Chemistry-for-the-IB-MYP-4-5-pdf

How important are biological
contributions to global systems?

HOW THE ATMOSPHERE INTERACTS
WITH THE WATER CYCLE

The water cycle (Figure 9.14) describes how water circulates around the Earth.
The driving force behind the water cycle is the thermal radiation from the Sun.
Sources of water vapour in the air include evaporation of water from oceans, seas
and lakes, from inside leaves of plants (via transpiration), and as a reaction product
of respiration and combustion (burning). Sinks for water vapour take the form of
precipitation, rain, hail or snow, caused when water vapour cools and condenses.
The first tiny droplets of water form clouds and eventually precipitate and gather
into streams and rivers and then on into lakes, seas and oceans.

Water is very good at dissolving substances due to its polar nature. As water
droplets move through the air, as clouds blown by wind or as precipitation, they
absorb substances like carbon dioxide from the air, to form carbonic acid, a weak
acid. On land, rain may wash soluble chemical fertilizers off farmland and add
nitrate ions (NO3−) and phosphate ions (PO43–) to the rivers; these ions are
present because of the use of artificial fertilizers such as ammonium nitrate and

501

ammonium phosphate. Storm water may also contain human waste, as well as
insoluble impurities such as grit, bacteria, oil, lead and dust from the exhaust fumes
of vehicles.

ACTIVITY: A longitudinal stream study

ATL

• Collaboration skills: Help others succeed; delegate and share responsibility
for decision making

• Organization skills: Set goals that are challenging and realistic

Safety: Wear gloves and wash hands after this activity. Assess risk factors such
as managing responses to cuts and grazes, avoiding sunburn and water
awareness.

To understand global impacts we need to begin with local impacts. In this activity,
you will use samples from a local water source to evaluate the way that local
effects can have a wider impact on ecosystems.
Plan a collaborative investigation of local water resources (lakes, rivers,
reservoirs, canals, ponds or streams) or from different locations along the same
source, ideally one which provides opportunities to compare evidence of
disturbance such as changes in nearby land use or waste discharge. Sampling
should include at least two or three of these local sources.
Predict the impact of these conditions on the physical, chemical or biological
variables you plan to investigate.
When reporting and analysing your results, link evidence of variation to possible
environmental impacts.

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion B:
Inquiring and designing and Criterion C: Processing and evaluating.

502

Field studies and the experimental method

Field studies require data to be collected outside an experimental setting, often in
natural environments. Because natural environments are complex, it may be
difficult to control variables, or repeat longitudinal studies. Results may show
‘correlations’, rather than direct ‘causal’ relationships between causes and
effects. A causal link means that one variable directly causes a change in another;
correlation means that the change in one variable occurs with a change in another
variable.
The scientific value of a field study is that it is an ‘authentic’ context. Field studies
augment designed investigations, for example by evaluating a model, and their
very complexity can reveal unexpected outcomes that suggest new directions for
research.

ACTIVITY: Investigating pollutants dissolved in
water

ATL

• Critical-thinking skills: Gather and organize relevant information to form an
argument

• Organization skills: Create plans to prepare for summative assessments
(examinations and performances)

What are the consequences of water’s ability to accrete soluble pollutants as a
result of the water cycle? How could these dilute solutions affect living or built
environments over time?
Design an investigation of how pollutants in water may interact with living plants
or minerals. Refer to the experiment investigation cycle in Figure 1.11. A risk
analysis and environmental impact analysis must be included in your plan.

Assessment opportunities

• This activity can be assessed using Criterion B: Inquiring and designing and
Criterion C: Processing and evaluating.

503

Take action: Do we need to drink bottled water?
ATL

• Collaboration skills: Encourage others to collaborate
• Information literacy skills: Make connections between various sources of

information
• As a class, collect the labels from bottles of mineral water (Figure 9.15).

504

1 Compare the dissolved substances listed on the different brands of mineral
water. Make a scientifically supported judgment about their contribution to
health.

2 ‘Food miles’ are the distance a consumable such as food travels from where it
has grown to where it is eventually purchased and consumed. These transport
costs contribute considerably to global emissions.
List the mineral water brands, from those which have the highest to lowest
environmental cost, according to this definition. Make a scientifically
supported judgment about the environmental cost of using imported and local
bottled mineral waters.

3 Most drinks are sold in plastic bottles made of PET (polyethylene
terephthalate), produced from crude oil (petroleum).
Evaluate whether replacing this material with an alternative carries fewer
environmental costs.

4 Present and promote your findings about local mineral water consumption and
the decisions you have made to improve the environmental impact of your
school and local community.

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion A:
Knowing and understanding and Criterion D: Reflecting on the impacts of
science.

505

HOW THE ATMOSPHERE INTERACTS
WITH NUTRIENT CYCLES

Nitrogen gas has no smell and is colourless. The molecule consists of two nitrogen
atoms covalently bonded together by a strong triple covalent bond. Nitrogen gas
forms about 80 per cent of the air and is very unreactive. Few living organisms can
use it directly, but it is an essential element in proteins, nucleic acids and
biological compounds found in all forms of life. Nitrogen, in the form of nitrates
(NO3−), is therefore often the limiting nutrient controlling the amount of matter
transferred in biological cycles such as food chains.
The main source of atmospheric nitrogen is from anaerobic, denitrifying bacteria
(Figure 9.16).

506

Nitrogen is removed from the atmosphere by several important processes, both
natural and anthropogenic. Nitrogen fixation involves nitrifying bacteria, which are
found in soil and in special nodules on legumes (plants like beans and clover).
Their enzymes can convert elemental nitrogen from the atmosphere into nitrates via
oxides of nitrogen; the nitrates then dissolve in water in the soil.

Direct reactions between nitrogen gas and oxygen to form nitrates can only occur at
very high temperatures. Examples include lightning strikes, and the reactions within
internal combustion engines, where the energy from the spark plug helps convert
some of the nitrogen in air to nitrogen oxides (NOx). However, modern catalytic
converters prevent the emission of these polluting nitrogen oxides in exhaust fumes
by converting them back to nitrogen gas before they leave the exhaust pipe.

507

The Haber process (Chapter 11) is an industrial process for ‘fixing’ nitrogen from
the air and making ammonia. A small amount of nitrogen gas is also extracted by
industrial purification using the fractional distillation of liquid air.
A Flash-based animated version can also be found here:
www.sumanasinc.com/webcontent/animations/content/phosphorouscycle.html

Phosphorus – an essential nutrient

Like nitrogen, phosphorus is an essential nutrient for all organisms. The amount of
available phosphorus limits how much matter can move through food chains.
The source of inorganic phosphate is from the release of phosphate ions (PO43–)
as a result of the weathering of rocks by rain, and they are also dispersed by water
(Figure 9.17). The quantities of phosphorus compounds in soil are generally small.
Outside the range of pH 4 to pH 8, inorganic phosphorus can be chemically bound
(adsorbed) to soil particles, making it less available to plants. Some species of
plants have relationships with other organisms, usually fungi, to help them extract
these limiting phosphates.

The sink for phosphates is plants, which absorb inorganic phosphate and are eaten
by animals. When organisms die and decay, or produce wastes, the phosphate is

508

returned to the soil. Organic forms of phosphate can be made available to plants by
bacteria that break down organic matter to inorganic forms of phosphorus, a
process known as mineralization.
If the plants are harvested and removed, or phosphate is the limiting nutrient in an
agricultural system, humans may apply phosphate fertilizers to replenish soils. This
additional phosphorus is often mined from non-renewable sources such as bird and
bat guano, sometimes thousands of years old.
Eventually, excess phosphate is leached into waterways and oceans, where it can
be incorporated into sediments over time. However, like the geological cycling of
organic carbon compounds, the extraction and use of phosphorus is effectively a
one-way process. Globally, phosphorus resources are being used much faster than
new supplies are forming.

ACTIVITY: Evaluating the links between cycles

ATL

• Critical-thinking skills: Gather and organize relevant information to formulate
an argument

• Transfer skills: Make connections between subject groups and disciplines

The environment is sometimes divided into abiotic (non-living) and biotic (living)
components.
1 Identify inorganic substances other than nitrogen and phosphorus that are likely

to have roles in both components of the environment.
2 Suggest, based on your scientific knowledge of the nitrogen and phosphorus

cycles, how all other elements are also likely to enter the food chain.
3 Suggest how plant growth connects nutrients such as phosphorus to the

atmosphere.

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion A:
Knowing and understanding.

509

To what extent do we need to change to
counter the predictions of
environmental models?

REPAIRING OZONE DEPLETION

The former Director General of the United Nations, Kofi Annan, was quoted as
saying that ‘perhaps the single most successful international agreement to date
has been the Montreal Protocol’. This famous example of international,
collaborative action engenders hope for a positive future for all. The 1987
Montreal Protocol has now been signed by every sovereign country.

In environmental science, a contaminant is a substance which does not occur
naturally but is introduced by human activity. It may or may not be harmful to living
organisms. Harmful contaminants are also pollutants. Chlorofluorocarbons (CFCs)
were used as coolant liquids in refrigerators and as the solvent in aerosol sprays.
They are volatile (low boiling point), non-flammable, non-toxic and were initially
believed to be almost inert (very unreactive). The ability of gases to mix freely and
convection currents in air meant that once released, these contaminants slowly
diffused and eventually reached the stratosphere.

The ozone ‘layer’

Ozone is a gas found mainly in the lower portion of the stratosphere, from
approximately 20 to 30 kilometres above the Earth, though the thickness varies
seasonally and geographically. Ozone (O3) is an allotrope of oxygen (O2) and is
formed when oxygen molecules in the stratosphere absorb ultraviolet radiation and
split into two reactive oxygen atoms. Some of the highly reactive oxygen atoms
called free radicals react with molecules of oxygen to form ozone molecules:

Ozone is a dangerous substance if inhaled at ground level because it can cause
respiratory problems, but it performs a vital role in the stratosphere. It absorbs
ultraviolet B radiation with a range of wavelengths between 290–320 nm,
preventing it from reaching the surface of the Earth. This extreme ultraviolet
radiation harms marine phytoplankton and land plants, affecting food chains. It can
also cause skin cancers and damage eyes, leading to cataracts.

510

How CFCs damage ozone

During the 1980s measurements from the ground and satellites suggested that CFCs
were causing the removal of ozone from the upper atmosphere. The ozone
concentration has been reduced most significantly in the atmosphere above
Antarctica. Ozone depletion is greatest there because of wind and cloud
conditions and is sometimes referred to as the ‘ozone hole’ (Figure 9.18). Parts of
the Arctic ozone layer also develop ‘thinning’ during spring, but not nearly to the
extent of the Antarctic.

Take action: Performing chemistry to educate and
entertain
ATL

• Critical-thinking skills: Consider ideas from multiple perspectives; identify
obstacles and challenges; identify trends and forecast possibilities
511

• The success of the Montreal Protocol has been attributed to many factors. When
the first countries signed in 1987, the science of ozone depletion was not
entirely certain. Therefore, the treaty was designed to be a flexible instrument,
in which controls could be increased or decreased as the science became
clearer. For example, when in 1991 it became apparent the danger from ozone
depletion had been underestimated, a special fund was established to support
the 142 signatories representing developing nations in meeting their obligations.
There were also incentives to industry to develop new, patentable alternatives
to CFCs, gases which could be readily identified in the atmosphere.
Collaboration continues to be an important feature of the regular meetings,
which were often addressed directly by scientists.

• You have seen in this chapter how human chemical industry can impact on our
global environment. Global impacts require global solutions – but it has not
been easy to communicate this across the world.

• A powerful way to communicate the message and educate people is to use
performance.

• In this activity, you will research two global initiatives to limit or control
environmental impacts. Then you will devise a performance to communicate
and educate the world.

1 List similarities between the Montreal Protocol and the Kyoto Protocol.
2 Outline how our lives would be different today if the Montreal Protocol had

not been agreed to.
3 Suggest why the management of anthropogenic greenhouse gas emissions will

be a greater global challenge than the management of ozone-destroying
emissions.
• In groups, devise a mime, short play, performance poem or other performance to
dramatize the importance of the Montreal and Kyoto Protocols.
• Perform your pieces to another class, or in a school assembly. Why not video
record them, and post online?

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion D:
Reflecting on the impacts of science.

512

Links to: Arts – Dance; Drama

How can expression through dance or mime deliver an understanding of dynamic
interactions?

A catalytic cycle

The intense ultraviolet radiation in the upper atmosphere breaks the weaker C–Cl
bonds of CFCs to release very reactive chlorine atoms, also known as chlorine
radicals (Cl·) because they contain an unpaired electron.
A chlorine atom (Cl·) reacts with an ozone molecule, taking an oxygen atom to form
an oxychlorine radical (ClO·) and leaving an oxygen molecule:

The oxychlorine radical (ClO·) can react with a second molecule of ozone to
produce another chlorine radical and two molecules of oxygen:

Each alternating reaction generates a molecule that destroys ozone. The cycle can
repeat and continue to destroy ozone molecules until it is removed by another
reaction.
In the parts of the stratosphere away from the Earth’s poles, two alternative
reactions also occur. They remove the oxychlorine and chlorine radicals by
converting them into stable chlorine nitrate and hydrogen chloride molecules:

The nitrogen dioxide and methane molecules act as natural sinks for oxychlorine
and chlorine radicals. Methyl radicals are very reactive, and quickly combine with
other radicals, including chlorine atoms.
The ozone hole contracts during winter and expands over Antarctica during spring,
in November. As the temperature increases, accumulated chloric(I) acid and
chlorine molecules undergo photolysis in sunlight to form chlorine radicals which
initiate the catalytic cycle for ozone depletion.

513

SOME SUMMATIVE PROBLEMS TO TRY

Use these problems to apply and extend your learning in this chapter. The problems
are designed so that you can evaluate your learning at different levels of
achievement in Criterion A: Knowing and understanding.
A copy of a periodic table should be available for reference.

THESE PROBLEMS CAN BE USED TO
EVALUATE YOUR LEARNING IN CRITERION A
LEVEL 1–2

1 Referring to Figure 9.19,
a state the names of the processes A, B, C and D
b list names of three fossil fuels that could be listed in the box marked ‘fuels’.

2 Referring to Figure 9.19,
a present the biological processes taking place in A and B as word equations
b suggest how these two processes balance the composition of the air.

3 Referring to Figure 9.19,
a interpret where animals obtained their carbon, directly and indirectly
b judge the effect of stopping process B.

514

THESE PROBLEMS CAN BE USED TO
EVALUATE YOUR LEARNING IN CRITERION A
LEVEL 3–4

4 Outline how carbon dioxide in the atmosphere contributes to the greenhouse
effect.

5 Air is passed over heated copper turnings using the apparatus shown in Figure
9.20. This experiment is used to determine the percentage (by volume) of
oxygen in air.
a Formulate an equation describing the reaction which occurs in the tube.
b Suggest how much the plunger in each of the two syringes will be depressed,
by the end of the reaction.
c State two gases that will be present in the apparatus after all the oxygen has
been removed.

6 Air is a raw material from which several useful substances can be separated.
They are separated in the following process. Dry and ‘carbon dioxide free’ air
is cooled under pressure. Most of the gases liquefy as the temperature falls
below –200 °C. The liquid mixture is separated by fractional distillation. The
boiling points of the gases left in the air after removal of water vapour and
carbon dioxide are given in the table below.

515

Gas Boiling point (°C)
Argon –186

Helium –269

Krypton –157

Neon –246

Nitrogen –196

Oxygen –183

Xenon –108

a Interpret and state which three substances in the liquid mixture will be the
first to change from liquid to gas as the temperature is slowly increased.

b Suggest why the air needs to be dried, and why carbon dioxide is removed
before it is liquefied.

c Make a scientifically supported judgment about which of the gases will not
have become liquid at –200 °C.

THESE PROBLEMS CAN BE USED TO
EVALUATE YOUR LEARNING IN CRITERION A
LEVEL 5–6

7 Describe how the composition of gases in the atmosphere interacts with the

a carbon cycle
b water cycle
c nitrogen cycle
d phosphorus cycle.

8 This question refers to the following information. Oxygen and nitrogen are both
slightly soluble in water.

The apparatus shown in Figure 9.21 was used to collect 400 cm3 of
dissolved air from water heated to boiling. This 400 cm3 of collected air was
passed over heated copper powder and its final volume was 264 cm3.

516

a Determine the volume of oxygen that was present in 400 cm3 of dissolved
air.

b Calculate the approximate percentages of oxygen and nitrogen in dissolved
air.

c Compare these results with the percentages of these gases in dry atmospheric
air and suggest reasons for differences.

d Suggest how the results of this experiment may differ if the water sample
used had been collected downstream from a sewage outlet which regularly
released large quantities of nutrients into the lake.

9 The simple apparatus used in Figure 9.22 could be used to compare the level of
the pollutant, sulfur dioxide (SO2), in the air at two different locations. A pump
sucks air through the apparatus. The air passes through a filter paper which
traps smoke particles. The air passes through potassium dichromate(VI)
solution. The time is measured until the indicator changes colour from orange
dichromate(VI) ions (Cr2O72–) to green chromium(III) ions (Cr3+).

517

Air sample Time taken for indicator to change colour (seconds)
Location A 98

Location B 575

Results of a comparative study of atmospheric pollution
a Suggest why the same pump needed to be used in the two experiments.
b Analyse the results and make a scientifically supported judgment about
which of the two locations has cleaner air.
c Suggest why this example of environmental sampling is not a scientific
experiment.

10 Analyse the information about the composition of the atmosphere of Mars.

518

Gas Composition of
Carbon dioxide atmosphere on Mars

95.3%

Nitrogen 1.7%

Argon 1.6%

Oxygen 0.2%

Other gases, including carbon monoxide, water 1.2%
vapour and other noble gases

a Compare and contrast the atmospheres of Mars and Earth.
b Scientists predicted the surface temperature on Mars by calculating its

distance from the Sun. The measured surface temperature is slightly higher
than the predicted value.

i Suggest which gas(es) in the table will cause the surface temperature of
Mars to increase.

ii The Martian atmosphere also appears to contain a small amount of
methane. Suggest possible source (s) for this gas on Mars.

THESE PROBLEMS CAN BE USED TO
EVALUATE YOUR LEARNING IN CRITERION A
LEVEL 7–8

11 Explain the difference between a ‘pollutant’ and a ‘contaminant’.

12 Chemists have confirmed that the depletion of the ozone layer is caused by a
class of compounds called chlorofluorocarbons, CFCs. An example is
dichlorodifluoromethane, CF2Cl2.

In the stratosphere, the dichlorodifluoromethane molecules absorb ultraviolet
radiation and undergo dissociation and release reactive atoms.

The bond energies between atoms in the molecule are given below.

519

Bond Energy needed (kJ mol−1)
C–F 492

C–Cl 324

C–H 414

a Describe the outer electron structure of a molecule of CF2Cl2 using a Lewis
structure diagram.

b Suggest which bond between atoms in dichlorodifluoromethane is most
likely to be broken during the dissociation process, and why.

c Predict and explain the relative environmental effects on a per molecule
basis of the following three CFCs:

i monochlorotrifluoromethane, CF3Cl
ii dichlorodifluoromethane, CF2Cl2
iii trichloromonofluoromethane, CFCl3.

d Bromomethane, CH3Br, is used as a pesticide in agriculture to fumigate
soils. However, hydrobromofluorocarbons (HBFCs) and bromocarbons are
also implicated in ozone depletion.

The energy required to break a C–Br bond is 285 kJ mol−1.

Evaluate whether HBFCs are likely to be more or less reactive than
CFCs.

11 ‘Lean burn’ engines are a type of car engine. This table shows information
about ‘lean burn’ engines.

Another way of reducing the amounts of carbon monoxide and nitrogen oxides
from cars is to use catalytic converters. The transition metal catalyst increases
the rate of conversion of carbon monoxide and nitrogen monoxide to carbon
dioxide and nitrogen.

520

a Explain why ‘lean burn’ engines produce smaller amounts of
i carbon monoxide
ii nitrogen oxides.

b Describe the reactions catalysed by the transition metal using a balanced
chemical equation.

c Identify the information you would require to be able to evaluate the
relative environmental advantages of using a ‘lean burn’ engine compared to
a ‘normal’ engine with a catalytic converter.

521

Reflection

In this chapter you reflected on the global systems on Earth, and their
interrelationships. All systems are dynamic, and geological and biological
processes have changed Earth’s atmosphere through time. The carbon, water and
nitrogen cycles involve the atmosphere directly, and the nutrient cycles such as the
phosphorus cycle affect the atmosphere indirectly through plant growth. Human
ingenuity and collaboration have successfully responded to the global
environmental challenge caused by CFC contamination. The challenge posed by
anthropogenic greenhouse warming caused by carbon dioxide pollution remains.

522

523

Change
Energy
Fairness and development

524

525

10 How can our energy resources be
accessed fairly?

Global exploitation of energy resources relies on energetic changes
in chemical reactions; global development depends on the fair and
equitable exchange of those resources.

IN THIS CHAPTER, WE WILL …

• Find out what happens during combustion.
• Explore

• the contribution of chemical fuels to global energy demands;
• why chemical reactions are feasible;
• how to predict whether a chemical reaction or physical change is likely to be

endothermic or exothermic.
• Take action by raising awareness of the dangers of home fires, and how to fight

them.

CONSIDER AND ANSWER THESE QUESTIONS:

Factual: What conditions help substances burn?
Conceptual: What happens to chemical energy during physical and chemical
changes? Why is energy transfer so important? Why are fossil fuels still so
attractive as an energy source? What is meant by ‘spontaneity’ in scientific terms?
Debatable: Should people in the developing world be allowed the same energy
consumption as ourselves?
Now share and compare your thoughts and ideas with your partner, or with the
whole class.

526

These Approaches to Learning (ATL) skills will be
useful …

• Communication skills
• Collaboration skills
• Organization skills
• Reflection skills
• Media literacy skills
• Critical-thinking skills
• Creative-thinking skills
• Transfer skills

KEYWORDS

disorder
excess
spontaneity

We will reflect on this learner profile attribute …

• Reflective – How important is reflection for planning change? How does
making mistakes and identifying misconceptions help you learn? Are the ways
in which we have supported others in the past also misguided? Will considering
the fairness of past energy usage around the world and consequences on our
world communities and to the environment lead to actions that make our world
equitable for all?

Assessment opportunities in this chapter …

• Criterion A: Knowing and understanding
• Criterion B: Inquiring and designing
• Criterion C: Processing and evaluating
• Criterion D: Reflecting on the impacts of science

527

528

‘PLAYING’ WITH FIRE!

Fires are often easy to light, but difficult to put out. But, how do fuels burn? Until
this question was answered, chemistry could not make much progress.

Phlogiston theory!

By the 17th century, scientists were beginning to realize that the burning of fuels and
the reactions of metals in air all had a common feature – they were all faster or
slower versions of the same type of reaction.
Johann Becher (1625–1682) and Georg Stahl (1660–1734) explained these
observations by the removal (loss) of a mysterious, undetectable substance called
phlogiston (Figure 10.2).

According to their theory:
• All combustible substances contain phlogiston, which is released into the air on

burning, along with caloric (thermal energy).
• Charcoal (carbon) leaves very little ash when it burns because it is almost pure

phlogiston. Charcoal is useful for smelting (Chapter 8) metals from their ashes
and earths (metal oxides) because charcoal restores the levels of phlogiston in
the ash.
• A candle flame goes out in a sealed container because the air becomes saturated
with phlogiston.
• Substances lose mass when they burn because they lose phlogiston; however,
phlogiston must have negative mass because metals gain mass when they react
with the air!
Lavoisier and Priestley (Chapter 1) disproved the phlogiston theory and replaced it

529

with a theory based on the law of conservation of mass and oxidation.
In May 1794, during the French Revolution, Lavoisier was arrested, accused of a
number of crimes and later beheaded. The judge ignored his defence with the
words, ‘The Republic has no need of men of science.’

DISCUSS

1 List similarities between phlogiston theory and the modern understanding of
oxidation.

2 Identify other examples of scientific ideas that have changed through time.
What caused these ideas to change?

3 Suggest a reply to the judge’s claim that ‘The Republic has no need of men of
science.’ Would a person living in an isolated, remote region of the world,
whose energy needs are entirely based on biomass, appreciate your argument?

Links to: History

It has been argued that the Enlightenment ideas introduced by scientific
philosophers (including Isaac Newton, Blaise Pascal and René Descartes) were
related to political ideas that became very important, such as liberalism,
nationalism and democracy. Without these ideas, the American and French
Revolutions would never have happened! Do you think science changes people’s
ideas about society? To what extent does society affect the progress of science?

530

What conditions help substances burn?

WHEN CANDLES BURN

You first observed a burning candle in Chapter 1. When solid fuels burn, two
changes of state are required. Long-chain hydrocarbons like candle wax (usually
around C30H62) melt, then the molten wax moves up the wick where the molten
wax vaporizes. The combustion reaction involves the wax vapour around the wick
(see Figure 10.3) to form carbon dioxide and steam.

531

ACTIVITY: Investigating a burning candle
ATL

• Organization skills: Plan strategies and take action to achieve personal and
academic goals; select and use technology effectively and productively

Refer to the inquiry cycle (Figure 1.11) to plan an experimental investigation into
the effect on combustion of a candle of different atmospheres, held in sealed
containers. (Figure 10.4). A risk analysis and an environmental impact analysis
must be included in your plan.

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion B:
Inquiring and designing and Criterion C: Processing and evaluating.

532

FLASH POINTS

For a fuel to combine with oxygen, its vaporized gas needs to mix with oxygen in
the air. The flash point of a fuel is the lowest temperature at which there is enough
vapour (gas) for the fuel to ignite.
Petrol vaporizes very easily at room temperature, and will ignite when a lit match
is held over the liquid. In a car engine, a spark plug (Figure 10.5) provides the
energy needed to start the reaction. Paraffin (kerosene) vaporizes less readily, so it
is harder to light. A spirit burner and wick help it vaporize.

533

ACTIVITY: Team research into flashpoints
ATL

• Collaboration skills: Help others succeed; encourage others to contribute;
negotiate effectively

1 Identify, using the Internet for a particular flammable substance, the
information needed to complete the table collaboratively.

534

Flammable substance Flash point/°C Boiling point/°C
Benzene

Butane

Ethanol

Propan-1-ol

Ethoxyethane (ether)

Glycerine

Paraffin

Petrol

Diesel

White spirit

2 Plot the relationship of the flash point and the boiling point against each other.
If a range of temperatures is given, consider using a box and whisker
representation.

3 Suggest why petrol, white spirit (Figure 10.7) and paraffin have ranges for
their flash points and boiling points.

4 Explain, with reference to intermolecular forces and changes of state, the
relationship between flash point and boiling point.

5 Suggest why is it more dangerous to be trapped in a bushfire (wild fire) in a
car with a nearly empty petrol tank than one that is nearly full of fuel.

535

Assessment opportunities

• In this activity you have practised skills that are assessed using Criterion A:
Knowing and understanding.

536

COMBUSTION PRODUCTS

Complete combustion

Most fuels contain carbon and hydrogen. During complete combustion, a fuel’s
carbon forms carbon dioxide and its hydrogen forms water. For example,

Incomplete combustion

In any chemical reaction, a limiting reactant is the substance that is completely
used up, and which therefore determines when the reaction stops.
During combustion, if oxygen is the limiting reactant, the fuel’s carbon atoms may
be released as small particles of black soot. This effect can be seen in ‘smoking’
candles, and on the surface of laboratory glassware that has been incorrectly heated
using the ‘safety’ flame of a Bunsen burner. These soot particles can also be a
major source of air pollution in cities that rely on cheap grades of coal for heating
and cooking.
Incomplete combustion can also form a dangerous alternative product:

Carbon monoxide (CO) has no colour and no smell, although household detectors
exist (Figure 10.6). If accidentally inhaled, it binds permanently with hemoglobin in
red blood cells, preventing the transport of oxygen via the circulatory system.
Faulty gas heating or using outdoor stoves in enclosed places can cause fatal
carbon monoxide poisoning. For this reason, all fuels should be completely
combusted to carbon dioxide.

EXTENSION

Explore further: What is the role of the choke and carburettor in the combustion
of fuel in a vintage car? How does ‘tuning’ a car engine reduce the percentage of
carbon monoxide present in a vehicle’s exhaust gases?

537

Why is energy transfer so important?

GETTING CHEMICAL REACTIONS
STARTED

Thermal energy is sometimes referred to as ‘heat energy’. However, heat is not a
form of energy, but rather a process by which energy is transferred from a hotter
object to a cooler one. There are two explanations for saying something is ‘heated’:

1 Energy has been transferred by convection or conduction and the thermal motion
or kinetic energy of the particles receiving the energy has increased. Some of the
energy will increase the speed at which the molecules move through space
(translation), but some energy will also be transferred to vibrations and rotations
if the particles involved are molecules.

2 Energy has been transferred by radiative heating in the form of infra-red
radiation, which often leads to an increase in the vibrational motion of
molecules.

The term ‘thermal energy’ in this chapter has been used to describe either mode of
heat transfer.

The energy needed to start a chemical reaction is known as the activation energy.
All reactions need some thermal energy to get them started, but some need so little
they can start at room temperature. Rusting and neutralization are examples of such
reactions.

To release thermal energy, fuels need to react with oxygen. However, most fuels are
safe at room temperature because they ‘burn’ so slowly that effectively no reaction
occurs. Like all chemical reactions, the rate increases at higher temperatures.
Combustion or burning can start only after the fuel has been ignited.

The barrier to the reaction is called the ‘activation energy’ (Figure 10.8) and it can
be overcome in different ways. For example, the ignition temperature of a fuel is
the lowest temperature at which it will spontaneously burn, and at which the
reaction releases enough thermal energy to keep it going. The barrier can also be
decreased by using a catalyst, which helps the reactants approach each other in
directions that optimize the probability they will be able to react. The catalyst itself
is not used up, but the rate of the reaction increases because less energy is required
to start the reaction and keep it going.

538

Figure 10.8 is an example of an energy or enthalpy level diagram, showing the
activation energy input as a positive increase in the system, and the release of
energy as a negative value.
Figure 10.9 is an example of an energy level diagram for the enthalpy of
combustion of a fuel, methane. The value ΔH represents the energy released in the
exothermic reaction (see Chapter 6). The diagram shows that thermal energy has
flowed from the chemicals (the system) to the surroundings. For this reason, it is
given a negative sign in the thermochemical equation of the reaction:

539

DISCUSS: Reflecting on the enthalpy of combustion

1 List six examples of fuels your family used during the year. From the context
where the fuel is used, suggest the relative size of its ‘activation energy’.

2 Suggest how Figure 10.8 would need to change if an enzyme lowered the
activation energy.

3 Explain the concept of activation energy by including kinetic energy of
colliding particles.

540

THE FIRE TRIANGLE

Fires need fuel, oxygen (usually from the air) and a high temperature to start the fire
(provide activation energy). These three variables make up the fire triangle (Figure
10.10).

DISCUSS

1 Identify the fire-fighting equipment that is present in your chemistry laboratory,
and describe how you would use it.

2 Describe, with reference to the fire triangle, how each of these fire-fighting
methods prevents fires spreading: a fire blanket, sand, water, carbon dioxide,
foam, halon (an inert organic compound) and powder.

3 Explain why water should not be used against oil, petrol or electrical fires.

Take action

• Complete a fire risk assessment of your school’s facilities, then write a report to
the Head of the School and Board of Directors, citing your commendations and
recommendations.

541

Why are fossil fuels still so attractive
as an energy source?

ACTIVITY: What makes a good fuel?
ATL

• Reflection skills: Consider ethical, cultural and environmental implications
Different fuels have different properties (Figure 10.11). Fuels which may once
have been popular in the past (Figure 10.12) may have fallen out of favour in
developed countries. Impurities in fuels and incomplete combustion may
contribute to air pollution and adverse health effects (Figure 10.13).

542

543

544

1 Identify, with reference to Figure 10.11, properties needed of a fuel intended
for use in
a transport
b camping
c long-term storage
d indoor use
e generating electricity for industry.

2 Compare and contrast advantages associated with each of the properties
listed in Figure 10.11.

3 Suggest how your fuel choice decisions would change if
a the air quality standards of your country were lower than surrounding
countries
b the ‘ideal’ fuel was not locally available
c your business was in a location where storage costs were very high.

4 Outline how the chemical qualities might be determined experimentally,
explaining measurements and observations you would record and how the
variables will be controlled. Describe the safety precautions you will take.

Assessment opportunities

• In this activity you have practised skills that can be assessed with Criterion D:
Reflecting on the impacts of science.

EXTENSION

Explore further: Fuels in the past. Interview your grandparents and ask them
about the fuels that were used for heating, cooking and transport when they were
your age. What chemical processes contribute to smog and what political
processes lead to oil crises?

545

Should people in the developing world
be allowed the same energy
consumption as ourselves?

ENERGY FOR A FAIRER WORLD

All living organisms, including humans, require food, shelter, water and warmth.
These complex needs are linked through physical–chemical and biological systems
(Chapter 9) where the common denominator is energy.
As an example of how energy availability interacts with a range of factors in
societies, consider the impact of the inefficient combustion (burning) of coal.
Manufacturing and industrial processes will be affected directly. For example,
large amounts of energy are needed to refine crude oil, extract aluminium, make
plastics and lime (calcium oxide), which is used for making concrete, directly
affecting productivity. Wasteful burning also causes air pollution (Figure 10.13),
indirectly affecting productivity. For example, people working in these industries
may suffer a larger number of sick days because of the quality of the air, and other
people will need to care for them.
Rich or developed countries have a long history of using cheap and plentiful fossil
fuels (coal, fuel oil and gas) and continue to use most of global energy supplies,
contributing unfairly to global, anthropogenic climate change (see Chapter 9). On
the other hand, as developing countries transform their industries, some of them
may be able to completely bypass inefficient and polluting technologies used in the
past.

546

ACTIVITY: Realizing a sustainable energy future for
all

ATL

• Critical-thinking skills: Gather and organize relevant information to form an
argument; interpret data; consider ideas from multiple perspectives; identify
trends and forecast possibilities

• Communication skills: Make inferences and draw conclusions; organize and
depict information logically

A sustainable energy future that is fair should recognize the right of every person
to benefit equally from the world’s energy resources. Each country has access to a
range of natural resources to meet some of its energy needs, and a proportion must
be met through chemical solutions.
Develop a 1200-word proposal to recommend strategies based on scientific
knowledge that could help to end ‘energy poverty’ in a developing country of
choice. Assume a five-year time frame and a hypothetical budget of 10 billion US
dollars.
• Include an assessment of the role of fuels as a resource for contributing to

energy needs, and an explanation of energy conversions involved in their use.
• Include an evaluation of the consequences of the scientific solution for resource

management, and how this may interact with a moral, ethical, social, economic,
political, cultural or environmental factor in the country you have selected.
Approximately half the length of your written presentation should concern this
reflection.
• You may wish to consider a presentation that includes headings and
subheadings, and refer to tables, diagrams, graphs, lists and appendices.
• All sources should be fully documented.

Assessment opportunities

• This activity can be assessed using Criterion D: Reflecting on the impacts of
science.

ACTIVITY: Exploring a simple fuel cell

547

ATL

• Creative-thinking skills: Create novel solutions to authentic problems

Fuel cells (introduced in Chapter 6) have been used since the 1960s in NASA’s
vehicles, submarines and the International Space Station (ISS). What are the
implications of extending their use?

Safety: Eye protection must be worn when using caustic solutions.

Materials and equipment

• safety goggles
• 6 V DC power supply
• leads with alligator clips
• 0.1 mol dm−3 sodium hydroxide solution
• 2 × test tubes
• beaker
• graphite electrodes
• high resistance voltmeter

Method

1 Put on safety goggles.
2 Set up the electrolysis cell shown in Figure 10.14(a).
3 Connect the cell to a 6 V DC power supply and let electrolysis continue until

the tube above the anode (positive electrode) is filled with oxygen and that
above the cathode (positive electrode) with hydrogen, as shown in (b) of the
diagram.
4 Disconnect the low voltage power supply.
5 The assembly is now ready to be a fuel cell. Connect a high resistance
voltmeter across the electrodes.

548

Reflecting on results

1 List evidence for a chemical reaction.
2 Identify the source for the current observed when the power supply was

disconnected.
3 Suggest contexts in which fuel cells might have advantages.
4 Evaluate the potential of this technology for solving energy poverty.

Assessment opportunities

• In this activity you have practised skills that can be assessed with Criterion C:
Processing and evaluating.

549

What happens to chemical energy
during physical and chemical changes?

SEE–THINK–WONDER

Your teacher will demonstrate the reaction shown in Figure 10.15 in a fume
cupboard, or you can observe the reaction on www.youtube.com/watch?
v=5Ib3MuOThTg.
Safety: Concentrated sulfuric acid is very dangerous and should only be handled
by a science teacher wearing gloves and glasses.
What do you see? What do you think about that? What does it make you wonder?

550