation state of C in CO2?

? – 4 = 0

? = +4Put the +!

Oxidation NumbersOxidation Numbers• The oxidation

numbers of atoms in an ion add up to the charge on the ion.

F -1

O -2

H +1

Cl -1

Oxidation state of S in SO4

2-?

? – 8 = -2

S = +6

ASSIGN AN OXIDATION NUMBER / STATE TO EACH ATOM IN :

Cl2   CO32-  

Ca2+   SO32-  

Al3+   ClO-

H2O   IO4-  

CO2   CH4  

ClF   MnO4-  

NO3-   Na2S4O6

 

CuCl   CuBr2 

N2  C2O4

2-  

BrF5  Mn2O3

 

SF6  CO  

S2-   BrF  

VCl2   Na2S  

NO2-   BrO3

-  

NH4+   H2SO4

 

SO42-   I-  

S2O32-   NH3

 

CCl4  Cr2O7

2-  

Cl(0)

Ca(+2)

Al(+3)

H(+1) O(-2)

O(-2) C(+4)

F(-1) Cl(+1)

O(-2) N(+5)

Cl(-1) Cu(+1)

N(0)

F(-1) Br(+5)

F(-1) S(+6)

S(-2)

Cl(-1) V(+2)

O(-2) N(+3)

H(+1) N(-3)

O(-2) S(+6)

O(-2) S(+2)

Cl(-1) C(+4)

O(-2) C(+4)

O(-2) S(+4)

O(-2) Cl(+1)

O(-2) I(+7)

H(+1) C(-4)

O(-2) Mn(+7)

Na(+1) & O(-2) S(+2.5)

Br(-1) Cu(+2)

O(-2) C(+3)

O(-2) Mn(+3)

O(-2) C(+2)

F(-1) Br(+1)

Na(+1) S(-2)

O(-2) Br(+5)

O(-2) & H(+1) S(+6)

I(-1)

H(+1) N(-3)

O(-2) Cr(+6)

Problems

• What is the oxidation number of each element?

• I2

• Cr2O3

• AlCl3• Na2SO4

• CaH2

Identifying Redox Reactions using oxidation number

0 +3 -2 0 +3 -2

• 2 Al + Fe2O3 2 Fe + Al2O3

• Al increases from 0 to +3, it is Oxidized!

• Fe decreases from +3 to 0, it is Reduced!

Problems

• Zn Zn2+

• Fe3+ Fe2+

• CaCO3 CaO + CO2

• AgNO3 Ag

• Do Practice Exercises

Rem. OXIDATION REDUCTION

= OXIDATION No. INCREASE DECREASE

Work out the oxidation number change for each of the following process and use it to decide whether it is an OXIDATION or a REDUCTION.

PROCESS PROCESS Ox. No.’s Ox. No.’s OxidationOxidation ReductionReduction

ClCl2 2 Cl Cl--

CaCa Ca Ca2+2+

NONO2 2 NO NO

33--

MnOMnO44- - Mn Mn2+2+

SOSO22 SO SO

442-2-

IOIO44- - I I

22

HH22SOSO4 4 S S2-2-

BrBr2 2 BrO BrO--

NHNH44+ + NH NH

33

CrCr22OO77

2- 2- Cr Cr3+3+

Cl(0) (-1)

Ca(0) (+2)

N(+4) (+5)

Mn(+7) (+2)

S(+4) (+6)

I(+7) (0)

S(+6) (-2)

Br(0) (+1)

N(-3) (-3)

Cr(+6) (+3)

NONE