Chemistry 2020 HSC exam pack

Marking guidelines are developed with the examination paper and are used by markers to guide their marking of a student's response. The table shows each question and the criteria with each mark or mark range. Sample answers may also be developed and included in the guidelines to make sure questions assess a student's knowledge and skills, and guide the Supervisor of Marking on the expected nature and scope of a student's response. They are not intended to be exemplary or even complete answers or responses.

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Chemistry HSC exam paper 2020
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Chemistry marking guidelines 2020
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HSC Marking Feedback

Select from the link(s) below to view feedback about how students performed in this year’s examination.

Use the feedback to guide preparation for future examinations. Feedback includes an overview of the qualities of better responses. Feedback may not be provided for every question.

Feedback on written examination

Sections I and II

Students should:

read the question carefully to ensure that they do not miss important components of the question
have a clear understanding of key words in the question and recognise the intent of the question and its requirements
plan the response to assist in the logical sequencing of information
integrate relevant scientific terms into their responses
engage with any stimulus material provided and refer to it in their response
show all working in calculations and include correct units and significant figures
present a logical and succinct response that addresses the question
review their response to ensure that it addresses the question requirements.

Question 21

In better responses students were able to:

name the alkane and write the chemical formula correctly
identify the parent ion peak at 44 and relate this to the molecular mass of propane
ignore the small peak at 45 (as the parent ion) and account for that peak as likely being from the uncommon C-13 isotope.

Areas for students to improve:

interpreting the Mass Spectrum carefully to distinguish between the base peak and the parent ion peak at 44
reading the question carefully to ensure the alkane is identified.

Question 22

In better responses students were able to:

logically outline some suitable procedures for testing cations and anions
accurately provide the results of flame tests
accurately provide the results of precipitation reactions
apply precipitation reactions to identify cations and anions
use flame tests to appropriately match the colour of the flame to the right cation
use a flow processing chart to outline a sequence of procedures of testing cations and anions.

Areas for students to improve:

writing a balanced net ionic equation to indicate a precipitation reaction
identify the source ions when conducting the precipitation reaction
applying their prior knowledge of the Preliminary module.

Question 23

In better responses students were able to:

make several references to the flowchart
explain multiple factors from different parts of the flow chart
explain the benefits of reducing transportation costs due to plant location, in terms of accessibility of reagents and/or transport to markets
explain the function of a catalyst as increasing the rate of reaction by lowering the activation energy.

Areas for students to improve:

analysing the stimulus rather than modifying existing knowledge to fit the question
misconstrued the oxidation of ethene as polymerisation or combustion
misunderstanding the use of water as a reagent in the hydrolysis of ethylene oxide assuming it is present as a coolant
clearly showing cause and effect between the chosen factor and why it was considered
differentiating between availability and accessibility of reagents.

Question 24

In better responses students were able to:

identify the correct name or structure
write correctly balanced equations
link the formation of soot to insufficient oxygen or molar ratio of oxygen in both equations when the same moles of diesel/biodiesel were used
include units in their response
show cause and effect for each explanation.

Areas for students to improve:

drawing the correct structural formula
ensuring soot is identified as C_(s)
balancing equations correctly
showing full working
ensuring effects relate to the identified cause
ensuring advantages and disadvantages are not merely identified/outlined.

Question 25

In better responses students were able to:

label the axes correctly with concentration as the independent variable thus the x axis and temperature change was the dependant variable thus the y axis
include units in their labels and the graph had clear data points with smooth line(s) of best fit
identify the equivalence point was between the 2.00ml and 3.00ml, thus they looked for the intersection of the lines of best fit in this region to determine the volume of citric acid used
calculate the moles of sodium hydroxide using the mole ratio of 3:1
calculate the concentration of sodium hydroxide.

Areas for students to improve in:

taking care with scale used and the lines of best fit drawn
reading the question more carefully and understanding what graph needs to be used to determine the concentration of citric acid, rather than using the data in the table
clearly showing the 3:1 mole ratio in calculations
drawing line/s of best fit on a graph if data is following an obvious trend.

Question 26

In better responses students were able to:

write a balanced chemical equation including states and equilibrium arrows
clearly articulate that both the forward and the reverse reaction rates are increased and describe why one increases more than the other
identify the reaction is endothermic, will make more products, and K will increase
clearly describe collision theory
avoid using Le Chatelier’s Principle.

Areas for students to improve in:

including all states
demonstrating a deep understanding of what occurs in an equilibrium reaction when temperature rises as per Collision Theory
ensuring that responses are clearly written in a logical sequence.

Question 27

In better responses students were able to:

link the acid-base reaction to the amine
add a proton to the amine
present a clear ICE table
clearly explain the assumption used
show all working with correct units.

Areas for students to improve in:

drawing clearly within the boxes provided, making sure that H does not look like N and that pencil lines are completely erased if then changed to ink
constructing structure diagrams for amines
including units in their response.

Question 28

In better responses students were able to:

engage with stimulus material before attempting an answer
realise the importance of the incorrect use of NaOH as a primary standard and an incorrect indicator in titrations
provide careful and thorough explanations of the reasons for inaccurate and unreliable results, not just make statements
understand equivalence/end points in titrations
have an understanding of the use of indicators in acid/base titrations.

Areas for students to improve in:

understanding what a primary standard is and what makes an acceptable primary standard
demonstrating an understanding of indicator ranges
appreciating different acid/base reactions.

Question 29

In better responses students were able to:

identify that structure A was a tertiary alcohol and then deduce the other 4 structures from this
recognise the difference between the Markovnikov and anti- Markovnikov reactions
include all relevant atoms.

Areas for students to improve in:

understanding the organic reactions as the Syllabus
knowing how to draw organic structures with correct atoms and bonds
reading the question more carefully as there were five different structures
logically following the sequence of reactions
drawing complete structural formulas containing all hydrogen atoms.

Question 30

In better responses students were able to:

use and explicitly refer to the spectroscopic data to put together the pieces of the puzzle
clearly show their thinking processes
annotate the spectroscopic data.

Areas for students to improve in:

learning how to draw organic molecules with the different functional groups listed in the syllabus
taking greater care with handwriting when drawing organic molecules eg oxygen O frequently looked like carbon C as students did not enclose the O, and H looked like N (nitrogen)
annotating the structure to clearly link it to H and C environments.

Question 31

In better responses students were able to:

outline a step for working out the initial amount of silver ion ( Ag ⁺ ) in a given solution
outline a step for working out the amount of thiocyanate ion (SCN^- (mole) reacting with the silver ion during the titration
outline a step for subtracting the reacting silver ion with thiocyanate ion from known initial silver ion (Ag⁺)
outline a step for finding concentration of the chloride ion (Cl^-)
outline a step for changing mol L^-1 in to mg L^-1 or a step for changing the mole of chloride into mass (mg or gram).

Areas for students to improve in:

understanding the steps of a back titration
converting concentration of chloride ions into g/L by multiplying with the molar mass of Cl instead of AgCl
calculating the number of moles of species that reacted when one of the solution is added in excess.

Question 32

In better responses students were able to:

relate molecular structure (polar/non polar) to the types of intermolecular force generated
correctly name each intermolecular force that applies to each molecular type
rank the strength of the intermolecular forces of attraction in order to compare the compounds boiling point
clearly outline the relationship between the intermolecular forces and their cumulative effect on the boiling point for each molecule in the table
ensure that the differences accounted for are in line with the molecules boiling point.

Areas for students to improve in:

understanding the difference between intra and intermolecular forces
providing correct terminology of each type of intermolecular force including hydrogen bonding, dipole-dipole and dispersion forces
linking chain length (number of carbons present) to the net strength of their dispersion forces.

Question 33

In better responses students were able to:

clearly show logical working
write a balanced chemical equation and extract the mole ratio of the reactants
use number of moles = concentration ∕ volume equation correctly, substituting given values
calculate calcium ions concentration using data from part a and part b
write the Ksp expression for calcium hydroxide with the correct value from the data sheet
substitute the calculated calcium ion concentration into the Ksp expression
calculate hydroxide ion concentration using algebra and the Ksp process
show full working for the calculation of pOH and pH.

Areas for students to improve in:

writing balanced chemical equations including states especially for dissociation reactions
including clear logical calculations for all steps
recognising Ksp and be able to write a Ksp expression
avoiding the confusion of Ksp with Keq expressions and hence recognising situations in which ICE tables are not required
calculating pH from the hydroxide ion concentration using pOH = -log [OH] and pH+pOH=14.

Question 34

In better responses students were able to:

clearly identify trends in the graph
explain the trends with the appropriate terms to each particular trend
write relevant correct equations for the ionisation of the named strong and weak acids.

Areas for students to improve in:

being explicit in the response by referring to the graph to identify the trends to be explained
explaining the effect of acid strength on pH relevant to the trend lines in the graph
explaining the effect of acid concentration on pH and relate this to the trend lines
being careful with the use of pronouns as there were two acids in the question.

Question 35

In better responses students were able to:

read and engage with the given mathematical equations in the stimulus part of the question
correctly write an equilibrium constant expression and carefully substitute into it, based on calculated concentrations
show working for all steps, including all algebra, and correctly use their calculator to determine numerical values.

Areas for students to improve in:

writing out the equilibrium constant expression prior to substituting values
recognising the difference between initial and equilibrium concentrations of reactants and products
performing numerical calculations with numerators, denominators and terms with indices.

Question 36

In better responses students were able to:

set all calculations out in a methodical manner
calculate mass of solution
know the specific heat capacity of the solution
sequence number of moles (n), energy (q) and ∆ H.

Areas for students to improve in:

using q correctly
making ∆ T the subject of the equation and rearranging equations
making simple mole calculations
realising c is not always water 4.18 x 10³ J kg^-1 K ^-1
differentiating between ∆ H and q.

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