Oxidation & Reduction

Reactions in which some elements change their oxidation number.

Oxidation Numbers

• An oxidation number is the charge an atom would have if electrons in its bonds belonged completely to the more electronegative atom.

ClO2

Cl can have several oxidation numbers: -1,+1,+3,+5,+7

Redox Reaction

• A chemical reaction in species change their oxidation number:

Na + Cl2 => NaCl

What’s the point ?

• Electrical production (batteries, fuel cells)

REDOX reactions are important in …

Rules

Determine the oxidation number of each atom in each of the following formulas:

(a) O2

(b) CH4

(c) NaCl

(d) SO4-2

(e) KMnO4

Practice Assigning Oxidation Numbers

NO2

N2O5

HClO3

HNO3

Ca(NO3)2

KMnO4

Practice Assigning Oxidation Numbers

Fe(OH)3

K2Cr2O7

CO32-

CN-

K3Fe(CN)6

Practice Assigning Oxidation Numbers

NO2N= +4, O = -2

N2O5 N = +5, O = -2

HClO3 H=+1, Cl=+5, O = -2

HNO3 H=+1, N = +5, O = -2

Ca(NO3)2 Ca=+2, N =+5, O= -2

KMnO4 K=+1, Mn=+7, O= -2

Practice Assigning Oxidation Numbers

Fe(OH)3 Fe =+3, O=-2, H=+1

K2Cr2O7 K=+1, Cr=+6, O=-2

CO32-

C=+4, O =-2

CN- C=+4, N=-5

K3Fe(CN)6 K=+1, Fe=+3, C=+4, N=-5

Lab: Corrosion of Iron• An important aspect of the use of some metals,

particularly of iron, is the possibility of corrosion.• Rust is apparently a hydrated form of iron(III)oxide

Fe2+(aq) + O2(g) + H2O(l) → Fe2O3•H2O(s)   

rust

Galvanic Corrosion• Galvanic corrosion is an electrochemical

process in which one metal corrodes preferentially to another when both metals are in electrical contact.

Zinc

What do we do with oxidation numbers?

• The oxidation numbers of atoms in an equation allow us to determine which element is oxidized and which is reduced in a redox reaction.

6Cd + 2 FeBr3 => 3 Cd2Br2 + 2 Fe

LEO

LEO says

GER!

LEO says

Loss of Electrons = Oxidation

Gain of Electrons = Reduction

GER!

Oxidation and Reduction

• Apply oxidation numbers to all particles and identify the oxidized and the reduced elements.

Mg + HCl => MgCl2 + H2

F2 + AlBr3 => AlF3 + Br2

• An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized.

• A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced

Zn + CuSO4 ZnSO4 + Cu

Using Oxidation Numbers

• An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized.

• A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced

Zn + CuSO4 ZnSO4 + Cu

Exercise

For each of the following reactions find the element oxidized and the element reduced

Cl2 + KBr KCl + Br2  Cu + HNO3 Cu(NO3)2

+ NO2 + H2O

HNO3 + I2 HIO3 + NO2

ExerciseFor each of the following reactions find the element oxidized and the element reducedCl2 + KBr KCl + Br2

Exercise For each of the following reactions find the element oxidized and the element reduced

Cu + HNO3 Cu(NO3)2 + NO2 + H2O

Oxidation-Reduction Reactions (Redox)

• All oxidation reduction reactions have element reduced and one one element oxidized.

Why is the Statue of Liberty green?

Replica – Ellis Island

• Acid Rain causes the copper to turn into copper oxide, copper sulfate, copper hydroxide or copper chloride because of the oxidation-reduction reaction and this is basically copper salts

or in other words, tarnish.

Things like statues, pennies,

or pipes made of Copper turn green

over a period of time.

This happens due to oxidation.

Corrosion of steel, whether in atmospheric, soil, water, or chemical exposure, annually costs the economy about 3.1% of the Gross Domestic Product (GDP). Estimates show 25-30% of corrosion could be eliminated if proper corrosion protection methods, such as: galvanizing, painting, stainless or weathering steel were employed.

Steel

• Steel is an alloy made by combining iron and another element, usually carbon. When carbon is used, its content in the steel is between 0.2% and 2.1% by weight, depending on the grade. Other alloying elements sometimes used are manganese, chromium, vanadium and tungsten.

Corrosion of Iron• When iron rusts a spontaneous redox

reaction occurs, between the oxygen and iron. If water is added the rusting occurs more rapidly.

Oxidation :

Fe(s) → Fe2+(aq) + 2e-          Equation 1

Reduction : O2(g) + 2H2O(l) + 4e- → 4OH-(aq)     Equation 2

4Fe(OH)2(s) + O2(g) + xH2O(I)→ 2Fe2O3•(x)H2O(s) Equation 5

RUST

  3Fe2+(aq) + 2Fe(CN)6 3-(aq) → Fe3[Fe(CN)6]2 (s)    Equation 6            Yellow                          Prussian blue

Corrosion of Iron

• Corrosion of iron is called rusting. Rust is Fe2O3x.H2O.

Many metals like iron, steel, polymers, ceramic etc are damaged due to some natural process.Corrosion is one of them.

http://tdwhs.nwasco.k12.or.us/staff/bfroemming/CorrosionIron.html