Chemistry SM-1232Week 11 Lesson 1

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademySpring 2008

Class Today• A few wikis started!!! GET ON IT! At least start the page.• Wiki quiz and homework due Wednesday. No exceptions.• Chapter 16 end today• Start Chapter 18 on Wednesday, • Test on chapter 15 and 16 next Friday, so everyone can have

lab.

Simpler definition

• LeoGer• Aka Leo the Lion goes Ger• Lose electrons = oxidation• Gain Electrons = reduction

Rules!• 1. Pure elements have an oxydation state of 0• 2. Any charged ion has an oxydation state equal to its

charge• 3. If a compound is neutral the sum of all oxydation

states equals 0• 4. If a compound is charged the sum of all oxydation

states equals the charge• 5. Group 1,2,3 will always be +1,+2,+3• 6. Non metals get oxidation numbers on the next

slide.

More Rules: Oxidation numbers

• This is list is also in order of precedence! • Fluorine oxidation is always -1• Hydrogen is +1• Oxygen is -2• Group 7 is -1• Group 6 is -2• Group 5 is -3

When in doubt we add in H2O and keep rebalancing

• CN- + MnO4-2 CNO- + MnO2

-

• 1. Assign oxidation numbers

When in doubt we add in H2O and keep rebalancing

• CN- + MnO4-2 CNO- + MNO2

-

• 1. Assign oxidation numbers• CN-, N=-3 total = -1, C=2• MnO42-, O=-2, 4O=-8, total =-2, Mn-8=-2, Mn=+6

• 2. Separate into half reactions

When in doubt we add in H2O and keep rebalancing

• CN- + MnO4-2 CNO- + MnO2

• 2. Separate into half reactions

• CN- CNO-

• MnO42- MnO2

-

• 3. Add water to make up the difference in unbalanced Os

When in doubt we add in H2O and keep rebalancing

• CN- CNO-• MnO42- MnO2

• 3. Add water to make up the difference in unbalanced Os• CN- + H2O CNO-

• MnO42- MnO2 + 2H2O

• 4. Add in the Hydrogen to make up the difference in unbalanced Hs

When in doubt we add in H2O and keep rebalancing

• 4. Add in the Hydrogen to make up the difference in unbalanced Hs

• CN- + H2O CNO- + 2H+

• 4H+ + MnO42- MnO2 + 2H2O

• 4a. Add in OH- to neutralize H+ to make water molecules

When in doubt we add in H2O and keep rebalancing

• 4a. Add in OH- to neutralize H+ to make water molecules• CN- + H2O + 2OH- CNO- + 2H+ + 2OH-

• 4H+ + MnO42- +4OH- MnO2 + 2H2O + 4OH-

• 4b. Cancel the waters on each side.• CN- + 2OH- CNO- + H2O• MnO4

2- +2H2O MnO2- + 4OH-

• 5. Check the oxidation state of carbon on the left and right as well as Manganese on the left and right. Figure out the electron flow.

When in doubt we add in H2O and keep rebalancing

• 5. Figure out the electron flow• CN- + 2OH- CNO- + H2O + 2e-

• 3e- + MnO42- +2H2O MnO2 + 4OH-

• 6. Balance the oxidation and reduction. What’s the lease common denominator between 2 and 3.

When in doubt we add in H2O and keep rebalancing

• 6. Balance the oxidation and reduction. What’s the lease common denominator between 2 and 3.

• CN- + 2OH- CNO- + H2O + 2e-

• 3e- + MnO42- +2H2O MnO2 + 4OH-

• 6! So, we need six electrons on both sides. We’ll multiply the top by 3 and the bottom by 2.

• 3CN + 6OH- 3CNO- + 3H2O + 6e-

• 6e- + 2 MnO42- + 4H2O 2 MnO2 + 8 OH-

• 7. We have to add these two half reactions together

When in doubt we add in H2O and keep rebalancing

• 7. We have to add these two half reactions together• 3CN + 6OH- 3CNO- + 3H2O + 6e-

• 6e- + 2 MNO42- + 4H2O 2 MnO2

- + 8 OH-

• 3CN + 2 MnO42- + H2O 3CNO- + 2MnO2

- + 2OH-

Activity Series

• Lab is all about the activity series. Some of you won’t have lab until next week.

• You’ll learn everything you need about this in Lab, so I’m skipping this section in class.

Batteries!

• Electrical current is the flow of electrons (negatively charged particles).

• Redox reactions require the flow of electrons, and we can use these reactions to make current!

Electrochemical Cell

Half Cells

• Zn is placed in Zn(NO3)2 to form a half cell

• Cu is placed in Cu(NO3)2 to form a half cell

Electrochemical Cell

Anode is the strip where oxidation occurs, labeled with a negative signCathode is the strip where reduction occurs and is labeled positive

The Bridge

• The bridge is a salt bridge that allows ions to flow to balance out the chares that are being generated at the anode and the cathode

Galvanic (Voltaic) Cells

• Cells that spontaneously produce electrical current are called Galvanic or Voltaic cells.

• Water is analogous. The amount of water that flows through a river determines the river current. The amount of electrons that flow determine the electrical current.

• Gravity forces water down hill to reduce potential energy. Voltage forces electrons from one atom to the next to reduce pot. En.

Dead Batteries

• You saw this in lab. The metal actually dissolves into solution. The metal containing the electrons has been transformed and no longer has those electrons. When all the metal is used up the battery dies. Rechargable batteries reverses the reaction and solid metal reappears.

Dry Cell Batteries

• Dry because there is no water.• Zinc Zn2+ + 2e- (oxidation, anode)• 2MnO2(s) + 2NH4

+ + 2e- Mn2O3(s) + 2NH3(g) + H2O(l) (cathode reduction)

• Alkaline battery:• Zn(s) + 2OH-(aq) ZnOH2(s) + 2e-• 2MnO2(s) + 2H2O + 2e- 2MnO(OH)(s) + 2OH-

(aq)

Dry Cell

Lead-Acid Storage Batteries (car)

Car Batteries

• 6 different cells each that have 2 volts• Pb(s) + SO4

2- PbSO4(s) + 2e-

• PbO2(s) + 4H+ + SO42- + 2e- PbSO4(s) + 2H2O

• Putting current through in the opposite direction reverses the reaction and Pb(s) is regenerated as is PbO2. So, the forward reaction can repeat later.

Fuel Cells

Fuel Cell

• H2(g) + 4(OH)- 4H2O + 4e-

• O2(g) + 2H2O + 4e- 4OH-

• Go ahead and combine them. What product do we make? Which is the reduction, which is oxidation? Which side will be the anode? Cathode? Which side gets the positive sign? Which gets the negative sign?

Silver Plating

Read all of chapter 16

• Work on your wikis• Homework due Friday• Read the start of Chapter 18 first 4 sections• Test on Friday.• Get your quizes and HW