Chpater 4 Oxidation-ReductionWhat 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 _________”Loss of Electrons is Oxidation Gain of Electrons is Reduction

GER

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.

Metal “ATOMS are Solid”* and Tend to Lose Electrons (LEO)

Metal IONS are (aq) and Tend to Gain Electrons (GER)*Some Ions can LEO more

How are Metal Atoms different from Metal Ions?

Yes, GER!

Zinc Metal and Oxygen Hyperlink

*Note: 1. Atoms are Mostly Empty Space! 2. Mercury is a Liquid at room temp.

The Activity SeriesWhat is the “Activity” of Metals? What do Metal Atoms Tend to do?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?

Yes. Al3+ can Gain Electrons from Ca or Ca will Lose Electrons to Al3+

Biggest Losers

Strongest Gainers

Reduction Potentials

Reduction Potential is the tendency of a species to gain electrons measured in Volts.

What is a Reduction Potential?

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).

Oxidation NumbersWhat 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 Numbers1. 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

* If Charge is 2+,

then Ox. No. is +2

Practice With Oxidation NumbersWhat is the Oxidation Number of Chromium in Dichromate Ion Cr2O7

2-

Cr2O72-

Individual Atoms

Total of Elements

-2

-14 = -2+12

+6

Identify the “Ionic Character”(Polarity) in the Covalent Bonds below:

Compare the Partial Charges to the Assigned Oxidation Numbers.

-2

-2

+1

+1

+1

+1

-3

+3Review: Add 3 H2O molecules forming Hydrogen Bonds. Use dotted lines for Hydrogen Bonds.

For each covalent bond determine whether an atom “feels” like it is: gaining (-1), losing (+1) or neutral (0) regarding shared electrons.

C in C2O42-

S in HSO4-

N in NO3-

N in NH4+

Cr in CrO42-

Cl in ClO3-

C in HCO3-

S in S2O32-

Determine the Oxidation Number of the following:

+3

+6

+5

-3

+6

+5

+4

+2

Oxidation-Reduction ReactionsWhat 3 things must always be Balanced in a Chemical Equation?Mass, Energy, and Charge must always balance.

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.

Balancing the Charge in a Simple Red-Ox Reaction:

Ex. 1. Write the Reaction that occurs when aluminum metal is added to an aqueous solution of copper(II)chloride.

Al (s) + Cu2+ (aq) Cu (s) + Al3+ (aq)

0 +2 0 +3

Oxidation: Al 3e- + Al3+

Reduction: Cu2+ + 2e- Cu

( ) x 2( ) x 3

2Al + 3Cu2+ 3Cu + 2Al3+

Identify the Oxidizing Agent and the Reducing Agent.

Oxdizing Agent is Cu2+(aq) and Reducing Agent is Al (s)

Voltaic CellsHow can Red-Ox Reactions be used to Generate an Electric Current?View the animation at Hyperlink 1 Construct a Voltaic Cell at Hyperlink 2Additional animation of Zn/Cu Cell at Hyperlink 3

The Electrode where Reduction occurs is the Cathode. “Trust the Red Cat.” Also “Trust An Ox” for the Anode.

Identifying the Main Parts of a Voltaic CellThe Cathode and 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.

Use this diagram to Build and Label a

Voltaic Cell

Voltmeter

Calculating the Voltage of a Voltaic CellHow 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 BatteryIdentify:

Oxidation ReactionZn 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.

Electrolytic CellsHow 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

Start with the Flow of Electrons from the Power Source.

cheap jewelry

- +

Electroplating involves an Electrolytic Cell.

AgNO3 (aq)

Ag (s)

Write the equation for Reduction.Ag+

(aq) + e- Ag(s)

What is the Charge of the Anode?The Anode is Pos. (+).

D.C. Battery

PH Video of Chrome Plating Hyperlink 1

Cool School Video of Electrolysis of Water then KI Hyperlink 3

PH Video of Electrolysis of Water Hyperlink 2

e-

Practice With Red-Ox ReactionsWrite a Balanced Red-Ox Reaction that could have occured in flask a.

2Ag+ + Cu 2Ag + Cu2+

Name one cation that could be present in flask b and explain your reasoning.

Na+ because it cannot Oxidize Copper.

Draw and Label an Electrolytic Cell that could be used to

Separate Na from Cl in NaCl?

Why is Solution a Blue?Cu2+(aq) produced

D.C. Power Supply

+ -

Molten (melted) NaCl

Na+ + e- Na02Cl- 2e- + Cl2

e-

More Review With Oxidation-Reduction

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.

Voltaic Cells: Chemical Energy Electrical Energy Electrolytic Cells: Electrical Energy Chemical Energy

Practice with a Voltaic CellDetermine:

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 CellDetermine:

Red-Ox Half-Reactions

Anode & Cathode

Flow of Electrons

Charge of Electrodes

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.

D.C. Battery

+ -

Practice with Electrolytic Cells

Determine:

Flow of Electrons

Red-Ox Half-Reactions

Anode & Cathode

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

+ -

Practice with Electrolytic Cells

Determine:

Flow of Electrons

Red-Ox Half-Reactions

Anode & Cathode

Determine what metal you would like to Electroplate onto a surface or determine what binary compound you would like to separate into elements.