1. Oxidation and Reduction Section 9.1

2. Electron Transfer Theory According to modern theory the gain of electrons is called reduction. the loss of electrons is called oxidation.

3. Redox Theory suggests: the total number of e - gained in a reaction must equal the total number of e - lost Reduction and oxidation are separate processes that occur simultaneously The chemical equations of the oxidation, reduction and the overall equation must be balanced.

4. Writing and Balancing Half- Reactions Write a balanced net equation for the reaction of copper with aqueous silver nitrate. Step 1: Write each half reaction (the metal reacting with the solution will undergo an oxidation – forms a + ion thus a loss of electrons, while the one in solution will undergo a reduction)

5. Example Continued… Step 2: Remember the number of electrons lost must equal the number of electrons gained. So we need to multiply the second reaction by two so that we will have 2e - in each equation

6. Example Cont… Step 3: Now add the two half-reaction equations Step 4: Cancel out like terms Step 5: Write net equation

7. Oxidation States An oxidation number is a positive or negative number corresponding to the oxidation state of an atom Using the following rules you can determine the oxidation number of other atoms in compounds (pg. 658)

8. Examples What is the oxidation number of sodium in sodium hydride (NaH)? - hydrogen in a hydride has an oxidation # of -1 - b/c there is only 1 Na, it must be +1 What is the oxidation number of sulphur in dihydrogen sulphide (H 2 S)? - hydrogen is always +1 - b/c there are 2 of them the overall is 2(+1) = +2 - b/c only 1 S, it must be -2

9. Examples Cont… What is the oxidation number of carbon in the carbonate ion (CO 2 2- )? - O has an oxidation number of -2 - there is an overall charge of 2- on the ion so we can set up a mini equation: - solve for x

10. Examples Cont… What is the oxidation number of sulphur in calcium sulphate ( ) We can assumed that Ca is in ionic form  its oxidation number is +2 So we can set up our equation and solve

11. Oxidation Numbers and Redox Reactions An oxidation is an increase in oxidation number A reduction is a decrease in oxidation number In redox reactions, oxidation numbers change.

12. Example: When methane burns, carbon dioxide and water form. Identify the oxidation and reduction in this reaction: First write the reaction: Now insert the oxidation numbers and arrows:

13. Seatwork: Pg. 653 #2 Pg. 656 #8-9 Pg. 659 #12, 13 Pg. 662 #18, 19, 20