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 Expected Coverage Achievement Merit Excellence
(a) (i) (ii)
The time taken would be longer (ie, slower reaction rate).
The time taken would be longer (ie, slower reaction rate).
THREE of:
• in (a) time taken would be longer in (i) and (ii)
• 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.
EITHER in (b) rate of reaction explained in terms of
frequency of particle collision for the effect of decreased concentration OR
in (b) rate of reaction explained in terms of energy of particle collision for the effect of decreased temperature.
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.
(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
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 Expected Coverage Achievement Merit Excellence
(a) (i) (ii)
NH4
+ + OH– C2H5COOH + OH-
THREE of:
• 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
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
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.
(b) See Appendix One.
See Appendix Two.
(c) 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.
CH3COOH is a weak acid and is only partially dissociated in water. Fewer protons are donated to water.
CH3COOH + H2O
H3O+ + CH3COO–
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.
a strong acid, CH3COOH is a weak acid OR NaOH is
basic/has 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.
Three Expected Coverage Achievement Merit Excellence (a) (i)
(ii)
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).
The colour of the solution becomes more brown.
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).
THREE of:
• in (a) BOTH colour changes correct
• in (a) temperature decrease causes a shift in the exothermic direction
• in (a) pressure decrease causes a shift to the side with the greater number of moles
• in (b) Kc
expression correct
• in (b) value calculated accurately.
In (a) ONE of temperature or pressure
explained in terms of equilibrium principles 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) equilibrium principles used to justify equilibrium shift both
qualitatively and quantitatively.
(b) (i) [SO3]2 / [SO2]2[O2] (ii) [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.
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.
