Modern Engineering Thermodynamics

15.3 Organic Fuels 597
ANSWERS SOMETIMES COME IN DREAMS!
In the early part of the 19th century, chemists were puzzled by the fact that it was possible to construct two seemingly
different compounds that had dissimilar physical properties yet identical chemical formulae. For example, ethanol
and dimethyl ether both have the same chemical formula, C 2 H 6 O, yet ethanol boils at 79°C while dimethyl ether boils at
−24°C. Materials that have the same chemical formula but dissimilar physical properties are called isomers. The isomer
puzzle was solved by the German chemist Friedrich August Kekulé (1829–1896) in a dream while dozing on the top deck
of a horse-drawn bus in London. In his dream, he realized that the atoms of a molecule could be arranged in different geometric
structures; and in 1858, he introduced the schematic notation, still used, in which bonds between atoms are represented
by lines drawn between their corresponding chemical symbols (e.g., H 2 as H—H). He was then able to show that
isomers were simply the result of different bonding patterns. For example, butane (C 4 H 10 ) has the isomers n-butane, which
boils at −0.5°C, and isobutane, which boils at −12°C. (The n prefix is always used to denote the normal, or chainlike, structure,
whereas the iso prefix is used to denote the branched structure.) These two isomer bonding patterns are shown in
Figures 15.2 and 15.3.
H H H H
C C C C
H H H
H C C C
H
H
H
H H H H
n-Butane
(normal butane)
(C 4 H 10 )
FIGURE 15.2
n-Butane and isobutane.
H H
H C H
H
Isobutane
(C 4 H 10 )
H
H H
C C
H H
H
Ethane (C 2 H 6 )
(alkane, saturated)
H
H
H
C C H C C H
H
Ethylene (C 2 H 4 )
(alkene, unsaturated)
Acetylene (C 2 H 2 )
(alkyne, unsaturated)
FIGURE 15.3
Ethane (alkane, saturated), ethylene (alkene, unsaturated), and
acetylene (alkyne, unsaturated).
Another 19th century hydrocarbon curiosity was the existence of the two classes, aliphatic (fatty) and aromatic (fragrant)
compounds. Aromatic hydrocarbons always had at least six carbon atoms and a smaller proportion of hydrogen atoms
than the aliphatic hydrocarbons. In 1865, Kekulé again found the solution in a dream. He envisioned a six-carbon chain
closing on itself to form a ring, like a snake biting its own tail, and he concluded that the aromatic compounds contain
such rings whereas the aliphatic compounds contain only straight chains.
Within the aliphatic group, the alkanes are characterized by having carbon atoms with single bonds between them, while
the alkenes have carbon atoms with double bonds between them, and alkynes have carbon atoms with triple bonds between
them. If all the bonds within an organic compound are single, then the compound is said to be saturated; but if multiple
bonds exist between any two carbon atoms in the compound, it is said to be unsaturated. Thus, the alkanes are all saturated
hydrocarbons, while all the remaining hydrocarbons are unsaturated.
Also, its combustion with excess air using 100.(x) percent theoretical air (i.e., 100.(x − 1) percent excess air),
where x ≥ 1.0, is
C n H m + xn+ ð m/4Þ½O 2 + 3:76ðN 2 ÞŠ ! nðCO 2 Þ + ðm/2ÞðH 2 OÞ
+ ðx − 1Þðn + m/4ÞðO 2 Þ
+ xð3:76Þðn + m/4ÞðN 2 Þ
(15.3b)
And its combustion in deficit air using 100.(y) percent theoretical air (i.e., 100.(1 − y) percent deficit air), where
ð2n + mÞ ÷ ð4n + mÞ ≤ y ≤ 1:0, is
C n H m + yn+ ð m/4Þ½O 2 + 3:76ðN 2 ÞŠ ! nð2y − 1Þ − mð1 − yÞ/2 ðCO 2 Þ
+ ð2n + m/2Þð1 − yÞðCOÞ + ðm/2ÞðH 2 OÞ
+ yð3:76Þðn + m/4ÞðN 2 Þ
(15.3c)
In Eq. (15.3c), it has been assumed that the hydrogen is much more reactive than the carbon, so it will take up all
the oxygen it needs to be converted into water. This leaves only the carbon subject to incomplete combustion.