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chem100-- November 2, 2001

Exam: back on Monday , comments then

copy with answers on the Web site

Chapter 8-- New Energy Sources

In one way, a misnomer

source 1: current sunlight and consequences

solar, biomass, hydroelectric and wind

source 2: old sunlight (fossil fuels)
source 3: nuclear stockpile

(geothermal resources)
(perhaps: mineral resources-- but
generally already used up as energy sources)

Basically we mainly repackage energy
We store energy that would be lost

sunlight --> electricity --> storage battery ---> lighting at night

We move energy

Niagara ---> electricity ----> power grid ---> to Ohio

We make energy portable

ZnO + C ---> Zn ----> battery---> flashlight

We change form

sunlight + water + catalyst ---> O₂ + H₂ (fuel)

We tap supplies we wouldn't use otherwise

wind farms (wind generators)
geothermal
(probably not in scope of chemistry course)

We also try to improve efficiency
We might try to reduce waste and reduce consumption

hopefully, retaining high standard of living

This is really an open ended tour of Problem Solving

Problem-- present energy uses are wasteful

doomed in long run (limited oil resources)
dangerous products (CO₂, SO₂, NO_x, CO)
geopolitical nightmare (who owns the oil?)

so the "problem" is complex, many parts

not simply solve a technical question or compute an answer
not obvious "an answer" exists
probably would get resolved unpleasantly by default

Challenge--

find better approaches to way we do things
consider alternative lifestyles

suburbs drive fuel consumption
building highways may be counterproductive
speed, size, convenience drive fuel inefficiencies

consider alternative technologies

probably not new scientific concepts
probably not revolutionary devices

new materials can resurrect old technologies

electric cars once exceeded gasoline vehicles
steam cars have always outperformed internal combustion

Really need a true solution-- (solutionS)

too often, solution creates its own nightmares
often shifts problem to others

agriculture and industry dried up the Aral Sea

irrigation and power dams have wiped out
fisheries and created deserts there

catalytic converter reduces hydrocarbon and CO, but increased NOx

A very real challenge-- how to evaluate "better"

evaluate net costs
dollar costs (include investment)

high efficiency furnace
= better fuel efficiency
but $500-1000 more
would I save $1-2 K over 20 year lifetime ?
borrow $1000 for 20 years costs $2-3 K

energy costs
other costs (e.g., global warming)

would nation be better off if my furnace uses less gas?

can be difficult -- comparing apples and oranges

Hydrogen as an Alternative Fuel

We can produce Hydrogen from water

Hydrogen is a great fuel

286 kJ / mole of H₂

There's lots of water; we get it back on combustion

BUT net energy = 0

simple physics-- conservation of energy
have to spend energy to split water

(break OH bonds)
combustion works because of OH bond

no new energy; actually, a net loss

process not really 100% efficient
energy cost to transport H₂ for use

Can decompose water with electrical current

that changes energy (electrical to portable fuel)
could be cleaner energy

perhaps a clean electrical source
(well run nuclear plant-- no fossil fuels)
clean engine (no CO, CO₂ or SO₂)
if pure O₂ used, no NOx either

awkward to use hydrogen for vehicles

1 mole of gasoline (C₈H₁₈)

molecular weight 96
heat of combustion: ______ kJ/mole
about 150 ml (small teacup)
simple liquid, simple gas tank

2 mole of H₂

molecular weight 2
heat of combustion-- 286 kJ/mole
gas -- 25 liters in simple balloon
250 ml at P=100 atm

need very heavy, strong steel tank
raises cost, weight, reduces auto's efficiency

probably not a great idea if energy source is Coal

produce CO₂ , some SO₂ to get electricity
(perhaps more CO₂ than gasoline for cars)

rest of process is environmentally friendly
global damage is done at first step
could still be local advantage

(less pollution where the car are used)

Analysis might miss some tradeoffs

heavy H₂ cylinders

heavy vehicle costs more fuel to move
on impact, H₂ escape = hazard

(overrated-- escaping gasoline = bigger hazard)

Hydrogen Economy (vs Oil/Coal Economy)

Actually H₂ changes several parts of the picture
Together, might be a better overall energy design (not true at present)

We can get H₂ from other fuels
like methane or methanol--

might not need big gas cylinders
(less efficient, but could be more convenient)

We can use H₂ in other ways (no flames, engines)

(typical car "wastes" fuel without producing motion)

Electrical Batteries and Fuel Cell

(discuss in a minute)
don't need to "idle" or warm up electrical motors

We can store H₂ in ways other than tanks

Hydride Storage

Metal Hydrides

A Few metals react with H₂ and form compounds

Ca + H₂ --> CaH₂ (s)

one mole is now 42 grams
a solid, perhaps 15 ml in size
could carry in a simple jar or tank
to use, add H₂O ---> Ca(OH)₂ + H₂
text, p310, used Li, LiH

better, find metals with weaker Metal-H bonds

think of the hemoglobin case

O₂ + HG ---> HG-O₂ if O₂ level is high
HG-O₂ ---> HG + O₂ if O₂ level is low
reversible process

M + H₂ ----> MH₂ at moderately high H₂ pressure

can "charge" a tank of M with H₂
keeps the H₂ if pressure kept high

MH₂ ---> M + H₂ as we release pressure

solid produces H₂ for use
Again, reversible reaction
equilibrium system

Chemical Energy Directly into Electrical Energy

We don't need to burn the fuel, producing heat

sometimes we can split a chemical reaction

Oxidation--sample loses electrons

(1, 2 or 3 electrons per atom, molecule)

Na ---> Na+ + e-
H₂ ---> 2 H+ + 2 e-
notice: oxidation doesn't need oxygen
really are balanced equations, if you count electrons

Since we don't collect and store electrons

Must have comparable use of these electrons as part of the overall process

Reduction-- sample gains electrons

Br₂ + 2e- ---> 2 Br^-
O₂ + 4e- ---> 2 O₂- ions

Oxidation and Reduction always occur simultaneously

2Mg + O₂ ---> 2MgO
Zn + 2HCl ---> ZnCl₂ + H₂

Mg burns ... Mg and O swap electrons
Zn dissolves (react) in HCl, swap electron

We often can separate the processes

electrons need a path
we provide a wire for the electrons

get useful electrical current

Zn + 2Ag⁺ ----> Zn²⁺ + 2 Ag + energy

could put Zn rod into Ag+ solution
rod dissolves and gets covered with Ag metal

electrons go directly between Zn and Ag

put Zn into its own solution (say Zn²⁺ ions)

reaction Zn ---> Zn²⁺ + 2e won't occur
because solution can't store electrons

put silver rod into Ag+ solution

reaction Ag+ + e ---> Ag
won't occur because it needs the electrons

connect a wire between the Zn and the Ag

now both reactions occur
electrons travel from Zn to Ag via wire
(if they travel through a small light bulb, it lights)

disconnect the wire, reactions stop
can use the power on demand
we also need ways for ions to travel between the solutions

This is said to be a voltaic or galvanic cell

(like a flashlight battery)

technically 1= cell, several = battery
word battery now used for single cells

components consumed as we generate electricity
process stops when we use up the materials

usually need coal, oil, electricity to produce Zn; not new energy source

Alternative is rechargeable battery

can regenerate the materials
can keep reusing the battery
need access to electrical power supply

Alternative is the fuel cell

actual reactants are not part of the cell
they are supplied from outside

often air provides oxygen
a tank provides H₂, CH₄ or Methanol fuel

could be used to power a car

also of alcohol powered computers

fast refill, vs. slow recharging

Some commercial house sized Fuel Cells

about size of refrigerator
perhaps cost about $10,000

Use natural gas or propane to produce your electricity
serious choice now

if you live off the electrical power grid

might cost $10,000 to get wires installed
might be subject to power outages
may be less expensive, quieter, more reliable than gasoline or diesel generators
even on campus-- many buildings have back-up emergency generators; need to run monthly

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