1. Oxidation-Reduction Reactions (REDOX reactions)
LO 3.8: The student is able to identify redox reactions and justify the identification in terms of electron transfer.

2. Oxidation-Reduction Reactions (REDOX reactions)
After completing this lesson, students will be able to:
Identify the oxidation numbers for all elements in a reaction.
Determine whether or not a reaction is redox.

3. Oxidation-Reduction Reactions (REDOX reactions)
Oxidation – the loss of electrons by a species; often, this involves a reaction with oxygen
Reduction – the gain of electrons by a species
To determine which species is oxidized and which species is reduced, we need to look at oxidation numbers.

4. Oxidation Numbers
Oxidation number – the charge or effective charge of a species
When a substance is oxidized, the oxidation number for one of its elements increases.
When a substance is reduced, the oxidation number for one of its elements decreases (it’s reduced!).
Reduction and oxidation occur at the same time. It is not possible to have one without the other!

5. Oxidation Numbers
Oxidation numbers are applied to individual elements, either by themselves, or within compounds and polyatomic ions.
SO42- does not have an oxidation number.
The S has an oxidation number.
The O has an oxidation number.

6. Rules for assigning oxidation numbers
Elements
The oxidation number is zero
Monatomic ions
The oxidation number is the charge of the ion
Compounds and ions
The sum of the oxidation numbers equal
the charge of the species

7. Rules for assigning oxidation numbers
Oxygen
In compounds, the oxidation number of oxygen is
usually -2 (exception is peroxide, where O is -1)
Hydrogen
Oxidation number is usually +1, but can be -1if acting as a negative ion (ex. NaH).
Halogens
Oxidation number is usually -1, but can be positive if combined with more electronegative elements like oxygen.
The oxidation numbers for other elements can be determined using deductive reasoning.

8. Determine the oxidation number for each atom/ion in these neutral compounds.
Ga2O3
PbO2
KBrO4
K2MnO4
Fe2(CrO4)3

9. Determine the oxidation number for each atom/ion in these polyatomic ions.
NH2-
IO3-
Al(OH)4-
H2PO4-
S2O32-

10. REDOX REACTIONS Basic Points
An oxidation number is property of individual atoms, either by themselves, as ions, or in compounds. Groups of atoms or ions do not get assigned an oxidation number.
Not all reactions are REDOX
If there is oxidation, there must be reduction!