Oxidation-Reduction Reactions “Redox” LEO SAYS GER

Oxidation and Reduction Reactions  Also known as redox or electron transfer reactions  One or more elements change oxidation numbers  All single replacement & combustion reactions  Some synthesis & decomposition reactions

How can I tell if it is redox?  If there is an element in the reactants or products, you know it must be redox.  In a redox reaction, electrons are lost and gained.  The element that loses electrons is oxidized.  The element that gains electrons is reduced. LEO says GER (OIL RIG)  If the oxidation numbers change during the reaction, it must be redox

Oxidation and Reduction (Redox)  Electrons are transferred  Spontaneous redox rxns can transfer energy  Electrons (electricity)  Heat  Non-spontaneous redox rxns can be made to happen with electricity  So what are oxidation numbers?

Trends in Oxidation and Reduction Active metals: Lose electrons easily Are easily oxidized Are strong reducing agents Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

What are oxidation numbers?  A tool to help determine which element is oxidized and which is reduced.  For ionic compounds, the oxidation number is the element’s charge.

Rules for Assigning Oxidation Numbers Rules 1 & 2 1.The oxidation number of any uncombined element is zero 2. The oxidation number of a monatomic ion equals its charge

Oxidation Reduction Reactions (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

Reducing Agents and Oxidizing Agents  The substance reduced is the oxidizing agent  The substance oxidized is the reducing agent Sodium is oxidized – it is the reducing agent Chlorine is reduced – it is the oxidizing agent

Rules for Assigning Oxidation Numbers Rules 3 & 4 3. The oxidation number of oxygen in compounds is The oxidation number of hydrogen in compounds is +1

Rules for Assigning Oxidation Number Rule 5 5. The sum of the oxidation numbers in the formula of a compound is 0 2(+1) + (-2) = 0 H O (+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation Numbers Rule 6 6. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge X + 3(-2) = -1 N O  X = +5  X = +6 X + 4(-2) = -2 S O

The Oxidation Number Rules - SIMPLIFIED 1. The sum of the oxidation numbers in ANYTHING is equal to its charge 2. Hydrogen in compounds is Oxygen in compounds is Charge on uncombined element is zero

Let’s try a few…….. ClO 2 ClO - N 2 O H 2 S

Oxidized or Reduced? 2Sr + O 2  2SrO 2Li + S  Li 2 S 2Cs + Br 2  2CsBr 3Mg + N 2  Mg 3 N 2

Half Reactions Fe 2+ + MnO 4 -  Fe 3+ + Mn 2+ Sn 2+ + IO 3 -  Sn 4+ + I - S 2- + NO 3 -  S + NO NH 3 + NO 2  N 2 + H 2 O

Not All Reactions are Redox Reactions Reactions in which there has been no change in oxidation number are not redox rxns. Examples:

Identifying Electron Transfer in Redox-Reactions 4Al(s) + 3O 2 (g)  2Al 2 O 3 (s)  Each Al atom loses 3 electrons (for a total of 12 electrons)  Each O atom gains 2 electrons (for a total of 12 electrons)

Oxidation-Reductions Rxns (cont’d) Always have both oxidation and reduction LEO says GER Lose e- = oxidized; Gain e- = reduced Split reaction into oxidation half-reaction and a reduction half reaction Half reactions include electrons

Oxidation Oxidation is the process that occurs when: Oxidation number of an element increases Element loses electrons Compound adds oxygen Compound loses hydrogen Half reactions have electrons as products

Reduction Reduction is the process that occurs when: Oxidation number of an element decreases Element gains electron Compound loses oxygen Compound gain hydrogen Half reactions have electrons as reactants

Predicting Products of Metal + Nonmetal Reactions Metal + Nonmetal  ionic compound Ionic compounds always solid unless dissolved in water In the ionic compounds, the metal is now a cation In the ionic compounds, the nonmetal is now an anion

Predicting Products of Metal + Nonmetal Reactions (cont’d) To predict direct synthesis of metal + nonmetal: 1.Determine the charges on the cation and anion from their positions on the periodic table. 2.Balance the charges to get the formula of the compound. 3.Balance the equation.