BCS Written Chemistry common mistakes
BCS Written Chemistry · Understand the traps and how to avoid them.
When asked to write the electron configuration of a transition-metal ion such as Fe2+ or Cu+, the candidate removes electrons from the 3d subshell before emptying the 4s subshell, giving Fe2+ as [Ar]3d4 4s2 instead of [Ar]3d6.
The aufbau order used to build neutral atoms (4s filled before 3d) is memorised as a fixed sequence, so candidates assume the reverse order of filling is the reverse order of removal. In fact 4s lies above 3d in energy once the 3d shell is occupied, so the highest-energy 4s electrons leave first.
What to do instead
Drill a fixed rule: for d-block cations, ionise from 4s before 3d. Practise ten ions (Fe2+, Fe3+, Cu+, Cu2+, Cr3+, Mn2+, Co2+, Ni2+, Zn2+, V3+) writing both the full and noble-gas configurations, then verify each by counting total electrons against the nuclear charge minus the ion charge.
On periodicity questions the candidate states a trend (for example 'ionisation energy increases across a period') but cannot explain the exceptions at Group 2-to-13 and Group 15-to-16 boundaries, and does not link the trend to effective nuclear charge, shielding, or electron configuration.
Trends are learned as slogans without the underlying Coulombic model, so the candidate has no mechanism to invoke when the trend reverses. The Be-to-B and N-to-O dips are the standard probes of whether the model is understood.
What to do instead
For each exception, write a two-line mechanism: state the configuration of the two species, identify whether the electron removed is from a filled subshell or a half-filled subshell, and state the effect on exchange energy and shielding. Build a one-page table of the four classical anomalies (Be/B, N/O, Mg/Al, P/S) with configurations and explanations, and rehearse writing them from memory.
When drawing a Lewis structure the candidate assigns formal charges loosely or stops at the first structure that fits the octet rule, without comparing competing resonance forms or checking that the sum of formal charges equals the overall charge.
Formal charge is treated as an optional decoration rather than the selection criterion for the best structure. Candidates also confuse formal charge with oxidation number, which leads to nonsensical structures for species such as CO, N2O, or the nitrate ion.
What to do instead
Adopt a fixed four-step routine: count valence electrons, draw the skeleton, distribute to satisfy octets, then compute formal charge on every atom and check the total. Where two structures compete, apply the standard ranking (minimise formal charges, place negative charge on the more electronegative atom). Work through CO, N2O, OCN-, NO3-, and SO4 2- using this routine until it is automatic.
The candidate predicts molecular shape using VSEPR counts of electron pairs but ignores lone-pair repulsion strength, and therefore assigns the wrong bond angle order (for example claiming the H-N-H angle in NH3 exceeds the H-C-H angle in CH4, or missing the distortion in H2O).
The counting step is learned but the ranking of repulsions (lone pair-lone pair > lone pair-bond pair > bond pair-bond pair) is not applied, and the candidate does not connect lone pairs to the compressed angle.
What to do instead
For every molecule, first draw the electron-pair geometry, then the molecular geometry, then explicitly state the repulsion ranking and predict the direction of angle compression. Compare CH4 (109.5), NH3 (107), H2O (104.5) as a single worked sequence and explain the monotonic decrease in one sentence. Repeat for SF4, ClF3, and XeF4 to cover lone pairs in axial versus equatorial positions.
In thermodynamics problems the candidate substitutes into the Gibbs equation or the first law without fixing sign conventions, producing a positive delta-H for an exothermic reaction or a positive delta-S for a system that becomes more ordered.
Signs are copied from memorised formulas rather than derived from the stated convention (heat released by the system is negative; work done by the system is negative). Candidates also mix system and surroundings quantities in the same equation.
What to do instead
Before any calculation, write the convention at the top of the answer: delta-H negative for heat released, w negative for work done by the system, delta-G negative for spontaneity. Then write the defining relation (delta-G = delta-H - T delta-S) and check the sign of each term against the physical picture. Practise twenty short sign-and-magnitude problems where only the sign is asked, to isolate the convention from the arithmetic.
When asked for the rate law or mechanism of a reaction, the candidate writes an order taken from the stoichiometric coefficients and does not distinguish molecularity of an elementary step from the overall order of the reaction.
The balanced equation is the most visible piece of information, so coefficients are assumed to control rate. The candidate has not internalised that the rate law is experimental and that only the rate-determining step contributes to the rate expression.
What to do instead
State explicitly in every answer: 'overall order is determined experimentally and cannot be read from the balanced equation'. For mechanism questions, identify the slow step, write its rate law, then eliminate intermediates using the steady-state or pre-equilibrium approximation. Work through the classic two-step cases (SN1, a catalysed decomposition, a pre-equilibrium substitution) and show the algebra for eliminating the intermediate.
In organic mechanism questions the candidate draws curved arrows that start from an atom rather than a bond or lone pair, does not check that the arrowhead lands on an atom that can accept electron density, and omits intermediates such as the carbocation in an SN1 or the enolate in an aldol condensation.
Arrows are treated as a diagram convention rather than a bookkeeping device for electron pairs. Without a habit of checking the electron count on each side of each step, the mechanism drifts into an implausible sequence.
What to do instead
Impose a discipline: every arrow starts at a lone pair or a bond and ends at an atom or a bond; after each step, count electrons and formal charges and confirm conservation. Always draw the full energy profile with the intermediate labelled. Rehearse five named mechanisms (SN1, SN2, E2, electrophilic addition to an alkene, aldol condensation) drawing every intermediate and every arrow until no step is skipped.
For coordination compounds the candidate predicts the geometry and magnetic moment from the oxidation state alone, without determining whether the ligand field is strong or weak, and therefore assigns the wrong spin state (for example predicting [Fe(CN)6]4- to be high spin).
The spectrochemical series and the pairing-energy argument are memorised as a list but not applied as a decision rule. The candidate also forgets to compute the d-electron count after removing ligands.
What to do instead
Use a fixed sequence: determine the metal oxidation state, count d electrons, classify the ligand as strong or weak field using the spectrochemical series, then decide high or low spin and hence the number of unpaired electrons. Practise [Fe(CN)6]4-, [Fe(H2O)6]2+, [Co(NH3)6]3+, and [Ni(CN)4]2- with full working, and compute the spin-only magnetic moment for each.
In instrumental analysis questions the candidate reports a result from a calibration curve without addressing the limit of detection, the linear range, or matrix effects, and treats a single measurement as if it carried no uncertainty.
Instrumental methods are taught as procedures (prepare, measure, read off the graph) rather than as measurement systems with defined performance characteristics. Candidates therefore omit the vocabulary of validation that the examiner is testing.
What to do instead
For every analytical method described, answer five standard prompts: what is the calibration strategy, what is the limit of detection and how is it defined, what is the linear range, what interferences or matrix effects exist, and how is the result validated. Build a comparison table for AAS, UV-visible spectrophotometry, and ion-selective electrodes covering these five prompts, and use the table as a checklist in the exam.
On environmental and industrial questions the candidate describes a treatment process or a pollution problem qualitatively, without giving the governing chemistry (for example the lime-soda softening reactions, the chemistry of arsenic removal by coagulation, or the pH dependence of a treatment step) or any quantitative standard.
These topics are read as general-awareness material rather than as applied chemistry, so the candidate cannot supply the equations, the operating conditions, or the regulatory limits that distinguish a chemistry answer from a newspaper summary.
What to do instead
For each major topic (water softening, arsenic and iron removal, effluent treatment, fertiliser manufacture, cement chemistry, air-pollutant control), write a one-page card containing the balanced equations, the key operating variable such as pH or dose, and the relevant national or WHO limit where known. If a specific limit cannot be recalled with confidence, state the principle and mark the figure as approximate rather than inventing a number.