WACE Chemistry ATAR common mistakes
WACE Chemistry ATAR · Understand the traps and how to avoid them.
When writing electron configurations for transition-metal ions, the candidate removes electrons from the 4s orbital before the 3d orbital (e.g. writes Fe2+ as [Ar]3d4 4s2 instead of [Ar]3d6), and fills 4s before 3d when building the neutral atom's configuration from the wrong energy ordering.
The candidate memorises the relative energies of 4s and 3d from the Aufbau diagram used for neutral atoms and applies the same ordering to ions. Once 3d is occupied, the 3d subshell lies lower in energy than 4s, so the highest-energy (4s) electrons are lost first. Many candidates never see this distinction made explicit.
What to do instead
Teach the rule as a two-step procedure: build the neutral atom using the standard filling order, then for cations remove electrons from the highest principal quantum number shell first (4s before 3d). Practise with Fe, Fe2+, Fe3+, Cu, Cu+, Cr and Cr3+, and have students check the total electron count against the nuclear charge each time.
The candidate predicts molecular shape by counting all electron pairs around the central atom, including lone pairs, as if they were bonding pairs, so H2O is described as linear or tetrahedral with a 109.5 degree bond angle, and NH3 is given 109.5 degrees rather than about 107 degrees.
The candidate has learned VSEPR as 'count pairs and match a shape' without separating electron-domain geometry (which counts lone pairs) from molecular geometry (which describes only atom positions). The two-step logic is compressed into one step and the lone-pair repulsion effect is lost.
What to do instead
Require a written scaffold for every VSEPR question: total valence electrons, Lewis structure, number of bonding pairs and lone pairs, electron-domain geometry, then molecular geometry and angle with the lone-pair compression stated explicitly. Have students compare CH4, NH3 and H2O side by side so the trend 109.5, 107, 104.5 degrees is explained by increasing lone-pair repulsion.
In limiting-reagent problems the candidate calculates moles of each reactant, divides each by its stoichiometric coefficient, and then selects the larger quotient as the limiting reagent, or simply picks the reactant with the smaller number of moles without dividing by the coefficient.
The candidate has memorised 'compare the ratios' without attaching meaning to the comparison. The quotient moles divided by coefficient represents how many times the reaction can run, so the smallest quotient, not the largest, is limiting. Without interpreting the quotient, the direction of the comparison is arbitrary.
What to do instead
Have students compute the 'reaction cycles available' from each reactant and state in words: this amount of A can support x runs, this amount of B can support y runs, therefore the smaller number limits. Follow every calculation with a mass-of-product check from the limiting reagent and a brief sentence identifying the excess reagent and how much remains unreacted.
The candidate writes a neutralisation or salt-preparation equation that is not balanced in charge or atoms, or omits states, so a reaction between a weak acid and a carbonate is written with H2CO3 as a product and no gas evolved, and titration calculations then use the wrong mole ratio.
The candidate treats equation writing as a formality before the calculation and rushes it. Weak-acid/carbonate and acid/metal-hydrogen carbonate reactions are also poorly rehearsed, so the candidate does not recognise that carbonic acid decomposes to CO2 and water.
What to do instead
Enforce a checklist applied to every equation: balance atoms, balance charge, add states, then check that the products are chemically sensible (gas evolution, precipitate, water formation). Drill the specific patterns acid + carbonate, acid + hydrogencarbonate, acid + metal, acid + base, and require the mole ratio to be read aloud from the balanced equation before any titration arithmetic.
When asked to identify the oxidant and reductant, the candidate labels the species that is oxidised as the oxidising agent, and assigns oxidation numbers to polyatomic ions by giving oxygen -2 in peroxides or hydrogen +1 in metal hydrides without checking the exception.
The language is genuinely confusing: the oxidising agent is itself reduced. Candidates learn the definitions as vocabulary rather than as paired processes, and oxidation-number rules are memorised as a list with the exceptions treated as trivia rather than as conditional rules.
What to do instead
Use a fixed written template for every redox question: oxidation number of each element before and after, the species whose number increases (oxidised, and it is the reductant), the species whose number decreases (reduced, and it is the oxidant), then the half-equations. Reinforce with the mnemonic-free statement 'the oxidant takes electrons, so it is reduced' and practise H2O2 and NaH as deliberate exception cases.
The candidate calculates an enthalpy change and reports it without sign or with the sign reversed, for example giving combustion of methane as positive because the calculation produced a positive number after subtracting in the wrong order, or omitting the sign when the reaction is endothermic.
The candidate treats Hess's law and bond-energy calculations as arithmetic to be completed rather than as a bookkeeping of energy flow. Sign conventions are applied mechanically and the candidate does not connect the final number back to whether energy is released or absorbed.
What to do instead
Require every thermochemistry answer to end with a sentence interpreting the sign in physical terms (negative means energy released to the surroundings, temperature would rise). Have students draw an energy-level diagram or cycle before substituting numbers, and insist that bond-breaking is always positive and bond-making always negative so the direction of each term is visible.
When a change is applied to an equilibrium system, the candidate states that the equilibrium position shifts but then claims the value of the equilibrium constant changes, or says a catalyst increases yield by shifting the position to the right.
The candidate conflates three separate ideas: the rate at which equilibrium is reached, the position of equilibrium, and the equilibrium constant. Because these are taught close together, the terms are used interchangeably, and the candidate has not been given a clear statement of what each one depends on.
What to do instead
Build an explicit three-column table for every disturbance (concentration, pressure, temperature, catalyst) showing the effect on rate, on position, and on K, with the rule that only temperature changes K. Have students justify each entry with collision-theory or thermodynamic reasoning rather than asserting the shift, and include a catalyst row that shows no change in position or K.
The candidate names an organic compound or identifies its functional group by counting carbon atoms incorrectly from a condensed or skeletal structure, missing that each vertex and line end is a carbon, or classifying an ester as an ether or an aldehyde as a ketone.
Skeletal and condensed formulae require an implicit-atom convention that is not automatic for candidates who learned structures as full displayed formulae. Aldehyde versus ketone and ester versus ether differ only in the position of one atom, so pattern-matching by overall shape fails.
What to do instead
Have students redraw every condensed or skeletal structure as a full displayed formula with all hydrogens shown before naming it. Teach functional-group recognition by the local environment of the carbonyl carbon (H attached means aldehyde, two carbons means ketone, O attached means ester) and drill paired examples such as propanal/propanone and ethyl ethanoate/methoxyethane.
In a titration or gravimetric calculation the candidate uses the burette reading or the mass of the wet precipitate directly, ignores dilution factors when an aliquot is taken, or reports an answer to more significant figures than the apparatus justifies.
The candidate follows the arithmetic sequence without tracking what each measured number represents. Aliquot and dilution steps are handled implicitly, and significant figures are treated as a formatting rule applied at the end rather than as a consequence of the precision of each measurement.
What to do instead
Require a labelled calculation map before any numbers: concentration and volume of the standard, moles of standard, mole ratio, moles of analyte, then back to the requested quantity, with each dilution or aliquot step written as an explicit multiplication. Set significant figures from the least precise measurement (burette to 0.05 mL, analytical balance to 0.0001 g) and mark the final answer for both value and precision.
When asked to explain a trend such as increasing first ionisation energy across a period or decreasing atomic radius down a group, the candidate states the trend correctly but gives no causal explanation, or attributes it to 'more protons' without mentioning shielding and the resulting effective nuclear charge.
The candidate has memorised the direction of trends as facts and has not been required to build the explanatory model of nuclear charge, shielding and distance. Under exam pressure the recall of the direction crowds out the explanation, and the candidate does not realise the explanation carries the marks.
What to do instead
Train a fixed explanatory sentence structure: compare effective nuclear charge (protons minus inner-shell shielding), then distance of the outer electron from the nucleus, then state the effect on the force experienced and hence on the energy required. Apply it to the period 3 trend, the group 1 trend and the group 18 anomaly, and have students mark each other's answers against the three required elements.