Chapter 8 - Energy From Electron Transfer Flashcards
What is a battery?
stored energy that is converted from chemical (potential) to electrical (kinetic) energy
What are galvanic cells?
cells that convert chemical energy to electrical energy (ex. normal battery)
What are electrolytic cells?
cells that convert electrical energy to chemical energy
What is electricity?
the flow of electrons
What are the parts to a full reaction?
half-reactions
- oxidation
- reduction
What is oxidation?
the loss of an electron
What is reduction?
the addition of an electron
OILRIG
Oxidation Is Loss, Reduction Is Gain
How do you make electricity?
- separate the half-reactions
- connect with a wire
- on way for reaction to go: electrons run through wire
- -> electron flow = electrical current
What are electrical conductors?
sites of reactions
- cathode
- anode
What is a cathode?
where reduction occurs, electrons are gained
What is an anode?
where oxidation occurs, electrons are lost
What is voltage?
the ease of anode release/cathode absorption –> bigger difference = greater voltage
units: volts
What is a salt bridge?
transfers ions, completes circuit between two solutions
What are desirable properties in a battery?
- high voltage (energy efficient)
- cheap
- long lasting
- safe handling/disposal
- small/light
What is a lead-acid battery?
true batter that consists of a series of 6 cells
- anode: Pb, cathode: PbO2
- stores electrical energy
What are the advantages/disadvantages of lead-acid batteries?
advantages:
- long lasting
- rechargeable
- power starter (ex. lights, radio, etc)
- alternator recharges
disadvantages:
- heavy
- toxic
What are ways we could improve batteries?
- Safer Disposal- phase out mercury/lead
- Lighter/More Dependable
- Rechargeable- Nickel-metal hydride, lithium-ion, replace alkaline
- Efficiency
What are hybrid cars?
Two engines (gas and NiMH/Li-ion)
- environmentally friendly, less pollutants
- range of gas engine
- other combos possible
- better gas mileage (in-city better than highway)
What are fuel cells?
chemical energy —> electrical energy (no fire/explosion)
BIG difference from batteries
-fuel and oxidant supplied
-“waste” expelled continuously
-half reactions are separate
-force H thru membrane (electrons thru wire)
-no flame, no solids needed
-little heat (efficient, less lost energy)
-product = water (environmentally friendly)
-unused H2 or O2 can be put back thru system
-40-45% efficient (compared to gasoline: 20-30%)
What are the advantages of fuel cell cars?
- no nitrogen oxides
- low/no CO2 emissions
- H2, methanol–> renewable resource? (biological sources)
- Engine: no/fewer moving parts = less/easier repair, longer lasting
- No recharging: fuel continuously provided, faster than recharging a battery
What are the disadvantages of fuel cell cars?
- slower reaction- not as much power in shorter amount of time
- catalysts are expensive
What are the chemical changes in a fuel cell?
H2 + 1/2 O2 –> H2O
Anode reaction: (Oxidation half-reaction)
H2(g) –> 2 H+(aq) + 2 e–
Cathode reaction: (Reduction half-reaction)
½ O2(g) + 2 H+(aq) + 2 e- –> H2O(l)
Energy output of fuel cells
143 kJ/g
compared to…
coal: 30 kJ/g
gas: 46 kJ/g
natural gas: 54 kJ/g
Sources of H2
- most plentiful element (93%)
- Very reactive (tied up in compounds)
- Must be extracted
From water?
572 kJ + 2H2O –> 2H2 + O2
-where do you get the energy?
From Methane/water?
165 kJ + CH4 + 2H2O –> 4H2 + CO2
-need a catalyst to be more efficient
H2 storage and transplant
- 12 L/g (approx 3 gal)
- cylinders: heavy/pressurized
- transport –> need more energy to transport the heavy H2
- liquify? –> would need to be -253 degrees C
- explosive!
What are possible options for H2 storage?
1) absorb onto C – Heat to release
2) React to make metal hydrides
2 Li + H2 –> 2 LiH
–>Reverse with water: LiH + H2O –> H2 + LiOH
3) Making ammonia:
N2 + 3H2 –> 2NH3 + energy
–>Reverse reaction: 2NH3 + energy –> N2 + 3H2
4) Storage in Nanotubes: tubes of C atoms, reusable, $$$
5) Photovoltaics: solar power
What are photovoltaics?
Solar Power
- sun –> lots of energy to earth
- renewable
- light/heat, not useful for work
- solar energy —photovoltaic cell—> electricity
- -> use this energy for H2 production?
What are semiconductors?
only conduct under specific conditions (ex. photon of right wavelength collides)
- silicon (4A): primary component of photovoltaic cells
- photon of 1100nm will knock electrons off
- visible light: more than enough energy
- problems: expensive refinement, low efficiency (max 28%) –> but energy is free/unlimited
Development of semiconductors
- non-crystalline silicon –> decreased $$, increased efficiency
- “doping” silicon: combine with other materials
- ->ex. As (5A): one more outer electron (n-type, “negative”), Ga (3A): one less outer electron (p-type, “positive”)
- easier to make current (lower energy photons, sandwich n-/p-type connected by wire)
Economics of Energy
Solar: cost dropping
Fossil Fuel: cost increasing
Solar Investments: increasing, currently 1% of global power, favored over nuclear
Installation of solar energy
Low-maintenance Build anywhere -Lots of space -Isolated areas -Undeveloped areas -No “network”
