Redox Reactions Part 2 Notes #12

Objective: Oxidation Numbers When aluminum or magnesium powder in fireworks are heated to high temperatures in the explosion, they burn with an intense white light. As elements burn, their oxidation numbers change. You will learn about how oxidation and reduction are defined in terms of a change in oxidation number.

What colors do fireworks produce? Yellow, green, blue, red…etc Certain elements are responsible in making those different colors when they are oxidized or reduced! Sodium- Yellow Barium- Green Copper- Blue or Blue-Green Strontium- Red

Oxidation Number An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. It describes how much a substance has been oxidized or reduced. It also allows scientists to keep track of electron transfer in a reaction. What are other examples where items are numbered to keep track of movement?

Oxidation Numbers The oxidation number of any element in the free or uncombined state is zero.

Oxidation Numbers

Rule 1: In Ionic Compounds, like NaCl and CaCl2, the oxidation numbers are the same as their ionic charges. NaCl: Na+1 and Cl-1  Na is +1 and Cl is -1 CaCl2: Ca +2 and Cl2-1  Ca is +2 and Cl is -1

Rule 2: Hydrogen has oxidation number of +1 unless found as a metal hydride where it is -1. HCl  hydrogen has +1 NaH or LiH  hydrogen is -1

Rule 3: Oxygen is always -2 except: in peroxides (H2O2) where it is -1. And with fluorine where it is +1.

Rule 4: The oxidation number of any element in the free or uncombined state is zero. Example: H2, Cl2, Cu, etc…

Rule 5: If the compound is neutral, the oxidation numbers should add up to equal ZERO. Positives and Negatives should cancel out!

Rule 6: For a polyatomic ion, the sum of oxidation numbers should equal the total charge of the polyatomic ion. NH4+1 Hydrogen: Always +1 (Rule # 2) . Total for Hydrogen: +4 Nitrogen: -3 because it must add up to +1.

Where to put them: Oxidation numbers are often written above the chemical symbols in a formula.

It can change! K2CrO4 Cr2O3 Cr oxidation number = +6

You try…

Identify Oxidation and Reduction Rxns An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction.

Demo When an iron nail is placed in a copper(II) sulfate solution, the iron reduces Cu2+ ions in solution and is simultaneously oxidized to Fe2+ The iron becomes coated with metallic copper.