Chemistry 2019 HSC exam pack

Students should:

• use the number of lines as an indication of the expected length of the response
• address the key words and provide a response that is coherent and logical
• directly answer all parts of the question
• use applied examples with specific details
• indicate the question being answered if using additional booklets
• well-rehearsed in using the data sheets provided
• show all working in calculations
• leave rounding until the end of the calculation.

Question 21

In better responses, students were able to:

• draw the correct structural isomer and name it (a)
• identify the compounds and state they were examples of ketones and aldehydes and that they were functional isomers (b)
• provide examples of chemical tests that could be used to distinguish between the ketone and aldehyde (c)
• include the name of a suitable reagent, the expected results and an explanation of the chemistry involved (c).

Areas for students to improve include:

• distinguishing between a structural isomer and a functional isomer (a)
• reading the question carefully to include chemical tests not physical tests.

Question 22

In better responses, students were able to:

• explain the reason for the equilibrium shift, not just describe the direction of the shift
• write an equilibrium equation that included the equilibrium arrow and the hydronium ion
• clearly link an experimental observation to a change in pH.

Areas for students to improve include:

• using the appropriate arrow when writing equilibrium equations
• identifying that a buffer consists of a weak acid on one side of the equation and the conjugate base on the other side
• understanding how a buffer system works
• using specific information regarding the effect of OH^- and H₃0⁺ on equilibrium rather than making general statements about Le Chatelier’s principle.

Question 23

In better responses, students were able to:

• use correct mass in mcΔT
• calculate the moles of ethanol correctly
• give the answer with correct significant figures and units.

Areas for students to improve include:

• using the mass of water and not ethanol in mcΔT
• knowing the formula for ethanol
• using atomic mass values from the periodic table.

Question 24

In better responses, students were able to:

• clearly explains the shape of the conductivity titration graph
• relate conductivity to the ions responsible and the manner in which they change during the addition of Ba(OH)₂
• identify conductivity as being non-zero at equivalence
• identify specific ions rather than a general ion statement
• write a complete balanced chemical equation
• convert concentration and volume to moles, use the balanced equation to convert moles of HCl to moles of Ba(OH)₂
• calculate the concentration
• include clear logical calculations for all steps.

Areas for students to improve include:

• relating the trend observed to the conductivity and the volume of added Ba(OH)­₂
• describing the reducing concentration of hydronium ions prior to equivalence
• identifying Ba²⁺ and Cl^- ions as being responsible for the lower level of conductivity at equivalence
• identifying specific ions rather than just general statements about ions increasing and decreasing
• recognising conductivity curves as against pH curves
• explaining why the curve has its shape rather than just describing the shape
• avoiding the use of c1v1=c2v2 or variants for titration calculations as this is designed for dilutions
• using volumes in L not mL for mole and concentration calculations
• using units for quantities
• writing formulae of common acids and bases
• distinguishing between equilibrium reactions and equilibrium arrow, and reactions which go to completion.

Question 25

In better responses, students were able to:

• graph the correct direction and shape of the change of the concentration for each chemical, based on whether they were reactants or products
• quantify the change for each chemical in the graph based on the stoichiometry of the equation
• identify the effect of carbon monoxide removal on the rate of collisions of reactants, as well as the forward rate of reaction
• compare the forward and reverse rates of reaction after the change and relate this to the shape of the carbon monoxide concentration curve.

Areas for students to improve include:

• relating the change of carbon monoxide to the effect on other chemicals in the reaction based on the equation
• recognising that the removal of carbon monoxide does not cause the sudden change in the concentration of other chemicals
• understanding the connection between frequency of collisions and overall rates of reaction
• understanding that Le Chatelier’s principle is a consequence of collision theory, not the cause.

Question 26

In better responses, students were able to:

• draw a clear structure of propanoic acid
• use the information in the question to link the reactivity with sodium carbonate producing bubbles to justify that the compound is a carboxylic acid
• correctly use information shown in three of the four spectra given to justify the structure of propanoic acid
• clearly show how two spectroscopic techniques can be used in the identification of an organic compound.

Areas for students to improve include:

• reading the question more carefully to ensure that they include all the necessary information in their response
• reading from the spectra provided and relating the values to the data sheet provided to determine the structure of the compound
• knowing the key use for each of the spectroscopic techniques and then indicating how multiple spectra work together in the identification of an organic compound.

Question 27

In better responses, students were able to:

• identify the correct water constant value
• rearrange the formula to show the K_b value
• write an equilibrium expression
• construct a clear ICE table showing initial, change and equilibrium concentrations
• use a pronumeral to indicate the unknown concentration
• provide an explanation of why 0.2M can be used for the concentration of the OCl^- ion at equilibrium
• write an equilibrium expression, substitute correct values from the ICE table and correctly calculates the OH^- ion concentrations
• show full working for the calculation of pOH and pH.

Areas for students to improve include:

• setting out the calculation, ensuring the correct values of K_a, K_b and K_w
• using K_w from the data sheet
• providing an explanation of the concentration of OCl ^-, appreciating the degree of ionisation is small and to assume the ion concentration is 0.2
• correctly recognising the concentration of each species at equilibrium
• avoiding the confusion between pH and pOH.

Question 28

In better responses, students were able to:

• address the key words by structuring a response with a clear assessment of both an advantage and limitation, using relevant scientific terminology and avoiding the use of colloquial terms
• articulate an advantage and clearly link it to a supporting equation
• articulate a limitation and clearly link it to a supporting equation
• construct correctly balanced equations
• provide an informed judgement based on the criteria they provided.

Areas for students to improve include:

• understanding the advantages and limitations of accepted acid–base models
• addressing the requirements of the question rather than state everything known about the models
• writing balanced equations correctly and legibly
• relating/linking a description made to the equation used.

Question 29

In better responses, students were able to:

• relate the K_sp values to the differences in solubility of the substances
• write correctly balanced ionic equations
• demonstrate their understanding of AAS relevant to detecting different metals in solution
• make the link between each different metal ion being able to absorb a wavelength that is specific to that ion
• construct graphs with correctly plotted points, a key to identify each ion, labelled axes (with correct units), appropriate scales and accurately ruled lines of best fit
• use the graph to find (interpolate) concentration values given the absorbance
• correctly convert concentration in molL^-1 to mass in milligrams with the correct orders of magnitude and units.

Areas for students to improve include:

• demonstrating a clear understanding of differences in solubility, for example, using K_sp values to support the response
• using relevant terminology for AAS rather than providing generalised information
• demonstrating a clear understanding of AAS
• answering ‘why’ rather than ‘how’ AAS works
• accurately plotting points on a graph and drawing lines of best fit rather than connecting the dots
• interpolating data from a graph
• correctly converting molL^-1 to mgL^-1
• not making careless mathematical or transcription errors such as putting the decimal point in the wrong place.

Question 30

In better responses, students were able to:

• use the data to compare the entropy and enthalpy of the two reactions, correctly recognising both reactions are exothermic (negative H) and moving towards order (negative S)
• explain the effect that the entropy and enthalpy has on the solubility of each salt
• identify the solubility of each salt by referring to the Gibbs free energy
• explain the overall effect on the reaction, indicating that the solubility of magnesium chloride is enthalpy driven and sufficient energy is present to overcome the entropy.

Areas for students to improve include:

• explaining what the numbers mean in terms of energy being released and entropy moving towards more order and avoiding reference to the magnitude of the numbers in terms of numerical size on a number plane
• making explanations clear and logical
• ensuring no part of the answer contradicts what is already written.

Question 31

In better responses, students were able to:

• write an equilibrium expression with correct substitution
• include an ICE table, identifying x or making the x approximation
• do a correct calculation with full working
• show an understanding of the K_eq value given
• understand that the concentration of products was initially zero.

Areas for students to improve include:

• avoiding calculator errors
• avoiding solving for quadratic equations, making x approximation if appropriate
• substituting correctly
• identifying where x is coming from
• noting that x is small.

Question 32

In better responses, students were able to:

• identify the three trends in the graph
• explain each of the trends
• use correct chemical terminology to identify the forces of attraction that caused each trend.

Areas for students to improve include:

• identifying and explaining the forces of attraction influencing boiling point
• understanding the difference between a hydroxyl group and hydroxide ion
• explaining what effects the strength of dispersion forces have on trends.

Question 33

In better responses, students were able to:

• provide a balanced chemical equation
• determine the moles of each reactant
• apply the mole ratio to work out the excess reactant and then solution concentration
• determine the pH.

Areas for students to improve include:

• writing a balanced equation
• using exact values throughout the calculation
• showing all working
• using the data from the periodic table provided.

Question 34

In better responses, students were able to:

• demonstrate a clear understanding of the reagents, conditions and chemical reactivity of the product for each step
• succinctly outline the reagents, conditions and chemical reactivity tests for each step
• identify acidified reagents for Step 2
• identify the change in colour for indicators/acidified reagents if used as chemical reactivity tests
• identify that concentrated sulfuric is used in esterification.

Areas for students to improve include:

• understanding reactions of haloalkanes
• addressing all parts of the question, including chemical reactivity
• using specific spectroscopic data to identify specific products, including the wavelengths when using the data
• naming organic compounds
• annotating the stimulus diagrams with the name of the compounds and reagents before writing a response.