NCEA L3 Chemistry common mistakes
NCEA L3 Chemistry · Understand the traps and how to avoid them.
Writing equations such as Mg + O2 -> MgO without balancing, or leaving ionic charges unbalanced in half-equations when answering structure and bonding or redox questions.
Candidates treat the equation as a label for the reaction rather than a quantitative statement, and under exam pressure they copy the word equation directly. In redox work, they balance atoms but forget that electrons lost must equal electrons gained, because they never formally assigned oxidation numbers.
What to do instead
Adopt a fixed routine: assign oxidation numbers first, split into half-equations, balance O with H2O and H with H+, then add electrons and multiply to equalise electron count before recombining. For any full equation, count each element on both sides before moving on. Practise ten redox equations from scratch, checking total charge balances as well as atoms.
Confusing the terms 'strong' and 'concentrated' (or 'weak' and 'dilute') when describing acids, and writing that a weak acid has a low pH or that dilution increases Ka.
Everyday language uses these words interchangeably, and candidates memorise 'strong acid = low pH' without separating the ideas of degree of ionisation (an intrinsic property) from amount of acid per litre (a concentration). They also assume any change shifts the equilibrium constant.
What to do instead
Write a one-line definition for each of the four terms and use them only in that sense. For a weak acid, state that it ionises partially, so [H3O+] is much less than the acid concentration. Remember Ka depends only on temperature; dilution changes the degree of ionisation and pH but not Ka. Test yourself with a table of 0.1 mol/L HCl, 0.1 mol/L CH3COOH and 0.001 mol/L HCl, predicting pH and conductivity for each.
Calculating pH or equilibrium concentrations by ignoring the ICE (initial-change-equilibrium) table, or by substituting initial concentrations into Ka or Kc instead of equilibrium values.
Candidates reach for a formula such as pH = -log[H+] and plug in the stated concentration, forgetting that for weak acids the equilibrium [H3O+] is not the initial acid concentration. They also drop the 'x' approximation without checking it is valid.
What to do instead
Always set up an ICE table with symbols before substituting numbers, and label which row you are using. For weak acids, solve the quadratic or state the approximation and justify it (x less than about 5% of the initial concentration). Finish by checking that calculated equilibrium values reproduce the given Ka or Kc.
Describing organic reaction types imprecisely, for example calling an addition reaction 'substitution', or failing to name the product of an oxidation or elimination reaction with the correct functional group and locant.
Candidates learn reactions as lists of reagents rather than as mechanisms classified by what happens to the functional group. They also neglect IUPAC numbering rules, so the locant of a new group is guessed rather than derived from the parent chain.
What to do instead
Build a classification chart: addition (two reactants give one product, alkene or alkyne), substitution (one group replaced, alkane or haloalkane or alcohol with HX), elimination (small molecule lost, forming a double bond), oxidation (primary alcohol to aldehyde to carboxylic acid; secondary to ketone), esterification and hydrolysis. For every product, redraw the carbon skeleton, number the longest chain, and check the functional group suffix before writing the name.
Drawing Lewis structures, shapes or polarity incorrectly: omitting lone pairs, giving a bent molecule a linear shape, or calling a molecule polar without considering symmetry of the bond dipoles.
Candidates count bonding pairs but not electron domains, so they apply VSEPR to the wrong total. Polarity is often decided from electronegativity differences alone, ignoring that identical dipoles can cancel in a symmetrical geometry.
What to do instead
Use a repeatable sequence: total valence electrons, draw the skeleton, place bonding pairs, then lone pairs, then count electron domains (bonding regions plus lone pairs) to get the shape. Decide polarity in two steps: are there polar bonds, and do their dipoles cancel by symmetry? Test on CO2, H2O, NH3, BF3 and CH4 until the reasoning is automatic.
Treating entropy and enthalpy as sufficient to predict spontaneity, for example claiming any exothermic reaction is spontaneous, or confusing ΔG with ΔH.
Candidates memorise 'exothermic reactions happen' from earlier levels and do not link spontaneity to the Gibbs relation. They also forget that temperature can reverse the sign of ΔG when ΔH and ΔS have the same sign.
What to do instead
Write ΔG = ΔH - TΔS every time and state the sign of each term before concluding. Learn the four sign combinations and the temperature at which the reaction becomes spontaneous. Remember ΔS is about dispersal of energy or matter, so an increase in gas moles or a solid dissolving generally raises it.
Mixing up reaction rate with equilibrium position, for example saying a catalyst increases yield, or that raising temperature shifts equilibrium and therefore changes the rate constant for both directions unequally without explanation.
The two topics are taught together and both involve temperature and concentration, so candidates transfer conclusions across contexts. They do not distinguish kinetics (how fast equilibrium is reached) from thermodynamics (where equilibrium lies).
What to do instead
Write two separate headings, 'rate' and 'yield', and answer each question under the correct one. State explicitly that a catalyst lowers activation energy for both directions and so speeds attainment without changing K. For temperature, use the Arrhenius idea that both rate constants rise but the endothermic direction rises more, shifting the equilibrium.
Using the wrong formula or units in thermochemical and electrochemical calculations, for example q = mcΔT with mass in grams but c in kJ, or forgetting to divide by moles when asked for ΔH per mole, or omitting the sign convention for ΔH.
Candidates focus on the arithmetic and lose track of units and of what the question asks per mole or per gram. Exothermic and endothermic signs are often applied from memory rather than from the direction of energy flow.
What to do instead
Write the unit next to every number in the working, including c in J g-1 K-1 and mass in g, then convert to kJ at the end. Always state the quantity being calculated in the formula itself, for example ΔH per mole of fuel. Decide the sign by asking whether heat is released to or absorbed from the surroundings, and state it in words alongside the value.
Answering 'explain' or 'discuss' questions with a single word or an unlinked statement, for example writing 'because it is more reactive' without naming the species, the trend and the reason.
Candidates treat explanation questions like recall questions and run out of marks because the assessment schedule rewards a chain of reasoning. Under time pressure they write the conclusion and omit the evidence or the link.
What to do instead
Use a three-part sentence structure: observation or fact, underlying principle, and consequence. For trends, name the species, state the trend across the period or group, and give the reason (effective nuclear charge, atomic radius, shielding, bond strength). Practise by marking your own answers against the verbs in the question (describe, explain, justify, compare) and counting the linked steps.
Ignoring significant figures, units and the required precision in numerical answers, or rounding intermediate values so the final answer is outside the accepted range.
Candidates treat the calculation as the whole task and do not check whether the answer is physically sensible or expressed to the precision implied by the data. They also round at each step to save time.
What to do instead
Carry full precision through the working and round only the final answer, using the least precise given value as a guide. Write the unit and, where relevant, state whether the value is reasonable (for example a pH between 0 and 14, or a percentage yield below 100). Build a final 30-second check into every calculation question.