The document discusses oxidation-reduction (redox) reactions. It defines oxidation as the loss of electrons and reduction as the gain of electrons. Redox reactions always involve both an oxidation and a reduction process. The substance undergoing oxidation is the reducing agent as it loses electrons, while the substance undergoing reduction is the oxidizing agent as it gains electrons. Assigning oxidation numbers to atoms allows identification of which substances are oxidized and reduced in a reaction.

1. “ Oxidation-Reduction Reactions” LEO SAYS GER Pre-AP Chemistry Charles Page High School Stephen L. Cotton

2. The Meaning of Oxidation and Reduction (called “redox”) OBJECTIVES Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons.

3. OBJECTIVES State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.

4. Oxidation and Reduction (Redox) Early chemists saw “ oxidation ” reactions only as the combination of a material with oxygen to produce an oxide. For example, when methane burns in air, it oxidizes and forms oxides of carbon and hydrogen,

5. Oxidation and Reduction (Redox) But, not all oxidation processes that use oxygen involve burning: Elemental iron slowly oxidizes to compounds such as iron (III) oxide, commonly called “rust” Bleaching stains in fabrics Hydrogen peroxide also releases oxygen when it decomposes

6. Oxidation and Reduction (Redox) A process called “ reduction ” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur simultaneously The substance gaining oxygen (or losing electrons) is oxidized , while the substance losing oxygen (or gaining electrons) is reduced .

7. Oxidation and Reduction (Redox) Today, many of these reactions may not even involve oxygen Redox currently says that electrons are transferred between reactants

8. Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:

9. LEO says GER : L ose E lectrons = O xidation Sodium is oxidized G ain E lectrons = R eduction Chlorine is reduced

10. LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent . - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent . Mg (s) + S (s) -> MgS (s) Mg is oxidized : loses e - , becomes a Mg 2+ ion S is reduced : gains e - = S 2- ion Mg is the reducing agent S is the oxidizing agent

11. Not All Reactions are Redox Reactions - Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples:

12. Corrosion Damage done to metal is costly to prevent and repair Iron , a common construction metal often used in forming steel alloys, corrodes by being oxidized to ions of iron by oxygen. This corrosion is even faster in the presence of salts and acids , because these materials make electrically conductive solutions that make electron transfer easy

13. Corrosion Luckily, not all metals corrode easily Gold and platinum are called noble metals because they are resistant to losing their electrons by corrosion Other metals may lose their electrons easily, but are protected from corrosion by the oxide coating on their surface, such as aluminum – Iron has an oxide coating, but it is not tightly packed, so water and air can penetrate it easily

14. Corrosion Serious problems can result if bridges, storage tanks, or hulls of ships corrode Can be prevented by a coating of oil, paint, plastic, or another metal If this surface is scratched or worn away, the protection is lost Other methods of prevention involve the “ sacrifice ” of one metal to save the second Magnesium, chromium, or even zinc (called galvanized) coatings can be applied

15. Oxidation Numbers OBJECTIVES Determine the oxidation number of an atom of any element in a pure substance.

16. Assigning Oxidation Numbers An “ oxidation number ” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative element

17. Rules for Assigning Oxidation Numbers The oxidation number of any uncombined element is zero . The oxidation number of a monatomic ion equals its charge .

18. Rules for Assigning Oxidation Numbers The oxidation number of oxygen in compounds is -2 , except in peroxides, such as H 2 O 2 where it is -1. The oxidation number of hydrogen in compounds is +1 , except in metal hydrides, like NaH, where it is -1.

19. Rules for Assigning Oxidation Numbers The sum of the oxidation numbers of the atoms in the compound must equal 0. 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

20. Rules for Assigning Oxidation Numbers The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its ionic charge. X + 3(-2) = -1 N O thus X = +5 thus X = +6 X + 4(-2) = -2 S O

21. Reducing Agents and Oxidizing Agents An increase in oxidation number = oxidation A decrease in oxidation number = reduction Sodium is oxidized – it is the reducing agent Chlorine is reduced – it is the oxidizing agent

22. Identifying Redox Equations In general, all chemical reactions can be assigned to one of two classes: oxidation-reduction, in which electrons are transferred: Single-replacement, combination, decomposition, and combustion this second class has no electron transfer, and includes all others: Double-replacement and acid-base reactions

23. Identifying Redox Equations In an electrical storm, nitrogen and oxygen react to form nitrogen monoxide: N 2(g) + O 2(g) -> 2NO (g) Is this a redox reaction? If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction; therefore, the reaction as a whole must be a redox. YES!

24. Using Oxidation-Number Changes Sort of like chemical bookkeeping, you compare the increases and decreases in oxidation numbers. start with the skeleton equation Step 1 : assign oxidation numbers to all atoms; write above their symbols Step 2 : identify which are oxidized/reduced Step 3 : use bracket lines to connect them Step 4 : identify substance oxidized and reduced.