Download Chemistry 91166 (2.6) Schedule

NCEA Level 2 Chemistry 91166 (2.6) — page 1 of 5
SAMPLE ASSESSMENT SCHEDULE
Chemistry 91166 (2.6): Demonstrate understanding of chemical reactivity
Assessment Criteria
Achievement
Achievement with Merit
Achievement with Excellence
Demonstrate understanding involves
describing, identifying, naming,
drawing, calculating, or giving an
account of chemical reactivity. This
requires the use of chemistry
vocabulary, symbols and
conventions.
Demonstrate in-depth understanding
involves explaining chemical
reactivity. This requires explanations
that use chemistry vocabulary,
symbols and conventions.
Demonstrate comprehensive
understanding involves
elaborating, justifying, relating,
evaluating, comparing and
contrasting, or analysing chemical
reactivity. This requires the
consistent use of chemistry
vocabulary, symbols and
conventions.
Evidence Statement
One
(a) (i)
(ii)
Expected Coverage
The time taken would be longer (ie,
slower reaction rate).
The time taken would be longer (ie,
slower reaction rate).
(b)
For a reaction to occur particles must
collide and these collisions must be
effective. For a collision to be effective
particles must have energy greater than
the activation energy (this is the minimum
energy that a particle needs to react
when it collides).
When the concentration is decreased
there is a decreased frequency of
collisions between particles so fewer
particles react. This means that the rate
of reaction is decreased.
When the temperature is decreased the
kinetic energy and hence the speed of
reacting particles decreases. This means
that particles collide less frequently. It
also means that the particles collide with
less energy (and less effective collision)
as fewer particles have energy greater
than the activation energy. The activation
energy (or energy of activation) is a
measure of the energy required to bring
about effective particle collisions in order
Achievement
Merit
THREE of:
EITHER
 in (a) time taken
would be longer
in (i) and (ii)
in (b) rate of
reaction explained
in terms of
frequency of
particle collision
for the effect of
decreased
concentration
 in (b) reactions
occur when
particles collide
 in (b) there are
fewer collisions
when the
concentration is
decreased
 in (b) activation
energy is the
minimum energy
required for
particles to
collide
 in (b) the
solution would
go cloudy more
slowly.
OR
in (b) rate of
reaction explained
in terms of energy
of particle collision
for the effect of
decreased
temperature.
Excellence
In (b) collision
theory used to
explain the rate of
reaction in terms
of BOTH
frequency and
energy of particle
collision, related
to the changes
made to the
reaction between
hydrochloric acid
and sodium
thiosulfate.
NCEA Level 2 Chemistry 91166 (2.6) — page 2 of 5
for reactants to form products).
In both instances (ie diluted solution,
lower temperature) the solution would
take longer to go cloudy as the collisions
between the hydrochloric acid and the
sodium thiosulfate would occur less
frequently and/or the collisions would be
less effective.
NØ
No response or does not address the question.
N1
Provides some writing but does not fulfil any statement from the Achievement criteria column.
N2
Any ONE statement from the Achievement criteria column.
A3
Any TWO statements from the Achievement criteria column.
A4
Any THREE statements from the Achievement criteria column.
M5
Any ONE statement from the Merit criteria column.
M6
Gives BOTH statements from the Merit criteria column.
E7
Only minor error – omission or additional information – from the Excellence criteria column.
E8
ALL the evidence from the Excellence criteria column.
Two
(a) (i)
(ii)
(b)
Expected Coverage
NH4+ + OH–
C2H5COOH +
THREE of:
OH-
See Appendix One.
See Appendix Two.
(c)
Achievement
0.1molL–1HCI, 0.01molL–1 HCI,
0.01molL–1 CH3COOH, 0.01molL–1
NaOH Highest pH.
(See Appendix Three)
HCl is a strong acid and is fully
dissociated in water. It donates its
protons readily to become completely
dissociated.
HCl + H2O  H3O+ + Cl–
The 0.1 mol L–1 HCl will produce 0.1
mol L–1 hydronium ion; the 0.01 mol
L–1 HCl will produce 0.01 mol L–1
hydronium ion.
Hence the 0.1 mol L-1 HCl will have
the lowest pH.
 in (a) BOTH
equations correct
 in (b) one part of
calculation
correct eg[H3O+]
= 2.47  10–2 for
HCl and [OH–] =
0.0534 for NaOH;
ONE pH correct;
ONE solution
correct for all
three values
 in (b) [H3O+] =
2.47  10–2 for
HCl and [OH-] =
0.0534 for NaOH
OR ONE pH
correct OR ONE
solution correct
for all three
values
 in (c) ONE
equation for
dissociation
written OR HCl is
Merit
In (b) BOTH pH
correct
AND
in (c) comparison
made between a
strong acid and
weak acid; or
between a dilute
acid and more
concentrated acid;
or between a
strong acid and a
strong base in
terms of proton
transfer and pH.
OR (FOR M6)
In (c) 2 out of the 3
comparisons with
at least one
equation correct
Excellence
In (c) discussion
explores the links
between [HCl],
[CH3COOH],
[NaOH] and degree
of proton transfer,
amount of [H3O+] or
[OH–] in solution
and hence the pH
of the solution;
including
calculation of a
strong acid or
strong base and
some equations.
NCEA Level 2 Chemistry 91166 (2.6) — page 3 of 5
CH3COOH is a weak acid and is only
partially dissociated in water. Fewer
protons are donated to water.
CH3COOH + H2O
H3O+ +
CH3COO–
a strong acid,
CH3COOH is a
weak acid OR
NaOH is
basic/has highest
pH.
The 0.01 mol L–1 CH3COOH will
produce fewer protons, and hence
lower [H3O+] than 0.01 mol L–1 HCl;
hence 0.01 mol L–1 HCl will have a
lower pH than 0.01 mol L-1
CH3COOH.
NaOH is a strong base and is fully
dissociated in water. It accepts
protons readily.
NaOH  Na+ + OH–
For NaOH the concentration of
hydroxide ion is greater than the
concentration of hydronium ion, [OH–]
> [H3O+]. This solution has the
highest pH.
NØ
No response or does not address the question.
N1
Provides some writing but does not fulfil any statement from the Achievement criteria column.
N2
Any ONE statement from the Achievement criteria column.
A3
Any TWO statements from the Achievement criteria column.
A4
Any THREE statements from the Achievement criteria column.
M5
Any ONE statement from the Merit criteria column.
M6
Gives BOTH statements from the Merit criteria column.
E7
Only minor error – omission or additional information – from the Excellence criteria column.
E8
ALL the evidence from the Excellence criteria column.
NCEA Level 2 Chemistry 91166 (2.6) — page 4 of 5
Three
Expected Coverage
(a) (i)
The colour of the solution becomes less
brown.
Decreasing the temperature causes an
equilibrium shift to favour the reaction
that releases energy / heat, ie shift in
the exothermic direction. This is the
reverse direction (favouring N2O4).
(ii)
(b) (i)
(ii)
Achievement
THREE of:
 in (a) BOTH
colour changes
correct
The colour of the solution becomes
more brown.
 in (a)
temperature
decrease
causes a shift in
the exothermic
direction
Decreasing the pressure causes the
equilibrium to shift to increase the
number of gas particles in the
equilibrium mixture, ie shift to the side
with the greater number of moles. This
is the forward direction (favouring NO2).
 in (a) pressure
decrease
causes a shift to
the side with the
greater number
of moles
[SO3]2 / [SO2]2[O2]
[SO3]2 / [SO2]2[O2] =
(0.7)2 / (0.5)2.(0.1) = 19.6
This value is less than Kc at equilibrium
(280) meaning that there are more
reactants than products at this stage of
the reaction. This means that the
equilibrium will move in the forward
direction to produce more product, SO3
and hence allow the mixture to reach
equilibrium This value will approach Kc
for this reaction.
 in (b) Kc
expression
correct
Merit
Excellence
In (a) ONE of
temperature or
pressure
explained in terms
of equilibrium
principles
In (b) equilibrium
principles used to
justify equilibrium
shift both
qualitatively and
quantitatively.
AND
in (b) explanation
of equilibrium shift
relates the
calculated value
to Kc value.
OR (FOR M6)
In (a) BOTH
temperature and
pressure
explained.
 in (b) value
calculated
accurately.
NØ
No response or does not address the question.
N1
Provides some writing but does not fulfil any statement from the Achievement criteria column.
N2
Any ONE statement from the Achievement criteria column.
A3
Any TWO statements from the Achievement criteria column.
A4
Any THREE statements from the Achievement criteria column.
M5
Any ONE statement from the Merit criteria column.
M6
Gives BOTH statements from the Merit criteria column.
E7
EITHER qualitative or quantitative discussion.
E8
EITHER BOTH qualitative or quantitative discussion.
NCEA Level 2 Chemistry 91166 (2.6) — page 5 of 5
Appendix One: Question Two (b)
[H3O+] (mol L-1)
[OH-] (mol L-1)
pH
2.47  10–2
4.05  10–13
1.61
[H3O+] (mol L–1)
[OH-] (mol L–1)
pH
1.87  10–13
0.0534
12.7
Appendix Two: Question Two (b)
Appendix Three: Question Two (c)
0.1 mol L–1 HCl
Lowest pH
0.01 mol L–1 HCl,
0.01 mol L–1
CH3COOH,
0.1 mol L–1 NaOH,
Highest pH