Oxidation and Reduction

Overview

• Oxidation and reduction reactions always occur together (redox reactions)

• You can’t have one without the other

• Includes: extracting metal from an ore

• Salt on roads– Salt accelerates the reaction between oxygen

and metal– Makes electron transfer easier

Oxygen

• Most abundant element on Earth

• 21% of our atmosphere (pure)

• 89% of Earth’s water (compound)

• 45.5% of Earth’s crust (compound)

• 2/3rd’s of the human body (compound)

• Foods (compound)

• Rust

Chemical Properties of Oxygen

• Combines with many elements and compounds

• Rust (reacts with metals)

• Other oxides (reacts with nonmetals)

• Methane (reacts with other compounds)– Bunsen burner

Hydrogen

• In the universe, 95% of all atoms are hydrogen

• Seldom found alone

• Hydrocarbons

• Hindenburg…

3 ways of looking at oxidation and reduction

• 1. oxidation is a gain of oxygen atoms, reduction is a loss of oxygen atoms

• 2. oxidation is a loss of hydrogen atoms, reduction is a gain of hydrogen atoms

• 3. oxidation is a loss of electrons, reduction is a gain of electrons– Most fundamental explanation, what we will

be dealing with the most

Oxidation Reduction

Gains oxygen Lose Oxygen

Lose Hydrogen Gains hydrogen

Lose Electrons Gains electrons

Oxygen

Hydrogen

e-

In relation to our lab…

• Remember that reduction is a loss of oxygen from a compound

• We converted iron ore to metallic iron– We removed oxygen from the iron(III) oxide

– 2Fe2O3 + 3C 4Fe + 3CO2

– Carbon was oxidized because it gained an oxygen

Leo the Lion!

• LEO the lion says GER– Loss of electrons is oxidation, gain of

electrons is reduction

Examples

• Is the reactant oxidized or reduced?

• Pb PbO3

• SnO2 SnO

• KClO3 KCl

• C2H6O C2H4O

• C2H2 C2H6

Pertaining to LEO…

• Mg + S MgS• Mg + S Mg2+ + S2-

• Magnesium is oxidized– Said to be the reducing agent– Substance in the reaction that loses electrons

• Sulfide sulfur atom is reduced – Said to be the oxidizing agent– Substance in the reaction that gains electrons

Oxidation Numbers

• A count of the electrons transferred or shared in the formation or breaking of chemical bonds

• You must assign each element in the reaction an oxidation number

• Follow a set of rules…

Oxidation Number Rules

1. The total of the oxidation numbers of all the atoms in a neutral molecule, an isolated atom, or a formula unit is 0– Examples

2. In their compounds, the Group 1A metals all have an oxidation number of +1, and the Group 2A metals have an oxidation number of 2+.– Examples

Rules Con’t

3. In its compounds, hydrogen has an oxidation number of +1 (except in metal hydrides such as NaH, where it is -1)– Examples

4. In its compounds, oxygen has an oxidation number of -2 (except in peroxides such as H2O2, where it is -1)– Examples

5. In their binary compounds with metals, Group7A elements have an oxidation number of -1. Group 6A elements have an oxidation number of -2, and Groups 5A elements have an oxidation number of -3.– Examples

Problems

• What is the oxidation number of each element?

• I2

• Cr2O3

• AlCl3• Na2SO4

• CaH2

Identifying Redox Reactions

0 +3 -2 0 +3 -2

• 2 Al + Fe2O3 2 Fe + Al2O3

• Al increases from 0 to +3, it is ______

• Oxidized!

• Fe decreases from +3 to 0, it is _______

• Reduced!

Problems

• Zn Zn2+

• Fe3+ Fe2+

• CaCO3 CaO + CO2

• AgNO3 Ag

• Do Practice Exercises

Oxidizing and Reducing Agents

• Now the confusing part…

• CuO + H2 Cu + H2O

• Cu goes from +2 to 0– Cu is reduced, therefore it is called an oxidizing

agent because it causes some other substance to be oxidized

• H goes from 0 to +1– H is oxidized, therefore it is called a reducing agent

because it causes some other substance to be reduced.

Identifying Agents in an Equation

• CuO + H2 Cu + H2O

Reduction: CuO is the oxidizing agent

Oxidation: H2 is the reducing agent

Some common oxidizing agents

• Oxygen!– Oxidized coal in electric power– Gas in automobiles– Wood in campfires– Food we eat

• Antiseptics– Hydrogen Peroxide– Benzoyl peroxide

• Disinfectants– Chlorine

Some common reducing agents

• Metals

• Antioxidants– Ascorbic acid is used to prevent the browning

of fruits by inhibiting air oxidation– Many antioxidants are believed to retard

various oxidation reactions that are potentially damaging to vital components of living cells