Chapter 6, Activity 8

Chapter 6, Activity 8 “Oxidation-Reduction Reactions” LEO SAYS GER

Activity 8The 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.

Activity 8The Meaning of Oxidation and Reduction (Redox) • OBJECTIVES • State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.

Activity 8The Meaning of Oxidation and Reduction (Redox) • OBJECTIVES • Describe what happens to iron and other metals when they corrode.

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” • 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.

MEMORY AIDS • LEO the lion goes GER • Loss of electrons is oxidation • Gain of electrons is reduction • OIL RIG • Oxidation is loss • Reduction is gain

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

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

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 Mg2+ ion Mg is the reducing agent S is the oxidizing agent S is reduced: gains e- = S2- ion

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

Trends in Oxidation and Reduction • Active metals: • Lose electrons easily • Are easily oxidized • Are strong reducing agents • Active nonmetals: • Gain electrons easily • Are easily reduced • Are strong oxidizing agents

Identifying Redox Equations • In general, all chemical reactions can be assigned to one of two classes: • oxidation-reduction, in which electrons are transferred: • Single-replacement, synthesis, decomposition, and combustion • this second class has no electron transfer, and includes all others: • Double-replacement and acid-base reactions

Chapter 6, Activity 8