Unit VII: Redox & ElectrochemistryReduction, Oxidation & Electrochemistry: Text Chapter

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Link to Redox Topic Outline

The Baghdad Battery is believed to be about 2000 years old. The jar was found in Khujut Rabu just outside Baghdad and is composed of a clay jar with a stopper made of asphalt. Sticking through the asphalt is an iron rod surrounded by a copper cylinder. When filled with vinegar – or any other electrolytic solution - the jar produces about 1.1 volts.

The Baghdad Battery

Image and info taken from http://www.smith.edu/hsc/museum/ancient_inventions/battery2.html on 8/11/09.

Lithium-ion Battery

Image taken from http://static.howstuffworks.com/gif/lithium-ion-battery-2.jpg on 8/11/09.

Lithium-ion batteries are a popular battery used today in laptops, cellphones and iPods. They are some of the most energetic rechargeable batteries currently available.

What are Oxidation & Reduction?Define the following:• Oxidation-• Reduction-

• Hint: “LEO” the lion says “GER”

The Browning of Fruit involves both oxidation and reduction.

Image taken from http://www.questforright.com/images0/vol1co3.jpg on 8/11/09.

Image taken from http://www.food-info.net/images/brownbanana.jpg on 8/11/09.

Redox Rxn AKA: (Reduction-Oxidation Reaction)

What?...A single reaction in which electrons are lost (oxidation) from one atom to another (reduction).

How?...Occur simultaneously, one cannot occur without the other.

Why?...Results from competition for electrons to increase stability.

Zinc Oxide animation

Rxn of Zinc in Cu(NO3)2 solution

Assigning Oxidation Numbers!Follow steps below in order!

1) Free elements are 0. ex: Na, Cl2, Fe

2) In any compound or ion, the element with the larger electronegativity (more non-metallic) is negative. ex: Cl in NaCl, O in Mg(ClO2)2

3) In compounds, Group 1 elements are +1, group 2 are +2 & Oxygen is “usually” -2.(PT)

4) Sum of oxidation states on all atoms in a compound or ion equals the charge on the compound(0) or ion (+or-). ex: NaCl(0), H3O+(+1)

Do practice examples!

Half-Reactions• Part of a rxn that involves only oxidation or

reduction.• Must be balanced by both mass and charge to

support the Laws of Conservation of Mass (Matter) and Charge.

Image taken from http://www.xtreme.hawaii.edu/research-projects/corrosion/parts/corrosion2_big.jpg on 8/11/09.

Corrosion of a Metal Fence

Corrosion- the gradual destruction of a metal or alloy as a result of chemical processes such as oxidation or the action of a chemical agent.

Reduction Half-Reactions• Oxidation number decreases (reduces) to show

gain of negatively charged electrons.• Electrons are written as reactant to show gain and

to balance the charge.

• Example: F20 + 2e- 2F-

Image taken from http://www.ruststopnorthamerica.com/images/Rusty_Car.jpg on 8/11/09.

Rusting is an example of corrosion that is caused by redox. Tarnishing of

a silver spoon is a redox rxn.

Image taken from http://www.countryliving.com/cm/countryliving/

images/Tarnished-Silver-Spoon-ENTERT0106-de.jpg on 8/11/09.

Oxidation Half-Reactions• Oxidation number increases to show loss of

negatively charged electrons.• Electrons are written as product to show their

loss and balance charge.• Example: Lio Li+ + 1e-

Image taken from http://www.askanddiscuss.com/blog/wp-content/uploads/2008/05/hair-dye.jpg on 8/11/09.

Coloring your hair involves oxidation and reduction.

Image taken from http://www.crawleyadjusting.com/images/shuttle.jpg on 8/11/09.

The 2003 Columbia Shuttle explosion is thought to have been partly caused from redox rxns that weakened the carbon panels on the outside of the shuttle.

Writing Balanced Half-Rxns1) Figure out oxidation numbers for each element.2) Figure out which is oxidized and which is reduced.3) Write half-rxns showing rxnts and products.4) Add in electrons on correct side of arrows to show

both gaining and losing of electrons.5) **Balance transfer of electrons by multiplying by

least common multiple. May not need to do this step if already balanced.

6) Determine the oxidizing and reducing agents.

Do Practice Examples!

Reducing Agent = Oxidized• Loses an electron and

causes something else to gain the electron and be reduced.

• Metals tend to be good reducing agents (oxidizers) because they have low electronegativities & small # of valence electrons.

Image taken from http://www.clprosser.co.uk/jpeg/metals.jpg on 8/11/09.

Oxidizing Agent = Reduced• Gains an electron and

causes something else to lose an electron and be oxidized.

• Nonmetals tend to be good oxidizing agents (reducers) because they have high electronegativities and large # of valence electrons.

Image taken from http://doemarie.com/images/other_nonmetals.jpg on 8/11/09.

Types of Redox Rxns1) Composition (Synthesis) Rxns:

2 Na + Cl2 2 NaCl

2) Decomposition (Analysis) Rxns:

2 HCl H2 + Cl2

3) Single Replacement Rxns:

Mg + 2 HCl MgCl2 + H2

4) Double Replacement (not usually redox)

BaCl2 + Na2SO4 NaCl + BaSO4

Balancing Redox Equations1) Assign oxidation #’s.

2) Who is reduced/oxidized??

3) Write half-rxns.

4) Balance half-rxns for mass and then charge.

5) Insert coefficient values from half-rxn into whole rxn.

6) Balance rest of whole rxn by inspection.

Do Practice Examples!!!

Electrochemistry• Branch of chemistry

that is the study of the relationship between electric forces and chemical reactions.

• Connects chemicals and electricity.

Image taken from http://store.apple.com/us/browse/home/shop_ipod/family/ipod_nano?mco=MTIyMDI on 8/11/09.

Electrochemistry definitions

Types of Electrochemical Cells

Electrochemical cells

Voltaic cells(aka Galvanic cells)

spontaneous

Electrolytic cellsNot spontaneous

Daniell cells(chemicals in beakers)

Dry cells(Common Batteries)

Lead-acid batteries(Auto Batteries)

Fuel cells

Taken from chemwiki.ucdavis.edu on 7/29/11.

Diagram of Electrochemical Cell

Label the following in the diagram at right.

• Wire explained3

• Salt Bridge explained 5, &6

– Cations are postive.– Anions are negative.

• Electrodes– Anode explained2

– Cathode explained4

• Half-cells• Voltmeter• Electrolytes

Does this diagram show a voltaic or electrolytic cell? How can you tell?

Parts of an Electrochemical Cell-explained#1

Standard Electrode Potential (Eo)• The potential

developed by a metal or other material immersed in an electrolyte solution relative to the potential of the hydrogen electrode, which is set at zero for a standard.

Why do think nonmetals like F, Cl and O are found at the top of chart with positive reduction potentials?

Standard Electrode Potentials and Determining Spontaneity

• Use Eo values for each half-reaction to determine overall potential of a reaction.

• Overall positive value spontaneous rxn

• Overall negative value not spontaneous rxn

• Redox rxns that reach equilibrium will have an Eo value equal to zero.

• Do Practice Examples! Would these two metals make a good battery? Zn and Cr+3

Image taken from http://periodictable.com

/Samples/024.11/s9s.JPG on 8/11/09.

Image taken from http://z.about.com/d/

chemistry/1/0/w/c/zinc.jpg on 8/11/09.

Using Ref Table J to determine spontaneity (voltaic or electrolytic)

• More active metals easily lose electrons (easily oxidized).

• Oxidizers (WIMPS) should be towards top of metal chart J and reducers (BULLYS) towards the bottom. If so, then spontaneous.

• If not (BULLY is towards top), then not spontaneous.

Using Ref Table J to determine spontaneity (voltaic or electrolytic)

• Nonmetals gain electrons (reduced) to form compounds.

• So more active nonmetals tend to more easily be reduced (BULLYS towards the top of chart J).

• Remember F is biggest bully (4.0 electronegativity)

Voltaic (Galvanic) Cells• Spontaneity?• Eo values?• Draw a diagram of a

voltaic cell including all connections and labeling parts and direction of e- and ion flow.

Taken from http://xprizecars.com/ images/Battery_9V.jpg on 8/11/09.

Galvanic Cell Animation

Electrolytic Cells (Electrolysis)• Spontaneity?• Eo values?• Draw a diagram of an

electrolytic cell including all connections and labeling parts and direction of e- & ion flow.

Taken from http://www.saskschools.ca/curr_content/chem30_05/ graphics/6_graphics/electrolysis_water.gif on 8/11/09.

Volumetric Electrolysis of Water

Electroplating• A type of electrolytic cell.• An electric current is used to

reduce metallic ions in solution and cause them to plate out onto an object.

• The object to be plated is made the negative electrode (cathode).

• The metallic ions are produced by oxidation at the positive electrode (anode).

Taken from http://www.rschrome.co.uk/250px-Electroplating-of-spoon.PNG on 8/11/09.

Electrochemistry Definitions

• Electrochemical cell- a system that contains 2 electrodes separated by an electrolyte, 2 types of electrochemical cells are voltaic and electrolytic

• Voltaic cell (galvanic cell)- spontaneous electrochemical cell that produces electrical energy from chemical energy

• Electrolytic cell- non-spontaneous electrochemical cell that requires electrical energy to produce a chemical rxn.

• Half-cells- separate containers that contain each half (oxidation or reduction) of a redox rxn.

• Salt bridge or porous cup- used to connect half-cells to allow for the migration of ions and prevent polarity

• Electrodes- a conductor used to establish electrical contact with a nonmetallic part of a circuit, 2 examples are anode and cathode.

• Anode- the electrode on whose surface oxidation takes place, anions migrate towards the anode and electrons leave the system at the anode.

• Cathode- the electrode on whose surface reduction takes place,