Chemistry 2007
Sample assessment instrument and indicative responses
Extended experimental investigation
This sample is intended to inform the design of assessment instruments in the senior phase of
learning. It highlights the qualities of student work and the match to the syllabus standards.
Criteria assessed
• Knowledge and conceptual understanding
• Investigative processes
• Evaluating and concluding
Assessment instrument
The response presented in this sample is in response to an assessment task.
This is an extended experimental investigation on the topic of energy. A range of fuels was
provided and students have a choice of fuels to use for the investigation. The length of the
investigation is four weeks.
A summary of the task provided by the school is presented below. The task sheet has not been
included.
•
•
•
•
•
•
•
Maintain a journal throughout the investigation.
Carry out research on the topic.
The task is part of an eightFormulate your hypothesis for the investigation.
week unit of work on energy.
Decide on a method for the investigation.
Order materials and fill in the Materials Requisition form and Risk Assessment Form.
Begin the investigation. Record all observations. Refine your experiment as required.
Gather, record and process valid data.
Write a scientific report under the headings provided.
The report headings are: results, discussion and conclusion. The discussion is to include an
evaluation and recommendations.
Journal extracts have been included with the scientific report to show where the descriptors of
the standards have been demonstrated throughout the investigation. The annotations and
notes help to explain the journal entry.
References have not been shown in this sample.
Instrument-specific criteria and standards
Indicative responses have been matched to instrument-specific criteria and standards; those
which best describe the work in this sample are shown below. For more information about the
syllabus dimensions and standards descriptors, see www.qsa.qld.edu.au/1952.html#assessment.
Knowledge and
conceptual
understanding
Investigative
processes
Evaluating and
concluding
2 | Chemistry 2007
Standard A
Standard C
The student work has the following
characteristics:
The student work has the following
characteristics:
•
comparison and explanation of complex
concepts, processes and phenomena
•
explanation of simple processes and
phenomena
•
linking and application of algorithms,
concepts, principles, theories and
schema to find solutions in complex and
challenging situations
•
application of algorithms, principles,
theories and schema to find solutions in
simple situations
The student work has the following
characteristics:
The student work has the following
characteristics:
•
formulation of justified significant
questions/hypotheses which inform
effective and efficient design,
refinement and management of
investigations
•
formulation of questions and
hypotheses to select and manage
investigation
•
assessment of risk, safe selection and
adaptation of equipment, and
appropriate application of technology to
gather, record and process valid data
•
assessment of ri sk, safe selection of
equipment, and appropriate application
of technology to gather and record data
•
systematic analysis of primary and
secondary data to identify relationships
between patterns, trends, errors and
anomalies
•
analysis of primary and secondary data
to identify obvious patterns, trends,
errors and anomalies
The student work has the following
characteristics:
The student work has the following
characteristics:
•
analysis and evaluation of complex
scientific interrelationships
•
description of scientific
interrelationships
•
exploration of scenarios and possible
outcomes with justification of
conclusions/ recommendations
•
description of scenarios and possible
outcomes with statements of
conclusion/ recommendation
•
discriminating selection, use and
presentation of scientific data and ideas
to make meaning accessible to intended
audiences through innovative use of
range of formats
•
selection, use and presentation of
scientific data and ideas to make
meaning accessible in range of formats
Sample assessment and indicative responses
Indicative response — Standard A
Written scientific report
The aim of this investigation is to see the relationship between the number of carbon atoms in a
straight chain molecule and the heat of combustion that is released.
Comments
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning accessible
to intended
audiences through
innovative use of a
table
RESULTS
Alkanol
Molar mass
Average H
(kJ/mol) to heat
200g of water by
º
15 C
Average H
(kJ/mol) to heat
200g of water by
º
30 C
Methanol
32
601
604
Ethanol
46
1134
1129
1-Propanol
60
1675
1671
1-Butanol
74
2231
2210
1-Pentanol
88
2740
2730
1-Hexanol
102
3192
3178
1-Heptanol
116
3812
3700
The primary data
has been
summarised into
one table.
Table 5 Heats of combustion released when heating 200g of water by 15 ºC and
º
30 C
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning accessible
to intended
audiences through
innovative use of a
graph
Graph 1 Heats of combustion values
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Comments
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning accessible
to intended
audiences through
innovative use of a
graph
Graph 2 Comparison of heats of combustion
DISCUSSION
systematic analysis
of primary data to
identify relationships
between patterns,
trends, errors and
anomalies
linking and
application of
concepts and
principles and
schema to find
solutions in complex
and challenging
situations
4 | Chemistry 2007
Table 5 summarises the results from the primary data in the journal. Graph 1
indicates there is a linear relationship between molecular weight and heats of
combustion. Graph 2 compares the heats of combustion of the two sets of primary
data, the values from the literature and the values calculated from the bond
dissociation energy.
Graph 2 compares the heat of combustion values from the bond dissociation
energy calculations, the literature values and the experimental values when 200g
0
0
of water is heated by 15 C and 30 C. The bond dissociation values do not take
into account the hydrogen bonding and the literature values are done under
different conditions as stated. The experimental values for all trials undertaken are
going to be lower than the other two values because of the methodology involved.
There will always be some heat loss to the surroundings in a simple set up.
Even though the amount of fuel burnt is different in each case, the heat of
combustion of a fuel should theoretically be the same. These results are
comparative, within the efficiency of the apparatus. There appears to be no real
difference in results for the different trials.
Sample assessment and indicative responses
Comments
Experimental values are below the literature values which could be due to the
methodology involved. A linear relationship is seen between the length of the
carbon chain and the heat of combustion released. The values differ from the
theoretical values. This may have been due to any of several factors:
•
•
•
•
comparison and
explanation of
complex concepts,
processes and
phenomena
•
•
•
Incomplete combustion may have occurred
Some evaporation may have occurred and the exact volume may not have
been 200ml
Errors in reading the thermometer, measuring the exact volume of water and
measuring the mass of the fuel
The apparatus (burner, stand, mat, glass beaker) may not have cooled down
to room temperature after each experiment.
The whole investigation and gathering of data was extended over several days
The readings may have been done by different people
All of the trials may not have been done in sequence
One of the main factors in this investigation is the inability to tell if all of the fuel
burnt has been converted into carbon dioxide and water as shown in Equation 2.
With such small amounts of fuel being used it is probable that all is burnt, so for
the purposes of this discussion it is assumed that complete combustion has
occurred. Originally it was proposed to use a solid fuel i.e hexdecanol but the first
trial on this substance produced soot and it could not be placed in the burner. It
was decided not to continue with this and use only liquids.
The products of the reactions of complete combustion of alkanols are carbon
dioxide and water. The equation for methanol to release 726 kJ of energy is
shown below:
CH3 OH (l) + 3/2 O2 (g)  CO2 (g) + 2H2 O (l) H = -726 kJ/mo l (Eqn 2)
Figure 1 Equation of the complete combustion of methanol.
If all of the fuel is not burnt during the reaction, incomplete combustion occurs. The
products are carbon monoxide and water. This will be impossible to ascertain in
this experiment The exact amount of heat released is then unknown. It is assumed
that complete combustion occurs in this investigation.
The incomplete combustion of methanol releasing an unknown amount of energy
is shown below:
CH3 OH (l) + O2 (g)  CO (g) + 2H2 O (l) H = -… kJ/mol (Eqn 3)
Figure 2 Equation of the incomplete combustion of methanol.
Alcohols are polar, because of the O-H bonds, allowing alcohol molecules to
attract each other through hydrogen bonds. Since oxygen atoms are much more
electronegative than hydrogen atoms, the O-H bond is especially polar. The
partially-negatively charged oxygen atom on one alcohol molecule is strongly
attracted to the partially positively charged hydrogen atom on another alcohol
molecule; this strong attraction results in stronger intermolecular forces between
alcohol molecules.
The hydrogen bonding between ethanol molecules is shown in Figure 4. These
polar bonds contribute to the substantial dipole moments. The bond between the
dipoles is a hydrogen bond. It is typically 197pm, depending on bond strength,
temperature and pressure. The O-H bond in alkanols is 0.96 Å. This distance is
sufficiently small that some hydrogen bonds may have a significant amount of
covalent or shared electron character.
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Comments
Figure 3 Hydrogen bonding in ethanol
The literature values used in Table 1 (see page 5 extract of journal) for
comparison are calculated from combustion experiments by the National
Bureau of Standards USA under the following conditions;
analysis and
evaluation of
complex scientific
interrelationships
•
the sample is burnt in a bomb of constant volume in pure oxygen at an
0
initial pressure of 30 atm and at 25 C
•
the number of grams of sample burnt is equal to three times the volume of
the bomb in litres
•
the number of grams of water placed in the bomb before combustion is
equal to three times the volume of the bomb in litres. Reactants and
products are in their standard states at one atmosphere pressure
(www.nist.gov/data/PDF files/jpcrd6.pdf). These conditions are impossible
to copy in an ordinary laboratory but the literature values are very
accurate.
Several factors are involved when considering the values in Table 1 (see
page 5 extract of journal):
•
The values calculated from bond dissociation energies do not consider
other energies between the molecules such as hydrogen bonding and van
der W aal’s forces
•
The molecules are different shapes and sizes and stability
The alcohols are polar molecules as shown in Figure 3. Oxygen has an
electronegativity of 3.44, higher than both carbon 2.55 and hydrogen 2.20. This
makes the oxygen atom very negative and the whole molecule very polar. The
journal entry on page 12 depicts the structure of the substances used. It can
be seen that in going from methanol to propanol, the shape changes from
linear to zig-zag. Molecules with three or more carbon atoms are this shape.
As the molecules become longer they have a greater capacity to link around
each other and more hydrogen bonding results as the molecules become
closer.
6 | Chemistry 2007
Sample assessment and indicative responses
Comments
Figure 4 Bond lengths in propanol.
The bond lengths between atoms shown in Table 2 (see page 6 extract from
journal) and in propanol in Figure 4 contribute to the overall shape of the
molecule. As the molecular weight increases the density increases. This
means the molecules are packed closer together. When combustion occurs
some heat will be needed to break these attractive forces between the
molecules.
exploration of
scenarios and
possible outcomes
with justification of
conclusions/
recommendations
Graph 3 Molecular weight vs relative polarity
Graph 3 shows that as the molecular weight increases the relative polarity of
the molecule decreases. This occurs because the carbon chain becomes
longer and each molecule has only one C-OH group at the end.
If the experiment was repeated, the methodology could be improved by having
a more efficient calorimeter e.g. some form of bomb calorimeter to retain as
much heat as possible. If the same spirit burner was used, then the cover
would need to be made of more heat proof material. A larger amount of fuel
could be combusted as long as the fuel did not boil. Boiling would require heat
and so results would be inaccurate.
CONCLUSION
A clear trend is seen. As the molecular weight increases so does the heat of
combustion released. The stated hypothesis was “as the carbon chain
increases in an alkanol, the heat of combustion released will increase”,
increasing from one in methanol to four in 1-butanol. The experimental values
show there is a linear relationship between the molecular weight and the heat
of combustion released.
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Comments
JOURNAL EXTRACTS
Page 4 Extract
The student has
started initial
planning of the
investigation and
considered topics
related to fuels
Initial idea or
question to
answer
Page 4 Extract
The structure of compounds is
researched to see the nature of
branching
The student is
linking concepts to
the research
question
Page 5 Extract
The student
researched
definitions
relevant to the
topic
The student
researched the
heat of
combustion
values for the
compounds from
different sources
8 | Chemistry 2007
Initial question — student now has
to research to see if this question
can be justified
Sample assessment and indicative responses
Comments
Page 5 Extract
These values will be compared
with experimental and bond energy
calculation values
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning accessible
to intended
audiences through
innovative use of a
table
This represents the
most recent data on
heats of
combustion
The student decided that using compounds that were branched would lead to
indistinguishable results. Research was also conducted on boiling points but this
was disregarded as an option.
Page 6 Extract
These values show the energy
stored within the molecule
Linking and
application of
concepts,
principles, theories
and schema to find
solutions in
complex and
challenging
situations
The values were
used to calculate
the bond
dissociation
energies of the
compounds
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Comments
linking and
application of
algorithms and
concepts to find
solutions in a
complex and
challenging
situation
Page 7 Extract
Calculating the heat of combustion from bond dissociation energy values.
All calculations are
based on this method
The student
understands the
term exothermic
and energy
diagrams
The stored chemical energy for
methanol is 653kJ
Page 7 extract shows that the bond dissociation energy calculations for the
branched materials were the same.
The student has
used secondary
data in the
decision making
process for the
design of the
investigation
Hypothesis/question
has been modified due
to research
formulation of
justified
significant
questions/hypoth
eses which inform
effective and
efficient design,
refinement and
management of
investigation
Page 10 Extract
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| Chemistry 2007
Sample assessment and indicative responses
Comments
Page 12 Extract
The student identified
the 3 dimensional
shapes of each of the
compounds
Methanol
CH3OH
linking and application
of concepts, principles,
theories and schema to
find solutions in a
complex and
challenging situation
1-pentanol
C5H11OH
Ethanol
C2H5OH
1-propanol
C3H7OH
1-hexanol
C6H13OH
1-butanol (n-butanol)
C4H9OH
1-heptanol
C7H15OH
Page 10 Extract
Several calorimetry methods were compared.
The student identified
the placement of the
thermometer, the
support of the
thermometer, heat loss
to the surroundings
and sufficient fuel in
the burner as issues in
the methodology
comparison and
explanation of complex
concepts, processes
and phenomena
This basic set up was
considered to be too inefficient
to gather accurate and valid
This bomb calorimeter is a
specialised piece of equipment
but has features that could be
incorporated into a set-up.
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Comments
Page 13 Extract
The efficiency of the
apparatus was tested
and found to be
51.21%. An initial trial
showed the efficiency
at 49.93%. The student
considered this too
low.
Application of
the algorithm
q = m x C x T
Mathematical check
was done to see how
much fuel was needed
to raise the
temperature of 100g of
0
water by 20 C. This
was 0.2812g.
Refinement of the cover
to minimise heat loss
effective and efficient
design, refinement and
management of the
investigation
The student now has a
planned course of
action for the
investigation
The student has
addressed the issues
associated with the
equipment
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| Chemistry 2007
Sample assessment and indicative responses
Comments
Page 12 Extract
The apparatus has a small
inlet to ensure that there is
sufficient oxygen for
combustion
The apparatus is designed to
ensure as much as possible
of the heat released goes
into heating the water
effective and efficient
design, refinement and
management of the
investigation
A description of the
experiment is provided
assessment of risk,
safe selection and
adaptation of
equipment
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Comments
Pages 17–20 Extracts
Only three examples of
primary data have been
shown.
Four pages of
primary data
was collected
Data from three trials was
collected each time.
Values considered out of range
were disregarded
appropriate
application of
technology to
gather, record
and process
valid data
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| Chemistry 2007
Sample assessment and indicative responses
Indicative response – Standard C
Written scientific report
The aim of this investigation is to see if the compound with the most carbon atoms will produce
the most heat of combustion.
Comments
selection, use
and
presentation of
scientific data
and ideas to
make meaning
accessible in
the form of a
table and a
graph
RESULTS
Alcohol
Molar
mass
Average H
(kJ/mol)
Average %
Efficiency
methanol
32
398.07
54.82
ethanol
46
674.37
49.33
propanol
60
1169.46
57.86
butanol
74
1519.97
56.8
Table 1 Heats of combustion
Graph 1 Heats of combustion vs molecular weight
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Comments
DISCUSSION
One of the observations made during the investigation was that the flames
were different colours. Some were yellow and some yellow and blue. This
may be due to the fact that the fuel was not burning completely. When all
the fuel burns in oxygen, complete combustion occurs. This is shown by
the equation below.
CH3 OH (l) + 3/2 O2 (g)  CO2 (g) + 2H2 O (l)
explanation of simple
processes and
phenomena
The processes have been
compared
Figure 1 Equation of the complete combustion of methanol.
When some of the fuel does not burn it is incomplete combustion. If all the
fuel does not burn, the products are carbon monoxide and water. It will be
impossible to tell what is happening in this experiment. It is assumed that
complete combustion occurs in this investigation. Only when all of the fuel
burns can the heat of combustion be found correctly.
CH3 OH (l) + O2 (g)  CO (g) + 2H2 O (l)
Figure 2 Equation of the incomplete combustion of methanol.
analysis of primary and
secondary data to identify
obvious patterns, trends
and anomalies
Table 1 shows the average heat of combustion found for each of the
compounds. These values were relatively accurate considering that heat
was lost to the atmosphere. The values for ethanol were lower than the
other compounds. The carbon chain increases from methanol to butanol
from one carbon to four carbons. Graph 1 shows that there is a linear
increase in the heat of combustion released as the carbons increase. The
value for ethanol has made the line slightly off straight. The value for
ethanol is only 49.33% efficient as opposed to the other values which are
above 50%.
The second trial for methanol was very much lower than the other two
trials even though more fuel was burnt i.e. 0.381 g of methanol as
opposed to 0.329 g in the first trial. The second trial was less efficient to
the other trials. Only 386.38 kJ was released compared to 406.56 and
401.28 kJ. In other trials when less fuel was burnt less heat of combustion
was released. This anomaly could be due to heat loss to the surroundings
and the overall inefficiency of the set up.
The student has identified
sources of error in the
investigation
description of scientific
interrelationships
16
| Chemistry 2007
The data confirms the hypothesis that as the carbon chain increases so
does the heat of combustion. As the carbon chain increases so does the
molecular weight. This is why heat of combustion was graphed against
molecular weight.
A source of error in the experiment was the heat loss to the surroundings.
This was impossible to stop. Also the beaker and the equipment became
hot as the experiment proceeded. So all of the heat would not have gone
into the water. The readings of the thermometer may not have been done
accurately. These were done on different days.
Sample assessment and indicative responses
Comments
Alcohols are polar. This is shown in Figure 3 below.
description of
scenarios and possible
outcomes with
statements of
conclusion/
recommendation
Figure 3 Polarity in alcohols
The OH group has a polar bond because the oxygen atom is more
electronegative than the hydrogen bond. This means that the oxygen has a
greater attraction for the electrons than the hydrogen. The oxygen is more
negative than the hydrogen so the bond is polar. The OH can then form
hydrogen bonds with other atoms. This may contribute to some of the
compounds having more heat of combustion than others.
If the investigation was to be repeated there are some recommendations that
could improve the results. The experimental trials should be conducted in a
controlled environment that has a maintained temperature as well as amount
of wind. The temperature of the water at the start should be the same to
ensure the change in temperature is about the same for each trial. This
would keep the temperatures low so the equipment would not get too hot.
This lowers the likelihood of evaporation and therefore loss of the mass of
water.
Further experiments could see if the amount of water heated had any effect
on the heat of combustion. Ambient temperatures could be raised or lowered
to see the effect.
CONCLUSION
The aim of the investigation was supported and the hypothesis achieved. It
was found that as the chain length increased the amount of heat of
combustion increased.
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Comments
JOURNAL EXTRACTS
Page 1 Extract
The student has
started initial
planning of the
investigation and
has considered
topics related to
fuels
Initial idea or
question to
answer
The student is
linking concepts
to the research
question
The concept of energy
within the compound
is researched
The student has
established the
availability of
fuels
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Page 2 Extract
| Chemistry 2007
Sample assessment and indicative responses
An initial question has been proposed —
the student has to research to see if this
is feasible to investigate
Comments
Page 2 Extract
The student has
researches the heat
of combustion
values for the
compounds from a
source
The set-up for the
experiment is established
formulation of a
hypothesis to select
and manage
investigation
Page 3 Extract
The student has a
planned course of
action for the
investigation
assessment of risk,
safe selection of
equipment
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Comments
The student has
considered other
aspects in the
management of
the investigation
Initial trials establish the set-up is
69.4% efficient
An initial trial
was conducted
to see if the set
up and
measurements
taken were
suitable
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| Chemistry 2007
Sample assessment and indicative responses
Comments
Page 4 Extract
application of
algorithms to find
solutions in a
simple situation
appropriate
application of
technology to
gather and record
data
Page 6 Extract
Trials were conducted heating
100g of water by 10 ºC
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