Chemistry 2022 HSC exam pack

Students should:

• read the question carefully to ensure that they do not miss important components of what is being asked
• have a clear understanding of key words in the question and recognise the intent of the question and its requirements
• attempt to engage with all questions
• 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, not rounding early and include correct units and significant figures
• recognise the importance of the Working Scientifically section of the syllabus.

Question 21

In better responses, students were able to:

• draw a structural formula of a straight chain haloalkane
• recognise that double bonds in hydrocarbons are more reactive than single bonds
• differentiate between an addition reaction and a substitution reaction
• apply the halogenation reaction of hydrocarbons.

Areas for students to improve include:

• drawing the correct structural formula showing all constituent atoms, including all hydrogen atoms in the structures
• recognising that normally carbon can only form four bonds with other atoms.

Question 22

In better responses, students were able to:

• demonstrate their knowledge of Brønsted–Lowry theory of acids and bases
• differentiate acidic substances from basic substances
• recognise reactions of acids and bases
• identify an acid or a base from a chemical equation
• demonstrate an understanding of conjugate acid/base pairs in solution.

Areas for students to improve include:

• writing acidic or basic ions
• pairing an acid with a base
• recognising hydrofluoric acid and the fluoride ion is a conjugate pair as is the hydrogen phosphate and phosphate ion combination.

Question 23

In better responses, students were able to:

• identify that a negative value of ΔH indicates the forward reaction is exothermic and that an increase in temperature will favour the endothermic reverse reaction (a)
• link reaction rates to catalyst use for both the forward and reverse reactions (b)
• state that fewer particle collisions cause a decrease in the rate of the reverse reaction and that the forward rate is unchanged (c)
• link changes in equilibrium to the increase in product, showing that NO concentration increases (c)
• describe collision theory and avoid using Le Chatelier’s Principle.

Areas for students to improve include:

• stating the impact on concentration of the species targeted in the question, rather than just identifying the equilibrium shift
• understanding that a catalyst impacts both the forward and reverse reaction rates equally
• recognising that catalysts can still affect systems that contain only gases
• understanding the difference between equilibrium position and Keq
• understanding the connection between the frequency of particle collisions, concentration, and overall rates of reaction
• knowing that Le Chatelier’s Principle is a consequence of collision theory, not the cause
• using specific terms when describing changes to reactions in equilibrium, for example, the terms forward reaction and reverse reaction cannot be used interchangeably with forward reaction rate and reverse reaction rate; replacing incorrect phrases of more collisions on the reactants side with more collisions favouring the forward reaction.

Question 24

In better responses, students were able to:

• interpret data from a graph and identify trends
• relate the increasing boiling points within a homologous series to increasing dispersion forces
• relate the energy required to separate molecules to the net strength of intermolecular forces
• relate molar mass to carbon chain length within an homologous series.

Areas for students to improve include:

• identifying correct intermolecular forces when explaining trends within an homologous series
• using cause and effect language to explain trends in physical properties
• correctly using intermolecular and intramolecular force terms
• distinguishing between intermolecular forces and covalent bonds
• identifying that boiling is a process that separates molecules by overcoming intermolecular forces rather than breaking covalent bonds.

Question 25

In better responses, students were able to:

• recognise HCl as a strong acid and HCN as a weak acid
• provide a balanced chemical equation for the ionisation of HCN using an equilibrium arrow and the ionisation of HCl using a one-way arrow
• relate the strength of each acid to its relative degree of ionisation, and hence its hydronium ion (or hydrogen ion) concentration
• identify that even though both acids have the same initial concentration, HCN has a lower concentration of hydronium ions (or hydrogen ions) than does HCl
• relate the resultant hydronium ion (or hydrogen ion) concentration of both acids to the pH, acknowledging that a lower hydronium ion (or hydrogen ion) concentration leads to a higher pH. Note that no calculations were necessary to successfully answer this question.

Areas for students to improve include:

• understanding the concept of strong and weak acids, and being able to recognise examples of each, including HCN as a weak acid
• ensuring familiarity with the pH scale and relating this to hydronium ion (hydrogen ion) concentration
• checking their responses to ensure that the correct adjective is used, for example, lower when they mean higher.

Question 26

In better responses, students were able to:

• use the stimulus material to calculate the combined mass of the hydrochloric acid and sodium hydroxide solutions, the average initial temperature, and from this the change in temperature
• substitute the correct values for mass and change in temperature appropriately into the q = mc∆T expression
• use the correct numerical value for specific heat capacity with units consistent for mass in either grams or kilograms as appropriate
• incorporate units in their calculation of q = mc∆T and demonstrate how those units could be mathematically cancelled out to produce a definitive value
• calculate the moles of water using the concentration and volume of either the sodium hydroxide or hydrochloric acid
• give their answer using the correct units with the negative sign to show enthalpy of neutralisation is exothermic.

Areas for students to improve include:

• showing all steps in their calculations and avoid introducing values into the response without showing how those values were derived
• substituting values with consistent units of measurement into a calculation of q = mc∆T
• recognising that enthalpy of neutralisation is exothermic and hence the value should be negative.

Question 27(a)

In better responses, students were able to:

• draw the expanded structural formula of both isomers (propan-1-ol and propan-2-ol)
• recognise the question was asking for a positional isomer (moving the –OH to a different position in the carbon chain).

Areas for students to improve include:

• understanding the difference between positional and functional isomers
• recognising that 'prop' is a 3-carbon chain and 'ol' is the -OH (alcohol) functional group.

Question 27(b)

In better responses, students were able to:

• recognise that propan-2-ol is symmetrical with two carbon environments, whereas propan-1-ol is not symmetrical and has three carbon environments
• relate the number of carbon environments to the number of peaks in the CNMR
• understand that one isomer will have more peaks compared to the other isomer.

Areas for students to improve include:

• using terms associated with spectroscopy, specifically CNMR, such as chemical shift and environments
• determining number of environments in an organic compound.

Question 27(c)

In better responses, students were able to:

• recall that oxidation of alcohols requires both an oxidising agent (Cr₂O₇^2-/K₂Cr₂O₇ or MnO₄^- /KMnO₄) and an acid catalyst (H⁺/H₂SO₄) and provides the correct formula or name
• write a chemical equation showing the products as a carboxylic acid and a ketone by using correct structural formulae (CH₃CH₂COOH) rather than using molecular formulae only (C₃H₆O₂)
• show the reaction conditions above the arrow rather than in a sentence.

Areas for students to improve include:

• understanding that oxidation of a primary alcohol progresses to a carboxylic acid rather than stopping at the intermediate step, producing an aldehyde.

Question 28(a)

In better responses, students were able to:

• identify the correct formula and/or name of the precipitate formed
• recognise that the brown precipitate is due to Fe (III) and not Fe (II).

Areas for students to improve include:

• providing the valency of the metal ion in the name or writing a correct chemical formula.

Question 28(b)

In better responses, students were able to:

• correctly calculate the percentage of iron to the correct number of significant figures
• demonstrate clear and logical steps and calculations
• calculate the moles of the correct substance.

Areas for students to improve include:

• applying the correct molar ratio; in this case, multiplying rather than dividing.

Question 29(a)

In better responses, students were able to:

• plot raw values from real data and recognise gaps in data
• display trends of exponential data by means of curve line of best fit
• interpolate data to obtain a desired value of a data set.

Areas for students to improve include:

• understanding IUPAC nomenclature
• drawing a curve line of best fit through a given data set
• reading units and signs from a graph.

Question 29(b)

In better responses, students were able to:

• provide a proper justification for differences between published data and experimental data
• measure and reliably compare the enthalpy of combustion for a range of alcohols.

Areas for students to improve include:

• exploring an idea or solving a problem which requires activities such as planning a course of action
• demonstrating a clear understanding of standard conditions.

Question 30

In better responses, students were able to:

• interpret the main features of each spectra
• draw the correct structure of the organic compound
• name the compound using IUPAC nomenclature
• relate the features of each spectra to the relevant part(s) of the molecule to justify the structure.

Area for students to improve include:

• interpreting mass spectra correctly to relate the parent peak to the molar mass of the compound and the base peak to the fragments such as CH₃C=O or (CH₃)₂CHO
• relating the hydrogen environments to the structure by explaining both integration and splitting patterns in the proton NMR spectra
• relating the carbon environments to the structure by explaining the deshielding caused by C=O
• linking information from each spectra in the justification.

Question 31(a)

In better responses, students were able to:

• apply solubility rules and concisely state the addition of NaI(aq) would result in a precipitate of AgI(s)
• demonstrate an understanding of equilibrium systems showing as the precipitate formed Ag⁺ ions were removed and explain the system would counteract this change by favouring the reverse reaction
• appreciate that the method is not suitable because precipitation results from not only the free silver ions but also silver ions from the silver complex
• demonstrate that as the reaction progressed, the reaction shifted, resulting in the complex breaking down, producing more silver ions with an evaluation that a precipitation titration is unsuitable because it would result in a higher concentration of silver ions.

Areas for students to improve include:

• engaging with the method described and evaluating it for its suitability
• understanding an equilibrium system.

Question 31(b)

In better responses, students were able to:

• use the provided data to identify that by being given the percentage of free ions they could also calculate the percentage of silver complex ions
• provide correct K_eq expression.

Areas for students to improve nclude:

• identifying that as equilibrium concentrations were given, an ICE table was not required to calculate the concentration of [NH₃].

Question 32

In better responses, students were able to:

• eliminate the outlier and correctly calculate the average volume of NaOH used in the first titration
• calculate the concentration of potassium hydrogen phthalate (KHP) using the 25 mL from the original 100 mL
• apply the 3:1 stoichiometric ratio in the second titration
• apply the dilution factor to arrive at the concentration of citric acid in the undiluted soft drink
• process all the information in the question to perform a multi-step calculation
• provide all steps in the calculation with full working
• use the appropriate mathematical relationships to calculate chemical amounts and concentrations
• identify that CO₂ in solution will react with NaOH, then link this to a greater volume of NaOH required and thus a higher calculated concentration of citric acid.

Areas for students to improve include:

• annotating calculations to show explicitly what quantity they are calculating in each step, for example, moles, or concentration
• checking for transcription errors with numerical data
• explaining by relating cause and effect
• understanding the acidic nature of carbon dioxide.

Question 33

In better responses, students were able to:

• design an organic synthesis process that could be done in a school laboratory by selecting specific identified reagents, conditions, equipment, hazards and risks that could be encountered in a school laboratory
• include a relevant balanced chemical equation, written with structural formulae
• use subheadings to refer to each of the factors indicated in the question
• use a labelled diagram, particularly if the name of the piece of equipment or process is forgotten
• link the yield to the reaction conditions such as how to manipulate equilibrium to maximise yield.

Areas for students to improve include:

• addressing all key parts of the question and including a chemical equation, especially the use of structural equations for organic chemicals
• avoiding the use of generic safety precautions without linking to the specific chemicals or equipment used, for example, only writing 'safety glasses' or 'PPE'
• planning their responses to address all parts of the question in the analysis in a logical and succinct manner.

Question 34

In better responses, students were able to:

• provide the Keq expression for the chemical reaction given including charges on ions
• identify that [OH^-] is stable when buffered
• calculate the [OH^-] given the pH
• import equilibrium concentrations into the correct Keq expression and solve for the unknown
• calculate the initial volume of sodium hypochlorite (NaOCl) required using a dilution calculation
• identify that at equilibrium, both OCl^- and HOCl species present were derived from the NaOCl originally added.

Areas for students to improve include:

• presenting a Keq expression when provided with a chemical equilibrium reaction
• using pH to calculate the pOH followed by the hydroxide ion concentration.

Question 35

In better responses, students were able to:

• find moles of all species, Sr(OH)_{2 (s)}, Sr(NO­ ₃)_{2 (aq)} and NaOH _(aq)
• subtract moles of solid from initial moles of Sr²⁺ and 2x for OH^-
• determine the molarity at equilibrium of Sr²⁺ and OH^-
• determine the correct Ksp by using correct equilibrium concentrations.

Areas for students to improve include:

• calculating moles of all species using correct units
• providing correct Ksp expression with correct indices
• providing clear subtraction of moles of solid from Sr²⁺ and 2x for OH^-
• converting moles to concentration values.

Question 36

In better responses, students were able to:

• distinguish between the two systems as open or closed
• distinguish the difference in evaporation between two systems; for the closed system the evaporation would equal condensation whilst the open system would eventually go to completion
• identify the forward reaction as endothermic
• identify entropy increasing but more in the open system than the closed system
• identify in equilibrium reactions that entropy and enthalpy remain constant.

Areas for students to improve include:

• understanding the enthalpy of reaction is endothermic and doesn’t change between two systems in the forward direction
• understanding that entropy increases when a liquid becomes a gas and does not decrease because the gas is removed
• understanding the difference in evaporation of the two systems was due to extent of evaporation and not in terms of speed
• understanding that in a dynamic equilibrium, evaporation and condensation would be equal
• understanding entropy initially increases for the equilibrium reaction and remains constant thereafter.