NCEA 91166: chemical reactivity (Level 2 Chemistry)

What NCEA 91166 assesses

NZQA limits chemical reactivity in the standard (version 2) to rates of reaction and equilibrium principles:

• Rates: the effect of concentration, temperature, surface area and a catalyst, explained with collision theory, including activation energy
• Equilibrium: the dynamic nature of equilibrium, and the effect of changes in temperature, concentration, pressure or adding a catalyst
• the significance of the equilibrium constant, Kc, for homogeneous systems, which may involve calculations
• acids and bases as proton transfer, and the properties of strong and weak acid and base solutions: conductivity, rate of reaction and pH
• calculations involving Kw and pH, restricted to strong acids and bases

The three grades go up in steps. Achieved describes, identifies, names, draws, calculates or gives an account of chemical reactivity. Merit explains chemical reactivity. Excellence elaborates, justifies, relates, evaluates, compares and contrasts, or analyses chemical reactivity. Every grade needs correct chemistry vocabulary, symbols and conventions, and Excellence needs them used consistently.

The NCEA 91166 exam in 2026 and 2027

NCEA 91166 is one of three Level 2 Chemistry externals, with 91164 (bonding) and 91165 (organic compounds). The 2026 exam is on Monday 16 November, in the afternoon session. NZQA's specifications for both years say the exam is on printed paper and 91166 is a 4-credit external.

• Three questions: you answer all three, and each has several parts. Questions may be set in contexts that are unfamiliar to you, and any context-specific formulae are given in the exam.
• Calculator: an approved scientific or graphics calculator is required.
• Resource booklet: it has the formulae you need, such as n = cV, and a periodic table with element symbols, atomic numbers and molar masses. Use the molar masses given with the question or on the periodic table.
• Calculations: show all working, round numerical answers to three significant figures and include the correct units.
• Conventions: symbols, nomenclature, spelling and formatting follow IUPAC conventions. The reference sheet for these is not provided in the exam.

How NCEA 91166 is marked

Each of three questions is marked N0 to E8 and added to a total out of 24. NZQA's report says that missing units and significant figures cost many Achieved candidates marks, and that Merit and above needed rates and equilibrium joined up in commercial contexts.

NCEA 91166 cut scores

The 2025 Achieved boundary of 9 was the highest in these five years, while 2021 was the lowest at 7. Merit and Excellence stayed at 14 and 19. The scores are where each grade begins, out of 24.

Cut scores 2025

2025 schedule: Achieved from 9, Merit from 14, Excellence from 19. Compared with 2024, Achieved went up by 1.

Cut scores 2024

2024 schedule: Achieved from 8, Merit from 14, Excellence from 19. The same as 2023.

Cut scores 2023

2023 schedule: Achieved from 8, Merit from 14, Excellence from 19. The same as 2022.

Cut scores 2022

2022 schedule: Achieved from 8, Merit from 14, Excellence from 19. Compared with 2021, Achieved went up by 1.

Cut scores 2021

2021 schedule: Achieved from 7, Merit from 14, Excellence from 19.

Worked examples at each grade

Achieved: pH and an equilibrium expression

Find the pH of 0.0100 mol L⁻¹ HCl and of 0.0100 mol L⁻¹ NaOH. Hydrochloric acid is a strong acid, so it fully ionises and [H₃O⁺] = 0.0100. For NaOH, [OH⁻] = 0.0100, and Kw = 1.0 × 10⁻¹⁴ gives [H₃O⁺] = 1.0 × 10⁻¹²:

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), write the equilibrium expression with the products on top and each concentration raised to the power of its coefficient:

Merit: a Kc calculation and what it means

At equilibrium [N₂] = 0.20, [H₂] = 0.60 and [NH₃] = 0.30 mol L⁻¹. Find Kc:

Give the units that follow from the expression and three significant figures, which NZQA's specification asks for. Merit interprets the size of Kc: a value above 1 means that, at this temperature, the mixture has more products than reactants, though not overwhelmingly. Kc changes only with temperature.

Excellence: integrating rates and equilibrium

The reaction to make ammonia is exothermic. Explain why a compromise temperature is used. Raising the temperature increases the rate, because a larger share of the particles can react when they collide, which gives more successful collisions each second. But the forward reaction gives out energy, so heating favours the reverse direction: the equilibrium shifts towards the reactants, Kc falls and the yield drops. A moderate temperature, a catalyst and a high pressure balance the rate and the yield.

A catalyst opens a different route for the reaction, one with a lower activation energy, so it speeds up the forward and reverse reactions equally. It does not change Kc or the position of equilibrium, and it does not supply energy (the report lists this as a common misunderstanding). A higher pressure shifts the position towards fewer moles of gas, here the 2 mol of NH₃ rather than the 4 mol of N₂ and H₂.

Strong and weak acids are compared in the same way. At equal concentration, HCl fully ionises, so it has a higher [H₃O⁺], a lower pH, a higher conductivity and a faster reaction with magnesium than ethanoic acid, which is only partly ionised. The same amount of each acid still neutralises the same amount of base.

Now try reading some reaction rate curves in our Reaction Kinetics 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 2 Chemistry track above is a short timed mock of 8 questions, not a full exam and not written by NZQA (NCEA Level 2 Chemistry). It covers chemical equilibria and kinetics, including the Maxwell-Boltzmann distribution, which support the rates and equilibrium parts of 91166. It does not include a topic on acids, bases and pH, which is a large part of 91166, so practise that with our pH Calculator and the NZQA past papers. It also covers topics from other standards, such as bonding and organic chemistry. For the whole course, see the NCEA Level 2 Chemistry guide.

Is NCEA 91166 changing?

NZQA's 2027 specification for this standard is the same as 2026. The new Year 12 qualification starts in 2029.

Common mistakes in NCEA 91166

• Thinking a catalyst supplies energy. NZQA's 2025 report lists this: it lowers the activation energy.
• Confusing electrons with ions in conductivity. Solutions conduct because of mobile ions.
• Wrong equilibrium expressions. Products over reactants, each to the power of its coefficient, and write the correct equilibrium arrows.
• Forgetting units and significant figures. The report says missing them cost Achieved candidates marks.
• Not linking surface area to collisions. More surface means more successful collisions.
• Not saying what happens when pressure changes. Name the side with fewer moles of gas.

Read next

New Zealand · NCEA Level 2NCEA Level 2 Chemistry: standards, exam date and practiceNCEA Level 2 Chemistry explained: bonding, organic and reactivity externals, the internals, the 2026 exam date, 2027 dates and worked examples.