OXIDATION REDUCTION REACTIONS

Oxidation Number Rules The oxidation number of any free,

uncombined element is zero. The oxidation number of an element in a

simple (monatomic) ion is the charge on the ion.

In the formula for any compound, the sum of the oxidation numbers of all elements in the compound is zero. In a polyatomic ion, the sum of the oxidation numbers of the constituent elements is equal to the charge on the ion.

Oxidation Number Rules

Hydrogen, H, in combined form, has the oxidation number +1, usually.

Oxygen, O, in combined form, has the oxidation number -2, usually.

Oxidation Numbers

assign oxidation numbers to each element in these compounds

NaNO3 K2Sn(OH)6 H3PO4

Oxidation Numbers

assign oxidation numbers to each element in these compounds

NaNO3 K2Sn(OH)6 H3PO4

+1 +5 -2

Oxidation Numbers

assign oxidation numbers to each element in these compounds

NaNO3 K2Sn(OH)6 H3PO4

+1 +5 -2 +1 +4 -2 +1

Oxidation Numbers

assign oxidation numbers to each element in these compounds

NaNO3 K2Sn(OH)6 H3PO4

+1 +5 -2 +1 +4 -2 +1 +1 +5 -2

Oxidation Numbers

assign oxidation numbers to each element in these polyatomic ionsSO3

2- HCO3- Cr2O7

2-

Oxidation Numbers

assign oxidation numbers to each element in these polyatomic ionsSO3

2- HCO3- Cr2O7

2-

+4 -2

Oxidation Numbers

assign oxidation numbers to each element in these polyatomic ionsSO3

2- HCO3- Cr2O7

2-

+4 -2 +1+4 -2

Oxidation Numbers

assign oxidation numbers to each element in these polyatomic ionsSO3

2- HCO3- Cr2O7

2-

+4 -2 +1+4 -2 +6 -2

Redox Reactions oxidation - loss of electrons

oxidation number increases reduction - gain of electrons

oxidation number decreases or reduces! oxidizing agents - chemical species that

gain electrons and oxidize some other species electron thieves they are reduced

Redox Reactions

reducing agents - chemical species that lose electrons and reduce some other species victims of electron thieves they are oxidized

Predicting reactions?

Activity Series

Allows us to predict if a certain metal will be oxidized by an acid or a particular salt.

Activity series is arranged in order of decreasing ease of oxidation

Metals at the top lose electrons easily Metals at the bottom are very stable Any metal can be oxidized by the ions of

the substances below it.

Balancing Redox Reactions

Balancing redox reactions.

We will use the Half-Reaction Method

Half Reaction Method Balancing redox reactions by half reaction

method rules: Write the unbalanced reaction Break into 2 half reactions -

1 for oxidation 1 for reduction

Mass balance each half reaction by adding appropriate stoichiometric coefficents in acidic solutions we can add H+ or H2O

in basic solutions we can add OH- or H2O

Half Reaction Method

Charge balance the half reactions by adding appropriate numbers of electrons

Multiply each half reaction by a number to make the number of electrons added equal to the number of electrons consumed

Add the two half reactions Eliminate any common terms

Half Reaction Method

Example: Tin (II) ions are oxidized to tin (IV) by bromine. Use the half reaction method to write and balance the net ionic equation.

reaction starting BrSnBrSn -+42

+2

Half Reaction Method

Example: Tin (II) ions are oxidized to tin (IV) by bromine. Use the half reaction method to write and balance the net ionic equation.

reaction halfoxidation e2Sn Sn

reaction starting BrSnBrSn-+4+2

-+42

+2

Half Reaction Method

Example: Tin (II) ions are oxidized to tin (IV) by bromine. Use the half reaction method to write and balance the net ionic equation.

reaction halfreduction Br 2e2Br

reaction halfoxidation e2Sn Sn

reaction starting BrSnBrSn

--2

-+4+2

-+42

+2

Half Reaction Method

Example: Tin (II) ions are oxidized to tin (IV) by bromine. Use the half reaction method to write and balance the net ionic equation.

Sn Br Sn Br starting reaction

Sn Sn e oxidation half reaction

Br e 2 Br reduction half reaction

Sn Br Sn Br balanced reaction

2+2

4+ -

2+ 4+ -

2- -

2+2

4+ -

2

2

2

Half Reaction Method

Example: Dichromate ions oxidize iron (II) ions to iron (III) ions and are reduced to chromium (III) ions in acidic solution. Write and balance the net ionic equation for the reaction.

reaction starting FeCrFeOCr +3+3+2272

Half Reaction Method

Example: Dichromate ions oxidize iron (II) ions to iron (III) ions and are reduced to chromium (III) ions in acidic solution. Write and balance the net ionic equation for the reaction.

reaction halfoxidation 1e+ FeFe

reaction starting FeCrFeOCr-+3+2

+3+3+2272

Half Reaction Method

Example: Dichromate ions oxidize iron (II) ions to iron (III) ions and are reduced to chromium (III) ions in acidic solution. Write and balance the net ionic equation for the reaction.

red. OH 7Cr 2 e 6H 14OCr

ox. 1e+ FeFe

rxnstart FeCrFeOCr

2+3-+2

72

-+3+2

+3+3+2272

Half Reaction Method

Example: Dichromate ions oxidize iron (II) ions to iron (III) ions and are reduced to chromium (III) ions in acidic solution. Write and balance the net ionic equation for the reaction.

red. OH 7Cr 2 e 6H 14OCr

ox. 1e+ FeFe 6

rxnstart FeCrFeOCr

2+3-+2

72

-+3+2

+3+3+2272

Half Reaction Method

Example: Dichromate ions oxidize iron (II) ions to iron (III) ions and are reduced to chromium (III) ions in acidic solution. Write and balance the net ionic equation for the reaction.

OH 7Cr 2Fe 6H 14OCrFe 6

red. OH 7Cr 2 e 6H 14OCr

ox. 1e+ FeFe 6

rxnstart FeCrFeOCr

2+3+3+2

72+2

2+3-+2

72

-+3+2

+3+3+2272

Half Reaction Method

Example: In basic solution hydrogen peroxide oxidizes chromite ions, Cr(OH)4

-, to chromate ions, CrO42-.

Write and balance the net ionic equation for this reaction.

OH8CrO 2OH 3OH2Cr(OH) 2

6OH+OH8CrO 2OH 3OH8Cr(OH) 2

OH2e2OH3

3e+OH4CrOOH4Cr(OH)2

solution) (basic CrOOHCr(OH)

22

422-

4

-2

2422

-4

--22

-2

24

-4

24224

Half Reaction Method

Half Reaction Method

Example: When chlorine is bubbled into basic solution, it forms chlorate ions and chloride ions. Write and balance the net ionic equation.

Cl 5+ OH 3ClO OH 6Cl 3

Cl 10+ OH 6ClO 2OH 12Cl 6

Cl 2e 2Cl 5

10e + OH 6ClO 2OH 12Cl 1

solution) (basic ClClOCl

23-

2

23-

2

-2

-23

-2

32

Half Reaction Method