Chpater 4 Oxidation-Reduction Yes, GER! What are Red-Ox Reactions? Some Examples of Reduction and Oxidation Reactions involve the Loss of Electrons and the Gain of Electrons by Atoms and Ions. Remember: “LEO the Lion says _________” GER Loss of Electrons is Oxidation Gain of Electrons is Reduction How are Metal Atoms different from Metal Ions? Metal “ATOMS are Solid”* and Tend to Lose Electrons (LEO) *Note: 1. Atoms are Mostly Empty Space! 2. Mercury is a Liquid at room temp. Metal IONS are (aq) and Tend to Gain Electrons (GER)*Some Ions can LEO more How is the Tendency to Gain and Lose Electrons Determined? Test Solid Metal Atoms with Ions in Aqueous Solutions Hyperlink Create an Activity Series for Metal Atoms. Zinc Metal and Oxygen Hyperlink

The Activity Series What is the “Activity” of Metals? What do Metal Atoms Tend to do? Biggest Losers Metal Atoms tend to lose valence electrons. Why are Lithium, Rubidium and Potassium at the Top of the Activity Series? Alkali Metal Atoms tend to Lose Electrons Easily. Why are Gold, Silver and Copper at the Bottom of the Activity Series? The “Coinage” Metals do not tend to undergo Oxidation. What Combinations of Atoms and Ions tend to result in RedOx Reactions? Solid Metal Atoms from “Above” with Aqueous Metal Ions from “Below.” Can Mg2+(aq) react spontaneously with Zinc(s)? No Reaction. Ions can only Gain Electrons from Atoms Above them in the Activity Series. Can Al3+(aq) Oxidize Calcium? Strongest Gainers Yes. Al3+ can Gain Electrons from Ca or Ca will Lose Electrons to Al3+

Reduction Potentials What is a Reduction Potential? Reduction Potential is the tendency of a species to gain electrons measured in Volts. How are Reduction Potentials related to the Activity Series? The most Active species (loss of electron) is at the bottom of the Reduction Potentials (gain of electrons). What are Reduction Potentials used for? Reduction Potentials are used to predict RedOx reactions and to determine Voltage of a Volatic cell (like a battery).

Rules for Assigning Oxidation Numbers * If Charge is 2+, then Ox. No. is +2 What are Oxidation Numbers (Oxidation States)? Assigned Values for keeping track of LEO and GER. An Oxidation Number can be similar to the Charge of an Ion* or it can be an exaggeration of the “Ionic Character” of a Covalent Bond. What changes in Oxidation Numbers indicate Oxidation and Reduction? A species is Reduced when its Oxidation Number is Reduced!! An Oxidation Number Increases during Oxidation!! Rules for Assigning Oxidation Numbers 1. Atoms in Elements = 0 Always!! 2. In Compounds: Monatomic Ions = Charge Fluorine = -1 Oxygen = -2*1 Hydrogen = +1*2 3. Assign All Others so that Sum of Ox #’s in a Compound is Zero. Sum of Ox. #’s in a Polyatomic Ion is Same as Charge. *1 Except: H2O2 and OF2 Why? *2 Except: Metal Hydrides Ex. NaH

Practice With Oxidation Numbers What is the Oxidation Number of Chromium in Dichromate Ion Cr2O72- Total of Elements +12 -14 = -2 Individual Atoms +6 -2 Cr2O72- Identify the “Ionic Character”(Polarity) in the Covalent Bonds below: Compare the Partial Charges to the Assigned Oxidation Numbers. +1 -2 For each covalent bond determine whether an atom “feels” like it is: gaining (-1), losing (+1) or neutral (0) regarding shared electrons. -2 -3 +1 +1 +3 Review: Add 3 H2O molecules forming Hydrogen Bonds. Use dotted lines for Hydrogen Bonds. +1

C in C2O42- S in HSO4- N in NO3- N in NH4+ Cr in CrO42- Cl in ClO3- Determine the Oxidation Number of the following: C in C2O42- S in HSO4- N in NO3- N in NH4+ Cr in CrO42- Cl in ClO3- C in HCO3- S in S2O32- +3 +6 +5 -3 +6 +5 +4 +2

Oxidation-Reduction Reactions What 3 things must always be Balanced in a Chemical Equation? Mass, Energy, and Charge must always balance. Balancing the Charge in a Simple Red-Ox Reaction: 1. Assign Oxidation Numbers. 2. Identify and Write the Oxidation and Reduction Half-Reactions. 3. Balance the number of Electrons Exchanged. 4. Combine the Balanced Reactions. Ex. 1. Write the Reaction that occurs when aluminum metal is added to an aqueous solution of copper(II)chloride. +2 +3 Al (s) + Cu2+ (aq)  Cu (s) + Al3+ (aq) ( ) x 2 Oxidation: Al  3e- + Al3+ Identify the Oxidizing Agent and the Reducing Agent. ( ) x 3 Reduction: Cu2+ + 2e-  Cu 2Al + 3Cu2+  3Cu + 2Al3+ Oxdizing Agent is Cu2+(aq) and Reducing Agent is Al (s)

Voltaic Cells Identifying the Main Parts of a Voltaic Cell How can Red-Ox Reactions be used to Generate an Electric Current? View the animation at Hyperlink 1 Construct a Voltaic Cell at Hyperlink 2 Additional animation of Zn/Cu Cell at Hyperlink 3 Identifying the Main Parts of a Voltaic Cell The Cathode and Anode: The Electrode where Reduction occurs is the Cathode. “Trust the Red Cat.” Also “Trust An Ox” for the Anode. The Flow of Electrons: Always From LEO to GER!! The Charge of the Electrodes: Electrons Flow from Neg. (-) to Pos. (+) The Salt Bridge: Allows Migration of Ions to Maintain Neutral Solutions. Voltmeter  Use this diagram to Build and Label a Voltaic Cell

Calculating the Voltage of a Voltaic Cell How can one use Reduction Potentials to predict RedOx reactions and calculate the Voltage of a cell? 1. Identify the atoms that could loose electrons and the ions that could gain electrons.* (Recall exceptions) 2. Search the table from the top for one of the ions. Write the reaction (gain of electrons) and the voltage. 3. Search the table from the bottom for the atoms. Write the reaction backwards (loss of electrons) and reverse the sign of the voltage. 4. Simply Combine the voltages. Do NOT multiply them by any factor. Positive Voltage indicates a spontaneous reaction. Neg…….

A Copper/Zinc Voltaic Cell - + Why do the Ions in the Salt Bridge Migrate as they do? K+ Replaces Cu+2(aq) “lost” as Cu(s). 2 Cl- Cancels the Zn2+ Produced in Ox. Determine the Pos. and Neg. Electrodes.

The Inside of a Battery Identify: Oxidation Reaction Cathode Zn  2e- + Zn2+ Cathode What is purpose of the Porous Cup? Same as the Salt Bridge Describe the Flow of Electrons. From Zinc toward Copper From Anode to Cathode What will cause this “battery” to become “dead?” It will run out of Copper ions or run out of zinc metal.

Identifying the Main Parts of an Electrolytic Cell Electrolytic Cells How is an Electrolytic Cell different from a Voltaic Cell? A Voltaic Cell uses Spontaneous Chemical Changes to Produce an Electric Current, but an Electrolytic Cell Uses External Energy and an Electric Current to Cause Non-spontaneous Chemical Changes. Identifying the Main Parts of an Electrolytic Cell Electroplating involves an Electrolytic Cell. Start with the Flow of Electrons from the Power Source. - + D.C. Battery Write the equation for Reduction. e- Ag+(aq) + e-  Ag(s) What is the Charge of the Anode? Ag (s) The Anode is Pos. (+). PH Video of Chrome Plating Hyperlink 1 cheap jewelry Cool School Video of Electrolysis of Water then KI Hyperlink 3 AgNO3 (aq) PH Video of Electrolysis of Water Hyperlink 2

Practice With Red-Ox Reactions Write a Balanced Red-Ox Reaction that could have occured in flask a. 2Ag+ + Cu  2Ag + Cu2+ Why is Solution a Blue? Cu2+(aq) produced Name one cation that could be present in flask b and explain your reasoning. Na+ because it cannot Oxidize Copper. + - D.C. Power Supply e- Draw and Label an Electrolytic Cell that could be used to Separate Na from Cl in NaCl? 2Cl-  2e- + Cl2 Na+ + e-  Na0 Molten (melted) NaCl

More Review With Oxidation-Reduction Voltaic Cells: Chemical Energy  Electrical Energy Electrolytic Cells: Electrical Energy  Chemical Energy Video of Zinc (s) in HCl (aq) then in Aqueous Copper Sulfate Video of Zinc in Aqueous Tin Chloride. Electrochem Unit Tutorial from Georgia Public Broadcasting. How Stuff Works: Batteries Describe these examples of Red-Ox Reactions.

Practice with a Voltaic Cell Determine: Red-Ox Half-Reactions Anode & Cathode Flow of Electrons Charge of Electrodes Which Electrode will increase in mass? Which Electrode will decrease in mass? Calculate the Standard Voltage.

Practice with a Voltaic Cell Determine: Red-Ox Half-Reactions Anode & Cathode Flow of Electrons Charge of Electrodes Which Electrode will increase in mass? Which Electrode will decrease in mass? Calculate the Standard Voltage. Determine: Red-Ox Half-Reactions Anode & Cathode Flow of Electrons Charge of Electrodes

Practice with Electrolytic Cells Determine what metal you would like to Electroplate onto a surface or determine what binary compound you would like to separate into elements. - + D.C. Battery Determine: Flow of Electrons Red-Ox Half-Reactions Anode & Cathode

Practice with Electrolytic Cells Determine what metal you would like to Electroplate onto a surface or determine what binary compound you would like to separate into elements. - + D.C. Battery Determine: Flow of Electrons Red-Ox Half-Reactions Anode & Cathode