REDOX Oxidation and Reduction Chapters 20 and 21

What is redox? Examples  Corrosion (Rusting)  Spoiling of Food  Voltaic Cells  Batteries  Electroplating

What is redox? Examples  Complete loss of electrons (ionic)  Shift of electrons away from an atom in a covalent bond  Gain oxygen  Loss of hydrogen  Increase in oxidation number  Complete gain of electrons (ionic)  Shift of electrons toward an atom in a covalent bond  Loss of oxygen  Gain of hydrogen  Decrease in oxidation number OxidationReduction

What is a redox reaction?  Take the following reaction, as an example. Mg + O 2 MgO2 2 What type of reaction is this? Synthesis But we can also define this reaction as a redox reaction.

What is a redox reaction?  Oxidation  When electrons are lost in a chemical reaction  Reduction  When electrons are gained in a chemical reaction  LEO the Lion says “GER”  GER – Gaining Electrons Recuction  LEO – Losing Electrons Oxidation

What is a redox reaction?  In our reaction:  Magnesium is being oxidized (losing electrons)  Oxygen is being reduced (gaining electrons)  The substance being reduced, is causing oxidation, and therefore, is the oxidizing agent  The substance being oxidized, is causing reduction, and therefore, is the reducing agent

Oxidation Numbers  Oxidation Numbers  Apparent numbers (charge) assigned to an atom of an element  Show whether the element is gaining or losing electrons in a reaction  Sometimes the oxidation number is not the same as the oxidation state (ex. Central atoms of a compound or molecule, that has a lower electronegativity than the surrounding atoms)

Oxidation Numbers  Rules for assigning oxidation numbers  The oxidation number for an atom in its uncombined or elemental form is 0. Ex. Na(s) = 0 Cl 2 (g) = 0  The oxidation number for a monoatomic ion is equal to the charge of the ion in its ionic form. Ex. Na +1 = +1Cl -1 = -1

Oxidation Numbers  Hydrogen is +1, except when it is bonded to a metal, to form a hydride. Then it is -1.  Oxygen is usually -2, unless it is bonded to a more electonegative atom, like fluorine, or in the form of a peroxide (H 2 O 2 )  The sum of the oxidation numbers in a neutral compound or molecule, must be equal to 0. The sum of the oxidation numbers for a polyatomic ion, must be equal to the charge on the ion.  Binary compounds of non-metals, the more electronegative atom gets the negative number.

Changes to oxidation numbers in Chemical Reactions  An increase in the oxidation number indicates oxidation (becoming more +)  A decrease in the oxidation number indicates reduction (becoming more -)  Which type of reaction (synthesis, decompostion, single displacement, double displacement) is not a redox reaction?  Examples

Practice  Page – 2  Page 638 3, 6, 7. a, c, e  Page 641 9, 10  Page , 16

Balancing – Using REDOX  Identifying the reaction as a REDOX Reaction is the first step  If the oxidation numbers change, then it is a redox reaction. Example  There are two ways to balance reactions  Using Oxidation Number changes  Using Half Reactions

Practice  Page  Page (but use the half reaction method)  Page 661