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 coal burns in air, it oxidizes and forms oxides of carbon and hydrogen, as shown in Fig. 22.1, p. 645
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 changes to a magnesium ion by losing 2 electrons, and is thus oxidized The sulfur atom is changed to a sulfide ion by gaining 2 electrons, and is thus reduced.
LEO says GER : L ose E lectrons = O xidation Sodium is oxidized G ain E lectrons = R eduction Chlorine is reduced
Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:
LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent . ( gets oxided) - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent .( gets reduced) Mg (s) + S (s) -> MgS (s) Mg is oxidized – loses e - S is reduced – gains e - Mg is the reducing agent S is the oxidizing agent
REDOX PROBLEM What is oxidized and reduced in this reaction? 2AgNO 3(aq) + Cu (s) ----Cu(NO 3 ) (aq) +2Ag (s) … and what are the oxidizing agents and reducing agents?
REDOX PROBLEM 2AgNO 3(aq) + Cu (s) ----Cu(NO 3)2(aq) +2Ag (s) +1 -1 0 +2 -1 0 Ag + went from +1 to 0, gained electrons so it got reduced, and is the oxidizing agent Cu went from 0 to +2 , lost electrons so it got oxidized, and is the reducing 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)
Look for the more electronegative element- it will “gain” electrons and so be reduced
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 – Figure 22.8, page 652 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