Intersection 14: The End

Outline

• Vitamin C

• Electrochemistry

• Final exam

Vitamin C

• Which fruit keeps best over a long period of time?

• Fruits known to sailors– Orange, lemon ~1400– Limes 1638– Shaddock (grapefruit) 1700

• Used lemon or lime juice preserved in brandy

From: Moore, Stanitski, and Jurs Chemistry: The Molecular Science 2nd Edition/

p.941

Exploding Batteries

• Cell phone Li/Co batteries– Li Li+ + e-

– Co+3 + e- Co+2

Increased energy densityLighter

• Use cobalt oxide which can lead to thermal runaway. Increased temperature and pressure cause case of battery to break and fumes are released. Lithium is exposed to oxygen and hydrogen in the air.

The other ½ of the battery:

• Co+3 + e- Co+2 (+1.92 V)

• What is the standard potential of the cell?

[Li Li+ + e- (+ V)]

• A lithium battery has a potential listed at 4V. Does this differ from the number you calculated? Why?

Fig. 19-25, p.953

Corrosion: A Case of Environmental Electrochemistry

• Facts about formation of rust– Iron does not rust in dry air: moisture must be present– Iron does not rust in air-free water: O2 must be present– The loss of iron and the deposition of rust often occur

at different places on the same object– Iron rusts more under acidic conditions (low pH)– Iron rusts more quickly in contact with ionic solutions– Iron rusts more quickly in contact with a less active

metal (such as Cu) and more slowly in contact with a more reactive metal (such as Zn)

Corrosion: A Case of Environmental Electrochemistry

O2(g) + 4 H+(aq) + 4 e- => 2 H2O(l) Eo = 1.23 V

Fe(s) => Fe+2(aq) + 2 e- Eo = 0.44

V

2 Fe(s) + O2(g) + 4 H+(aq) => 2 H2O(l) + Fe+2

(aq)

Eo = 1.67 V

2Fe2+(aq) + ½O2(g) + (2+n)H2O(l) Fe2O3.nH2O(s) + 4H+(aq)

Corrosion: A Case of Environmental Electrochemistry

• Sacrificial cathode

Fig. 19-27, p.955

• During the reconstruction of the Statue of Liberty, Teflon spacers were placed between the iron skeleton and the copper plates that cover the statue. What purpose do these spacers serve?

Cu+2 + 2e- Cu 0.153 VFe+2 + 2e- Fe -0.44 V

Picture from: musee-ampere.univ-lyon1.fr/

Ampere

The Units of Electrochemistry

• Coulomb

– 1 coulomb equals 2.998 x 109 electrostatic units (eu)

– eu is amount of charge needed to repel an identical charge 1 cm away with unit force

– Charge on one electron is -1.602 x 10-19 coulomb

Problem: An aluminum ion has a +3 charge. What is this value in coulombs?

magnitude of charge is same at that of e-, opposite sign3 x 1.602 x10-19 = 4.806 x 10-19 coulomb

Key Point: electrons or ions charges can be measured in coloumbs

The Units of Electrochemistry

• Ampere

– Amount of current flowing when 1 coulomb passes a given point in 1 second

– Units of Amperes are Coulombs per second

– Current (I) x time (C/s x s) gives an amount of charge.

Problem: How much current is flowing in a wire in which 5.0 x 1016 electrons are flowing per second?

The charge transferred each second = (5.0 x 1016 electrons/sec) x (1.602 x 10-19 coulomb/electron)= 8.0 x 10-3 coulombs/sec = amps

The Units of Electrochemistry

• Ampere

– Amount of current flowing when 1 coulomb passes a given point in 1 second

– Units of Amperes are Coulombs per second

– Current (I) x time (C/s x s) gives an amount of charge.

– We can express electron or ION flow in amps!

Problem: If 1 mol Al+3 ions passes a given point in one hour, what is the current flow?

1 hour = 3600 seconds; Al+3 charge is 4.806 x 10-19 coulomb

1 mol Al +3 ions 6.022 x 1023 Al +3 ions 4.806 x 10-19 coulomb 1 hour

Hour 1 mol Al +3 ions 1 Al+3 ion 3600 sec= 80 C/s = 80 A

Hybrid Cars

2004 Toyota PriusGasoline and Electric Engines

2004 Honda Civic Gasoline and Electric Engines

Electrochemistry and the Honda Civic

The battery in the 2004 Honda Civic is rated for 6.0 Ampere-hours. If the electric motor draws 42 amps to accelerate from 0 to 60 mph, and the time required is 15 seconds.

The Honda Civic battery is a nickel metal hydride battery. The half-reaction for generating the electrical current is:

MH + OH- → M + H2O + e-

How much metal (assume its nickel) is produce on accelerating from zero to 60?

42 C 15 sec 1 mole e- 1 mol Ni 58.69 g Ni 0.38 g Ni

sec 96, 487 C 1 mole e- 1 mol Ni

Hybrid Cars: Can We PowerUsing Sunlight and Fuel Cells?

SpectruM2005 Michigan Solar Car Team

Photovoltaics and Battery

Some Needed Data• The Toyota Prius can generate ~ 100 Amps

– Let’s assume our commute requires on average at ~ 20% max current for 1 hour.

• A 1 m2 solar cell array can generate 3 amps in full summer sunlight

Questions:1. If the electrical power was generated with a fuel cell, how

much hydrogen would be consumed? How much volume would be required at 1 atm pressure to hold the hydrogen?

2. Assume you can place 10 m2 of solar cells on your car. How long would it take to generate the needed hydrogen assuming full summer sunlight?

1. Hydrogen Consumption

At the anode (where oxidation occurs): 2 H2 (g) + 4 H2O (l) → 4 H3O+ (aq) + 4 e-

At the cathode (where reduction occurs) O2 (g) + 2 H2O (l) + 4e- → 4 OH- (aq)

Overall reaction: 2 H2(g) + O2 → 2 H2O (l)

Average 20% of maximum power = 20 amps20 amps x 3600 secs = 72,000 coulombs

72,000 C 1 Faraday 1 mol electrons 2 mol H2 0.37 mol H2

96,487 C 1 Faraday 4 mol electrons

V = nRT/P = (0.37 x 0.08206 x 300) / 1 = 9 Liters

2. Hydrogen Generation

A 1 x 1 m2 solar panel generates a peak current of 3 amps.Our car with 10 m2 of solar panels could generate 30 amps.

Electrolysis of water generates hydrogen with the following half-reaction:

At the cathode (where reduction occurs): 4 H3O+ (aq) + 4 e- → 2 H2 (g) + 4 H2O (l)

0.37 mol H2 4 mol e- 1 Faraday 96, 487 C sec 1 hour 0.66 h

2 mol H2 1 mol e- 1 Faraday 30 C 3600 sec

Issues Arising

• Can generate hydrogen from water quickly enough to supply average demand by fuel cell.

• Important since solar cells cannot supply peak current flow used in Prius of ~ 100 amps.

• Where to store hydrogen??– As water is good. High density storage.– Molarity??– ~110 M

• As gas is bad, requires too much space (~ 9 L)– Need good storage materials!! (Active research in Yaghi group)

Final Exam• Friday, December 16th 8:10 am-10:00am

• East Hall 1360 • Same rules:

– 75 minutes indiv/ 30 minutes team– No programmable calculators– No additional notes, etc.

• Topics (65 points)– ~17 electrochemistry– ~12 atoms, elements periodic trends– ~17 chemical bonding; thermochem– ~19 aqueous chem, equilibrium and acids/bases– (15-20 points team/ 45-50 individual)

• Don’t forget to check out of lab tomorrow.

• Review: Thursday Dec 15 1-3 pm Room:

• Return exams: Bring a self-addressed, stamped (*3) large envelope (manila with clasp) to exam

Evaluations

• Please fill out both 125 and 130 evaluations and put them in the correct envelopes!

• Constructive comments very helpful!

Thanks for a great semester!