Download NZIC 2012 - Rangiora High School

NZIC 2012
Assessment Schedule for Chemistry 2.6 AS 91166
Chemistry: Demonstrate understanding of chemical reactivity
While the writers of this assessment have worked to compile a resource that meets NCEA requirements, it has no official status and teachers may wish to adjust questions and the
assessment schedule as they see fit.
Q
ONE
(a)
(b)
Evidence
As the concentration of HCl increases, the time taken for the
cross to disappear decreases. A higher concentration of HCl
means there are more HCl molecules per unit volume to react at
any one time. This increases the total number of collisions per
second (frequency of collisions) to give a faster rate of reaction.
As the temperature increases, the rate of reaction increases.
This is because as temperature increases, the molecules have
more kinetic energy / higher energy. Particle collisions are more
effective in producing a reaction. There are more effective /
successful collisions because more particles have enough kinetic
energy to overcome the activation energy for the reaction,
resulting in an increased rate of reaction.
As a minor effect particles are moving faster. There will be an
increase in the frequency of collisions between particles.
Achievement
Achievement with Merit
THREE of:
An increase in the HCl
concentration increases the
rate of reaction/time taken
decreases
Increase in rate of reaction
explained in terms of:
EITHER
More particles per unit
volume
OR
Increase in frequency of
collisions
OR
More particles therefore
more collisions.
States the trend, i.e. that
reaction rate is increased.
More of the collisions will
be effective since more
particles have sufficient
energy to reach the
activation energy required.
OR
Links temperature to more
of the collisions being
effective / successful
Molecules moving faster
gives more collisions per
second
Achievement with
Excellence
Increase in rate of
reaction explained in
terms of:
BOTH
More particles per unit
volume
AND
Increase in frequency of
collisions.
Relates rates of reaction
to Collision Theory in
terms of an increase in
temperature results in
particles having more
kinetic energy / higher
energy and moving
Links temperature increase faster.
to molecules which are
AND
moving faster, so
Collisions are more
increased frequency of
likely to have sufficient
collisions
energy to overcome the
activation energy
barrier so more
successful / effective
collisions.
N1: One correct
N2: Two correct
A3: Meets criteria
A4: All four correct
M5: Meets criteria for (a)
or (b)
M6: Meets criteria for (a)
and (b)
E7: (a) correct or minor
error in (b)
E8: (a) and (b) correct,
but minor error allowed
in (a)
Q
TWO
(a)
(b) (i)
(ii)
Evidence
[H3O+] / mol L-1
0.0255
7.08 x 10–5
6.33 x 10–14
HOBr + H2O
Achievement with Merit
Achievement with
Excellence
Recognises the solution has
an increased concentration
of OH-.
Full explanation, including
equation for OBr – ions
reacting with water to
form OH-, and [OH-] >
[H3O+].
Full explanation,
including equation for
OBr – ions reacting
with water to form OH-,
and [OH-] > [H3O+].
Classifies all acids
correctly/One acid correctly
classified with reason.
Uses concentration to
determine strength for
both strong (HA or HC)
and weak acids (HB).
Fully explains acid
strength for all THREE
acids.
Achievement
[OH-] / mol L–1
pH
3.92 x 10–13
1.41 x 10–10
0.158
–
OBr + H3O+
1.59
4.15
13.2
OBr – reacts with water / accepts a proton to produce OH-:
OBr – + H2O ⇌ HOBr + OH–
THREE of:
Three calculations correct
Correct equation
The concentration of hydroxide ions, OH-, has increased so
[OH–] > [H3O+]. The resulting solution is therefore basic.
(c) (i)
HA is a strong acid since a 0.100 mol L–1 and [H3O+] = 0.100
mol L–1 (pH = 1) HA reacts completely with water.
If HB was a strong acid, its pH would be 2 (-log 0.0100 = 2).
Since HB has a pH of 2.95, reaction of HB with water is
incomplete so HB is a weak acid.
Like HA, HC is also a strong acid since a 0.000100 mol L-1
solution gives a pH = 4 [H3O+] = 0.0001 mol L–1
(ii)
The electrical conductivity of a solution depends on the number
of ions present. The concentration of ions in solution can be
calculated from the pH of the acid.
Conductivity depends on
number of ions present in
solution.
Uses pH to compare
conductivity for two acids.
Fully explains
conductivity for all
THREE acids
Since HA is a strong acid, it reacts completely with water giving
a 0.1 mol L–1 solution, so its conductivity will be high.
HA + H2O  A– + H3O+
Since HB is a weak acid, reaction with water is incomplete
giving a solution that is ~ 10–3 mol L–1 in ions so its
conductivity will be lower than that of HA.
HB + H2O ⇌ B– + H3O+
Since HC is a strong acid, it reacts completely with water.
However, concentration of ions is 10–4 mol L–1 so it is a poor
conductor compared to HA and HB.
HC + H2O  C– + H3O+
N1: One correct
N2: Two correct
A3: Meets criteria
A4: Four correct
M5: One correct
M6: Two correct
E7: (b) correct plus
(c)(i) or (c)(ii)
E8: (c) correct
Q
THREE
(a) (i)
(ii)
Evidence
Achievement
Achievement with Merit
Achievement with
Excellence
THREE of:
The increase in [N2O4] causes the equilibrium to turn a lighter
brown.
An increase in pressure will cause the equilibrium to shift
towards the side with the least number of gaseous particles.
There are two gas molecules on the left and one on the right, so
the equilibrium will shift towards the products / in the forward
direction to therefore minimise the change / decrease the
pressure.
Describes both
observations/both shifts
correctly described
Explains both observations
by linking them to the
appropriate shifts and the
species causing the colour
change.
TWO shifts correctly
described.
The equilibrium turns a darker brown.
The increase in [N2O4] causes the equilibrium to shift towards
the reactants / in the reverse direction to use up some of the
extra N2O4 and therefore minimise the change. This causes an
increase in [NO2], so
Kc expression correct
(b) (i)
Kc = [N2O4] / [NO2]2
Kc = [N2O4] / [NO2]2
164 = [N2O4] / (0.0185)2
[N2O4] = 0.0561 mol L-1
Correct substitution but
minor numerical error in
calculation, e.g. [NO2] not
squared.
Calculation correct
(ii)
(iii)
A temperature increase causes the equilibrium to shift in the
endothermic direction to absorb some of the extra heat energy.
This causes this equilibrium to shift towards to reactants/LHS.
As a result, the [NO2] will increase compared to [N2O4], so the
Kc value will decrease.
Identifies equilibrium shifts
in endothermic
direction/[NO2] will
increase compared to
[N2O4]
Relates the shift in
equilibrium position to
changes in [NO2] and
[N2O4], but does not
correctly link to the
change in Kc.
Fully explains the
effect of heat on the
equilibrium position
and how the relative
[NO2] and [N2O4]
affect the value of Kc.
M5: One correct
M6: Two correct
E7: (b) correct with
minor detail missing in
(iii)
E8: (b) correct
N1: One correct
N2: Two correct
A3: Meets criteria
A4: Four correct
Judgement Statements:
Achieved
Merit
Excellence
Score: 7 - 12
Score: 13 - 18
Score: 19 - 24