“Oxidation-Reduction Reactions” LEO SAYS GER Pre-AP Chemistry Charles Page High School Stephen L. Cotton

Oxidation and Reduction (Redox) A process called “reduction” is the opposite of oxidation, and originally meant the loss of oxygen from a compound Oxidation and reduction always occur simultaneously The substance gaining oxygen (or losing electrons) is oxidized, while the substance losing oxygen (or gaining electrons) is reduced.

Oxidation and Reduction (Redox) Today, many of these reactions may not even involve oxygen Redox currently says that electrons are transferred between reactants Mg + S → Mg 2+ + S 2- The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is oxidized to Mg 2+ The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is reduced to S 2- (MgS)

Leo the Lion! LEO the lion says GER –Loss of electrons is oxidation, gain of electrons is reduction LET'S LOOK AT HALF REACTIONS

Oxidation and Reduction (Redox) Each sodium atom loses one electron: Each chlorine atom gains one electron:

LEO says GER : LEO says GER : Lose Electrons = Oxidation Sodium is oxidized Gain Electrons = Reduction Chlorine is reduced

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 7 Identify each of the following half reactions as: 1) oxidation or 2) reduction __A. Sn(s)Sn 4+ (aq) + 4e − __B. Fe 3+ (aq) + 1e − Fe 2+ (aq) __C. Cl 2 (g) + 2e − 2Cl − (aq) Learning Check

General, Organic, and Biological Chemistry Copyright © 2010 Pearson Education, Inc. 8 Identify each of the following as: 1) oxidation or 2) reduction 1 A. Sn(s)Sn 4+ (aq) + 4e − 2 B. Fe 3+ (aq) + 1e − Fe 2+ (aq) 2 C. Cl 2 (g) + 2e − 2Cl − (aq) Solution

LEO says GER : LEO says GER : - Losing electrons is oxidation, and the substance that loses the electrons is called the reducing agent. - Gaining electrons is reduction, and the substance that gains the electrons is called the oxidizing agent. Mg (s) + S (s) → MgS (s) Mg is oxidized: loses e -, becomes a Mg 2+ ion S is reduced: gains e - = S 2- ion Mg is the reducing agent S is the oxidizing agent

Oxidation and Reduction (Redox) It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds? Oxygen is highly electronegative, so, in water: the oxygen gains electrons (is reduced and is the oxidizing agent), and the hydrogen loses electrons (is oxidized and is the reducing agent)

Other rules to remember: Flourine is ALWAYS -1. Other halogens are going to be -1, EXCEPT when bonding with oxygen. Group I elements have an oxidation state +1 Group II elements have an oxidation state of +2

Water as a product If the H[+1] was taken from a reactant compound, it already had a +1 charge. If O[-2] was taken from a reactant compound, it already had -2 charge. Therefore, there will be no change in the oxidation number of the elements in H and O when water is made and the H 2 O will have a total oxidation number of zero.

OxidationReduction Gains oxygenLose Oxygen Lose HydrogenGains hydrogen Lose ElectronsGains electrons Oxygen Hydrogen e-e-

Not All Reactions are Redox Reactions - Reactions in which there has been no change in oxidation number are NOT redox reactions. Examples: