Oxidation reduction reactions

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Oxidation reduction reactions

  1. 1. “ Oxidation-Reduction Reactions” LEO SAYS GER Pre-AP Chemistry Charles Page High School Stephen L. Cotton
  2. 2. The Meaning of Oxidation and Reduction (called “redox”) <ul><li>OBJECTIVES </li></ul><ul><ul><li>Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons. </li></ul></ul>
  3. 3. <ul><li>OBJECTIVES </li></ul><ul><ul><li>State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent. </li></ul></ul>
  4. 4. Oxidation and Reduction (Redox) <ul><li>Early chemists saw “ oxidation ” reactions only as the combination of a material with oxygen to produce an oxide. </li></ul><ul><ul><li>For example, when methane burns in air, it oxidizes and forms oxides of carbon and hydrogen, </li></ul></ul>
  5. 5. Oxidation and Reduction (Redox) <ul><li>But, not all oxidation processes that use oxygen involve burning: </li></ul><ul><ul><li>Elemental iron slowly oxidizes to compounds such as iron (III) oxide, commonly called “rust” </li></ul></ul><ul><ul><li>Bleaching stains in fabrics </li></ul></ul><ul><ul><li>Hydrogen peroxide also releases oxygen when it decomposes </li></ul></ul>
  6. 6. Oxidation and Reduction (Redox) <ul><li>A process called “ reduction ” is the opposite of oxidation, and originally meant the loss of oxygen from a compound </li></ul><ul><li>Oxidation and reduction always occur simultaneously </li></ul><ul><li>The substance gaining oxygen (or losing electrons) is oxidized , while the substance losing oxygen (or gaining electrons) is reduced . </li></ul>
  7. 7. Oxidation and Reduction (Redox) <ul><li>Today, many of these reactions may not even involve oxygen </li></ul><ul><li>Redox currently says that electrons are transferred between reactants </li></ul>
  8. 8. Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:
  9. 9. LEO says GER : L ose E lectrons = O xidation Sodium is oxidized G ain E lectrons = R eduction Chlorine is reduced
  10. 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. 11. Not All Reactions are Redox Reactions - Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples:
  12. 12. Corrosion <ul><li>Damage done to metal is costly to prevent and repair </li></ul><ul><li>Iron , a common construction metal often used in forming steel alloys, corrodes by being oxidized to ions of iron by oxygen. </li></ul><ul><ul><li>This corrosion is even faster in the presence of salts and acids , because these materials make electrically conductive solutions that make electron transfer easy </li></ul></ul>
  13. 13. Corrosion <ul><li>Luckily, not all metals corrode easily </li></ul><ul><ul><li>Gold and platinum are called noble metals because they are resistant to losing their electrons by corrosion </li></ul></ul><ul><ul><li>Other metals may lose their electrons easily, but are protected from corrosion by the oxide coating on their surface, such as aluminum – </li></ul></ul><ul><ul><li>Iron has an oxide coating, but it is not tightly packed, so water and air can penetrate it easily </li></ul></ul>
  14. 14. Corrosion <ul><li>Serious problems can result if bridges, storage tanks, or hulls of ships corrode </li></ul><ul><ul><li>Can be prevented by a coating of oil, paint, plastic, or another metal </li></ul></ul><ul><ul><li>If this surface is scratched or worn away, the protection is lost </li></ul></ul><ul><li>Other methods of prevention involve the “ sacrifice ” of one metal to save the second </li></ul><ul><ul><li>Magnesium, chromium, or even zinc (called galvanized) coatings can be applied </li></ul></ul>
  15. 15. Oxidation Numbers <ul><li>OBJECTIVES </li></ul><ul><ul><li>Determine the oxidation number of an atom of any element in a pure substance. </li></ul></ul>
  16. 16. Assigning Oxidation Numbers <ul><li>An “ oxidation number ” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. </li></ul><ul><li>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 </li></ul>
  17. 17. Rules for Assigning Oxidation Numbers <ul><li>The oxidation number of any uncombined element is zero . </li></ul><ul><li>The oxidation number of a monatomic ion equals its charge . </li></ul>
  18. 18. Rules for Assigning Oxidation Numbers <ul><li>The oxidation number of oxygen in compounds is -2 , except in peroxides, such as H 2 O 2 where it is -1. </li></ul><ul><li>The oxidation number of hydrogen in compounds is +1 , except in metal hydrides, like NaH, where it is -1. </li></ul>
  19. 19. Rules for Assigning Oxidation Numbers <ul><li>The sum of the oxidation numbers of the atoms in the compound must equal 0. </li></ul>2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H
  20. 20. Rules for Assigning Oxidation Numbers <ul><li>The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its ionic charge. </li></ul>X + 3(-2) = -1 N O thus X = +5 thus X = +6 X + 4(-2) = -2 S O
  21. 21. Reducing Agents and Oxidizing Agents <ul><li>An increase in oxidation number = oxidation </li></ul><ul><li>A decrease in oxidation number = reduction </li></ul>Sodium is oxidized – it is the reducing agent Chlorine is reduced – it is the oxidizing agent
  22. 22. Identifying Redox Equations <ul><li>In general, all chemical reactions can be assigned to one of two classes: </li></ul><ul><ul><li>oxidation-reduction, in which electrons are transferred: </li></ul></ul><ul><ul><ul><li>Single-replacement, combination, decomposition, and combustion </li></ul></ul></ul><ul><ul><li>this second class has no electron transfer, and includes all others: </li></ul></ul><ul><ul><ul><li>Double-replacement and acid-base reactions </li></ul></ul></ul>
  23. 23. Identifying Redox Equations <ul><li>In an electrical storm, nitrogen and oxygen react to form nitrogen monoxide: </li></ul><ul><li>N 2(g) + O 2(g) -> 2NO (g) </li></ul><ul><li>Is this a redox reaction? </li></ul><ul><ul><li>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. </li></ul></ul>YES!
  24. 24. Using Oxidation-Number Changes <ul><li>Sort of like chemical bookkeeping, you compare the increases and decreases in oxidation numbers. </li></ul><ul><ul><li>start with the skeleton equation </li></ul></ul><ul><ul><li>Step 1 : assign oxidation numbers to all atoms; write above their symbols </li></ul></ul><ul><ul><li>Step 2 : identify which are oxidized/reduced </li></ul></ul><ul><ul><li>Step 3 : use bracket lines to connect them </li></ul></ul><ul><ul><li>Step 4 : identify substance oxidized and reduced. </li></ul></ul>

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