OXIDATION – REDUCTION REACTIONS Chapter 19

Summarize the oxidation and reduction process ( including oxidizing and reducing agents.

Understand that “oxidation” is defined as the process of losing electrons, “reduction” is defined as process of gaining electrons –A substance that is “oxidized” has lost electrons –A substance that is “reduced” has gained electrons –When a substance is oxidized, it “gives” electrons to another substance, causing that substance to gain electrons or be reduced. A substance that causes another substance to be reduced is called a “reducing agent” Any substance that is oxidized is a reducing agent

Oxidation-reduction reactions 2Na + Cl 2 2NaCl

Definition Reactions in which one or more electrons are transferred. Also called redox reactions

Oxidation states Also called oxidation numbers Provides a way to keep track of electrons during oxidation-reduction reactions –Especially redox reactions involving covalent bonds. Covalent bonds Ionic bonds

Oxidation states Covalent compounds – the imaginary charges the atoms would have is the shared electrons were divided equally between identical atoms bonded to each other or, for different atoms, were all assigned to the atom in each bond that has the greater attractions. Ionic compounds – it is equal to the ion charge

RULES The oxidation number of any free element is zero A monatomic ion is the same as its charge Fluorine is -1 when it is in a compound Oxygen is -2. (exception – peroxides - O 2 is -1) Hydrogen is +1 in its covalent compounds and -1 when combined with a metal The sum of oxidation numbers in compounds = 0 The sum of oxidation numbers in PAI = charge

Writing oxidation states vs. numbers Oxidation numbers –Write the charges on ions as n+ or n- Oxidation states –+n or -n

Assign oxidation states CO 2

Noninteger oxidation states Fe 3 O 4

Characteristics of oxidation- reduction reactions Oxidation –Is an increase in oxidation state (loss of electrons) Reduction –Is a decrease in oxidation state (gain of electrons)

Oxidizing agent - electron acceptor Reducing agent - electron donor When the oxidizing or reducing agent is named, the whole compound is specified.

Sample problems Identify the atoms that are oxidized and reduced, and specify the oxidizing and reducing agents. 2Al(s) + 3I 2 (s) → 2AlI 3 (s) 2PbS(s) +3O 2 (g) → 2PbO (s) + 2SO 2 (g) →

Balancing redox reactions Half-reaction method

Steps Handout 1. Write formula equation (ionic equation) 2. Assign oxidation numbers. Delete substances containing only elements that do not change oxidation states. 3. Write half-reactions for oxidation. Balance the atoms. Balance the charge

4. Write the half-reaction for reduction Balance the atoms Balance the charge 5. Conserve charge 6. Combine the half-reactions and cancel out anything common to both sides. 7. Check that elements and charges are balanced.

Sample problem Write a balanced equation for: H 2 S + HNO 3 H 2 SO 4 + NO 2 + H 2 O

Uses of oxidation / reduction Aging Photography 2Ag Br - 2Ag + Br 2

Uses of oxidation / reduction Metallic iron must be separated and purified from iron ore to make steel 2Fe O CO 2Fe 0 + CO 2

Uses of oxidation / reduction Bleaches can be used to remove stains from clothing. Stains are not actually removed from the fabric. The bleach reacts with the stain to form a colorless compound. OCl- + colored stain molecule Cl- + oxidized colorless stain molecule

Uses of oxidation / reduction The alcohol in beverages, hair spray, and mouthwashes is ethanol. Ethanol is a volatile liquid. Drinking an alcoholic beverage results in a level of gaseous ethanol in the breath proportional to the level of alcohol in the bloodstream. Breathalyzer test – redox reaction

Uses of oxidation / reduction Green color copper gets when exposed to humid air Tarnish of silver Luminol emits cool light when it is oxidized.

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