AP Chemistry Update

Today in class students (hopefully) increased their understanding of Chapter 4 material. The answers to Example #11 and Example #12 from the chapter 4 review guide were discussed. You can watch the PowerPoint Notes by clicking here. Just fast forward to the end to see Examples #11 and #12. Then we moved on to a discussion of Section 4.4 in the textbook (Oxidation-Reduction Reactions). Here are some important points about oxidation numbers:

• The oxidation number of an element in its elemental state is zero
• The oxidation number for a monoatomic ion is simply the charge on the ion
• The O.N. for alkali metals in compounds is +1
• The O.N. for alkaline earth metals in compounds is +2
• The O.N. for fluorine in compounds is -1
• The O.N. for oxygen in compounds is usually -2 (except peroxides or OF₂)
• The O.N. for hydrogen in compounds is usually +1 (except metal hydrides)
• In a binary covalent compound, the element that is more electronegative will have an O.N. equal to the charge it normally forms as a monoatomic ion
• The sum of the O.N. values is zero in a neutral chemical formula
• In a polyatomic ion, the sum of the O.N. values is equal to the charge on the ion

Now that you understand oxidation numbers, here is a procedure to help you balance a half-reaction:

• Step 1 = Balance the atoms of the element that is being oxidized or reduced
• Step 2 = Use electrons to balance the change in oxidation number ("OIL RIG" will remind you that oxidation is the loss of e^– and reduction is the gain of e^–)
• Step 3 = Use H⁺ ions (or OH^– ions) to balance the overall charge on both sides
• Step 4 = Use H₂O molecules to balance the H atoms and the O atoms

We did not get a chance to go over all of the homework problems in class today. I'll be talking more about that on Monday. The homework for Monday is to do the AP Multiple Choice Questions related to Chapter 4, which can be found here.

E-mail me with any questions. Have a good weekend.