Oxidation-Reduction Reactions LEO SAYS GER

Remember Oxidation Numbers

Oxidation Numbers What is an oxidation number: An oxidation number is a positive or negative number assigned to an atom to keep track of electron transfers & electron sharing

Rules for Assigning Oxidation Numbers The oxidation number of any uncombined element is zero. 2.The oxidation number of a monatomic ion equals its charge. +1 -1 2Na + Cl2 2NaCl

Rules for Assigning Oxidation Numbers 3. The oxidation number of O in compounds is -2, Exceptions: in hydrogen peroxide: H2O2 where it is -1, in compounds with the more electronegative fluorine F2O +1 -2 +1 -1 -1 +2 F2O H2O H2O2 2(+1) + 1(-2) = 0 2(-1) + 1(+2) = 0 2(+2) + 2(-2) = 0

Rules for Assigning Oxidation Numbers 4. The oxidation number of H in compounds is +1, Exception: in metal hydrides it is -1. +1 -2 +1 -1 H2O NaH Warning! Don't get too bogged down in these exceptions. In most of the cases you will come across, they don't apply!

Rules for Assigning Oxidation Numbers 5. For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0. +1 -2 +2 -2 +1 Ca(OH)2 H2O (+2) + 2(-2) + 2(+1) = 0 Ca O H 2(+1) + (-2) = 0 H O

Rules for Assigning Oxidation Numbers 6. For a polyatomic ion, the sum of the oxidation numbers in the formula is equal to the ionic charge of the ion ? -2 ? -2 SO42- NO3- X + 3(-2) = -1 N O X + 4(-2) = -2 S O THUS X = +6 THUS X = +5

Oxidation and Reduction We already learned why salt is spread on roads during winter months Lowers the freezing point of water preventing the buildup of slippery ice But…what about the affect of the salt that clings to the metallic parts of cars? RUST The corrosion of metal is one example of an oxidation-reduction reaction

Oxidation and Reduction Defined old definition - substance oxidized reacts with oxygen new definition – loss of electrons Reduction old definition – substance reduced loses oxygen new definition – gain of electrons

Oxidation and Reduction Until now our view of a reaction: Mg + S  MgS Reaction viewed as Redox: Mg + S → Mg2+ + S2- The magnesium atom changes to a more stable magnesium ion by losing 2 electrons, and is thus oxidized The sulfur atom is changed to a more stable sulfide ion by gaining 2 electrons, and is thus reduced.

Oxidation and Reduction Mg + S MgS The over all process is represented as the two component processes below: Mg Mg2+ + 2e- S + 2e- S2- Oxidation – loss of electrons Reduction – gain of electrons

Example 2: Oxidation and Reduction Each sodium atom loses one electron – oxidation Each chlorine atom gains one electron – reduction

LEO says GER Gain Electrons = Reduction Lose Electrons = Oxidation Sodium is oxidized Gain Electrons = Reduction Chlorine is reduced

But there are two Cl atoms So, if Cl20 + 2e-  2Cl-1 Then, 2Na0  2Na+1 + 2e- ___________________

Terminology for Redox Oxidation Reduction Oxidizing Agent Loss of electrons Increase in oxidation number Increase in oxygen Oxidizing Agent Electron acceptor Species that is reduced Reduction - Gain of electrons - Decrease in oxidation number - Decrease in oxygen Reducing Agent - Electron donor - Species that is oxidized

You can’t have one… without the other! Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation GER!

Practice Problem Identify these processes as either oxidation or reduction. S2- S + 2e- Zn2+ + 2e- Zn Oxidation – is loss Reduction – is gain

Practice Problem Determine what is oxidized and what is reduced in each reaction. 2Na + S Na2S 4Al + 3O2 2Al2O3 Na is oxidized (Na1+ S2-) S is reduced Al is oxidized (Al3+ O2-) O is reduced

Redox in Covalent Compounds It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds? (complete electron transfer does not occur) Consider: 2H2 + O2 2H2O Oxygen is highly electronegative In H there is a shift of bonding electrons away from H Hydrogen is oxidized In O there is a shift of electrons toward O Oxygen is reduced loss gain