NCEA L2 Chemistry common mistakes
NCEA L2 Chemistry · Understand the traps and how to avoid them.
When asked to explain a trend in a property across a period or group (e.g. atomic radius or ionisation energy), the candidate simply restates the trend ('it decreases') or names the factor ('more protons') without stating the direction of the effect or linking it to the observed property.
NCEA L2 explanation questions are marked against a three-part structure: state the trend, give the chemistry reason, and then explain how that reason produces the trend. Candidates who have memorised the trend learn the first part only, and under exam pressure they write the trend and stop, believing the point has been made.
What to do instead
Practise writing every periodic-trend answer as a three-sentence chain: (1) the trend with direction, (2) the structural cause (e.g. 'the nuclear charge increases while the outer shell stays the same'), (3) the consequence ('so the outer electrons are held more tightly, requiring more energy to remove, so ionisation energy increases'). Check each answer contains the words 'because' and 'so/therefore' before moving on.
When writing a balanced equation the candidate produces correct formulas but wrong coefficients, or leaves an equation unbalanced because the formulas 'look right', and does not check atom counts on both sides.
Balancing is treated as a formality after the 'real' chemistry of writing formulas. Candidates also confuse subscripts (part of the formula) with coefficients (the balancing number) and change a formula to make it balance, which is a different and more serious error.
What to do instead
Adopt a fixed routine: write formulas first, then count each element in a table under reactants and products, then place coefficients, then recount. Never alter a subscript once written; if the equation will not balance, the formula is wrong. Do ten equations a week, including one with a polyatomic ion (e.g. sulfate or nitrate) kept intact as a unit.
The candidate calculates an amount in moles correctly but then uses the wrong stoichiometric ratio, or forgets to convert to the ratio at all, when moving between reactants and products.
The mole calculation is taught as a sequence of formulas, and the ratio step sits in the middle where it is easy to skip. Candidates who have memorised n = m/M and n = cV treat the calculation as finished once n is found, so they carry that number straight through to the answer.
What to do instead
Write the balanced equation at the top of every calculation and draw arrows from the given substance to the wanted substance with the ratio written on the arrow. Then state explicitly 'ratio is 2:1, so moles of product = moles of reactant ÷ 2'. Markers award this step, so writing it down also earns credit even if the arithmetic slips.
In titration and solution calculations, the candidate uses the volume in mL directly in n = cV, or mixes units (concentration in mol L⁻¹ with volume in mL), producing answers out by a factor of 1000.
The formula is taught with the unit requirement stated once, and candidates under time pressure copy numbers from the burette reading or the pipette volume without converting. The error is invisible because the arithmetic itself is correct.
What to do instead
Before substituting, write every quantity with its unit and convert to L and mol L⁻¹ on the same line: 'V = 24.6 mL = 0.0246 L'. Build the habit of writing the conversion explicitly rather than doing it mentally, and sanity-check the answer: a titration of about 25 mL of roughly 0.1 mol L⁻¹ solution should give an amount in the low thousandths of a mole, not tenths.
When asked to identify the limiting reactant, the candidate compares the masses of the two reactants, or compares the moles directly, without dividing by the coefficients from the balanced equation.
Mass feels like 'amount' in everyday reasoning, and the coefficient step is the least intuitive part of the topic. Candidates who do convert to moles often stop there, assuming the smaller number of moles is automatically limiting.
What to do instead
Use a fixed two-column comparison: for each reactant compute moles ÷ coefficient, and the smaller value identifies the limiting reactant. Always write the balanced equation first so the coefficients are visible. Practise one example where the reactant with the smaller mass is in excess, so the method, not the shortcut, is what you trust.
The candidate names an organic product or describes a reaction type incorrectly because they have not distinguished between addition, substitution and elimination, or has drawn a structural formula with the wrong number of bonds on a carbon.
The three reaction types are learned as vocabulary lists rather than as consequences of the bonding in the reactant. Alkenes and alkanes look similar in condensed formulas, and candidates do not consistently check that every carbon has four bonds.
What to do instead
For each reaction, first classify the reactant (alkane, alkene, haloalkane, alcohol) and state what it can do: alkenes add, alkanes substitute under UV light, haloalkanes substitute or eliminate depending on conditions. Then draw the structure and count bonds on every carbon before naming. Keep a one-page table of reactant, reagent/condition, reaction type and product class.
When describing an acid–base reaction, the candidate labels the acid and base by formula familiarity rather than by proton transfer, and cannot identify the conjugate pair in the equation.
Everyday labels ('HCl is an acid, NaOH is a base') work for strong species but fail for weak acids, water acting as an acid or base, and salts such as sodium carbonate. Candidates do not trace which species loses and which gains a proton.
What to do instead
For every equation, draw an arrow from the species losing H⁺ to the species gaining it, then label the two conjugate pairs. Include water and hydrogencarbonate examples so the pattern is practised on amphiprotic species. Write the definition (proton donor/acceptor) at the top of the answer before labelling anything.
The candidate calculates pH from the concentration of a weak acid using [H⁺] = c, treating it as a strong acid, or reports pH without checking whether the acid is strong or weak.
The strong-acid calculation is simple and well drilled, and questions often supply a concentration in the same format. The distinction is in the wording, and candidates read for numbers rather than for the identity of the acid.
What to do instead
Before calculating, write 'strong' or 'weak' next to the acid name and note whether Ka or a degree of dissociation is supplied. If the acid is weak, the hydrogen ion concentration is less than the acid concentration, so the pH must be higher than the strong-acid value; use that as a check. Practise comparing the same concentration of HCl and ethanoic acid side by side.
In redox work, the candidate assigns oxidation numbers inconsistently (for example, assuming oxygen is always −2 or hydrogen always +1) and so cannot identify what is oxidised and what is reduced.
The rules are memorised as absolutes, and the exceptions (oxygen in peroxides, hydrogen in metal hydrides) and the treatment of polyatomic ions are not practised. Candidates then guess which species changes.
What to do instead
Write oxidation numbers above every atom in the equation, apply the rules in order (element = 0, monatomic ion = charge, sum = overall charge), and identify the change by comparing before and after. State oxidation and reduction in terms of electron loss and gain and as changes in oxidation number, since both forms are asked for. Include one peroxide or hydride example deliberately.
When asked to explain a rate or equilibrium shift, the candidate says 'the reaction speeds up' or 'it shifts right' without naming the collision or concentration reason, and does not link the change to the effect on particle collisions or on both forward and reverse rates.
The observable ('faster', 'more product') is what candidates remember, while the explanatory mechanism is the part that carries the achievement and excellence marks. Under time pressure the observable is written and the mechanism is dropped.
What to do instead
Use a sentence template: 'Increasing X means more particles per unit volume, so collisions between reactant particles are more frequent, so the rate of the forward reaction increases.' For equilibrium, state which rate increases first, then how the system responds, then the observable change. Practise writing the full chain for concentration, temperature, pressure and surface area changes.