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.
OBJECTIVES State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.
OBJECTIVES Describe what happens to iron when it corrodes.
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.
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
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.
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)
Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:
LEO says GER : LEO says GER : Lose Electrons = Oxidation Sodium is oxidized Gain Electrons = Reduction Chlorine is reduced
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
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)
Not All Reactions are Redox Reactions Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples:
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
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
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