Unit II D.4
(Pages 136-138)
The book says this:
1. Reduction occurs when an atom gains one or more electrons.
2. Oxidation occurs when an atom or ion loses one or more electrons.
These changes happen during a chemical reaction. We have done several experiments that were redox reactions.
Examples:
Copper metal reacts with silver nitrate to form silver metal and copper nitrate:
Cu + 2 Ag(NO3) -> 2 Ag + Cu(NO3)2.
Our problem: How do you tell which atoms have gained or lost electrons?
Answer: We need to learn how to figure out each atom's oxidation number.
Rules for assigning Oxidation Numbers:
1. A "free element" (one that is not in a compound with other elements) is neutral; its oxidation number = 0.
2. Your periodic chart can tell you the oxidation number (charge) for most elements in compounds.
3. Some elements can have several possible oxidation states; for these, you need to calculate the oxidation state (charge) in each case.
Back to our equation:
Cu0 + 2 Ag+1(N+5O-23) -> 2 Ag0 + Cu2+(N+5O-23)2.
The oxidation state for each atom is shown. Our classroom discussion will explain each one.
Notice that the oxidation states for the nitrogen (N) and oxygen (O) atoms don't change in this reaction. They are therefore sometimes called spectator ions. Your book sometimes leaves them out altogether, as in the equations on page 137.
These notes are intended to be handed out to students.