1
Assessment schedule 2008
AS 90310 [Chem 2.6] Describe thermochemical and equilibrium principles
One (a)
(b)
(c)
(d)
One(e)
Criteria
Endothermic
Endothermic
Exothermic
Endothermic
n = m/M = 1.05g / 58.0gmol-1 = 0.0181mol
0.0181mol x -2874kJmol-1 = 52.0kJ = 52029J (52000J)
T = E / m.s = 52029J / (500g x 4.18) = 24.9oC
One(f)
480kJ / -2874kJmol-1 = 0.167mol
m = nM = 0.167mol x 58gmol-1 = 9.69g
Two (a)
(b)
Dynamic equilibrium is when the rate of forwards reaction is equal to the rate of
backwards reaction.
Concentration unchanging and closed system are not expected with this answer.
By walking outside there is an increase in the “concentration” of UV.
An increase in the concentration of reactants causes the equilibrium to shift to make
more products, as this acts to minimize the increase in reactants. This is Le
Chatalier’s Principle.
As silver is a product, more of it now exists, and the glasses are darker as a result.
By going inside, the UV is removed, causing the equilibrium to shift to the reactants
side, making more AgCl and less dark Ag.
A
M
E
Three correct
Two steps in
calculation
correct (error
can carry
forward).
One step
correct with
error carried
through.
Gives the
description
of dynamic
equilibrium
Describes
one
equilibrium
shift
Temperature
change value
correct.
Both steps
correct with
units.
Explains
why a shift
occurs.
Discusses
both shifts
with the
changes
made to the
system.
2
Three(a) Expect six CO, three O2, one CO2 (based on two CO2’s converting) or similar.
When temperature is increased, the equilibrium acts to minimize this increase.
Endothermic reactions (in this case backwards reactions) occur.
This decreases the concentration of CO2 and increases the concentration of CO and O2.
(b)
Expect no change drawn (therefore four CO, two O2 and three CO2).
A catalyst has no effect on the equilibrium position / concentration of the species.
The catalyst does speed up the rates of forwards and backwards reactions.
(c)
Expect two CO, one O2 and five CO2 (based on two CO’s and one O2 converting).
When the pressure is increased the system acts to reduce that pressure.
The equilibrium will shift to the side with the fewest gaseous particles; in this case the
forwards reaction is favoured.
This increases the concentration of CO2 and decreases the concentration of CO and O2.
Four(a)
[CO2 ]2
Kc 
[CO]2 [O2 ]
(b)
A high value of Kc like 2.48 x 106 suggests a very high concentration of products
(CO2) and low concentrations of reactants (CO and O2), because a large numerator in
the Kc expression causes a large Kc value.
(c)
(d)
Five(a)
[ N 2O5 ]2
Kc 
[ NO2 ]4 [O2 ]
NO2 will be circled.
With a very small Kc value in this equilibrium, it means that the denominator of the
Kc expression is large, therefore a high concentration of NO2 (and O2) and a small
concentration of N2O5.
4 x 10-13
0.0250
12.4
-11
-4
1.58x10
6.31x10
10.8
0.00568
1.76x10-12
2.2
Has drawn
two correct
sketches
Has two of
(a), (b) or (c)
correct,
including the
sketches
OR
OR
Any one of
(a), (b) or (c)
fully correct.
Has all three
correct with
appropriate
detail.
All three
correct but
lack detail.
Kc
expression
correct OR
description
of what it
means.
All parts
correct.
Kc
expression
correct OR
description
of what it
means.
All parts
correct.
Two correct
Four correct
Six correct
3
Five(b)
Six(a)
(b)
NaOH is a strong base, and NH3 is a weak base.
A strong base, when it dissolves completely, creates the same concentration of
hydroxide ions as the concentration of the solution. NaOH  Na+ + OHA weak base, when it completely dissolves, only partially dissociates with water,
creating a low concentration of hydroxide ions in water. NH3 + H2O <-> NH4+ + OH-.
It is the concentration of hydroxide ions that creates the pH, not the concentration of
the solution.
A high concentration of hydroxide ions leads to a much higher pH, therefore NaOH
has a higher pH.
HCl + NaHCO3  NaCl + H2O + CO2. (State symbols not required)
Because the same mass of NaHCO3 was used for each reaction the amount of
substance is the same. Therefore with acid in excess the same volume of gas would be
produced.
The HCl has a higher concentration of H+ ions available at any one time to react with
the NaHCO3, therefore there is a greater chance that an effective collision occurs,
making the rate greater. The reaction will be completed sooner.
The fruit acid has a lower concentration of H+ ions available at any one time, therefore
there is a lesser chance that an effective collision occurs, making the rate slower.
Volume of CO2 (mL)
(e)
Fruit acid - - - HCl ________
Time (s)
Has
identified
strong vs
weak
Explains the
link between
strong/weak
and
hydroxide
concentration
OR link
from OH- to
pH
Correct
Describes a
difference or
similarity
between the
two
reactions.
Explains
both a
difference
and a
similarity.
Diagram of
rate clearly
has the fruit
acid line
below the
HCl line
OR
Same
volume of
gas clearly
shown.
Both
comparative
lines and
same final
volume
correct.
Fully
labeled.
Discusses in
full as per
evidence or
similar.
Equations
necessary.
Discusses
why the
difference
and
similarity
exist, with
links to the
acid strength.
4
Seven(a) Carbonic anhydrase is a catalyst / catalytic enzyme.
It lowers the activation energy required for reaction to occur / provides an alternative
reaction pathway / provides a surface to react on / orientates reactants for correct
collision.
(b)
Because the particles are of different sizes, there is a greater surface area on 1mm
particles than 2mm and then 3mm.
By having a greater surface area, more surface of the egg shell is available to collide
with the acid at any one time.
An increase in collisions means a greater chance that effective collisions occur,
meaning that the production of CO2 will be faster.
(c)
A higher temperature means that the acid particles are traveling with greater kinetic
energy.
A faster particle has a better chance of overcoming activation energy when it collides,
so there is a greater chance that the collisions are effective.
If more collisions are effective then production of CO2 will be faster.
(a) is correct
OR
Aspects of
either (b) or
(c) are
correct.
Either (b) or
(c) are
completely
correct
OR
Incomplete
explanations
for both are
given.
All parts (a),
(b), (c) are
correct with
appropriate
detail.
Judgement statement
Achievement
Achievement with Merit
Achievement with Excellence
SEVEN questions answered correctly.
EIGHT questions answered correctly, including
at least FIVE at Merit level.
Minimum of 7 x A.
Minimum of 5 x M + 3 x A.
EIGHT questions answered correctly, including
at least THREE at Excellence level and THREE
at Merit level.
Minimum of 3 x E + 3 x M + 2 x A.