SCH4U - Chemistry 12 (2024-25) - A

Summary

• Electrochemistry involves chemical reactions in which electrons are transferred between reactant molecules
• Reactions involving electron transfer are either oxidation or reduction reactions.  We use the abbreviation redox to describe these reactions
• Oxidation is a reaction in which electrons are lost by the reactant species
• The species that loses electrons is known as the reducing agent
• Reduction is a reaction in which electrons are gained by the reactant species
• The species that gains electrons is known as the oxidizing agent
• Spectator ions are those that appear as both a reactant and a product, they can be removed from the net ionic equation
• In a redox reaction, we can identify which species are reduced and oxidized by comparing their charges on either side of the reaction arrow
• Half-reaction equations take the net ionic equation for a redox reaction and split them into oxidation half-reactions and reduction-half reactions

Oxidation Numbers

• Oxidation numbers are used to identify the degree of oxidation of a species in an oxidation-reduction reaction
• The rules for determining oxidation numbers are as follows:

1. An oxidation number of zero is given to an atom in its elementary state.

2. The oxidation number of a monoatomic ion is the same as its charge

3. The oxidation number for hydrogen is +1 in all compounds (except metal hydrides (metal + hydrogen) where it is -1)

4. The oxidation number for oxygen is -2 in all compounds (except peroxides, H₂O₂ in which it is -1)

5. All other oxidation numbers are assigned so that the sum of the oxidation numbers equals the net charge on the molecule or ion.  Therefore, for neutral molecules, the net sum of the oxidation numbers must equal zero; whereas for ions the net sum of the oxidation numbers must equal the charge of the ion.

• To determine the oxidation numbers for atoms in a molecule, we first apply rules #1-4 above than we apply rule #5 for all other atoms.