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The Finish Line is in site… Electrochemistry. “Oxidation-Reduction Reactions” LEO SAYS GER.

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Presentation on theme: "The Finish Line is in site… Electrochemistry. “Oxidation-Reduction Reactions” LEO SAYS GER."— Presentation transcript:

1 The Finish Line is in site… Electrochemistry

2 “Oxidation-Reduction Reactions” LEO SAYS GER

3 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.

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

5 OBJECTIVES Describe what happens to iron when it corrodes.

6 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.

7 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

8 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.

9 Oxidation and Reduction (Redox) Today, many of these reactions may not even involve oxygen Redox currently says that electrons are transferred between reactants Mg + S → Mg 2+ + S 2- The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg 2+ The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S 2- (MgS)

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

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

12 LEO says GER : 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

13 Oxidation and Reduction (Redox)  It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds?  In water, we learned that oxygen is highly electronegative, so: the oxygen gains electrons (is reduced and is the oxidizing agent), and the hydrogen loses electrons (is oxidized and is the reducing agent)

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

15 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

16 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

17 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

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