Chemistry (2007)
Sample assessment instrument and student response
Extended experimental investigation:
Electrochemistry
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.
Extended experimental investigation to an open topic
Task: Conduct an experimental investigation into a complex chemical phenomenon.
The investigation will consist of:
•
review of relevant scientific literature
•
preliminary experiment that explores a chemical concept
•
extended experiment that explores this chemical concept in more depth; this may be a
development of the preliminary experiment or an entirely new experiment.
Time: 4 weeks.
Instrument-specific criteria and standards
Student responses have been matched to instrument-specific criteria and standards; those which
best describe the student 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.
Standard A
Knowledge and
conceptual
understanding
Investigative
processes
Evaluating and
concluding
The student work has the following characteristics:
•
reproduction and interpretation of complex and challenging electrochemistry concepts,
theories and principles
•
comparison and explanation of complex electrochemistry concepts, processes and
phenomena
•
linking and application of electrochemistry and solubility concepts, principles and
theories to find solutions in complex and challenging situations
The student work has the following characteristics:
•
formulation of justified significant hypotheses which inform effective and efficient
design, refinement and management of investigations
•
assessment of risk, safe selection and adaptation of equipment, and appropriate
application of technology to gather, record and process valid data
•
systematic analysis of primary and secondary data to identify relationships between
patterns, trends, errors and anomalies
•
analysis and evaluation of complex scientific interrelationships
•
exploration of scenarios and possible outcomes with justification of conclusions/
recommendations
•
discriminating selection, use and presentation of scientific data and ideas to make
meaning accessible to intended audiences through innovative use language and tables
Note: Colour highlights have been used in the table to emphasise the qualities that discriminate
between the standards.
2 | Chemistry 2007: Sample student assessment and responses Subtitle
Student response — Standard A
The annotations show the match to the instrument-specific standards.
Corrosion of nails
Introduction
The rusting of iron is the result of a redox reaction that takes place
between iron, oxygen and water, in a process called galvanic corrosion.
The iron is oxidised and the oxygen is reduced, to produce ferrous ions
and hydroxide as follows:
reproduction and
interpretation of
complex and
challenging
electrochemistry
concepts and
principles
𝐹𝑒(𝑠) → 𝐹𝑒 2+ (𝑎𝑞) + 2𝑒 −
2𝐻2 𝑂(𝑙) + 𝑂2 (𝑔) + 4𝑒 − → 4𝑂𝐻 − (𝑎𝑞)
To produce an overall reaction:
2𝐹𝑒(𝑠) + 2𝐻2 𝑂(𝑙) + 𝑂2 (𝑔) → 2𝐹𝑒 2+ (𝑎𝑞) + 4𝑂𝐻 − (𝑎𝑞)
(Lancashire, 2008)
The iron ions and the hydroxide ions, react to produce Fe(OH) 2 which,
alongside other similar compounds, such as iron oxide (Fe2O3), is the
‘rust’ that is observed (Atkins, 2011, p. 62). In this redox reaction, the
anode and cathode can be identified using the two indicators:
phenolphthalein and potassium ferricyanide.
Phenolphthalein, typically clear, turns a bright pink colour in the presence
of OH− ions in solution. Thus pink appears where the reduction reaction
occurs (the cathode). Similarly, potassium ferricyanide turns Prussian
2+
blue when it reacts with Fe in the solution to produce iron (II)
hexacyanoferrate (III) (Spence, 2007, p. 71):
2
𝐹𝑒 2+ (𝑎𝑞) + 𝐾3 𝐹𝑒(𝐶𝑁)6 (𝑎𝑞) → 𝐹𝑒 3+ (𝑎𝑞) + 𝐾[𝐹𝑒(𝐶𝑁)6 ]2 (𝑎𝑞) (De, 2003)
explanation of
complex
electrochemistry
concepts,
processes and
phenomena
As a result, sites that turn blue indicate where the iron is being oxidised,
and thus identify the anode. The anode is usually found at each end of the
object, or at any position at which the oxidation reaction can readily take
place and is indicated by blue spots. The remaining length of the nail is
surrounded by pink, indicating the reduction reaction that takes place in
the water surrounding the nail.
This corrosion process can be prevented through the use of a sacrificial
anode. In this case, a more reactive metal (i.e. a metal that is more readily
oxidised) is wrapped around the iron. This metal is then oxidised rather
than the iron, and rusting is prevented (Wong, 2012). Zinc coating is often
used to protect iron in a process known as galvanisation, as it acts as a
sacrificial anode in the place of iron and thus protects it from corrosion. In
these instances, the blue indicator will not be activated, as the iron has
2+
not been ionised into Fe ; however, the solution will still turn pink, as the
−
OH has been produced through the reaction with the sacrificial anode
and the surrounding environment (Zekan, 2012). In the same way, when a
less reactive metal is wrapped around iron, the iron will rust in the place of
the more reactive metal.
Queensland Studies Authority April 2013 | 3
linking and
application of
electrochemistry
and solubility
concepts, principles
and theories to find
solutions in
complex and
challenging
situations
The common ion effect refers to the lowering of the solubility of a
substance due to the presence of a common ion in the solution (Spence,
2007, p. 575). This is because there is a point of saturation at which a
solution can absorb no more of a given solute. When an ion is already
present in the solution, the amount of this ion that can be added by
dissolving is limited. For example, the solubility of sodium chloride in
+
sodium nitrate solution is limited, due to the common ion, sodium (Na ). In
order for corrosion to occur, the ion of the metal being oxidised must
dissolve. Thus, by increasing the concentration of this ion in the solution,
the degree to which the metal corrodes is expected to be lowered. This
will be tested for sacrificial anodes identified in this experiment.
Aim
To investigate how the use of sacrificial anodes and manipulation of ion
concentration influences the corrosion of iron.
Hypothesis
formulation of
justified significant
hypotheses
It is predicted that when a more reactive metal is wrapped around a nail,
the nail will not be corroded. When a less reactive metal is wrapped, the
nail will be subject to corrosion. With the inclusion of a common ion in the
agar it is expected that the corrosion of the corresponding metal will be
limited.
Apparatus
2.5 g magnesium sulfate heptahydrate
0.1% phenolphthalein (0.1 g to 50-50 water-alcohol mixture)
2.9g zinc sulfate heptahydrate
0.1M potassium ferricyanide, K3Fe(CN)6
Agar
30 × petri dish
60 × iron nail
4 × Silver strip
4 × Copper strip
4 × Tin strip
8 × Zinc strip
4 × Aluminium strip
8 × Magnesium ribbon
4 | Chemistry 2007: Sample student assessment and responses
Method
effective and
efficient design,
refinement and
management of
investigations
selection and
adaptation of
equipment to
gather valid data
1. 250mL of distilled water was poured into a 400mL beaker and was
heated to boiling. The heat was then turned off and 5.0g of agar was
added, and stirred into the mixture until it was dissolved.
2. 10 drops of 0.1 M potassium ferricyanide and 5 drops of 0.1%
phenolphthalein solution were added to the solution. The agar was left
to cool.
3. An iron nail was sanded to remove any surface corrosion or
galvanisation and placed in the petri dish.
4. The agar was added to the petri dish. After 24 hours, observations
were made of the petri dish and photographs were taken and
recorded in Table 3 (see Appendix).
5. Steps 3 – 4 were repeated with the iron nail being wrapped in silver,
copper, tin, zinc, aluminium and magnesium strips. All metal strips
were sanded before being used. Observations were recorded in Table
4 (see Appendix).
6. Steps 1 – 4 were repeated, with 5.9 g of zinc sulfate (ZnSO4), to
create a 0.1M solution both with an iron nail and with the nail wrapped
in a zinc strip. Observations were recorded in Table 5 (see Appendix).
7. Steps 1 – 4 were repeated, with 5.5g of magnesium sulfate (MgSO4),
to create a 0.1M solution and both with an iron nail and with the nail
wrapped in a magnesium strip. Observations were recorded in Table
6 (see Appendix).
Results
Table 1: Summary of results – corrosion of metals
Metal tested
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning accessible
to intended
audiences through
innovative use of
tables
Corrosion
Silver
No
Copper
Yes
Tin
Yes
Iron
Yes
Zinc
Yes
Aluminium
No
Magnesium
Yes
NOTE: Bold type indicates unexpected results
See Table 3 (Appendix) for detailed notes and photographs. Coloured
shading is used in this table to identify the metal under consideration.
Queensland Studies Authority May 2013 | 5
Table 2: Summary of results – sacrificial anodes
Added Salt
none
selection and
adaptation of
equipment, and
appropriate
application of
technology to
record valid data
Sacrificial anode
Metal oxidised
Silver
Iron
Copper
Iron
Tin
Iron
None
Iron
Zinc
Zinc
Aluminium
Aluminium
Magnesium
Magnesium and
iron
None
Iron
Zinc
Iron
None
Iron
Magnesium
Magnesium and
iron
Zinc sulfate
Magnesium
sulfate
NOTE: Bold type indicates unexpected results
Electrochemical
equation
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝑍𝑛 → 𝑍𝑛2+ + 2𝑒 −
𝐴𝑙 → 𝐴𝑙 3+ + 3𝑒 −
𝑭𝒆 → 𝑭𝒆𝟐+ + 𝟐𝒆−
𝑴𝒈 → 𝑴𝒈𝟐+ + 𝟐𝒆−
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 −
𝑭𝒆 → 𝑭𝒆𝟐+ + 𝟐𝒆−
𝑴𝒈 → 𝑴𝒈𝟐+ + 𝟐𝒆−
See Tables 4-6 (Appendix) for detailed notes and photographs
Discussion
Sacrificial anodes
Table 1 lists the metals tested in order of increasing activity (i.e. the least
reactive metals to the most reactive metals.) Table 2 shows the outcome
of using these metals as sacrificial anodes with iron.
As expected, metals higher on the electrochemical series, when used as
sacrificial anodes, did not prevent the oxidation of iron (Refer to Table 2).
Furthermore, in the case of copper and silver, the presence of the second
metal appeared to increase the corrosion of iron, an unexpected result
(Refer to Table 4; Red and Orange sections).
6 | Chemistry 2007: Sample student assessment and responses
systematic analysis
of primary and
secondary data to
identify
relationships
between patterns,
trends, errors and
anomalies
formulation of
justified significant
hypotheses which
inform effective and
efficient refinement
of investigations
comparison and
explanation of
complex processes
and phenomena
systematic analysis
of primary and
secondary data to
identify
relationships
between patterns,
trends, errors and
anomalies
Similarly, metals lower in the electrochemical series (more reactive
metals) appeared to act as sacrificial anodes, corroding before the iron
nail did, generally meeting the expected the results (Refer to Table 2). In
the case of zinc, corrosion was evident on the surface of the zinc, while
the iron nail remained intact, with zinc acting as an effective sacrificial
anode (Refer to Table 4; Blue). Aluminium was similarly effective, despite
evidence of corrosion on the face of the aluminium not being observed;
the pink colouration indicated that an oxidation reaction had occurred
(Refer to Table 4; Purple). Given the absence of blue colouration, it could
2+
be inferred that that it was not the iron that was oxidised (as Fe was not
produced) and thus it must be the aluminium that acted as a sacrificial
anode. Magnesium produced unexpected results, with both the
magnesium and iron undergoing oxidation (Refer to Table 4; Pink). It was
expected that only the magnesium would corrode and act as a sacrificial
anode, thus protecting the iron.
In an attempt to explain the aluminium and magnesium results, all metals
were left for 24 hours, to observe if the metals corroded in unexpected
ways independently. In this test, aluminium did not appear to corrode at
all (Refer to Table 4; Purple). This is because when aluminium begins the
oxidation process, it forms aluminium oxide, which does not significantly
change the appearance of the aluminium but protects it from further
corrosion (Shwartz, 2000). This explains the limited pink produced by the
aluminium tests, as the oxidation process was limited. However, this
makes the absence of iron corrosion surprising, as it would be assumed
that once the aluminium was entirely oxidised the iron would be the next
to be oxidised. However, it is possible that the aluminium oxide layer not
only protected the aluminium, but acted to slow the corrosion reaction,
and thus protected the iron simultaneously.
The magnesium, however, corroded quickly, as expected (Refer to Table
2; Pink). Thus, the corrosion of iron, despite the presence of magnesium,
can be said to be due to magnesium’s high reactivity. It is possible that
the magnesium was entirely oxidised very quickly, as it was observed that
very little of the magnesium remained after 24 hours (Refer to Table 2;
Pink). Thus, once the magnesium was no longer available as a sacrificial
anode, iron replaced magnesium in the redox reaction.
Finally, in this test it was expected that the concentration of pink in the
various tests would match with the metal’s position in the electrochemical
series. Instead, in each of the tests in which corrosion occurred, the
intensity of pink was consistent (Refer to Table 1). It is expected that this
is due to the long time frame (24 hours) over which the dishes were left,
which resulted in even the least reactive metals having sufficient time to
corrode to such a degree that the pink was at its maximum intensity.
Queensland Studies Authority May 2013 | 7
Common ion effect
linking and
application of
electrochemistry
and solubility
concepts,
principles and
theories to find
solutions in
complex and
challenging
situations
As predicted, when zinc sulfate was added to the agar, the corrosion of zinc
was reduced, resulting in an increase in iron corrosion (Refer to Table 5). In
the test, the agar solution that resulted was observed to be white and
cloudy, indicating that despite the low concentration of zinc sulfate, it had
failed to dissolve. It is surprising that the zinc sulfate did not dissolve, given
that the substance has a solubility of 57.7g/100mL in water at 25°C and yet
failed to dissolve only 2.9g of the substance (Norkem, 2011). It is expected
that one of the components of the agar, such as the high concentration of
sodium chloride, interfered with the zinc dissolving and thus lowered its
solubility. This is because not only is there a maximum amount of a given
ion that can be dissolved in water, there is also a maximum amount of any
substance that the water molecules can dissolve. Thus, the sodium chloride
limited the amount of any additional substance that could be dissolved.
analysis and
evaluation of
complex scientific
interrelationships
Regardless, the result produced a strong blue colouration, suggesting the
corrosion of the iron nail, with very little zinc corrosion evident. This
suggests that the zinc strip was unable to act as the sacrificial anode, given
the zinc already present in the agar and thus was unable to prevent the
corrosion of iron. This is because, for the corrosion process to take place,
the zinc ions must dissolve in the water. Given the agar was a saturated
zinc solution (and thus no further zinc could dissolve), the oxidation reaction
could not take place. The results of this test suggest that another possible
means to prevent the corrosion of iron in water is to increase the
2+
concentration of Fe ions in the solution, which will prevent the oxidation
reaction from taking place.
comparison and
explanation of
complex
concepts,
processes and
phenomena
analysis and
evaluation of
complex scientific
interrelationships
However, the same test using magnesium sulfate in the agar produced very
different results (Refer to Table 6). Indeed, the magnesium, once again
acted as a sacrificial metal, experiencing significant corrosion and
preventing the bulk of the iron corrosion. However, in this experiment the
agar was not a saturated magnesium solution, as the entire volume of
magnesium salt dissolved. Magnesium sulfate has an even higher solubility
in water than zinc sulfate at 71g/100mL at 20°C (Hill Brothers, 2010). Thus,
the magnesium ions would still have been able to dissolve and the redox
reaction would still have been possible.
An anomaly in the experiment was the lack of pink in the experiment
involving the zinc sulfate (Refer to Table 5). In this test the phenolphthalein
failed to react, while the potassium ferricyanide reacted strongly. It is
expected that the inclusion of zinc sulfate resulted in unforseen reactions
within the agar that prevented the phenolphthalein from being an effective
indicator. Despite this, the test was successful, providing the expected
results, with the blue providing a sufficient result from which to infer that
rusting had taken place.
The most significant error in the experiment was a failure to regulate the
metal strip size. This means that it is more difficult to compare the various
successes of each metal as a sacrificial anode, given that some metals had
a greater mass which was available to react. It is possible that magnesium
would have been an equally effective sacrificial anode as zinc if the two
metals had shared the same dimensions. In future, a consistent metal strip
size would prevent this error and allow comparison of the various sacrificial
anodes more accurately.
exploration of
scenarios and
possible
outcomes
An extension to the experiment would be to use increasingly large pieces of
magnesium to test whether its failure to protect the iron from corrosion was
due to its being entirely consumed, or whether other factors are at play.
Alternative methods of corrosion protection could also be tested, such as
testing how effective painting iron nails is at preventing the redox reaction.
8 | Chemistry 2007: Sample student assessment and responses
Conclusion
justification of
conclusions
By using observing the electrochemical cell produced between an iron nail
and another metal, it was found that metals that are more reactive will
protect the iron nail from corrosion, in keeping with the hypothesis. It was
also found that the inclusion of a common ion in solution can prevent the
corrosion of the corresponding metal.
Bibliography
Atkins, P 2011, Reactions: The Private Life of Atoms, Oxford University
Press, Oxford
De, A 2003, A Text Book of Inorganic Chemistry, New Age International,
Ninth Edition, New Edition
Hill Brothers, 2010, Magnesium sulfate heptahydrate: MSDS, accessed 22
October 2012, http://www.hillbrothers.com/msds/pdf/n/magnesiumsulfate.pdf
assessment of
risk, safe
selection of
equipment
Lancashire, R 2008, Iron Chemistry, University of West Indies, accessed 15
October 2012, http://wwwchem.uwimona.edu.jm/courses/iron.html
Norkem, 2011, Zinc Sulfate heptahydrate: MSDS, accessed 22 October
2012, http://www.healthyhooves.eu/pdffiles/msds/Zinc%20Sulfate.pdf
ScienceLab.com, 2012, Agar-agar MSDS, accessed 21 October 2012,
http://www.sciencelab.com/msds.php?msdsId=9922809
ScienceLab.com, 2012, Phenolphthalein TS MSDS, accessed 21 October
2012, http://www.sciencelab.com/msds.php?msdsId=9926477
ScienceLab.com, 2012, Potassium ferricyanide MSDS, accessed 21
October 2012, http://www.sciencelab.com/msds.php?msdsId=9927405
Shwartz, M 2000, why aluminium doesn’t rust, Stanford News Service,
accessed 22 October 2012,
http://news.stanford.edu/pr/00/aluminum511.html
Spence, R et. al. 2007, Chemistry: A Contextual Approach, Heinemann,
Melbourne, Australia
Wong, I 2012, Teacher's Training, accessed 15 October 2012,
http://berryberryeasy.com/2011/02/berry-event-no-3-effective-chemistrypractical-course-johor-2010-kursus-amali-berkesan-kimia-johor-2010-johorbahru-jabatan-pendidikan-negeri-johor/
Zekan, 2012, Corrosion Results, accessed 15 October 2012,
http://www.dynamicscience.com.au/tester/solutions/chemistry/redox/rusting
%20agar%20results.htm
Queensland Studies Authority May 2013 | 9
Appendix
Figure 1: Mass of zinc sulfate for 0.1M solution
𝑛
𝑐=
𝑣
𝑛
0.1 =
0.1
𝑛 = 0.01 mol
𝑚 = 𝑛𝑀
𝑚 = 0.01 (65.41 + 32.07 + 4 × 16 + 7[2(1.008) + 16]
𝑚 = 0.01 × 287.592
𝑚 = 2.9 g
Figure 2: Mass of magnesium sulfate for 0.1M solution
𝑛
𝑐=
𝑣
𝑛
0.1 =
0.1
𝑛 = 0.01 mol
𝑚 = 𝑛𝑀
𝑚 = 0.01 (24.31 + 32.07 + 4 × 16 + 7[2(1.008) + 16]
𝑚 = 0.01 × 246.492
𝑚 = 2.5 g
10 | Chemistry 2007: Sample student assessment and responses
Note: Some multiples of trials, risk assessment forms and MSDS data
sheets included in the student work have been omitted for brevity.
selection and
adaptation of
equipment, and
appropriate
application of
technology to
record and
process valid data
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning
accessible to
intended
audiences
through
innovative use of
tables
Queensland Studies Authority May 2013 | 11
selection and
adaptation of
equipment, and
appropriate
application of
technology to
record and
process valid data
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning
accessible to
intended
audiences
through
innovative use of
tables
12 | Chemistry 2007: Sample student assessment and responses
selection and
adaptation of
equipment, and
appropriate
application of
technology to
record and
process valid data
discriminating
selection, use and
presentation of
scientific data and
ideas to make
meaning
accessible to
intended
audiences
through
innovative use of
tables
Queensland Studies Authority May 2013 | 13