Answer Key
Chem 165 Spring2014
Chem 165 Problem Set #1 due at the start of class Monday, April 7
1. (a) Problem 12.51 (text p. 567). Show your work; the numerical answer is on p. A.68.
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g)
Using the philosophy of “Average Bond Enthalpies” (which I do not agree with, see below),
2 C–C and 8 C–H and 5 O=O bonds are cleaved while 8 H–O bonds and six C=O bonds are
formed. Using values in Table 12.5 (p. 550),
∆H° ≅ [2(348) + 8(413) + 5(498)] – [8(463) + 6(728)]
= [696 + 3304 + 2490] – [3704 + 4368] = 6409 – 8072 = –1,663 kJ/mol
(b) Now calculate the heat of reaction from the real values, using Hess’ Law (Section 12.6, for
instance Figure 12.17 and equation 12.12 [which are equivalent]). The best available values
for heats of formation, of propane and the other compounds, can found in the National
Institutes of Standards and Technology (NIST) online Chemistry Webbook,
http://webbook.nist.gov/chemistry/. Note that the reaction makes H2O gas, not liquid, so be sure
to use the heat of formation of water in the right phase.
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (g)
The heat of formation of O2 (g), as a pure element, is by definition zero.
∆H° = 3∆H°f(CO2) + 4∆H°f(H2O (g)) – [∆H°f(C3H8) + 5∆H°f(O2)]
= 3(–393.5) + 4(–241.8)] – [–104.7 + 5(0)]
= –1180.5 – 967.2 + 104.7 = –2,043 kJ/mol
(i) Without looking it up, what is the standard heat of formation of O2?
(ii) How do the NIST values compare with those in Appendix D of our text?
The heat of formation of O2 (g), as a pure element under standard conditions, is by definition
zero.
The NIST values and those in Appendix D are pretty close, but not the same. The values get
revised every so often, and the NIST ones are up to date.
(c) How different are the answers to (a) and (b), in percentage terms? The reason for the
difference is average bond energies are only a fair approximation of the actual bond
dissociation energies (BDEs). For instance, see the table of BDEs of C-H bonds below.
name
Bond
BDE
methane
H3C–H
440
ethane
CH3CH2–H
422
propane
CH3CH2CH2–H
422
propane
(CH3)2CH–H
412
isobutane
(CH3)3C–H
400
(i) Why are there two entries for propane?
There are two different entries because there are two different kinds of C–H bonds.
(d) A common reaction of alkanes is their transfer of a hydrogen atom to an oxygen radical RO•.
R–H + RO• à R• + ROH
This is a key reaction in flames, for instance. We abbreviate the alkane as RH, where R is an
alkyl group, so one example of an oxygen radical would be methoxyl, CH3O•. Oxygen
radicals are very unstable compounds, which cannot be isolated or “put in a bottle” as
chemists say. The word “radical” implies (in organic chemistry parlance) that the species has
an unpaired electron, and does not obey the octet rule. In methoxyl, there are only seven
electrons about the oxygen. This is why oxygen radicals often react to form ROH, which
obeys the octet rule.
It has been found that oxygen radicals react at very different rates with different alkanes and
different C–H bonds within an alkane. Predict which alkane in the table above is most
Answer Key p. 2
Chem 165 Spring2014
reactive (reacts fastest), which is least reactive, and which kind of C–H bond in propane is
most reactive. Briefly explain your reasoning. You can assume that all RO• species react the
same way independent of what the R group is (this is a pretty good assumption).
The reaction shown involves cleaving a C–H bond and making an O–H bond. If all the ROH
compounds have essentially the same properties, then the differences in the enthalpy of
reaction will be the differences in the C–H bond strengths. The tertiary C–H bond in isobutane
(2-methyl-propane is its IUPAC name) is the weakest, so its reaction is most exothermic, most
favorable, and goes the fastest. The reaction with methane is the slowest, because methane
has the strongest C–H bond. RO• preferentially reacts with the secondary C–H bonds in
propane because they are the weaker bonds.
2. Problem 7.8.
3. Problem 7.12.
(a) 2,3-dimethyl-1,3-butadiene.
(b) trans,trans-2,4-hexadiene
(c) 2,2-dimethylbutane
(d) 2-methylpropene (not 2-methyl-1-propene because all propenes are 1-propene).
4. Draw the Lewis dot structure with all of the electrons for CHCl3 (chloroform) and sketch the
three-dimensional structure. What is the hybridization of the carbon atom?
(i)
H
H
:
C
: :
:
: Cl
Cl :
Cl
Cl
Cl
:
:
Cl :
C
3D structure
Lewis Dot structure
tetrahedral, sp3
hybridized C
Answer Key p. 3
Chem 165 Spring2014
5. For the following organic compounds, draw line structures (structures with carbons just
indicated as the vertices between line segments, without the letter “C,” and hydrogen atoms
not indicated):
(i) all the linear chain isomers of pentene – and name each isomer
(ii) all the isomers of pentene that are branched (not a linear chain) and still are alkenes
(iii) the five alkanes that have the same molecular formula as pentene, not worrying about
optical isomers
(iv) Which of these five alkanes from part (iii) are chiral? Are any of the pentenes chiral?
1-pentene
2-methyl-1-butene
trans-2-pentene
cis-2-pentene
3-methyl-1-butene
3-methyl-2-butene
cyclopentane, methylcyclobutane, ethylcyclopropane, and cis- and transdimethylcyclopropane are all cyclic alkanes with the formula C5H10 and are therefore isomers
of (noncyclic) pentenes.
None of the alkenes are chiral. Of the alkanes, only the trans-1,2-dimethylcyclopropane is
chiral.
(v) C6H5CH=CH2 (an ethylene with one hydrogen replaced by a phenyl group, called
styrene). Draw this molecule and give the molecular formula (CxHy) for styrene.
C 8H 8
Answer Key p. 4
Chem 165 Spring2014
6. (a) Draw the line structure of cholesterol (you can find it on the web and in the textbook). What
is its molecular formula?
(b) What do you think cholestane is? What is its empirical formula?
(a) This drawing is from Wikipedia,
http://en.wikipedia.org/wiki/Cholesterol
which also gives the molecular formula, C27H46O. You
can also find a drawing in Figure 7.33 in the text.
(b) Cholestane is the alkane with the same carbon
skeleton (no functional groups). It therefore has one
less oxygen (no alcohol) and two more hydrogen
atoms (from hydrogenating the alkene): C27H48.
7. Fats are tri-esters with long hydrocarbon chains, as described in on page 331. Consider the line
structure drawn for a major fat component of sunflower oil shown in the middle of that page,
containing three C18 fatty acids, called linoleic acid.
(a) There are potentially a number of geometric isomers of linoleic acid; which one is found in
biology? (A little searching on the web or in organic chemistry books will tell you this.)
Only cis double bonds are found in biological fats.
(b) When linoleic acid esters are “partially hydrogenated” in an industrial process (p. 331), two
isomers of the mono-alkene are formed. Which one has been in the news recently as being
bad for you? (You’ll see some products advertised as containing “none” of this.)
The artificial hydrogenation can isomerize the remaining double bond, so the product contains
both cis and trans fatty acids. There has been much in the news about how bad “trans” fats
are for you. This is what they are referring to, the trans alkene isomer of the partially
hydrogenated material.
8. (a) The heat of combustion of heptane and isooctane are –4501 kJ/mol and –5100 kJ/mol,
respectively. Isooctane (2,2,4-trimethylpentane, pronounced iso-octane) is a pretty good
model for the complex mixture of hydrocarbons that is gasoline. The density of these two
liquids (essentially the same for all liquid hydrocarbons) is 0.7 g/mL (kg/dm3). What is the
heat of combustion of one gallon of the two liquids? Are they essentially the same or quite
different? Why?
Answer Key p. 5
Chem 165 Spring2014
They are very similar because these solutions are basically closely-packed CH and CC
bonds. Looked at not from a molecular but from a less detailed perspective, isooctane and
heptane are really not very different, the first one with a H/C ratio of 2.25, the latter 2.29.
(b) The U.S. Congress (our representatives) mandated that all gasoline sold in the U.S. contain a
significant quantity of biomass-derived ethanol. Such “renewable” ethanol in the U.S. is
typically made from corn kernels, which is very inefficient because we throw away most of
the biomass of the corn. [In Brazil, however, they are making ethanol much more efficiently
from sugar cane.] Calculate the heat derived from combustion of a gallon of ethanol. Ethanol
has a liquid density of 0.789 g/mL and a heat of formation of –276 ± 2 kJ mol-1. Assume that
you start with liquid ethanol and make gaseous products (what would be alternative
assumptions?)
Start with the balanced equation for combustion of ethanol and put the heats of formation (all
in kJ mol-1) underneath:
C2H5OH + 3 O2
à
2 CO2 + 3 H2O
∆H˚f:
–276.2
0
2(–393.5)
3(–241.8) = –1,236.2 kJ mol-1
One gallon of ethanol is 3.7854 Liters and therefore contains 2,987 g of ethanol and 64.83
moles. (Since the density is only given to three significant figures, I prefer not to list any
answer to more than three or four “sig figs,” but it would be OK for you to carry more as long
as the final answer doesn’t have more than three sig figs.)
Combustion of one gallon of ethanol thus yields –8.01 x 107 J (–8.01 x 104 kJ).
Alternative assumptions would be to take the heat of formation of gaseous ethanol (fuel
injectors in most cars vaporize the material before combustion). One could also assume that
the product is liquid water, although in an ethanol flame the product clearly is gaseous water.
An ethanol flame also clearly makes CO2 and H2O at high temperature, so ∆H˚f is not correct
because the “˚” implies 298 K. Strictly speaking, one should measure or estimate the
temperature of the gases and calculate the heat used to get them to that temperature (using
the heat capacities of CO2 and H2O). Note that Henry’s law only holds at a fixed temperature.
However, the specific heats of gases aren’t very large so heats of formation don’t vary that
much with temperature and people typically just use the ∆H˚f values. All of these reasons,
Answer Key p. 6
Chem 165 Spring2014
however, imply that one shouldn’t report the results to too many significant figures: I’d
recommend 8.0 x 107 J.
(c) Compare the heat of combustion of a gallon of ethanol to that of a gallon of isooctane
(~gasoline). Do you think you can drive a car as far on a gallon of ethanol as on a gallon of
isooctane?
The energy density of ethanol is only 68.5% of that of isooctane, according to my calculations.
So no, you can’t drive a car as far on ethanol as on isooctane. You get poorer “gas mileage” in
gasoline with ethanol added.
(d) Nobel Laureate George Olah has written extensively about using methanol instead of
ethanol, and DuPont and BP and others are advocating for bio-derived butanol as a better fuel
than ethanol. Without calculating anything – just looking at the molecules and the products
of combustion and trying to see a pattern – can you guess what the order of energy density is
for the four molecules isooctane, ethanol, methanol, and butanol. Energy density is the heat
of combustion per gallon.
The energy density varies with the amount of oxygen – the more oxygen the material has,
the less energy that is derived from combustion with O2. The oxygen in the material becomes
part of the CO2 or water that is formed on combustion, but this part doesn’t generate much
heat since the C–O and O–H bonds are already present in the fuel. So the order is:
isooctane > butanol > ethanol > methanol
9. Look over the article “Net Air Emissions from Electric Vehicles: The Effect of Carbon Price and
Charging Strategies” S. B. Peterson, J. F. Whitacre, and J. Apt, Environ. Sci. Technol. 2011, 45,
1792-1797. The standard reference format is Journal Name publication year, volume, page
numbers (in this case, the full journal name is Environmental Science and Technology).
You can probably get this article in a printed copy of the journal in the UW Engineering
Library, but it’s much easier to download it from the Web. You can look up “Environ. Sci.
Technol.” in the “Journals” section of the UW libraries catalog and one of the entries will have a
link to “Full text available …” But much simpler is to just Google the journal title (or even the
abbreviated title) and that will take you to the ACS Publications web site (http://pubs.acs.org/).
At that site you’ll find a “Citation” link where you can enter the volume and page number for
this article.
Access to this journal (and most scientific journals) is not free – UW has purchased an online
subscription (your tuition $ at work!). If you access the journal from a UW computer or by
modem into a UW computer, the site will know automatically that you are a subscriber. If you
want to connect from a non-UW computer (e.g., from home), follow the instructions at:
http://www.lib.washington.edu/help/connect.html.
(a) This is a long article and I don’t expect you to read all of it. Try to answer the questions
below just from skimming the article, focusing on the Abstract, Introduction and
Conclusions. First, in a single sentence, what broad question(s) do the authors address?
The paper address the environmental impacts of the effects of switching a significant fraction
of our transportation fleet from fossil fuels to plug-in electric vehicles.
(b) The TV ads for the Chevy Volt and Nissan Leaf claim that this plug-in electric vehicles are
“zero emission.” Is that correct?
No, that’s mostly crap. The energy has to come from somewhere, and in the U.S. extra
electricity production is probably mostly from power plants burning coal or natural gas. (Here
in the Northwest, we are blessed with inexpensive power from hydroelectric plants, which
Answer Key p. 7
Chem 165 Spring2014
provide most of the baseline electricity needs. But any additional power we need will have to
come from other sources.)
(c) What’s NOX?
NOX is a shorthand for a mixture of the gaseous oxides of nitrogen. Sometimes I think this
means only NO and NO2, other times it can also include N2O5/HNO3 (nitric acid) [N2O5 + H2O
à 2 HNO3]. See http://en.wikipedia.org/wiki/NOx or
http://www.epa.gov/oaqps001/nitrogenoxides/
(d) Based on this paper, do you think it is easy to predict the environmental effects of switching
from petroleum-powered to “electric-powered” vehicles?
There are two types of answers to this general question. I think it’s pretty difficult to predict in
detail what the effects will be. That depends on a lot of factors. But pretty much all of the
scenarios that the authors ran resulted in less CO2, less NOx, and more SO2.
(e) Very roughly, what’s an “SO2 allowance” and why does it have price? Are there CO2
allowances? Why or why not?
There are limits on SO2 emissions, a total “allowance” of emissions. Companies that generate
SO2 emissions can trade those allowances, so that those facilities that can cut their emissions
easily can benefit by selling their allowances. Thus the market efficiently allocates the
allowances (at least in principle). See, for instance,
http://www.epa.gov/airmarkt/trading/factsheet.html or
http://new.evomarkets.com/index.php?page=Emissions_Markets. There is no such allowance
system for CO2 in this country because our elected politicians have decided not to institute
such a system. In general, some Democrats favor such a system while all Republicans
oppose it.
9. (a) Natural gas is viewed as a cleaner and less greenhouse-gas generating fuel than oil. In very
general layperson’s terms, why is that?
Natural gas makes significantly less CO2 per unit energy. In very rough terms, this is because
more of the energy from combusting hydrocarbons is in the making of water rather than CO2.
Natural gas is mostly methane, CH4, so two waters are made per CO2. Oil and gasoline have
roughly the empirical formula CH2, so only one water is made per CO2. Coal has the rough
empirical formula CH, so it makes even less water.
In terms of cleanliness, coal is an ugly material with lots of crap in it, including mercury,
arsenic, chlorides that form corrosive HCl upon combustion, and many other elements. “Clean
coal” is a bit of a misnomer – the emissions and slag can be cleaned up but that’s expensive.
Oil, as a liquid, is easier to clean up (to “refine” in a “refinery”). All crude oil is treated to
remove much of the sulfur, nitrogen, etc. Natural gas has the least amount of impurities in it by
virtue of it being a gas: only a limited number of compounds are gases at ambient
temperatures. There’s often some He in natural gas (this is where commercial He comes
from), some H2S (“sour gas”) and other obnoxious stuff, but they are relatively easy to
remove.
If you want a serious answer to this question, look at the article “Life-Cycle Greenhouse Gas
Emissions of Shale Gas, Natural Gas, Coal, and Petroleum” by A. Burnham* et al., Environ.
Sci. Technol. [an ACS journal] 2012, 46, 619–627, dx.doi.org/10.1021/es201942m. This “doi”
[digital object identifier] is the easiest way to search for an article – just put it into Google.
Answer Key p. 8
Chem 165 Spring2014
(b) There has been an huge increase in natural gas production in the U.S. in the last few years,
because of new technology (“hydraulic fracturing” or “fracking”) called to obtain gas trapped
in underground shale rocks (“shale gas”). The article “Petrochemicals” posted on the course
website with this homework describes the business perspective on this, at least from two
years ago. (i) What are “Petrochemicals”? (ii) U.S. production of ethylene is at least 10
million tons per year. Given the price difference quoted between naphtha-derived ethylene
and ethane-derived ethylene in this article, what is the order of magnitude of the money that
is involved in switching substantial ethylene production from naphtha to ethane? (iii) This
switching is not so simple, and in some cases new chemical plants will need to be built. A
typical chemical plant or oil refinery costs roughly $1B, and will run for ca. 50 years. If you
were the CEO [Chief Executive Officer] of a major chemical/oil/natural gas company, on
what basis would you decide to build a new plant? Look at the princes over time in the
Figure in the article; would this make you nervous?
(i) Petrochemicals are chemical derived from petroleum or natural gas.
(ii) the quoted price difference is 18.5 cents/lb. So if 10% of annual ethylene production
shifted, that would be 107 tons x 2,000 lb/ton x 18.5 ¢/lb ≅ four billion dollars per year.
(iii) $4B is a lot of cash, and Shell has just recently announced plans to build a new natural
gas “cracker” to make ethylene. But the price of ethane has varied by about a factor of three
just over the last four years, so how can you predict the value you’ll get from a plant that will
last 50 years? I guess this is one of the reasons why the CEOs of the major oil and chemical
companies make the big bucks.
(c) Please read the following “opinion pieces” in the “popular [non-scientific] press.” The first,
A Model for Reducing Emissions (March 19, 2013) is in the New York Times Business
Section, which (for a business section) is pretty liberal. The second is from the editors of The
Economist magazine, which is more on the conservative side (but in my view clear thinking,
rather than the anti-science perspective of some of the right wing in the U.S.). The third is an
“op ed” meaning it appeared on the page opposite the editorials. Please read them online at
the urls below, but rough pdf versions have also been posted on the course website.
http://www.nytimes.com/2013/03/20/business/us-example-offers-hope-for-cutting-carbon-emissions.html?ref=eduardoporter
http://www.economist.com/news/science-and-technology/21574461-climate-may-be-heating-up-less-response-greenhousegas-emissions?frsc=dg|a
http://www.nytimes.com/2013/03/14/opinion/global/the-facts-on-fracking.html?emc=eta1&_r=0
For another view, see: http://www.chevron.com/deliveringenergy/naturalgas/shalegas/.
Overall, what do you think?
I found the New York Times and Economist articles very interesting. Perez’s column seems
very convincing to me. As part (b) above indicates, there are huge amounts of money
involved. Money is the only thing that is really going to move the market, to make companies
and people change their behavior. And in our capitalist society, which I very much believe in, it
should be financial issues that lead companies to make billion dollar investments. If we as a
society want to influence the decisions that companies and people make, we need to do that
through government action, through a carbon tax (my preference) or tax credits or a ‘cap-andtrade’ system (as the European’s have done, with limited success).
I hadn’t seen the temperature data in the article in the Economist. That magazine is usually
very dependable, so I have no reason to doubt it, but I wonder why I haven’t seen this
elsewhere. Still, I completely agree with the basic point that our response to climate change
has to be in proportion to the anticipated problems. My sense is that globally we are still on
the “doing too little” side of the fence.
The Chevron web sight makes little effort to be “fair and balanced” (in the famous phrase from
Fox News). Chevron is investing in this area and I believe is a major producer. The
Answer Key p. 9
Chem 165 Spring2014
perspective of this site is that they are making money and we as consumers are benefiting
from cheap natural gas, so what’s the problem?
10. Read the article “Teaching Creative Science Thinking” in Science 2011, 334, 1499-1500.
Science magazine is the official publication of the American Association for the
Advancement of Science (AAAS). It publishes articles that report important new scientific
results, as well as perspectives, news, and articles on other topics. As above, you can search
for the journal online, or go to the journal homepage (http://www.sciencemag.org/) and
follow the “Advanced” link to enter the citation.
As a science student, what do you think of this article? Do you agree that science teaching in
schools is in need of improvement? What about at UW? Would you like to try some of these
things in 165?
A lot of this article rings true to me. But you (Chem 165 student) are probably a better judge of
this than I am.