REDOXOxidation 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

Oxidation Reduction

What is a redox reaction?

Take the following reaction, as an example.

Mg + O2 MgO2 2

What type of reaction is this?SynthesisBut 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 Cl2(g) = 0

The oxidation number for a monoatomic ion is equal to the charge of the ion in its ionic form.

Ex. Na+1 = +1 Cl-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 (H2O2)

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 634 1 – 2

Page 638 3, 6, 7. a, c, e

Page 641 9, 10

Page 643 15, 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 649 19 Page 654 25 (but use the half reaction

method)

Page 661