CHAPTER 22 “Oxidation-Reduction Reactions” LEO SAYS GER OIL REG or

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

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

Rules for Assigning Oxidation Numbers 1)The oxidation number of any uncombined element is _zero__. 2)The oxidation number of a monatomic ion equals its charge.

Rules for Assigning Oxidation Numbers 3)The oxidation number of _oxygen in compounds_ is -2, except in peroxides, such as H 2 O 2 where it is -1. 4)The oxidation number of hydrogen in compounds is +1, except in metal hydrides, like NaH, where it is -1.

Rules for Assigning Oxidation Numbers 5)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

Rules for Assigning Oxidation Numbers 6)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

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