Chemistry 2020 HSC exam pack

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.