Key Take-aways From the HSC Chemistry Exam

by Science Ready November 14, 2019

Don’t forget knowledge from Year 11 Chemistry

Introduction to Quantitative Chemistry

Yes, you need to know everything from Module 2 which includes calculation of moles, concentration, volume, pressure. Don’t forget situations which involve limiting reagents.

Question 33 from the HSC 2019 exam requires knowledge from Module 2, 3 and 6.

Enthalpy, entropy and Gibbs energy

These three topics are the core focus of Module 4: Drivers of Reactions. You will need a strong understanding of the theory as well as how these concepts are related to the spontaneity of chemical reactions.

Question 30 from the 2019 HSC Chemistry exam requires students to apply the equation `DeltaG=DeltaH-TDeltaS` in the context of solubility equilibria of ionic compounds (which is a Module 5 topic). In this question, the spontaneity (`DeltaG`) of each reaction is an indication of each compound’s solubility.

Sample answer

Both the dissolution of magnesium fluoride and magnesium chloride are exothermic at 298 K which means the process is enthalpically favourable. However, the magnitude of `DeltaH` of magnesium’s chloride is much larger than that of magnesium fluoride. This means it is more favourable.

Similar to enthalpy, the entropy change of dissolution is negative for both compounds, indicating that the process is entropically unfavourable. However, the entropy change (`DeltaS`) for magnesium fluoride is more negative than that of magnesium chloride, so it is more unfavourable.

For salts to be soluble (`DeltaG < 0`) the negative entropy change needs to be overcome by an even larger negative `DeltaH` value (`DeltaG=DeltaH-TDeltaS`). Evidently, magnesium chloride is soluble in water at 298K while magnesium fluoride is not.

Understand equilibrium systems in terms of collision theory (not just Le Chátelier’s principle).

This is one of the biggest differences between the old and new HSC Chemistry syllabus.

In the old syllabus, Le Chatelier’s principle was always the ‘go-to’ principle when explaining concentration changes after a change has been imposed on an equilibrium system. However, in the new syllabus, questions can specifically ask for the collision theory as the basis of an explanation.

Question 25 (2019 HSC)

Using collision theory, explain the change in the concentration of CO after time T.

Sample answer:

Removal of CO reduces the collision rate between the reactants of the reverse reaction. According to collision theory, this causes the forward reaction rate to become greater than the reverse reaction rate, resulting in a gradual increase in [CO].

As [CO] increases, the collision rate increases and so does the reverse reaction rate. This continues until the dynamic equilibrium is re-established after which [CO] stays constant.

Be familiar with both acid AND base chemistry

This applies to both Module 5: Equilibrium and Acid Reactions and Module 6: Acid and Base Chemistry.

In Module 5, familiarise yourself with both acid dissociation constant K_a and base dissociation constant K_b. Spend time understanding their relationship and how to determine one using the other.

In Module 6, familiarise yourself with the concept of pH and pOH as well as their relationship. While learning this part of Module 6, be sure to use the opportunity to re-consolidate the concept of K_a and K_b as they are closely related to pH and pOH.

Question 27 from the 2019 HSC Chemistry Exam exemplifies this:

In part (b), students are expected to calculate pOH from [OH^–], which should be first determined from the base dissociation expression. After this, pH can be determined by subtracting the value of pOH from 14.

A bigger focus on organic chemistry

This is not unexpected since the new HSC chemistry syllabus has an entire module dedicated to organic chemistry.

The new HSC chemistry exam reflects this change as at least 23 out of 100 marks encompasses organic chemistry concepts. This number increases to 32 if spectroscopy (NMR, IR, MS) questions that use organic molecules are included.

In addition, expect to see more organic chemistry as the last question of the exam, not just in the HSC but in your school trial exam.

The last question (Question 34) for the 2019 HSC is not as difficult as we predicted. However, regardless of its difficulty, these types of synthesis-related questions can easily determine whether you achieve a Band 5 or Band 6 in chemistry.

Analysing graphs and tables in HSC Chemistry.

Before the 2019 HSC Chemistry exam, NESA did inform students and teachers that the new syllabus will contain more graphical and data analysis to encourage students to think critically instead of memorising concepts.

At least 15 questions out of the total 34 in the 2019 HSC Chemistry exam require students to analyse a form of data.

As expected, a sizeable portion of these questions contains concepts from Module 8: Applying Chemical Ideas.

Students and teachers will need to dedicate more time learning and teaching this section of the curriculum, particularly because of three reasons:

Module 8 is typically taught towards the end of Year 12, during which students may experience difficulties managing study time.
This module may not be tested extensively in your school’s trial exam which means you get less opportunity to check your understanding.
Analysing graphs and data is not a skill you can develop in a short time span, it requires consistent practice.

If you are sitting the HSC Chemistry exam in 2020, what this means for you is that you need to pay more attention to the pace at which your teacher is going through the syllabus.

If your class is behind, you may want to consider studying ahead to allow more time for learning Module 8 content and more importantly, developing graphic and data analytical skills.

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