AP Chem This is our last unit!

A closer look at a selected reaction: 2 lost 2 gained Oxidation: the loss of electrons by an atom or ion during a chemical reaction Reduction: the gain of electrons by an atom or ion during a chemical reaction Oxidation and Reduction occur together in a reaction. Oxidation-reduction reactions are known as Redox reactions.

Oxidation Numbers (state) +, -, or neutral (0) numbers that can be assigned to atoms Used to identify how many electrons are being lost or gained by an atom/ion when they form bonds

Assigning Oxidation Numbers: Rules 1. Free or Uncombined Elements (elements not bonded to another element) ALWAYS have an oxidation number of 0 2. Ions have the same oxidation number as the charge on the ion

Assigning Oxidation Numbers: Rules 3. When combined with another element: Group 1 metal ions ALWAYS have an oxidation number of +1 Group 2 metal ions ALWAYS have an oxidation number of +2 Fluorine ion ALWAYS has an oxidation number of -1 4. Oxygen usually has an oxidation number of -2

Assigning Oxidation Numbers: Rules Hydrogen’s oxidation number is different depending on whether it’s in an ionic or covalent compound H bonded with a nonmetal (covalent): the oxidation number will be +1 H bonded with a metal (ionic): the oxidation number will be -1

Assigning Oxidation Numbers: Rules 6. The oxidation numbers must add up to the charge of the compound (0) or polyatomic ion

Assigning Oxidation Numbers: Practice +1 -1 -1 +2 -2 +1

Assigning Oxidation Numbers: Practice +1 +1 +6 -2 -2 +5 +4 -2 -2 +6 -2

In a redox reaction, one element loses electrons while another element gains electrons. xidation osing s lectrons is osing (electrons) xidation eduction aining s lectrons is aining (electrons) eduction

Writing Half Reactions +2 +2 2 2 Ca  Ca2+ + 2e- Cr2+ + 2e-  Cr CHARGES MUST BE BALANCED!

Writing Half Reactions +2 +2 2 2 Mg  Mg2+ + 2e- Zn2+ + 2e-  Zn CHARGES MUST BE BALANCED!

CHARGES MUST BE BALANCED!