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) 2/3 rd ’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

OxidationReduction Gains oxygenLose Oxygen Lose HydrogenGains hydrogen Lose ElectronsGains electrons Oxygen Hydrogen e-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 –2Fe 2 O 3 + 3C  4Fe + 3CO 2 –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  PbO 3 SnO 2  SnO KClO 3  KCl C 2 H 6 O  C 2 H 4 O C 2 H 2  C 2 H 6

Pertaining to LEO… Mg + S  MgS Mg + S  Mg 2+ + S 2- 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 H 2 O 2, 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? I 2 Cr 2 O 3 AlCl 3 Na 2 SO 4 CaH 2

Identifying Redox Reactions Al + Fe 2 O 3  2 Fe + Al 2 O 3 Al increases from 0 to +3, it is ______ Oxidized! Fe decreases from +3 to 0, it is _______ Reduced!

Problems Zn  Zn 2+ Fe 3+  Fe 2+ CaCO 3  CaO + CO 2 AgNO 3  Ag Do Practice Exercises

Oxidizing and Reducing Agents Now the confusing part… CuO + H 2  Cu + H 2 O 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 + H 2  Cu + H 2 O Reduction: CuO is the oxidizing agent Oxidation: H 2 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