Redox Reactions

What is oxidation?What is reduction?

What is REDOX?• chemical reactions in which atoms have their

oxidation state changed• family of reactions that are concerned with the

transfer of electrons between species

Rusting is an example of a redox reaction.

REDOX• term comes from the two concepts of reduction

and oxidation• Oxidation is the loss of electrons or an increase

in oxidation state • Reduction is the gain of electrons or a decrease

in oxidation state• Oxidation State = the hypothetical charge that an

atom would have if all bonds to atoms of different elements were 100% ionic

Assigning Oxidation Numbers#1. All free, uncombined elements have oxidation

number of 0.• Includes diatomic elements such as O2

#2. All monoatomic ions have the same oxidation # as the charge on the ion.

Mg = +2 Cl = -1

#3. All combined Hydrogen has oxidation # = +1. (except hydrides)

Assigning Oxidation Numbers#4. All combined Oxygen has oxidation # = -2.

(except peroxides)

#5. Polyatomics = sum of oxidation numbers of elements in ion equals the charge on that speciesEx. Sulfate SO4

2¯

O = -2. There are four oxygens for -8 total. Since -2 is left over, the S must = +6.

#6. Group I = +1 Group II = +2

EXAMPLESH2SO4

H = O =S =

MoO4 -2

Mo = O =

+1 -2 +6

-2+6

Determining REDOX Species

4 Fe 2 Fe2 + 1 e-

Was an electron gained or lost?

An electron was produced or lost by Fe.

LEO .. Losing electrons is oxidation.

Fe was oxidized.

1 e- + 3 O2 2 O3

Was an electron gained or lost?

An electron was gained by O2.

GER .. Gaining electrons is reduction.

Oxygen was reduced.

Oxidation and Reduction ALWAYS occur together. One cannot occur without the other.

How does this occur?By oxidizing and reducing agents• Oxidizing Agent = substance that removes an

electron from another species– Ex . oxygen

• Reducing Agent = substance that donates an electron to another species– Ex. Iron, metals

Full Rxn: 4Fe + 3O2 → 2Fe2O3

Half Rxn: Fe0 → Fe3+ + 3e−

Half Rxn: O2 + 4e− → 2 O2−

Iron is the reducing agent

Oxygen is the oxidizing agent

• Iron (Fe) has been oxidized because the oxidation number increased. (0 +3)

• Iron (Fe) is the reducing agent because it gave electrons to the oxygen (O2).

• Oxygen (O2) has been reduced because the oxidation number has decreased. (0 -2)

• Oxygen (O2) is the oxidizing agent because it took electrons from iron (Fe).

Full Rxn: 4Fe + 3O2 → 2Fe2O3

Half Rxn: Fe0 → Fe3+ + 3e−

Half Rxn: O2 + 4e− → 2 O2−

Cl2 (g) + 2NaBr (aq) 2NaCl (aq) + Br2 (g)

1. Assign Oxidation Numbers

OO+1 +1-1 -1

2. Is it REDOX? Did oxidation numbers change?

3. LEO goes GERWho is oxidized?Who is reduced?Who is the oxidizing agent?Who is the reducing agent?

Bromine

Bromine

ChlorineChlorine

Example