KSEAB II PUC Chemistry common mistakes
KSEAB II PUC Chemistry · Understand the traps and how to avoid them.
When balancing a redox equation in acidic or basic medium, the candidate balances atoms first and then simply adds H⁺ or OH⁻ without tracking electron loss and gain, so the final equation has unequal charge on the two sides (e.g., left side +6, right side +2).
The candidate treats balancing as an atom-counting exercise and forgets that in ionic equations charge must also balance; the half-reaction method is memorised as a sequence of steps rather than understood as conservation of electrons.
What to do instead
Always split into oxidation and reduction half-reactions, balance O with H₂O and H with H⁺ (or add OH⁻ in basic medium at the end), then equalise electrons before adding. After combining, verify that the sum of charges on both sides is identical and that the number of electrons cancelled equals the change in oxidation number. Practise two acidic and two basic examples and write the charge check explicitly on the answer sheet.
The candidate writes the IUPAC name of a branched or polyfunctional organic compound by numbering the chain from the end that gives the substituent the lowest number, ignoring the principal functional group, or by choosing the longest chain without checking whether it contains the principal group and the maximum number of multiple bonds.
School practice often drills 'lowest locant for substituents', and the candidate applies that rule mechanically. The priority order of functional groups (COOH > ester > amide > CN > CHO > C=O > OH > NH₂ > alkene/alkyne > halide) and the rule that the parent chain must contain the principal group are learned as a list but not applied.
What to do instead
Before writing any name, do a fixed four-step routine: (1) identify the principal functional group by priority, (2) select the longest chain containing it and the maximum number of double/triple bonds, (3) number to give the principal group the lowest locant, then multiple bonds, then substituents, (4) assemble alphabetically with correct suffixes. Practise ten compounds that mix OH, CHO, COOH and halogen on the same skeleton.
The candidate predicts the shape of a molecule such as SF₄, ClF₃, XeF₄ or BrF₅ by counting only bond pairs, or by treating lone pairs as if they occupy positions identical to bond pairs, and so gives tetrahedral or octahedral geometry instead of see-saw, T-shaped, square planar or square pyramidal.
VSEPR is remembered as 'electron pairs repel', but the candidate does not apply the stronger repulsion of lone pairs and does not distinguish electron-pair geometry from molecular geometry. The candidate also forgets to compute the steric number correctly for expanded octets.
What to do instead
For each molecule, draw the Lewis structure, compute the steric number (bond pairs + lone pairs on the central atom), assign the electron-pair arrangement, then remove lone-pair positions and name the resulting molecular shape. Memorise the standard table for steric numbers 2 to 6 with 0, 1, 2 and 3 lone pairs, and state the bond angle trend explicitly (lone pair–lone pair > lone pair–bond pair > bond pair–bond pair).
In numerical problems on equilibrium, pH, buffer or solubility product, the candidate substitutes concentrations directly into Ka, Kb or Ksp without checking whether the degree of dissociation is small, or omits the common-ion contribution, and reports an answer that fails a mass-balance check.
The approximation x ≪ C is used as a shortcut without verifying it, and the candidate does not set up an ICE (initial–change–equilibrium) table, so the change in concentration is applied inconsistently to products and reactants.
What to do instead
Always write an ICE table, define x, write the equilibrium expression in terms of C and x, then test whether x/C < 5% before dropping x. For buffers, use the Henderson–Hasselbalch form and keep the common ion in both numerator and denominator. Finish by checking that the computed pH lies in a chemically sensible range and that all concentrations are positive.
When writing the structure and IUPAC name of a coordination compound, the candidate places the metal first in the formula, assigns the wrong oxidation state because it ignores the charge on the complex ion, or orders ligands alphabetically instead of following the ligand-order rules, and writes the name with the cation and anion in the wrong sequence.
The candidate confuses the formula-writing convention (metal first inside the coordination sphere) with the naming convention (ligands alphabetically before the metal), and does not use the overall charge of the complex to deduce the metal's oxidation state when the counter-ion is present.
What to do instead
Use a fixed procedure: (1) identify the coordination sphere and its overall charge from the counter-ions, (2) let the metal oxidation state be x and solve from the sum of ligand charges, (3) name ligands alphabetically with the correct prefixes (di, tri, bis, tris) and suffixes (-o for anionic ligands, -ate for the metal in anionic complexes), (4) write the cation name before the anion name. Practise with K₄[Fe(CN)₆], [Co(NH₃)₄Cl₂]Cl and [Pt(NH₃)₂Cl₂].
The candidate answers 'which species is aromatic' or 'which is more stable' by counting double bonds or by visual inspection of the ring, without checking Hückel's rule (4n+2 π electrons in a planar, cyclic, fully conjugated system) or the stability order of carbocations and carbanions.
Aromaticity is reduced to 'alternating double bonds in a ring', and the candidate does not count the π electrons contributed by heteroatoms or by charges, so pyrrole, furan, cyclopentadienyl anion and tropylium ion are misclassified.
What to do instead
For every ring, count the π electrons contributed by each atom (lone pair in a p orbital contributes 2; a charge may add or remove 2), verify planarity and cyclic conjugation, and test 4n+2. For stability questions, rank carbocations by the number and strength of electron-releasing groups (tertiary > secondary > primary, benzylic/allylic resonance) and check for aromatic stabilisation of the ion.
In reaction-mechanism questions on haloalkanes and alcohols, the candidate labels every substitution as SN1 or SN2 by the substrate alone, ignoring the nature of the nucleophile, the solvent and the leaving group, and then predicts the wrong stereochemical outcome (inversion versus racemisation).
The candidate memorises 'tertiary = SN1, primary = SN2' as a slogan and does not connect the mechanism to the rate law, the role of polar protic versus polar aprotic solvent, and the stability of the carbocation intermediate.
What to do instead
For each reaction, state the substrate class, the nucleophile strength and concentration, the solvent polarity and the leaving group, then decide the mechanism from those four factors. Link the mechanism to the observed rate law and stereochemistry: SN2 gives inversion with second-order kinetics, SN1 gives racemisation with first-order kinetics and carbocation rearrangement. Practise a table of six substrate/nucleophile/solvent combinations.
When asked to compare boiling points, melting points or solubility of hydrides, halides or oxides across a period or down a group, the candidate gives a single-factor answer such as 'hydrogen bonding' or 'molar mass' without identifying which intermolecular force dominates and without checking the trend against the actual data.
The candidate has learned that H₂O, HF and NH₃ have hydrogen bonding but applies the label indiscriminately, and does not distinguish London dispersion, dipole–dipole and hydrogen-bonding contributions or consider lattice enthalpy and hydration enthalpy for ionic compounds.
What to do instead
For each comparison, list the intermolecular forces present in both species, decide which is stronger and why (electronegativity, molecular size, surface area, presence of N–H/O–H/F–H), then check the prediction against known values. For ionic solids, compare lattice enthalpy and hydration enthalpy rather than molar mass alone. Write the reasoning as a two-sentence chain rather than a single keyword.
In electrochemistry and kinetics numericals, the candidate uses E°cell = E°cathode − E°anode with the wrong sign convention, or applies the Nernst equation without converting concentration to the correct power, and in kinetics substitutes time in minutes into a rate constant expressed in s⁻¹ or uses the wrong integrated rate law for the order.
Sign conventions for electrode potentials and the direction of the cell reaction are memorised inconsistently, and the candidate does not check units or the order of the reaction before choosing an integrated rate equation.
What to do instead
Write the cell diagram, identify anode and cathode, and use E°cell = E°cathode(red) − E°anode(red) with both values as reduction potentials. In the Nernst equation, match the number of electrons transferred to the stoichiometric coefficients and keep concentrations in mol L⁻¹. In kinetics, first determine the order from the units of k or from a plot, then select the integrated law and convert all times to seconds before substituting.
In practical-chemistry questions, the candidate describes a salt analysis or titration procedure in vague terms ('add reagent, observe') without naming the confirmatory test, the expected observation, the ionic equation, or the indicator and endpoint colour change, and so loses the marks allocated to observation and inference.
Laboratory work is often learned as a sequence of steps performed in class rather than as a set of test–observation–inference triples, and the candidate does not connect the qualitative test to the underlying ionic reaction.
What to do instead
Build a one-page table for each cation and anion with three columns: test (reagent and conditions), observation (colour, precipitate, gas, smell), and inference with the ionic equation. For titrations, record the indicator, the colour change at the endpoint, the concordant titre values and the calculation formula. Rehearse by writing the full inference for two cations and two anions from memory without looking at notes.