C H E M I S T R Y

Chapter 4Chapter 4Reactions in Aqueous SolutionReactions in Aqueous Solution

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

2Fe2O3(s)4Fe(s) + 3O2(g) Rusting of iron:an oxidation of Fe

4Fe(s) + 3CO2(g)2Fe2O3(s) + 3C(s)Manufacture of iron: a reduction of Fe

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

1. An atom in its elemental state has an oxidation number of 0.

Rules for Assigning Oxidation Numbers

Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor.

Na H2 Br2 S Ne

Oxidation number 0

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

2. A monatomic ion has an oxidation number identical to its charge.

Na1+

+1

Ca2+

+2

Al3+

+3

Cl1-

-1

O2-

-2

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

b) Oxygen usually has an oxidation number of -2.

H O1-

-2+1

HH Ca

-1-1 +2

3. An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion.

a) Hydrogen can be either +1 or -1.

OH O

-1+1 -1

HH O

+1+1 -2

H

+1

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

H Cl

+1 -1

c) Halogens usually have an oxidation number of -1.3.

ClCl O

+1+1 -2

Oxidation-Reduction (Redox) Oxidation-Reduction (Redox) ReactionsReactions

Cr2O72-

-2+1 x

4. The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion.

x = +6

2(x) + 7(-2) = -2 (net charge)

H2SO3

x -2

x = +4

2(+1) + x + 3(-2) = 0 (net charge)

ExampleExampleDetermine the oxidation number for

each atom in the following compounds/molecules◦CO2

◦CCl4

◦CoSO4

◦K2O2

Identifying Redox ReactionsIdentifying Redox Reactions

Oxidation: losing one or more electrons increasing in oxidation number Reducing agent

2Fe23O2(g)+4Fe(s) O3 (s)

+30

oxidation

reduction

-20

Reduction: gaining one or more electron decreasing in oxidation number

Oxidizing agent

Identifying Redox ReactionsIdentifying Redox Reactions

Reducing Agent

Causes reductionLoses one or more electronsUndergoes oxidationOxidation number of atom increases

Oxidizing Agent

•Causes oxidation •Gains one or more electrons•Undergoes reduction•Oxidation number of atom decreases

ExampleExample

Identify each of half reactions as 1) oxidation or 2) reduction.

__A. Sn(s) Sn4+(aq) + 4e−

__B. Fe3+(aq) + 1e− Fe2+(aq)

__C. Cl2(g) + 2e− 2Cl-(aq)

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Writing Oxidation and Reduction Writing Oxidation and Reduction ReactionsReactions

Write the separate half oxidation and reduction reactions for the following equation.

2Cs(s) + F2(g) 2CsF(s)

3 Na(l) + AlCl3(l) 3 NaCl(l) + Al(l)

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The Activity Series of the The Activity Series of the ElementsElements

2Ag1+(aq) + Cu(s)2Ag(s) + Cu2+(g)

Cu2+(aq) + 2Ag(s)Cu(s) + 2Ag1+(g)

Which one of these reactions will occur?

The Activity Series of the The Activity Series of the ElementsElements

Elements that are higher up in the table are more likely to be oxidized.

Thus, any element higher in the activity series will reduce the ion of any element lower in the activity series.

The Activity Series of the The Activity Series of the ElementsElements

2Ag1+(aq) + Cu(s)2Ag(s) + Cu2+(g)

Cu2+(aq) + 2Ag(s)Cu(s) + 2Ag1+(g)

Which one of these reactions will occur?

ExampleExamplePredict whether the following

redox reactions will occurred or not. If so, predict the products

◦Zn(s) + FeCl2(aq)

◦Ni(s) + Mg(NO3)2(aq)

Redox TitrationsRedox Titrations

If the unknown concentration is the potassium permanganate solution, MnO4

1-, it can be slowly added to a known amount of oxalic acid, H2C2O4, until a faint purple color persists.

Titration: A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known.

5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

Redox TitrationsRedox Titrations

5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

A solution is prepared with 0.2585 g of oxalic acid, H2C2O4. 22.35 mL of an unknown solution of potassium permanganate are needed to titrate the solution. What is the concentration of the potassium permanganate solution?

Calculation Set upCalculation Set up

Moles ofH2C2O4

Mass ofH2C2O4

Moles ofKMnO4

Molarity ofKMnO4

Mole Ratio

Molarity of KMnO4

Molar Mass of H2C2O4

ExampleExampleA 0.0484M standard solution of potassium

permanganate was titrated against 25.00mL of an iron (II) sulfate solution. The equivalence point, as indicated by a faint pink color, was reached when 15.50mL of potassium permanganate solution had been added. Calculate the concentration of the iron (II) sulfate solution