Oxidation and Reduction Section 9.1

Electron Transfer Theory According to modern theory the gain of electrons is called reduction. the loss of electrons is called oxidation. In metallurgy reduction = producing metals from ore and oxidation = reaction with oxygen (combustion, corrosion) LEO the lion says GER

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.

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)

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

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

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) Oxidation # are assigned arbitrarily, they are not electric charges. The pos/neg sign comes before the number

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 (H2S)? - 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

Examples Cont… What is the oxidation number of carbon in the carbonate ion (CO22-)? - 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

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

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.

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:

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