NZ Scholarship Chemistry common mistakes
NZ Scholarship Chemistry · Understand the traps and how to avoid them.
Writing a full descriptive answer to an 'assess/discuss/evaluate' question: describing the chemistry correctly but never weighing evidence, comparing competing explanations, or stating a justified judgement at the end.
Candidates rehearse recall and explanation in practice, so under time pressure they default to telling the marker everything they know rather than answering the command word; Scholarship questions deliberately use high-order verbs to discriminate at the top band.
What to do instead
Before writing, underline the command word and draft a one-sentence judgement as a working thesis. Structure the answer as claim, evidence, counter-consideration, resolution, and finish with an explicit evaluative sentence. In practice, rewrite one past descriptive answer per week into an evaluative one and check that the final paragraph contains a comparison or a limitation, not new description.
Treating equilibrium, kinetics, and thermodynamics as interchangeable explanations: claiming a large K means a fast reaction, or that a spontaneous reaction proceeds quickly, or that a catalyst shifts the equilibrium position.
These topics are taught in separate chapters and the links between them are rarely tested in school internal assessment, so candidates carry loose verbal associations into an exam that specifically probes the boundaries between them.
What to do instead
Build a three-column comparison table (what it measures, what it does not tell you, what changes it) for K, Ea/rate, and ΔG. For each past question, label which framework is being asked about before answering. Practise a set of deliberately confusable statements and write one line each explaining why the statement is wrong.
Using ΔG sign alone to declare a reaction 'will happen', ignoring activation barriers, and failing to reconcile a positive ΔG with an observed reaction driven by coupling, concentration changes, or precipitation.
Standard conditions are assumed silently, and candidates forget that ΔG° applies at unit activity and that a thermodynamically unfavourable step can be pulled forward by a favourable coupled process or by removal of product.
What to do instead
Always state the conditions attached to ΔG° and distinguish it from ΔG under actual concentrations using Q. When a reaction is observed but ΔG° is positive, explicitly identify the driving term (Q far from 1, coupled hydrolysis, precipitation, gas evolution) in one sentence. Practise calculating ΔG at non-standard concentrations to see how the sign can flip.
Presenting calculated numerical answers with no discussion of assumptions, uncertainties, or the significance of the number, so a correct calculation earns process marks but no Scholarship-level credit.
School assessment rewards the correct value; candidates therefore treat the number as the endpoint rather than as evidence, and they rarely state assumptions such as ideal behaviour, complete reaction, or negligible heat loss.
What to do instead
After every calculation, add two sentences: one naming the dominant assumption and one stating the direction and rough size of the error it introduces. Where possible, compare the result with an independent estimate or a known value. Include significant figures consistent with the data and comment when a result is suspiciously precise.
Describing organic reaction mechanisms as remembered sequences of arrows without explaining why the electron flow occurs, and failing to use curly arrows to show heterolytic bond breaking, nucleophile/electrophile roles, and intermediate stability.
Mechanisms are often memorised as pictures, so candidates reproduce the arrows for the taught example but cannot transfer the reasoning to an unfamiliar substrate or predict the major product from first principles.
What to do instead
For each mechanism, annotate the arrows with the reason (bond polarisation, lone pair availability, carbocation or anion stability, leaving-group ability). Practise predicting products for substrates not seen before and justify regiochemistry with carbocation stability or steric arguments. Redraw each mechanism from a blank page rather than copying.
Confusing the language of bonding models: describing a delocalised molecular-orbital picture and a localised Lewis structure in the same paragraph, or using hybridisation terminology inconsistently with observed geometry and resonance.
Multiple models are taught for the same species and candidates do not appreciate that each model has a domain of validity, so mixed vocabulary leaks into answers and undermines the explanation.
What to do instead
For a set of species (benzene, carbonate, ozone, an amide, a carbocation), write parallel explanations in Lewis/resonance, hybridisation, and MO language, then state which model best explains the observed property (bond lengths, planarity, reactivity). In the exam, choose one model, name it, and stay inside it.
Ignoring the quantitative treatment of acids and bases: assuming strong-acid formulas apply to weak acids, neglecting the autoprotolysis of water at low concentration, or mis-handling buffers and polyprotic species.
Candidates learn pH as a formula rather than as an equilibrium problem, so they do not check whether the approximation (neglecting x, neglecting water) is valid, and they treat buffer questions as arithmetic rather than as competing equilibria.
What to do instead
For every acid-base calculation, first write the full equilibrium set, then state and test each approximation before simplifying. Practise cases at 10^-7 M and below, mixtures, and diprotic acids where the second dissociation matters. For buffers, explain the shift using Le Chatelier or the Henderson-Hasselbalch derivation rather than quoting the equation.
Writing unbalanced or non-conserved redox half-equations in complex media, and failing to combine them with correct electron, charge, and atom balance, especially in acidic versus basic conditions.
Candidates balance atoms but forget charge, and they do not systematically add H2O and H+ or OH- for the medium, so the final cell equation or potential calculation is built on an incorrect foundation.
What to do instead
Adopt a fixed procedure: balance the skeleton, balance O with H2O, balance H with H+, balance charge with electrons, then convert to basic conditions by adding OH- to both sides and cancelling water. Check total charge on each side as a final step. Practise with permanganate, dichromate, and disproportionation examples in both media.
Describing structure and bonding in solids and complexes without linking to observable properties: naming a lattice type or geometry but not using it to explain conductivity, melting point, colour, magnetism, or solubility.
Candidates learn descriptive facts about crystal structures and coordination compounds as isolated items, so they cannot use the model as an explanatory tool when a question asks them to account for a trend or an anomaly.
What to do instead
For each structure type and complex, write a two-way card: property to explanation and explanation to predicted property. Include band gaps, delocalisation, ligand-field splitting, high-spin versus low-spin, and chelate effects. Practise questions phrased as 'account for the difference between X and Y' and always name the structural feature responsible.
Treating data interpretation as reading off a trend: describing what a graph or table shows without identifying anomalies, proposing a chemical explanation for the deviation, or evaluating the reliability of the data set.
Scholarship papers include unfamiliar data and expect candidates to reason from it, but candidates default to summarising the obvious pattern and stop before the analytical step that earns the top marks.
What to do instead
Use a fixed routine for every data question: state the trend, quantify it, flag the outlier or discontinuity, propose a chemical cause, and comment on precision or whether the data can discriminate between competing models. Practise with unfamiliar tables where the expected answer is not the headline trend.