Redox Reactions

13.1a Electron Transfer Theory

Overview

• oxidation and reduction rxtns always occur together (redox rxtn)– you can’t have one without the other

• examples– extracting metal from an ore– electroplating of metals– salt on roads (corrosion)

• salt accelerates the reaction between oxygen and metal• makes electron transfer easier

3 perspectives of oxidation/reduction

• 1) oxygen– oxidation gains and reduction loses

• 2) hydrogen– oxidation loses and reduction gains

• 3) electrons (modern view)– oxidation loses and reduction gains– most fundamental explanation, what we will

be working with the most

Sample Rxtn.

We convert iron ore to metallic iron using carbon. What is being oxidized?

2Fe2O3 + 3C 4Fe + 3CO2

– we removed oxygen from the iron(III)oxide and the carbon gained oxygen

– carbon was oxidized because it gained an oxygen

Sample Rxtns. con’t

Fe2O3 (s) + 3 CO(g) 2 Fe (s) + 3CO2

(g)

SnO2 (s) + C(s) Sn(s) + CO2 (g)

CuS(s) + H2(g) Cu(s) + H2S(g)

Oxidation Reduction

Gains oxygen Lose Oxygen

Lose Hydrogen Gains hydrogen

Lose Electrons Gains electrons

Oxygen

Hydrogen

e-

Redox with ElectronsRedox with Electrons

many reactions may not even involve oxygen redox currently refers to electrons being

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-

•Mg is the reducing agent and S is the oxidizing agent

• agents cause the other half of the rxtn. to occur

(MgS)

Examples

• Is the reactant oxidized or reduced?

• Pb PbO3 oxidized • SnO2 SnO reduced• KClO3 KCl reduced• C2H6O C2H4O oxidized• C2H2 C2H6 reduced

Homework

Textbook p. 559 #1-4

Redox Reactions

13.1a1 Electron Transfer Theory

Leo the Lion!

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

electrons is reduction

LEO says GERLEO says GER

0 1

Na Na e

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

0 1

Cl e Cl

Chlorine is reduced

LEO says GER con’tLEO says GER con’t - 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

Oxidizing and Reducing Agents

CuO + H2 Cu + H2O

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

oxidizing agent 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 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

• A) H2(g) + Cl2 (g) 2HCl(g)• H2 oxidized Cl2 reduced

• B) S(s) + Cl2 (g) SCl2(g)• S oxidized Cl2 reduced

• C) N2 + 2O2 2NO2

• N2 oxidized O2 reduced

• D) 2Li + F2 2LiF• Li oxidized F2 reduced

• E) H2 + S H2S• H2 oxidized S reduced

Examples

Half-Cell Reactions

The overall reaction:

2Fe + 3Cl2 2Fe3+ + 6Cl-

may be written as the sum of two half-cell reactions:

2Fe 2Fe3+ + 6e- (oxidation)

3Cl2 + 6e- 6Cl- (reduction)

Complete redox reactions can be expressed as the sum of two half reactions, one reduction and one oxidation.

Homework

Textbook p.564 #7-11

LSM 13.1B,C summary