QCE Chemistry common mistakes
QCE Chemistry · Understand the traps and how to avoid them.
When drawing or interpreting a Lewis structure, the candidate stops at a plausible skeleton and never counts total valence electrons to check that every atom satisfies its octet (or duet for H) and that formal charges are minimised.
The candidate treats structure drawing as pattern-matching from familiar examples rather than as an electron-bookkeeping exercise, so an odd-electron or expanded-octet species is drawn by habit without verification.
What to do instead
Adopt a fixed four-step routine: total valence electrons, place single bonds, complete octets on terminal atoms, then place any remainder on the central atom and compute formal charge for each atom. Practise on species with an odd total electron count and on period-3 species where the octet can expand, and always finish by writing the formal charges explicitly.
When predicting the shape and polarity of a molecule, the candidate counts lone pairs on the central atom as bonding directions, or determines the electron-domain geometry and then reports it as the molecular shape without removing lone pairs.
VSEPR is learned as a naming exercise rather than as a procedure with a distinct electron-geometry and molecular-geometry stage, so the candidate collapses the two stages into one.
What to do instead
Force the two-stage answer: first state the number of electron domains and the electron-domain geometry, then state how many are lone pairs and derive the molecular shape from that. For polarity, add a separate step where each bond dipole is drawn as an arrow and the vector sum is judged against the molecular shape, not against the number of polar bonds alone.
When asked to explain a trend in melting point or boiling point across a series of substances, the candidate states only 'stronger intermolecular forces' without naming the type of force and without linking it to a specific structural feature such as molar mass, molecular shape, or the presence of an O-H or N-H bond.
The candidate has memorised the phrase as a safe answer and does not realise that the marking scheme awards the point for the identification of the force type and the structural cause.
What to do instead
Write explanations as a chain of three explicit links: structural feature, therefore force type and relative strength, therefore energy needed to separate particles. For example, a longer unbranched chain gives greater dispersion forces because of greater polarisability, so more energy is required to overcome them. Compare substances pairwise and name the dominant force in each before comparing.
When describing an organic reaction mechanism, the candidate draws curly arrows that start from an atom rather than from a bond or a lone pair, or that end on an atom rather than on the atom or bond being formed, and omits the resulting charges.
The candidate copies the overall shape of a remembered mechanism diagram without understanding that an arrow represents the movement of an electron pair from an electron-rich site to an electron-poor site.
What to do instead
Before drawing any arrow, label the nucleophile or electron-rich site and the electrophile or electron-poor site. Draw each arrow tail exactly on a lone pair or in the middle of the bond being broken, and each head on the atom or bond being formed. After each step, check that the total charge is conserved and that every intermediate has a complete octet where possible.
When calculating an equilibrium constant or an equilibrium concentration, the candidate substitutes the initial concentrations into the equilibrium expression instead of the equilibrium concentrations, or omits the change row of an ICE table entirely.
The candidate treats K as a ratio of whatever numbers are given in the question, and the distinction between initial and equilibrium composition is not operationally secure.
What to do instead
Always construct an ICE table with initial, change, and equilibrium rows, and define the change variable in terms of the stoichiometric coefficients. Only substitute the equilibrium row into the K expression. Check the answer by confirming that the calculated equilibrium concentrations, when substituted back, reproduce the given K, and that the direction of shift is consistent with the size of K.
When predicting the effect of a change on an equilibrium system, the candidate says the equilibrium 'shifts to the right' but cannot state what happens to the value of K, or claims that adding a catalyst or changing pressure always changes K.
Le Chatelier's principle is applied as a verbal rule about direction without a firm grasp that only temperature changes the value of K for a given reaction.
What to do instead
Separate two questions in every answer: which direction does the system shift, and does K change. State explicitly that concentration and pressure changes alter the position of equilibrium but leave K unchanged at constant temperature, that a catalyst changes only the rate of approach to equilibrium, and that temperature changes alter K because the forward and reverse activation energies change unequally.
When identifying a conjugate acid-base pair or writing an equation for a weak acid, the candidate uses a single arrow for a weak acid ionisation, or pairs an acid with an unrelated base that differs by more than one proton.
The candidate relies on recognising familiar formulas rather than applying the proton-transfer definition, and does not distinguish strong from weak electrolytes in the arrow convention.
What to do instead
Apply the test explicitly: a conjugate pair must differ by exactly one H and one unit of charge. For weak acids and bases, use the equilibrium arrow and write the Ka or Kb expression alongside. Practise by taking any given species and generating its conjugate partner in both directions before writing the full equation.
When balancing a redox equation in acidic or basic solution, the candidate balances atoms but not charge, or adds H2O and H+ to the wrong side, and forgets to convert to OH- for basic conditions.
The half-equation method is applied as a sequence of memorised steps without the charge-balance check that makes the method self-correcting.
What to do instead
Use the half-equation procedure with an explicit charge check after every addition: balance the key element, balance O with H2O, balance H with H+, then balance charge with electrons. Multiply the half-equations so electrons cancel, add and cancel, and only then, for basic conditions, add OH- to both sides to neutralise H+ and cancel the resulting water. Verify that atoms and total charge balance in the final equation.
When asked to identify an unknown organic compound from combined data such as infrared, mass, and nuclear magnetic resonance spectra, the candidate interprets one spectrum in isolation and proposes a structure that contradicts the others.
Each technique is studied as a separate topic, so the candidate never builds the habit of using the data sets as mutual constraints.
What to do instead
Work through a fixed order: use the mass spectrum for molecular mass and fragmentation clues, infrared for the presence or absence of specific functional groups, and the number and splitting of NMR signals for the number and environment of hydrogen or carbon environments. Reject any candidate structure that fails any one technique, and state which piece of evidence supports which part of the structure.
When describing a synthesis pathway, the candidate proposes a sequence of reactions without checking functional-group compatibility, such as oxidising a molecule that also contains an unprotected group that would react, or using conditions that would also attack another part of the molecule.
The candidate focuses on whether each individual step is a known reaction and does not evaluate the whole sequence as a design problem with selectivity and protecting-group requirements.
What to do instead
Before writing the route, list every functional group present at each stage and mark which ones would react under the proposed conditions. Where a conflict exists, insert a protection or deprotection step or reorder the sequence. Finish by checking that the route starts from the stated starting material and reaches the stated target with the correct carbon skeleton and functional groups at each stage.