NCEA 91390: thermochemical principles and the properties of particles and substances (Level 3 Chemistry)

What NCEA 91390 assesses

NZQA limits the content of the standard (version 2) to:

• Properties of particles: electron configurations of atoms and ions of the first 36 elements in s, p, d notation; periodic trends in atomic radius, ionisation energy and electronegativity, and atomic and ionic radii; Lewis structures and shapes with up to six electron pairs about the central atom; polarity; and the attractive forces between atoms, ions and molecules, including temporary and permanent dipoles and hydrogen bonding
• Properties of substances: melting point, boiling point and solubility, and enthalpy and entropy changes in relation to spontaneity (entropy calculations are not required)
• Thermochemical principles: specific heat capacity, phase changes, ΔcH, ΔfH, ΔrH, ΔvapH, ΔsubH and ΔfusH, and Hess's law, including ΔrH = ΣΔfH(products) − ΣΔfH(reactants), with related calculations

The three grades go up in steps. Achieved describes, identifies, names, draws and gives an account of the properties of particles and substances and of thermochemical principles, with related calculations. Merit makes and explains links between them. Excellence elaborates, justifies, relates, evaluates, compares and contrasts, or analyses those links. Every grade needs correct chemistry vocabulary, symbols and conventions, and Excellence needs them used consistently. NZQA's 2025 report says that at Excellence you need to apply ideas in unfamiliar contexts and read each question for what it asks, not for what you hoped it would ask.

The NCEA 91390 exam in 2026 and 2027

NCEA 91390 is one of three Level 3 Chemistry externals, with 91391 (organic compounds) and 91392 (equilibrium in aqueous systems). The 2026 exam is on Friday 20 November, in the afternoon session. NZQA's specifications for both years say the exam is on printed paper and 91390 is a 5-credit external.

• Three questions: you answer all three. The format is the same for each Level 3 Chemistry standard, and the questions may be set in contexts that are unfamiliar to you.
• Calculator: an approved scientific or graphics calculator is required.
• Resource booklet: it has the formulae you need, such as n = cV, a periodic table with symbols, atomic numbers and molar masses, and a list of nine complex ions with their formulae.
• Calculations: show all working, use an appropriate number of significant figures (usually three) and include the correct units.
• Practical work: NZQA says you should be familiar with relevant practical work, and the content is set by the explanatory notes of each standard.
• Not assessed: the concept of resonance, Hund's rule and the stability of half-filled sublevels. The specific heat capacity of water, 4.18 J g⁻¹ °C⁻¹, is provided when required.

How NCEA 91390 is marked

Three questions, each N0 to E8, give a total out of 24. NZQA's report says the 2025 paper covered shape and polarity, entropy and spontaneity, vaporisation enthalpies, Hess's law, enthalpy of formation and a calorimetry calculation, so most of the evidence is explanation backed by numbers.

NCEA 91390 cut scores

Achieved started as high as 9 in 2022 and 2023 and as low as 7 in 2024. Merit ranged from 14 to 15. The scores are where each grade begins, out of 24.

Cut scores 2025

2025 schedule: Achieved from 8, Merit from 14, Excellence from 19. Compared with 2024, Achieved went up by 1.

Cut scores 2024

2024 schedule: Achieved from 7, Merit from 14, Excellence from 19. Compared with 2023, Achieved came down by 2 and Merit came down by 1.

Cut scores 2023

2023 schedule: Achieved from 9, Merit from 15, Excellence from 19. The same as 2022.

Cut scores 2022

2022 schedule: Achieved from 9, Merit from 15, Excellence from 19. Compared with 2021, Achieved went up by 1 and Merit went up by 1 and Excellence came down by 1.

Cut scores 2021

2021 schedule: Achieved from 8, Merit from 14, Excellence from 20.

Worked examples at each grade

Achieved: electron configurations and shapes

Write the electron configuration of Fe²⁺. Iron has 26 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Iron loses its 4s electrons before any 3d electrons, so Fe²⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶.

Now the shapes of BrF₃ and SF₆. In BrF₃, bromine has three bonding pairs and two lone pairs, five regions of electron density, so the electron geometry is trigonal bipyramidal. The lone pairs sit in the equatorial positions, giving a T-shaped molecule, and the Br-F dipoles do not cancel, so it is polar. SF₆ has six bonding pairs and no lone pairs, so it is octahedral, and the six equal dipoles cancel, so it is non-polar.

Merit: calorimetry and Hess's law

A solid is dissolved in 100 g of water, and the temperature rises by 6.5 °C. Dissolving 0.050 mol of the solid caused this. Using c = 4.18 J g⁻¹ °C⁻¹:

The negative sign shows the reaction is exothermic: the water gained the energy the system lost. Check q in the Specific Heat Calculator. Now Hess's law. For CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), with ΔfH values (kJ mol⁻¹) of −74.8 for CH₄, −393.5 for CO₂ and −285.8 for H₂O(l):

Merit completes two or more steps correctly: multiplying the 2H₂O by 2, subtracting the reactants, and keeping the signs. NZQA's 2025 report says Merit candidates applied Hess's law correctly and compared the strength of intermolecular forces.

Excellence: spontaneity and a justified comparison

Is N₂(g) + 3H₂(g) → 2NH₃(g) spontaneous at room temperature, and why? The reaction goes from 4 mol of gas to 2 mol, so the entropy of the system decreases. The reaction is exothermic, so it releases energy to the surroundings, which increases the entropy of the surroundings. At a low temperature the surroundings' increase is large enough to outweigh the system's decrease, so the total entropy increases and the reaction is spontaneous. The report says Excellence candidates justified spontaneity from the entropy changes of both the system and the surroundings, and recognised the effect of losing 2 mol of gas.

For a comparison, why does Br₂ have a higher enthalpy of vaporisation than Cl₂? Both are non-polar molecules held by temporary dipoles. Bromine has the larger electron cloud, which is more easily distorted, so its temporary dipoles are stronger. Separating its molecules takes more energy, so its enthalpy of vaporisation is greater. Intermolecular forces act between the molecules, not within them, and the covalent bonds do not break.

Now predict molecular shapes in our Chemical Bonding course, then press Mark to check your answer in the course:

Exam answers

The answers and a worked mark scheme come with today's paper once it is marked. We do not publish official or past paper answers. Every question on the daily paper is our own.

Use the daily paper as a practice exam

The Level 3 Chemistry track above is a short timed mock of 8 questions, not a full exam and not written by NZQA (NCEA Level 3 Chemistry). It covers Born-Haber cycles and entropy, which support the enthalpy and entropy part of 91390. It has no topic on electron configurations, shapes and polarity or periodic trends, which are a large part of 91390, and it includes NMR, transition metals and electrode potentials, which are not part of it. For the shapes, use our Chemical Bonding course.

Is NCEA 91390 changing?

The 2027 specification matches 2026: resonance, Hund's rule and half-filled sublevel stability are still not assessed. Year 13 moves to the New Zealand Advanced Certificate of Education in 2030.

Common mistakes in NCEA 91390

• Linking entropy to the wrong change. NZQA's 2025 report says many candidates focused on the loss of a solid, not the loss of two moles of gas.
• Writing that forces exist inside a molecule. Intermolecular forces exist between molecules.
• Forgetting shielding. Explain periodic trends with the number of energy levels, nuclear charge, repulsion from inner energy levels and attraction to the valence electrons.
• Stating trends without reasons. The report lists stating a periodic trend but not the factor behind it as a Not Achieved error.
• Calculation slips. Give units and significant figures, and keep track of the sign.

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