11.3
6. When the test tube has cooled, remove it from the clamp and pour out the
contents onto a piece of paper. Observe and describe the final solid
product.
Analysis
(a) Interpret your Evidence, including hypotheses about possible products.
Section 11.2 Questions
Understanding Concepts
1. What is petroleum and why is it an important resource?
2. Is petroleum renewable? Provide your reasoning.
3. State and briefly describe the three main processes used in the
refining of petroleum to produce gasoline.
4. Are the fractions obtained in the fractional distillation of petroleum pure substances? Provide an example to support your
answer.
5. What are some useful products, other than gasoline, of the
refining operation?
6. How does the size of a hydrocarbon molecule relate to its boiling
point? What is the explanation for this relationship?
Making Connections
7. We depend on hydrocarbons as a fuel for a wide range of technological applications. Imagine that our supply of hydrocarbons
was suddenly removed. What would be some of the consequences? List at least 10.
11.3
Combustion of Hydrocarbons
You get up in the morning to reasonably warm surroundings, have a hot shower,
cook breakfast, and catch a bus to school. All of these activities directly or indirectly are made possible by the combustion of fossil fuels. Less obvious to us is
our dependence on the combustion of fossil fuels for the production of most
things that we possess: our clothes, shelter, books, and entertainment. Our way
of life depends on the combustion of hydrocarbons. If the sources of hydrocarbons were suddenly, or even gradually, cut off (as they eventually will be), our
way of life would dramatically change.
Complete Combustion of Hydrocarbons
Combustion is a very common hydrocarbon reaction, the one that is most
familiar to us. In ideal or complete combustion, a hydrocarbon reacts with
oxygen to produce carbon dioxide gas and water vapour. All hydrocarbons burn
in air provided they are warm enough to vaporize. If the vaporization temperature (boiling point) is low and sufficient oxygen is available, the hydrocarbon
burns so rapidly it forms an explosive mixture with air. Ninety-five percent of
petroleum ends up being used as fuels in combustion reactions to produce
energy. For example,
complete combustion: the reaction of
an element or compound with oxygen to produce the most common oxides, for example,
carbon dioxide, sulfur dioxide, nitrogen
dioxide, and water
2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g) + energy
Hydrocarbons 519
DID YOU KNOW ?
Cold Vapour
The white trails behind jet aircraft are formed
from condensed water vapour generated by
the combustion of jet fuel. These are not
unlike the vapour trails left by automobiles
on a cold day. Sometimes vapour trails will
abruptly end as the jet leaves a cold air mass
and enters a warm air mass.
Recall that in hydrocarbon combustion reactions, the physical states of reactants are given for SATP conditions (before any reaction), ignoring the fact that
a hydrocarbon must be vaporized to begin the reaction. The water produced is
always shown as water vapour, as it normally remains gaseous even after cooling.
So much carbon dioxide is produced by burning fossil fuels that human
activities are measurably increasing the concentration of this gas in our atmosphere. Many scientists are concerned that this may be contributing to an increase
in the average temperature of Earth. Of course, all the energy produced by
hydrocarbon combustion eventually transfers to the atmosphere. For example,
most Canadians know that winter temperatures are always several degrees higher
in and around large cities than they are in rural areas, due to emitted heat from
buildings, people, and vehicles.
The exploitation of fossil fuels has its consequences. On the one hand, inexpensive fossil fuels have contributed to our high standard of living. On the other
hand, we may be paying dearly for this good fortune. Environmental problems
such as global warming, rising costs of scarce resources, and shortages of raw
materials for the petrochemical industry are some of the disadvantages of our
dependency.
Practice
Understanding Concepts
1. List a major use and some minor uses of hydrocarbons.
2. Write the reaction equation showing the complete combustion in a
jet engine of tetradecane, C14H30(l), a hydrocarbon substance found in
jet fuel.
3. When the combustion reaction for one mole of a hydrocarbon is balanced with whole numbers, four moles of carbon dioxide and five
moles of water vapour have formed. Write the molecular formula for
this hydrocarbon substance.
4. State some risks and benefits of our reliance on the combustion of
fossil fuels.
Making Connections
5. Airplanes used by major airlines carry a large load of jet fuel. In the
event of a crash, especially on takeoff, airports need to be equipped
to deal with a jet fuel fire. Research the Internet to answer the following questions.
(a) What substances are used to fight jet fuel fires outside of the
airplane?
(b) Fire spreading inside the aircraft is another major concern of firefighters. What is done if this occurs?
(c) Some consumer groups are concerned about the availability of
suitable firefighting equipment and personnel trained to use it at
Canadian airports. Does the airport near your home or one that
you have used have firefighting equipment?
Follow the links for Nelson Chemistry 11, 11.3.
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Reflecting
6. How would your life be different without hydrocarbons? What would
you notice first, and what are some longer-term effects?
520 Chapter 11
11.3
Global Warming
The chemical products of the combustion of hydrocarbons can be variable. With
sufficient oxygen present, the combustion of hydrocarbons such as methane
produces carbon dioxide and water vapour:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Carbon dioxide and water vapour are the two most important greenhouse
gases. These gases absorb heat radiation (long wavelength infrared) from Earth
and emit some of this radiation back to Earth (Figure 1). The term greenhouse
effect refers to the trapping of the thermal energy in the atmosphere, producing
the same effect as the glass or plastic panels of a greenhouse. The greenhouse
effect is an important natural process that has made Earth habitable. Without
this process, Earth would be cooler, and life, if there were any, would be confined
to the warm rock beneath the surface. An important current issue is the concern
about an enhanced greenhouse effect in which additional carbon dioxide and
other greenhouse gases, such as methane, accumulate in the atmosphere and
increase the heating effect. The resulting increase in average global temperature,
called global warming, is considered by many scientists to be the most crucial
environmental problem in the world today.
(c) long wavelength
infrared radiation
emitted by
Earth
CO2
(d) Some of the
infrared radiation
is trapped by
greenhouse
gases.
(a) Sun’s radiation
(b) short wavelengths
absorbed
global warming: the increase in the
average temperature of Earth’s atmosphere
(e) As the concentration
of greenhouse gases
increases, more
infrared radiation
is trapped, and
the atmospheric
temperature increases.
Earth
Although scientists do not completely understand the cycle of carbon
dioxide on a global scale, there is little doubt that the concentration of carbon
dioxide in the atmosphere is steadily increasing (Figure 2, page 522). This
increase in CO2(g) concentration is largely attributed to the combustion of fossil
fuels: coal, oil, and natural gas. Can an increasing concentration of CO2(g) be
absorbed in the biosphere? If not, what effect will this have on the global temperature? Temperature increases of only a few degrees can cause a melting of polar
ice caps, a rise in sea levels, and significant climate changes. The lack of complete
Figure 1
Earth’s surface absorbs sunlight, then heats up
and radiates longer wavelength infrared radiation into the atmosphere. Greenhouses gases,
such as carbon dioxide, absorb this longer
wavelength radiation, thereby increasing the
temperature of the atmosphere.
Hydrocarbons 521
understanding of the natural carbon cycle means that we cannot be very certain about any predictions about global warming. Many people, including
some scientists, believe that the threat of the greenhouse effect is minimal.
Earth’s temperature has fluctuated in the past, for example, during the ice ages.
It is possible that, independent of human interference, the temperature of
Earth is increasing naturally. It is also possible Earth is in the midst of a cooling
trend and that the human-generated greenhouse effect is preventing another
ice age. Models of the atmosphere are complex but inadequate for making precise predictions. The capacity of the oceans to absorb higher levels of carbon
dioxide is not known, and the effect of an increased concentration of atmospheric carbon dioxide on plant growth is not predictable.
370
360
Alert, NWT
Figure 2
Annual concentrations of CO2, Mauna Loa,
Hawaii (1959–1996), and Alert,
Northwest Territories (1975–1996)
CO2 Concentration (ppm)
350
340
330
320
Mauna Loa, Hawaii
310
300
Pre-industrial CO2
290
280
0
Source: Carbon Dioxide Information Analysis
Center, Environment Canada
’59 ’61 ’63 ’65 ’67 ’69 ’71 ’73 ’75 ’77 ’79 ’81 ’83 ’85 ’87 ’89 ’91 ’93 ’95 ’97
Year
Although the rate and extent of global warming are difficult to predict, it
seems reasonable to reduce the production of greenhouse gases to avoid upsetting the delicate balance of the biosphere. We can create technologies to switch
from high-carbon fuels to low-carbon fuels and use conventional fuels more efficiently. We can also practise energy conservation and exploit energy sources that
do not produce carbon dioxide, such as solar energy, wind power, fuel cells, and
photovoltaic cells.
Canada, with about 0.5% of the world’s population, produces about 2% of
the world’s total greenhouse gas emissions. In 1997, Canada was one of over 160
countries to adopt the Kyoto Protocol, which commits industrialized countries
to binding targets for reduction in greenhouse gas emissions. The emissions of
six gases will be affected: carbon dioxide, methane, nitrous oxide, and three halocarbons used as substitutes for chlorofluorocarbons (CFCs). As part of the agreement, Canada has committed to reducing its 1990 level of greenhouse gas
emissions by 6% by the year 2012. Countries that do not meet their own emission targets can strike deals with nations that do better than required and buy
their excess “quota,” a process known as “carbon trading.” In 1997, Canada
emitted about 682 Mt of carbon dioxide, making us the eighth largest greenhouse gas emitter in the world, and the fourth largest on a per capita basis.
Several factors contribute to Canada’s greenhouse gas emissions:
522 Chapter 11
11.3
• Canada’s population is distributed over an enormous area, creating significant transportation needs for people and goods.
• Canada’s northern climate places a heavy demand on energy consumption.
• A large part of Canada’s economy is resource based, which means a high
demand for energy, for example, in smelting ore.
If Canada continues with “business as usual,” it will fall far short of its target.
Canada will need innovative technological solutions to slow the trend of climate
change and to adapt to changes we cannot avoid. Equally important are changes
in the attitudes that lead us to squander energy resources.
Practice
Understanding Concepts
7. Is the greenhouse effect useful or harmful? Explain briefly.
Answers
8. How is carbon dioxide recycled in the biosphere?
11. (b) 2.74 g
9. How might Canada be affected if global warming effects become
significant?
(d) 7.33 g
10. Explain how a nation could benefit from “carbon trading.”
11. Coal (assume pure carbon) and natural gas (assume pure methane)
are two fossil fuels widely used in electric power-generating stations.
(a) Write the chemical equation for the complete combustion of natural gas.
(b) Calculate the mass of carbon dioxide produced for 1.00 g of
methane consumed.
(c) Write the chemical equation for the complete combustion of coal.
(d) To obtain the same energy as 1.00 g of natural gas, you need
about 2.00 g of coal. Calculate the mass of carbon dioxide produced for 2.00 g of carbon consumed.
(e) Which fuel, coal or natural gas, is better in terms of carbon
dioxide produced? Justify your answer.
(f) Suggest a reason why coal is still widely used in electric powergenerating stations, even in places where natural gas is available.
Making Connections
12. The Kyoto Protocol takes effect during the period 2008–2012. One
way in which countries like the United States and Canada can meet
their targets is the use of “emissions trading.” What does this term
mean and how does this trading work?
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Explore an
Issue
Take a Stand: How Can Technology
Help Canadians Reduce CO2 Gas Emissions?
As Canada strives to meet its target under the Kyoto Protocol, various
technologies are being considered to decrease our emission of greenhouse gases. Technological fixes are not the only answer; changes in
lifestyle can have a huge impact on how efficiently we use energy.
Working in small groups, research one of the technologies being
DECISION-MAKING SKILLS
Define the Issue
Identify Alternatives
Research
Analyze the Issue
Defend a Decision
Evaluate
Hydrocarbons 523
recommended as a means of reducing the production of greenhouse
gases, for example:
• more fuel-efficient cars
• high-efficiency natural gas furnaces
• improved insulation in buildings
• improved mass transportation
• home energy-saving technologies (e.g., set-back thermostats)
(a) Use library and Internet resources to collect information about
one of the technologies.
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(b) Before people will change their lifestyles or switch to a new
technology, they have to be convinced that there is a benefit.
Identify a target group and prepare an advertisement (poster,
video, audio, or Web ad) designed to promote the technology
you researched.
(c) In a separate report, identify your target group, explain why
you think your ad will be effective, and summarize any risks
and benefits of the technology that do not appear in the ad.
INQUIRY SKILLS
Questioning
Hypothesizing
Predicting
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
Investigation 11.3.1
Combustion of a Hydrocarbon
All hydrocarbons burn, but the characteristics of the combustion depend on
many variables. The purpose of this investigation is to determine some characteristics of the combustion of hydrocarbons. Complete the Analysis section of
the lab report.
Question
What are the characteristics of the combustion of paraffin wax and methane?
Experimental Design
Two hydrocarbons—paraffin wax, C25H52(s), and methane—are burned.
Observations of the burning are made to determine the characteristics of the
combustion.
Materials
eye protection
apron
paraffin candle on dish or tin lid
matches or lighter
large beaker (e.g., 1000 mL)
2 smaller beakers (e.g., 400 mL)
timer or stopwatch
index card
bent glass tubing
laboratory burner (connected to natural gas supply)
beaker tongs
paper towels
524 Chapter 11
11.3
Procedure
Part 1: Paraffin Candle
1. Light the candle in an area with little or no draft.
2. Record as many observations as possible about the candle flame.
3. Quickly move an index card into the middle of the candle flame and
remove it before it ignites. Record observations of the underside of the
card.
4. Invert the large beaker over the candle and determine the time the candle
continues to burn.
5. Examine the inside of the beaker and note any deposits.
6. Repeat steps 4 and 5 using the smaller beaker.
7. Remove the beaker and relight the candle. Place the short end of the glass
tubing close to the wick and gently blow air up into the candle flame
(Figure 3). Note any changes in the flame. Relight the candle if it goes out
and repeat as many times as necessary to be sure of the changes observed.
8. Place the short end of the glass tubing in the dark part of the flame just
above the wick (Figure 4). Hold it there for several minutes and describe
the vapour coming out the other end of the tubing.
9. When it looks like the maximum quantity of vapour is coming out of the
glass tubing, try lighting the vapour and observe. You may have to try this
several times.
10. Move the end of the tubing that is in the flame up near the tip of the
bright yellow region of the flame. Observe the appearance of the vapour,
and again try lighting it.
11. Remove the glass tubing from the flame and let it cool for a few minutes.
Record your observations of the outside and inside of the tube.
Long hair should be tied back
and loose sleeves should be
contained by sleeve protectors or a lab coat.
Be careful when handling
hot glassware.
Wash and dry the glass
tubing before use. Do not
share with others.
blow air
Figure 3
With a little practice, you should be able to
gently blow air into the candle flame without
extinguishing the flame.
Part 2: Laboratory Burner
12. Connect the lab burner to the natural gas supply and make sure that the air
vents are closed.
13. Light the burner and record your observations of the flame.
14. Clean the smaller beaker with paper towel and fill it to the half to threequarters level with cold water.
15. Using beaker tongs, hold the beaker over the flame. Remove the beaker and
observe and record any deposits on the outside of the beaker.
16. Slowly open the air vents on the burner and record changes in the flame.
17. With the burner adjusted to obtain the usual pale blue flame, repeat steps
14 and 15.
Analysis
(a) Describe the characteristics of the candle flame, including colour and temperature of different parts of the flame.
(b) What interpretation can be made about the times to extinguish the candle
in steps 4 and 6?
(c) For a normal yellow candle flame, what is one obvious product of the combustion? State any Evidence for other products.
(d) When extra air is supplied to the candle flame (step 7), the colour of the
flame changes. Provide an explanation.
Figure 4
Hold the glass tubing by the end, away from
the candle, and keep the short end in the
darkest region of the flame, just above the
wick.
Hydrocarbons 525
(e) Based on your Evidence from steps 8 to 11, what is actually burning in a
candle flame?
(f) How does the combustion in a laboratory burner compare with that in a
candle?
(g) In one or two sentences, summarize the combustion characteristics of the
paraffin wax in a candle.
(h) In one or two sentences, summarize the combustion characteristics of natural gas in a laboratory burner.
Incomplete Combustion of Hydrocarbons
incomplete combustion: the reaction
of an element or compound with oxygen to
produce some oxides with less oxygen content than the most common oxides, for
example, carbon monoxide and nitrogen
monoxide
Ideally, the combustion of hydrocarbons produces only carbon dioxide and
water. If, however, there is insufficient oxygen available, the combustion of
hydrocarbons can also produce carbon monoxide and/or carbon. This is called
incomplete combustion.
The larger the hydrocarbon molecule, the more likely there is to be incomplete combustion. This is why incomplete combustion of gasoline (e.g., octane)
is more of a technological and environmental problem than for natural gas
(methane). In each of the following chemical equations, the amount of oxygen
reacting with one mole of octane is different. Note that less oxygen means less
carbon dioxide and more carbon monoxide or carbon produced.
Complete combustion:
25
C8H18(l) + O2(g) → 8 CO2(g) + 9 H2O(g)
2
Some incomplete combustions:
C8H18(l) + 12 O2(g) → 7 CO2(g) + CO(g)+ 9 H2O(g)
23
C8H18(l) + O2(g) → 6 CO2(g) + 2 CO(g)+ 9 H2O(g)
2
C8H18(l) + 5 O2(g) → CO(g) + 7 C(s) + 9 H2O(g)
Incomplete combustion of a fuel such as octane is undesirable for several
reasons. It decreases the fuel economy of the automobile, because less energy is
produced, and the spark plugs may become fouled with carbon. Incomplete
combustion also produces toxic carbon monoxide. Let’s review the properties of
carbon monoxide: It is a colourless, odourless gas that is usually found in concentrations of a few parts per million in indoor and outdoor air. Parking garages
may have levels in the range of 10–20 ppm. Up to 30 ppm is considered a safe
level of continuous exposure. Automobile emissions in large cities during rush
hours cause health problems. If there is no wind and/or there is a temperature
inversion, then headaches (a symptom of mild carbon monoxide poisoning)
become common.
Automobile emissions of carbon monoxide have been reduced in newer
models; however, older models that are not kept tuned emit carbon monoxide at
a toxic level. Parking in a car with the motor running for extended periods of
time can result in carbon monoxide poisoning if there are leaks in the exhaust
system. Faulty exhaust systems have been the cause of deaths of children and
other passengers in the back seat.
526 Chapter 11
11.3
Carbon monoxide poisoning can happen in our homes too: Backdrafts
down the furnace chimney when you are using a fireplace can result in toxic
levels of carbon monoxide in a house. Using a briquette barbecue inside the
home can also result in toxic levels of carbon monoxide. Emergency spaceheating of homes with kerosene heaters can also be lethal. Many homes now have
carbon monoxide detectors. Outside the home, every year there are reports of
campers dying after using barbecues to heat their tents.
The symptoms of carbon monoxide poisoning include headache, fatigue,
and unconsciousness. Be aware of situations where incomplete combustion may
occur and watch for the symptoms. Some chemical knowledge could save your
life or the lives of your family and friends.
Practice
Understanding Concepts
13. If you were looking for evidence of incomplete combustion, describe
what you would expect to find while examining the flame of a furnace, the exhaust pipe of a car, and emissions from a smokestack.
Answer
17. 1.5 × 10–6 %
14. How do the products of complete combustion and incomplete combustion differ?
15. Without additional information, why is it difficult to write a balanced
chemical equation for the incomplete combustion of a hydrocarbon?
16. Heptane is a component of gasoline. Write a balanced chemical equation for the
(a) complete combustion of heptane;
(b) incomplete combustion of heptane, assuming equal amounts of
the two carbon oxides as the only carbon-containing products.
17. If the concentration of carbon monoxide from an untuned car engine
is 15 ppt, what is the percentage (parts per hundred) of carbon
monoxide in the gases at the exhaust pipe?
Making Connections
18. In some Canadian cities a bylaw requires homes to be equipped with
carbon monoxide detectors (Figure 5). How do these detectors work?
Do they have to be checked periodically? In a brief report, explain
whether you would be in favour or against such a bylaw if you
owned a home.
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19. A carbon tax has been suggested as a way of reducing carbon
dioxide emissions. Its proponents believe that people who pay more
for a resource will be more likely to conserve that resource. If a
carbon tax were added differentially to various fuels, in your opinion,
which hydrocarbons should be taxed the most?
Figure 5
A typical home carbon monoxide detector
Reflecting
20. List as many ways as you can think of for use of the word “burn” in
our daily lives. Do all of these uses convey the same meaning as the
term “combustion”?
Hydrocarbons 527
Section 11.3 Questions
Understanding Concepts
1. What are some of the consequences of incomplete combustion?
Write a sentence each from scientific, technological, economic,
and environmental perspectives.
2. Write the complete and one possible incomplete combustion
equation for the following reactions:
(a) butane in a barbecue lighter
(b) nonane in gasoline
(c) C24H50(s) in candle wax
Applying Inquiry Skills
3. Complete the Analysis and Evaluation sections of the following
report.
Question
What are the products of combustion of natural gas?
Evidence
A beaker of cold water moved through the flame condenses a clear,
colourless liquid, which when tested with cobalt chloride paper
turns the paper from blue to pink. No black deposit is observed.
An Erlenmeyer flask inverted over the flame is then stoppered.
When some limewater is added to the flask and shaken, the limewater turns cloudy.
Analysis
(a) According to the Evidence collected, what is the answer to
the Question?
Evaluation
(b) Evaluate the Evidence collected. For example, is it possible to
determine whether the combustion was complete?
(c) Suggest an improvement to this experiment to be more certain about the type of combustion.
11.4
Alkanes and Cycloalkanes
Why is octane rating based on a chemical called 2,2,4-trimethylpentane when
octane is just a simple string of eight carbon atoms? Why is the chemical in a
butane lighter called methylpropane or isobutane rather than just butane? What is
the proliferation of names all about? The answer to these questions is that, so far,
your study of hydrocarbons has been restricted to an emphasis on the first 10
straight-chain alkanes: methane through decane. In order to look at the real impact
of hydrocarbons on our everyday life, we now look at branched and cyclic alkanes.
However, the restriction to aliphatic hydrocarbons remains; aromatic hydrocarbons other than the parent aromatic, benzene, are left for your next course.
An analysis of hydrocarbon samples from crude oil and natural gas reveals
evidence that needs to be explained. Careful fractional distillation of the samples
in an organic chemistry laboratory reveals precise fractions that have identical
empirical formulas but different physical and chemical properties. For example,
two fractions with the same empirical formula of C4H10(g) have different boiling
528 Chapter 11