WBCHSE HS Chemistry common mistakes
WBCHSE HS Chemistry · Understand the traps and how to avoid them.
Writing the electronic configuration of Cu, Cr, or their ions strictly by the Aufbau (n+l) filling order and ignoring the extra stability of half-filled and fully filled d subshells, so Cu is given as [Ar]3d9 4s2 or Cr as [Ar]3d4 4s2.
Candidates memorise a single filling sequence and treat the exchange-energy stabilisation of d5 and d10 as an exception to be rote-learned rather than understood, so it is forgotten under exam pressure or misapplied to ions where the 4s electrons are removed first.
What to do instead
Practise a fixed two-step routine: build the neutral atom with the d5/d10 adjustment, then remove 4s electrons before 3d when forming cations (Fe is [Ar]3d6 4s2 but Fe2+ is [Ar]3d6, not [Ar]3d4 4s2). Work through Cu, Cr, Fe, Fe2+, Fe3+, Cu+, Cr3+ and Mn2+ in writing until the pattern is automatic.
Drawing a plausible Lewis structure and then jumping straight to a hybridisation label (sp, sp2, sp3) without counting sigma bonds and lone pairs on the central atom, so species like SF4, XeF4, ClF3, or the H2O molecule get the wrong geometry and bond angle.
Candidates learn hybridisation as a vocabulary list linked to a few familiar molecules and never practise applying VSEPR to unfamiliar or hypervalent species, so the reasoning chain (count electron domains, minimise repulsion, then name shape and angle) collapses.
What to do instead
Use a rigid worksheet for every species: total valence electrons, Lewis structure, number of bond pairs and lone pairs on the central atom, then electron-domain geometry, then molecular shape, then expected angle with the lone-pair compression stated. Drill SF4 (see-saw, ~102 degrees), XeF4 (square planar), ClF3 (T-shaped), NH3 (107 degrees) and H2O (104.5 degrees) until the sequence is reflexive.
Applying Le Chatelier's principle mechanically to every disturbance, for example claiming that adding an inert gas at constant volume shifts the equilibrium, that a catalyst changes the equilibrium position, or that adding a solid reactant shifts K.
The principle is memorised as a slogan ('stress causes shift to oppose stress') without the underlying condition that only changes in partial pressures or concentrations of species actually present in the equilibrium expression matter, so candidates cannot distinguish cases.
What to do instead
Before answering, write the equilibrium constant expression and ask which terms actually change. Practise a table of disturbances (inert gas at constant V, inert gas at constant P, catalyst, pure solid added, temperature change, volume halving) and state for each whether K changes, whether Q changes, and the direction of shift, with the reason in one line.
Treating the Henderson-Hasselbalch equation, buffer capacity and salt hydrolysis as one undifferentiated formula pool, so candidates compute the pH of a buffer of a weak base and its salt using the acid form, or forget that the degree of hydrolysis of a salt of a weak acid and weak base is independent of concentration.
The many pH formulas (weak acid, weak base, salt of weak acid, salt of weak base, salt of weak acid and weak base, buffer) are memorised as a list without the derivation logic, and candidates cannot tell which approximation is valid for a given salt type.
What to do instead
Classify the salt first (strong-strong, strong-weak, weak-strong, weak-weak), then write the hydrolysis equation and derive the expression from Kw, Ka and Kb each time rather than recalling it. Check the answer for plausibility: a salt of a strong acid and weak base must give pH below 7. Practise one problem of each of the four salt classes plus one acidic and one basic buffer.
Balancing redox equations by trial and error on atoms only, ignoring charge balance, or mixing the ion-electron method with the oxidation-number method midway so the final equation balances atoms but not electrons.
Candidates are taught both methods but not told to commit to one and verify, and in acidic versus basic medium they forget the step of adding H2O and H+ or OH- to balance oxygen and hydrogen, so half-reactions are left unbalanced in charge.
What to do instead
Adopt a fixed checklist for every redox balance: split into half-reactions, balance atoms other than O and H, balance O with H2O, balance H with H+ (or with H2O and OH- in basic medium), balance charge with electrons, equalise electrons, add, then cancel. Finish by verifying that total charge is identical on both sides. Do at least one permanganate and one dichromate balance in both acidic and basic media.
Using the integrated rate law or half-life formula appropriate to first order for a zero-order or second-order reaction, or computing the order from a data table by inspection of concentration changes rather than by testing the rate law.
First-order kinetics dominates textbook practice, so candidates default to the first-order expression even when the units of k (mol L-1 s-1 for zero order, L mol-1 s-1 for second order) clearly indicate otherwise, and they do not use unit analysis as a check.
What to do instead
Always read the units of k first: they uniquely identify the order. Then test the data by computing k from the appropriate integrated form for successive intervals; if k is constant, that order is confirmed. Memorise the half-life expressions for zero, first and second order and note that only the first-order half-life is independent of initial concentration.
Confusing molarity with molality, or forgetting that colligative properties depend on the number of particles, so a 0.1 m NaCl solution is treated as having the same depression of freezing point as 0.1 m glucose, or the van't Hoff factor is omitted entirely for electrolytes.
The formulas for relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure are learned without the i factor, and candidates do not connect the definition of the property to the count of dissolved species.
What to do instead
For every colligative problem, first write the dissociation equation and determine i (2 for NaCl, 3 for CaCl2, 2 for K2SO4, 1 for glucose). Then apply the formula with i included and state the assumption of complete dissociation. Practise a comparison question ranking several solutions of equal molality by freezing point, justifying each step.
Writing the products of organic reactions without tracking the mechanism, for example giving the Markovnikov product for HBr addition to an alkene in the presence of peroxide, predicting SN1 for a primary halide with a strong nucleophile, or failing to distinguish SN1 from SN2 stereochemistry.
Reaction products are memorised as isolated facts without the underlying carbocation stability, steric access and nucleophile/base strength reasoning, so any variation in conditions (peroxide, polar protic solvent, tertiary substrate, bulky base) breaks the memorised mapping.
What to do instead
For each reaction, state three things before writing the product: the substrate class (methyl, primary, secondary, tertiary, aryl), the reagent type (strong or weak nucleophile, strong or bulky base), and the solvent. Then decide the mechanism (SN1, SN2, E1, E2) and derive the product with its stereochemistry. Practise the same substrate with different reagents to see the branch points.
Naming and drawing coordination compounds with the wrong order of ligands, wrong oxidation state of the metal, or wrong isomer count, for example writing the formula of tetraamminecopper(II) sulfate incorrectly or missing the geometric isomers of a square planar MA2B2 complex.
IUPAC nomenclature rules for coordination compounds (ligands alphabetical, anionic before neutral, oxidation state in Roman numerals) are learned as a list but not practised on unfamiliar complexes, and isomer counting is done by drawing one structure and stopping.
What to do instead
Learn the naming sequence as a fixed template: cation then anion, ligands alphabetically, metal with oxidation state in parentheses, and the whole complex in square brackets. For isomer counting, systematically draw all arrangements of ligands on the coordination geometry (octahedral, square planar, tetrahedral) and eliminate duplicates by rotation. Practise [Co(NH3)4Cl2]+, [Pt(NH3)2Cl2] and [Cr(H2O)4Cl2]+ until the count is reliable.
Answering practical chemistry questions from memory of a textbook procedure rather than from the logic of the test, for example naming the confirmatory test for a chloride without stating the observation, or failing to explain why a particular reagent is added before another in a qualitative analysis scheme.
Practical work is often studied as a list of tests and observations without the sequence logic (group separation by precipitation, then confirmatory tests), and candidates do not practise writing observations in the required form (colour, precipitate, gas, smell).
What to do instead
For each ion, write the group reagent, the precipitate formed, its colour, and the confirmatory test with its observation, then explain why earlier groups must be removed before later ones (to avoid interference). Practise writing observations in full sentences with colour and state changes, and rehearse the reasoning for one full group separation scheme end to end.