HSC Chemistry common mistakes
HSC Chemistry · Understand the traps and how to avoid them.
Writing ionic equations with spectator ions left in, or with unbalanced charges across the arrow.
Candidates copy the full molecular equation and delete only some ions, or they balance atoms but never check that total charge is equal on both sides.
What to do instead
After writing any ionic equation, run two checks in order: (1) count atoms of each element on both sides; (2) sum the charges on each side and confirm they match. Then cross out every ion that appears unchanged on both sides. Practise by converting five full equations to net ionic form and marking each spectator explicitly.
Drawing Lewis structures that exceed the octet on period-2 atoms or omit lone pairs on the central atom.
Candidates place all available electrons as bonding pairs first and treat lone pairs as leftovers, rather than assigning electrons to satisfy octets before forming extra bonds.
What to do instead
Use a fixed procedure: count total valence electrons, draw single bonds, complete outer atoms' octets, then place remaining electrons on the central atom, and only form multiple bonds if the central atom is short of an octet. Finish by checking formal charges; a structure with formal charges larger than ±1 on period-2 atoms is usually wrong.
Predicting molecular shape from electron-domain geometry without subtracting lone pairs, e.g. calling water linear or ammonia trigonal planar.
Candidates memorise the shape names against the number of electron domains and forget that lone pairs change the observed shape even though they occupy a domain.
What to do instead
Always state the number of bonding pairs and lone pairs separately before naming a shape, then apply the lone-pair adjustment (four domains with one lone pair gives trigonal pyramidal, with two gives bent). Draw the lone pairs on the diagram itself so the domain count is visible.
Explaining boiling point or solubility trends by saying 'it has hydrogen bonding' without identifying which atom donates the hydrogen bond and which atom accepts it.
The phrase is memorised as a label for strong intermolecular forces, so candidates apply it to any molecule containing H, including alkanes and hydrogen halides without N–H, O–H or F–H bonds.
What to do instead
For every hydrogen-bonding claim, write the donor (an H attached to N, O or F) and the acceptor (a lone pair on N, O or F) explicitly, and compare molecules by counting hydrogen bonds per molecule and by molecular size. If no N–H, O–H or F–H bond is present, state that dispersion forces dominate and rank by molar mass and shape.
Treating 'like dissolves like' as a rule about polarity alone and predicting that any polar solute dissolves in water, ignoring ion formation and lattice energy.
Candidates learn a simplified slogan and do not connect dissolution to competition between solute–solute, solvent–solvent and solute–solvent forces, so they miss cases such as insoluble carbonates or slightly soluble hydroxides.
What to do instead
Answer solubility questions by naming the three force sets that must be overcome and formed, then judge which side wins. For ionic solids, explicitly weigh lattice energy against hydration energy and note that high charge and small ionic radius raise lattice energy. Support every prediction with a specific structural feature of the solute.
Confusing enthalpy change with activation energy, or claiming a reaction is 'spontaneous because it releases heat'.
The two quantities both appear on energy profile diagrams, and candidates read the height of the curve without distinguishing the peak height from the difference between reactant and product levels.
What to do instead
On every profile, label three separate quantities: activation energy (reactants to peak), enthalpy change (reactants to products, with sign) and, where relevant, the reverse activation energy. State that spontaneity depends on both enthalpy and entropy via free energy, and give one counterexample such as an endothermic dissolution that occurs spontaneously.
Drawing an energy profile for an exothermic reaction with products drawn above reactants, or with a second hump that is lower than the first in a two-step mechanism without justification.
Candidates sketch the curve quickly from memory and do not link the vertical position of each species to its actual energy or to the rate-determining step.
What to do instead
Before drawing, decide the sign of enthalpy change and write it beside the diagram, then place products accordingly. For multi-step mechanisms, make the highest peak correspond to the slowest step and ensure every intermediate sits in a trough above the reactants. Check the finished curve against the labelled values.
Predicting reaction outcomes using a single factor such as 'the stronger acid reacts' without considering concentration, temperature or the full reaction pathway.
Candidates treat qualitative rules as absolute and do not check whether the conditions in the question match the assumptions behind the rule.
What to do instead
For each prediction, write the conditions given in the question and test whether the rule's assumptions hold. Where a rule is used, state it explicitly and note its limits, for example that acid strength governs extent of ionisation while concentration governs amount of acid present. Where two pathways are possible, compare their activation energies and the stability of products.
Reporting a yield or atom economy calculation without identifying the limiting reagent, or mixing up the two quantities.
Both are percentage calculations from a balanced equation, so candidates substitute numbers into whichever formula they remember without checking which reactant runs out first or what each formula measures.
What to do instead
Always convert all reactant amounts to moles, divide by stoichiometric coefficients and name the limiting reagent before any yield calculation. Write the two formulae side by side: percentage yield compares actual and theoretical product mass, while atom economy compares the molar mass of desired product with the total molar mass of all products. Label which one the question asks for.
Describing a synthesis or analysis method without stating the purpose of each step, the observations expected, or how the product is confirmed.
Candidates reproduce a memorised procedure as a list of actions and omit the reasoning, so they cannot justify choices such as solvent, temperature or purification technique when the question asks why.
What to do instead
Write each step in a three-part form: action, purpose, and expected observation or confirmation test. For example, state why a particular solvent is chosen (to dissolve one reagent but precipitate the product), what a positive confirmatory test looks like, and how purity is checked. Practise by explaining an unfamiliar procedure rather than recalling a familiar one.