| |
Chemistry 1002 Chapter 12Fossil Fuels |
|
Fossil Fuels
What is a "fossil fuel?" Remains of old
old old old dead things (fossils) which can be used (usually burned)
for energy. Like most matter fossil fuels come in three forms:
solid (coal), liquid (oil), and gas (natural gas).
Currently world uses close to 400 quad(rillion BTU)s
energy per year. Boil water from 400 billion Olympic-sized swimming
pools. In US: 4 pools/yr/person. Natural gas reserves: about
4900 quads (13 year supply). Rarest & cleanest of big three
fossil fuels. Coal: 29,000 quad reserves (80 year supply), dirtiest
fossil fuel. Oil: 5,900 quads reserves (16 year supply). Discuss
more details Chap. 13.
Energy
Fossil fuels produce energy when burned in oxygen.
Process involves breaking bonds of fuel molecules (costs energy)
and creating bonds between atoms of fuel molecules and atoms of
oxygen from the air (releases energy).
"Oxidized" fuels (mostly carbon dioxide and
water) have much less energy content than unburned fuels. Unburned
fuels lose high energy content as heat when they burn. Amount
of heat generated when fuel burns called "heat of combustion."
Fuel (hi E) + O2 Æ
CO2 + H2O + heat
Can calculate heat of combustion by subtracting amount
of energy it takes to break fuel molecule bonds from amount of
energy released when carbon dioxide and water made.
Must put E in to get more E out.
Organic Chemistry
The chemistry of carbon compounds.
Notice position of C on Periodic Table.
Upper right area of table along with H (hydrogen) forms
the "organic block."
These atoms all have medium to high electronegativity.
Must share electrons to achieve octet config. around atoms.
No transfer of electrons possible from one atom to another because
all atoms too electron greedy.
Carbon catenates (bonds to self) because it has exactly
mid-range electronegativity. Can't decide whether it prefers
to steal or dump electrons so it is more suited to share than
any other atom on the table.
Catenation results in "chains" of carbon
atoms.
Hydrogens (or other atoms) attached in such a way that
carbon always has a valence of four (all C have 4 bonds). Can
have double & triple bonds between carbon atoms.
Hydrocarbons
Compounds made of hydrogen and carbon only.
Alkanes: single bonds only; alkenes:
at least one C-C double bond; alkynes: at least 1 C-C triple
bond; cycloalkanes: rings (circles) of carbons with only
single bonds; cycloalkenes or cycloalkynes (rare): double or triple
C-C bond connects at least two ring carbons together;
aromatics: made of special kinds of rings known as "benzene"
rings.
Alkanes
Can be either straight-chain or branched.
AKA "saturated" hydrocarbons. Saturated
means no double or triple bonds (only single bonds). Saturated
hydrocarbons can be made from alkenes or alkynes by "saturating"
the double and triple bonds with as much hydrogen (H2)
as they can react with.
Each molecule of H2 destroys one
C-C bond in one molecule of hydrocarbon. Hydrogen only reacts
with double or triple C-C bonds; leaves all single bonds alone.
Except for cycloalkanes, all alkanes have twice as
many hydrogen atoms as carbon atoms plus two extra hydrogens (CnH2n+2).
Each ring or double bond gives molecule 2 less H's, and each
triple bond gives molecule 4 less H's.
ALKANE NOMENCLATURE
Organic nomenclature works best when names are constructed
or deconstructed backwards.
Names of first ten linear open-chain (not cyclic) alkanes
given on previous page. To name branched alkanes:
1. Count the number of carbons in the longest catenated carbon
chain and use corresponding linear alkane name as suffix
(9 carbons - nonane).
2. Number the carbons in longest chain in one of two possible
directions. Add up carbon numbers where branches are attached.
nonane
3. Add up numbers of carbons where branches are (two branches
count twice). Left-to-right numbering totals: 4 + 5 + 7 + 7 =
23.
4. Number the carbons in opposite direction & add up branch
numbers.
nonane
Right-to-left numbering totals: 3 + 3 + 5 + 6 = 17.
5. Numbering which gives lowest total is correct (right-to-left).
6. Branches called "substituents." They are named
like linear hydrocarbons with same number of carbons, but the
suffix "yl" is added to their names. Collect their
names under numbered structure and put their position numbers
in front of them with a dash (ie. 5-ethyl).
7. Organize substituents in alphabetical order; place names
& numbers before chain name (suffix). Substituents with same
name not named over and over - use "di," "tri,"
"tetra," etc. Numbers separated from other numbers with
commas & letters separated from numbers with dashes. Letters
are run together into single large words.
ALKENE NOMENCLATURE
1. Carbon chain for suffix must contain the C-C double
bond. Number longest chain thru C=C & give C=C
carbons lowest possible numbers.
2. Suffix of name is formed by replacing "ane" of
longest chain alkane name with "ene." Next put number
of lowest-numbered carbon involved in double bond in front of
this name with a dash.
3. Now finish up by doing steps 6 & 7 in alkane nomenclature
section.
Notice that "di" in 5,5-dimethyl doesn't
count when substituents are listed in alphabetical order.
NAMING ALKYNES
Follow the procedure for alkenes but use "yne"
in name rather than "ene."
NAMING CYCLIC HYDROCARBONS
Put "cyclo" in front of the suffix name.
Suffix name and numbering count only the carbons contained in
the ring.
INTERPRETING HYDROCARBON NAMES
To deconstruct a hydrocarbon name simply work the above
procedures in the same order but create structures from
names rather than names from structures. Remember: always work
backwards.
PROBLEM 18b
4,4-dimethyl-5-ethyloctane:
PROBLEM 18c
2-methyl-2-hexene:
ISOMERS
Different chemicals whose molecules have the same molecular
formula (same number and kinds of atoms) but different atom arrangements
(which atoms are attached to which and how) known as "isomers."
Classes of isomers: structural, stereo, geometric,
and optical.
Example "structural" isomers:
Example "geometric" isomers:
Stereoisomers can be either
geometric isomers or "optical" isomers. Optical isomers
will show up in Chapter 15 (Biochemistry).
CIS AND TRANS GEOMETRIC ISOMERS
"cis" (same side) and "trans" (across
from or opposite side) normally used to describe two functional
groups attached to different positions on an alkene.
Aromatic Compounds
A hydrocarbon with 6 carbons catenated to each other
in a circular arrangement (known as a "ring") which
has alternating single and double bonds between the carbons has
the special name "benzene."
Benzene has special and peculiar properties. C-C single
and double bonds are hybrid between 2 structures.
Each C-C bond in benzene is equivalent. Neither single
nor double.
In some ways (bond length) C-C bonds behave like 1.5
bonds. However, benzene is much harder to burn than any
other kind of hydrocarbon.
Bonding & chemical behavior of benzene is really
unusual.
Because of unusual behavior and bond lengths of benzene
we cannot draw normal bonds between carbons in this molecule.
To indicate that a special and unusual kind of bonding happens
between the carbons in benzene we usually draw the molecule with
a circle rather than single and double bonds.
Benzene the simplest of a class of compounds all having
six-membered rings with the wierd kind of bonding benzene has.
These compounds have one or more of the hydrogen atoms in benzene
replaced by various "functional groups". These compounds
called "aromatic" compounds because often have unusual
smells (sometimes obnoxious, sometimes fruity). Special kind
of bonding between carbon atoms here compounds also called aromatic.
PROBLEM 28
What is difference between benzene and cyclohexane?
| Benzene
| Cyclohex. |
| C-C bonds
| aromatic | single
|
| shape
| flat | 3-D
|
| formula
| C6H6
| C6H12
|
Cyclohexane has two possible 3-D shapes (called "boat"
and "chair;" see pg. 356); benzene is flat as a pancake.
AROMATIC NOMENCLATURE
Text (p 359) has other examples.
Alcohols and Ethers
Alcohols and ethers used in gasoline formulations to
make gasoline burn more smoothly (slower) and more cleanly (less
unburned hydrocarbon "smoke").
Alcohols and ethers are hydrocarbons which have one
or more CH2 fragments replaced by oxygen atoms
Alcohols have C-O-H bonding pattern and ethers have
C-O-C pattern. C has 4 bonds, H has 1, and O has 2 bonds.
Petroleum
Composed mostly of straight-chain, branched, and aromatic
hydrocarbons. Most of these hydrocarbons contain more than 6
carbon atoms. Some contain hundreds of carbons. Very complex
hydrocarbon mixture.
PETROLEUM TECHNOLOGY
Petroleum refining: Separation of a variety of different
hydrocarbon mixtures with different boiling points from petroleum
by distillation. Larger hydrocarbons have higher boiling points
than smaller ones. "Straight-run" gasoline is made
this way.
Catalytic cracking: More commonly crude petroleum is
distilled while reacting with a catalyst that helps break down
("crack") large hydrocarbon molecules (make smaller
ones).
Catalytic reforming: A particular petroleum fraction
(typically gasoline) is redistilled from a different catalyst
which turns linear hydrocarbons into branched hydrocarbons. Alternatively,
catalyst may be selected that turns alkane (saturated) molecules
into alkenes, alkynes, and aromatics (unsaturated). Reforming
makes a slower-burning gasoline.
Oxygenation: Gasoline is mixed with alcohols and/or
ethers so that it will burn smoother (slower).
Reformulation: Catalyst is chosen which saturates hydrocarbon
molecules (opposite of reforming). Oxygenation is then done to
slow down burn rate of gasoline.
Reforming makes slower-burning gasoline by creating
slower-burning hydrocarbons and reformulation & oxygenation
use (or make) fast-burning hydrocarbon mixture and slow down burn
rate with additives.
Octane Rating
Problem 39.
How is octane rating determined?
Based on amount of knocking in a high-compression high-performance
engine running fuel-rich mixtures (too little air in carbueretor).
Fuel mixtures of heptane and "isooctane" (2,2,4-trimethylpentane)
used for developing scale. Engine performance given value of
0 for pure heptane fuel (0% isooctane, poor fuel) and 100 for
pure isooctane. Ratings > 100 if fuel better than isooctane
(superior fuel).
Why do oxygenated compounds, unsaturated hydrocarbons,
branched hydrocarbons and aromatics burn slower (higher
octane) than straight-chain hydrocarbons (Prob. 37)?
When C-H bonds are broken and replaced by O-H and O-C
bonds more energy released than when C-C bonds broken and replaced
by twice the number of C-O bonds.
Calculate 2 O + C-H Æ
C-O + H-O
Compare 2 O + C-C Æ
2 C-O
H is tastier for the demon oxygen; releases more energy
than C when burned. The more saturated a hydrocarbon is (the
more H relative to C) the hotter (faster) it burns. Saturated
hydrocarbons burn hotter (faster) than unsaturated hydrocarbons
and super-stable aromatics.
Oxygen-containing compounds are already partly "burned."
They have less E to release if burned totally.
Interestingly oxygen-containing compounds initiate
more easily than hydrocarbons & burn more completely even
though they burn slower.
Branched hydrocarbons have H's more accessible than
C's to demon oxygen. O gobbles up H's, is then stuck with slower
C atoms. May leave soot.
Compare:
H-CÙC-H + 2.5
O2 Æ 2 CO2
+ H2O
H3C-CH3 + 3.5
O2 Æ 2 CO2
+ 3 H2O
Natural Gas
Made mostly of methane (CH4).
Burns completely (cleanly) because oxygen doesn't need to break
any C-C bonds. Even if oxygen is messy eater, once all H's have
burned off left with one lonely carbon; duck soup. Maximum H/C
ratio (4:1). Burns hot (also tends to drive reaction to completion).
Only fuel cleaner than CH4 is H2.
Oxygen loves this stuff; no wasted carbons here! Hydrogen technology
may provide energy in future. Problem is where do we get energy
to make hydrogen?
Bad news is natural gas is in most limited supply of
all fossil fuels. Good news is it can be made from biomass, and
even from coal (coal abundant, but making natural gas a tad costly).
Coal
Coal is most abundant and dirtiest of fossil fuels.
Contains sulfur (burned sulfur-PU!). Also coal has lots of aromatic
rings, very low H/C ratio (oxygen will play fussy eater with it),
and C atoms are so thoroughly bonded to neighbors all around them
that its tough for O to break all bonds necessary to completely
burn the stuff. Will leave lots of unburned residue (soot).
Coal burns hotter than liquid fuels because even though
H/C is low and each average burned bond releases less heat than
liquid fuels, coal is very dense (more stuff and more bonds to
burn in smaller space). This means a certain volume of it will
release lots more heat when it burns. Intense (concentrated)
heat can melt metal. Useful historically for making steel.
COAL PROCESSING
In today's environmentally-conscious world doesn't
make sense to burn coal without extensive cleanup (sulfur) and
special apparatus to make sure it burns completely (need high
oxygen concentration; either pure oxygen or high pressure air).
Often makes more sense to turn coal into something cleaner if
you want to burn it.
Coal gasification: Older process produces CO (carbon
monoxide) plus H2 mixture known as "synthesis
gas." This mixture exciting because it can be converted
into methanol. Methanol is a "gateway" chemical which
can be converted into every other substance ordinarily made from
petroleum (ie. gasoline & organic "petrochemicals").
Newer gasification process produces natural gas directly
from coal.
Coal liquefaction: Coal + H2Æ
gasoline
Alcohol Motor Fuels
Alcohols added to fuels make them burn cooler, smoother
(slower), and cleaner (more completely). Also colder burn produces
less nitrogen oxides (they're unstable; nitrogen only burns at
high temperatures).
Conventional engines designed to burn gasoline need
to be redesigned to burn alcohol ; can't just burn high alcohol
content fuel in normal cars without causing higher level of wear
and tear.
Cars designed to burn either high-alcohol fuels or
gasoline are called Flexible-Fueled Vehicles (FFV's, Problem
45).
FUEL DESIGNATIONS
(Prob. 45)
M (methanol) or E (ethanol) followed by a percentage
number. Remainder of fuel is gasoline. Currently available:
E85 (85% ethanol, 15% gasoline), M85 (85% methanol,
15% gasoline), E100 (100% ethanol), M100 (100% methanol), and
"gasohol."
Gasohol is 10% ethanol and 90% gasoline. Not called
E10 because it was introduced before the "E" and "M"
designations were created. Supposedly burns OK in conventional
gasoline engines (mostly gasoline), but some users complain of
increased engine wear burning gasohol in conventuional engines.
Comments?
Last Revised : Sunday, October 5, 1997
Copyright © 1997
Louisiana State University, Department of Chemistry.
All rights reserved.
http://www.chem.lsu.edu/lucid/courseinfo/chem1002/ch12.html
