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Chemistry 2007
Sample assessment instrument and indicative responses
Supervised assessment
December 2009
Purposes of assessment 1
The purposes of assessment are to:

promote, assist and improve student learning

inform programs of teaching and learning

provide information for those people — students, parents, teachers — who need to know about the progress
and achievements of individual students to help them achieve to the best of their abilities

provide information for the issuing of certificates of achievement

provide information to those people who need to know how well groups of students are achieving (school
authorities, the State Minister for Education and Training and the Arts, the Federal Minister for Education).
It is common practice to label assessment as being formative, diagnostic or summative, according to the major
purpose of the assessment.
The major purpose of formative assessment is to help students attain higher levels of performance. The major
purpose of diagnostic assessment is to determine the nature of students’ learning, and then provide the
appropriate feedback or intervention. The major purpose of summative assessment is to indicate the achievement
status or standards achieved by students at a particular point in their schooling. It is geared towards reporting and
certification.
Syllabus requirements
Teachers should ensure that assessment instruments are consistent with the requirements, techniques and
conditions of the Chemistry syllabus and the implementation year 2007.
Assessment instruments 2
High-quality assessment instruments 3 :

have construct validity (the instruments actually assess what they were designed to assess)

have face validity (they appear to assess what you believe they are intended to assess)

give students clear and definite instructions

are written in language suited to the reading capabilities of the students for whom the instruments are
intended

are clearly presented through appropriate choice of layout, cues, visual design, format and choice of words

are used under clear, definite and specified conditions that are appropriate for all the students whose
achievements are being assessed

have clear criteria for making judgments about achievements (these criteria are shared with students before
they are assessed)

are used under conditions that allow optimal participation for all

are inclusive of students’ diverse backgrounds

allow students to demonstrate the breadth and depth of their achievements

only involve the reproduction of gender, socioeconomic, ethnic or other cultural factors if careful consideration
has determined that such reproduction is necessary.
______________________________________________________________________________
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2
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QSA 2008, P–12 Assessment Policy, p. 2.
Assessment instruments are the actual tools used by schools and the QSA to gather information about student achievement, for
example, recorded observation of a game of volleyball, write-up of a field trip to the local water catchment and storage area, a test of
number facts, the Senior External Examination in Chinese, the 2006 QCS Test, the 2008 Year 4 English comparable assessment
task.
QSA 2008, P–12 Assessment Policy, pp. 2–3.
Chemistry 2007
Chemistry 2007
Sample assessment instrument and indicative responses
Supervised assessment
Compiled by the Queensland Studies Authority
December 2009
About this assessment instrument
The purpose of this document is to inform assessment practices of teachers in schools. For this
reason, the assessment instrument is not presented in a way that would allow its immediate
application in a school context. In particular, the assessment technique is presented in isolation
from other information relevant to the implementation of the assessment. For further information
about those aspects of the assessment not explained in this document, please refer to the
assessment section of the syllabus.
This sample provides opportunities for students to demonstrate:

comparing and explaining complex concepts, processes and phenomena

linking and application of algorithms, concepts, principles, theories and schema to find
solutions in complex and challenging situations.
This sample assessment instrument is intended to be a guide to help teachers plan and develop
assessment instruments for individual school settings.
Sample assessment and indicative responses Supervised assessment
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Assessment instrument
The student work presented in this sample is in response to assessment items which are subsets or
parts of an assessment instrument.
Question 1:
1. Most swimming pools use 'chlorine' as the sanitizing agent to kill bacteria and viruses. There
are a variety of ways of generating the 'chlorine' in pool water ranging from; the direct bubbling in
of Cl2 gas, the addition of OCl2, the electrolysis of NaCl in saltwater pools, to the most widely used
method, the addition of hypochlorite compounds. The active chemical produced in each method is
HOCl or hypochlorous acid.
Once in the water, an equilibrium is established between the strong oxidant HOCl and the weaker
OCl- ion.
HOCl + H2O  H3O+ + OCl-
Ka = 3.0 x 10-8
This equilibrium system is pH dependent and the following graph shows how the concentrations
of HOCl and OCl- change with the pH.
The level of 'free chlorine' (ie; HOCl + OCl-) present in a pool is most commonly measured using
a pool testing kit. This involves adding a special chemical to a water sample and checking if the
colour produced shows that the amount of chlorine is in or outside of the accepted range.
The amount of free chlorine can be determined more accurately by carrying out a redox titration
reaction on a sample of pool water. The titration involves two simultaneous redox reactions as
shown below;
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OCl-(aq) + 2I-(aq) + 2H+(aq)  Cl-(aq) + I2(aq) + H2O(l)
(equation 1)
2S2O32-(aq) + I2 (aq)  S4O62-(aq) + 2I-(aq)
(equation 2)
Chemistry 2007
A solution of known concentration of thiosulphate ions (S2O32-) is reacted with a sample of pool
water containing OCl- ions and when the reaction is complete the number of moles of OCl- can be
calculated.
Refer to the above information in answering the following questions:
A Year 12 student invites some of her friends over for a pool party. The pool is regularly
maintained by the student’s parents and so the pH level was at 7.5 and the 'free chlorine' at the
recommended level of 2ppm (ie; [HOCl] + [OCl-] = 3.84 x 10-5M) before the students used the
pool.
After the party was over the student tested the water and found that the pH was 8.5 and the 'free
chlorine' level was 0 – 0.5ppm.
The student also collected a 100mL sample of the pool water to carry out a titration to check if the
chlorine level was as low as indicated by the test kit. A volume of 38mL of 5.0 x 10-6M
thiosulphate solution was found to react with the OCl- ions in the pool water sample.
(a) Only OCl- is shown as reacting with thiosulphate ions in equations (1) & (2) for the titration.
Explain how titrating for the OCl- ion only, enables the amount of 'free chlorine' to be determined
and then show that the concentration of ‘free chlorine’ in the pool water is 9.5 x 10-6 mol/L.
(b) Show that the ratio of [HOCl] : [OCl-] agrees with what is indicated in the graph when the pH
is 8.5, and then determine the concentration of both HOCl and OCl- that are present.
Question 2:
In the equilibrium reaction;
MnO4– (aq) + H2O2 (aq) + 6H+ (aq)  4H2O (aq) + Mn2+ (aq) + O2 (g)
the MnO4– species has a purple/pink colour and is the only coloured species.
An experiment to observe the effects of heat on this equilibrium reaction gave the following data:
Temperature (K)
Observations
273
Pale pink, almost clear
283
Pale pink
293
Pale pink
323
Slightly deeper colour than 293K
373
Deep pink, nearly purple
Determine whether the reaction as written was exothermic, endothermic or not possible to predict,
and justify your decision.
Sample assessment and indicative responses Supervised assessment
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5
Instrument-specific criteria and standards
Schools draw instrument-specific criteria and standards from the syllabus dimensions and exit
standards. Schools will make judgments about the match of qualities of student responses with the
standards descriptors that are specific to the particular assessment instrument. While all syllabus exit
descriptors might not be assessed in a single assessment instrument, across the course of study,
opportunities to demonstrate all the syllabus dimensions and standards descriptors must be provided.
The assessment instrument presented in this document provides opportunities for the demonstration
of the criterion:

Knowledge and conceptual understanding:
o comparison and explanation of complex concepts, processes and phenomena
o
linking and application of algorithms, concepts, principles, theories and schema to find
solutions in complex and challenging situations.
This document provides information about how the qualities of student work match the relevant
instrument-specific criteria and standards at standards A and C. The standard A and C descriptors
are presented below. The complete set of instrument-specific criteria and standards is on page 11.
Knowledge and
Standard A
Standard C

Explains complex concepts and processes
relating to swimming pool equilibria.

Explains the simple aspects of concepts and
processes relating to swimming pool
equilibria.

Links and applies algorithms and concepts
to find a solution to a complex and
challenging problem on pool chemistry.

Applies algorithms and concepts to find a
solution to the simple aspects of a problem on
pool chemistry.

Links and applies Le Chatelier’s Principle to
find a solution to a complex and challenging
equilibrium problem.

Applies Le Chatelier’s Principle to find a
solution to the simple aspects of an
equilibrium problem.
conceptual
understanding –
explains concepts,
processes and
phenomena
Knowledge and
conceptual
understanding –
applies algorithms,
principles and
concepts
Knowledge and
conceptual
understanding –
links and applies
concepts and
principles
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Chemistry 2007
Sample student responses: Standard A
Standard descriptors
Student response A
(a) The overall reaction that occurs during the titration is:
OCl-(aq) + 2H+(aq) 2S2O32-(aq)  Cl-(aq) + H2O(l) ) + S4O62-(aq)
where OCl-(aq) reacts with S2O32-(aq) in the ratio of 1:2.
Explains complex concepts and
processes relating to swimming
pool equilibria.
As the OCl-(aq) is used up in the reaction its concentration
decreases in the pool water sample. However, OCl-(aq) is part of
the equilibrium system, HOCl + H2O  H3O+ + OClDecreasing the concentration of OCl-(aq) causes the system to
oppose this change and so the reaction moves to the right
producing more OCl-(aq) but decreasing the concentration of HOCl.
This continues until all of the HOCl is changed to OCl-(aq).
Thus the titration enables the determination of the concentration of
OCl-(aq) , which will be equal to the combined concentrations of
OCl-(aq) and HOCl that were present in the pool water eg the free
chlorine concentration.
In the actual titration, Vol of S2O32 = 38ml = 0.38L and [S2O32] =
5.0x10-6M.
Therefore, moles S2O32 = conc S2O32 x vol S2O32
n S2O32 = [S2O32] x Vol of S2O32
= 5.0x10-6 x 0.38
= 1.9 x 10-6 moles
Links and applies algorithms and
concepts to find a solution to a
complex and challenging problem
on pool chemistry.
From the equation for the titration, n S2O32 = 2 x n OClTherefore, n OCl- = n S2O32 /2
= 1.9x 10-6 /2
= 9.5 x 10-7 moles
Volume of pool water = 100ml = 0.100L
Therefore, [ OCl- ] = 9.5 x 10-7 /0.100
= 9.5x 10 -6 M
(b) When pH = 8.5, the graph indicates that [HOCl] : [OCl-] = 1:9
At pH = 8.5, [H3O+] = 10-8.5
= 3.16 x 10-9 M
Ka for HOCl = 3.0 x 10-8
Links and applies algorithms and
concepts to find solutions to
complex and challenging problems
on pool chemistry
HOCl + H2O  H3O+ + OClKa = [H3O+] x [OCl-] / [HOCl]
Therefore, [HOCl] / [OCl-] = [H3O+] / Ka
= 3.16x10-9 /3.0 x 10-8
= 0.11
This is equivalent to 1:9
The free chlorine concentration at ph = 8.5 is 9.5 x 10-6 M
Now, [HOCl] + [OCl-] = 9.5 x 10-6
Sample assessment and indicative responses Supervised assessment
eqn 1
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7
Sample student responses: Standard A
And [HOCl] / [OCl-] = 0.11
Therefore, [HOCl] = 0.11 x [OCl-]
eqn 2
Substitute eqn 2 into eqn 1
0.11 x [OCl-] + [OCl-]
Links and applies algorithms and
concepts to find solutions to
complex and challenging problems
on pool chemistry
1.11 x [OCl-]
[OCl-]
= 9.5 x 10-6
= 9.5 x 10-6
= 9.5 x 10-6 / 1.11
= 8.6 x 10 -6 M
And [HOCl] = 9.5 x 10-6 - 8.6 x 10 -6
= 9.0 x 10-7 M
OR [OCl-]
= 0.9 x 9.5 x 10-6
= 8.6 x 10 -6 M
And [HOCl] = 0.1 x 9.5 x 10-6
= 9.0 x 10-7 M
In this method, the ratio of [HOCl] : [OCl-] when pH =8.5
represents the only accurate way of knowing the individual
concentrations of both substances after equilibrium has been reestablished in the pool.
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Chemistry 2007
Sample student responses: Standard C
Standard descriptors
Student response C
(a) By titrating the OCl-(aq) ion, the amount of free chlorine is able to be
determined because it is able to show how much chlorine ia able to
react with any micro-organisms. When it reaches the equivalence
point, it is the amount of Cl- readily available.
Explains the simple aspects of
concepts and processes
relating to swimming pool
equilibria.
OCl-(aq) + 2I-(aq) + 2H+(aq)  Cl-(aq) + I2(aq) + H2O(l)
(1)
2S2O32-(aq) + I2 (aq)  S4O62-(aq) + 2I-(aq)
(2)
The two equations show the chloride ions present. This is equal to HOCl
and the OCl-. This changes as the titration proceeds.
In the actual titration, Vol of S2O32 = 38ml = 0.38L and [S2O32] = 5.0x10M.
6
Therefore, moles S2O32 = conc S2O32 x vol S2O32
Applies algorithms and
concepts to find a solution to
the simple aspects of a problem
on pool chemistry.
n S2O32 = [S2O32] x Vol of S2O32
Sample assessment and indicative responses Supervised assessment
= 5.0x10-6 x 0.38
= 1.9 x 10-6 moles
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9
Sample indicative responses: Question 2
Question 2
Standard descriptors
Student response A
MnO4– (aq) + H2O2 (aq) + 6H+ (aq)  4H2O (aq) + Mn2+ (aq) + O2 (g)
Links and applies Le Chatelier’s
Principle to find a solution to a
complex and challenging
equilibrium problem.
Standard descriptors
The data provided shows that as the temperature increases from 273k
to 373K, the solution shows an increasing intensity in the purple/pink
colours. This indicates that the concentration of the MnO4– (aq) ion is
increasing.
According to Le Chatelier’s principle, as the temperature of an
equilibrium system increases the system responds by seeking to
remove the extra energy. Thus the endothermic reaction is the one
that is favoured. In this case the reaction to the left or towards the
reactants. This means that the energy term is on the right hand side of
the equation as written. Thus the equation as written is exothermic,
Student response C
MnO4– (aq) + H2O2 (aq) + 6H+ (aq)  4H2O (aq) + Mn2+ (aq) + O2 (g)
Applies Le Chatelier’s Principle to
find a solution to the simple
aspects of an equilibrium
problem.
The data provided shows that as the temperature increases from 273k
to 373K, the solution goes from pale pink to deep pink. This indicates
that the concentration of the MnO4– (aq) ion is increasing.
According to Le Chatelier’s principle, as the temperature of an system
increases the system releases heat in the form of energy.
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Chemistry 2007
Instrument-specific criteria and standards
Q1(a)
(KCU2)
Explains
concepts,
processes
and
phenomena
Q1 (a)
(KCU3)
Applies
algorithms,
principles
and
concepts
Q1 (b)
(KCU3)
Applies
algorithms,
principles
and
concepts
Q2 (KCU3)
Links and
applies
concepts &
principles
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Chemistry 2007
Standard A
Standard B
Standard C
Standard D
Standard E
 Explains
complex
concepts and
processes
relating to
swimming
pool
equilibria.
 Explains
concepts and
processes
relating to
swimming
pool
equilibria.
 Explains the
simple
aspects of
concepts
and
processes
relating to
swimming
pool
equilibria.
 Describes the
simple
aspects of
processes
relating to
swimming
pool
equilibria.
 Identifies
isolated
simple
aspects
about
swimming
pool
equilibria.
 Links and
applies
algorithms
and concepts
to find a
solution to a
complex and
challenging
problem on
pool
chemistry.
 Links and
applies
algorithms
and concepts
to find a
solution to a
complex or
challenging
aspect of a
problem on
pool
chemistry.
 Applies
algorithms
and
concepts to
find a
solution to
the simple
aspects of a
problem on
pool
chemistry.
 Applies
algorithms to
find a solution
to some
simple
aspects of a
problem on
pool
chemistry.
 Applies an
aspect of an
algorithm to
find a
solution to
some basic
part of a
problem on
pool
chemistry.
 Links and
applies
algorithms
and concepts
to find
solutions to
complex and
challenging
problems on
pool
chemistry.
 Links and
applies
algorithms
and concepts
to find
solutions to
complex or
challenging
aspects of
problems on
pool
chemistry.
 Applies
algorithms
and
concepts to
find a
solutions to
the simple
aspects of
problems on
pool
chemistry.
 Applies
algorithms to
find a solution
to some
simple
aspects of
problem(s) on
pool
chemistry.
 Applies an
aspect of an
algorithm to
find a
solution to
some basic
part of a
problem on
pool
chemistry.
 Links and
applies Le
Chatelier’s
Principle to
find a
solution to a
complex and
challenging
equilibrium
problem.
 Links and
applies Le
Chatelier’s
Principle to
find a
solution to a
complex or
challenging
equilibrium
problem.
 Applies Le
Chatelier’s
Principle to
find a
solution to
the simple
aspects of
an
equilibrium
problem.
 Applies Le
Chatelier’s
Principle to
find a solution
to some of
the simple
aspects of an
equilibrium
problem.
 Applies an
aspect of Le
Chatelier’s
Principle to
find a
solution to
some basic
part of an
equilibrium
problem.