Question 1

What is NCEA AS 91390 about?

Accepted Answer

You need to show you understand how and why chemical reactions happen by explaining the particles involved, the energy changes, and the forces between molecules. This standard asks you to link ideas about entropy, heat energy, intermolecular forces, and periodic trends to explain real chemical behaviour rather than just recalling facts.

Question 2

What do students need to do for AS 91390?

Accepted Answer

1. Draw and label Lewis structures of molecules and explain their shapes using electron density regions 2. Explain the polarity of molecules and justify differences in bond angles between similar molecules 3. Use entropy changes in the system and surroundings to explain why reactions happen spontaneously 4. Describe and compare the strength of intermolecular forces (temporary dipoles, hydrogen bonds) and link this to particle properties like electron cloud size 5. Calculate enthalpy changes using Hess' Law and relate periodic trends (ionisation energy, atomic radius, ionic radius) to nuclear charge, electron shielding, and energy levels

Question 3

What's the difference between Achieved, Merit, and Excellence for AS 91390?

Accepted Answer

Achieved: You draw Lewis structures and name molecular shapes correctly, identify entropy changes, write electron configurations properly, and recognise that intermolecular forces exist between molecules rather than within them. Merit: You justify why a molecule is polar, explain entropy changes in both the system and surroundings, successfully apply Hess' Law, compare intermolecular force strengths, and link electron cloud size to temporary dipole strength. Excellence: You explain the spontaneity of reactions by considering entropy changes in both the system and surroundings together, comprehensively compare intermolecular forces with detailed reasoning, justify periodic trends using nuclear charge, energy levels, and shielding effects, and complete all calculations with correct units and significant figures.

Question 4

What mistakes do students typically make in AS 91390?

Accepted Answer

1. Confusing intermolecular forces (between molecules) with intramolecular forces (within molecules) — examiners specifically noted candidates talked about forces inside molecules when they should describe forces between them 2. Focusing on the loss of a solid reactant when explaining entropy rather than recognising the major impact of losing two moles of gas on disorder in the system 3. Forgetting to mention shielding (repulsion from inner electron shells) when explaining periodic trends like ionisation energy and atomic radius — you need to link nuclear charge, energy levels, and electron shielding together

Thermochemical principles · AS 91390

9,101 students sat this in 2024 · 78% passed

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