Cambridge IAL Chemistry common mistakes
Cambridge IAL Chemistry · Understand the traps and how to avoid them.
When asked to explain why a value differs from the 'expected' one, candidates write 'human error' or 'experimental error' as the whole explanation.
They have memorised that experiments have errors but never learned to classify them (systematic vs random, apparatus vs technique) or to link a named error to the direction of the deviation.
What to do instead
Practise a fixed template: name the faulty step, state the type of error, then state the direction of effect on the final value (e.g. 'heat lost to surroundings means the measured temperature rise is too small, so the calculated enthalpy of combustion is less exothermic than the data book value'). Rehearse this on heat-loss, incomplete-transfer, and endpoint-overshoot scenarios until the direction argument is automatic.
In titrations and enthalpy calculations, candidates average all burette readings including a rough titration, or use the initial and final readings of a single titration instead of subtracting correctly.
They treat every number in the table as data to be averaged, and they confuse the 'titre' (final minus initial for one run) with the raw readings.
What to do instead
Adopt a strict procedure: compute each titre as final minus initial for every row, discard the rough value and any titre not within 0.10 cm3 of the others, average only the concordant values, and quote the mean to two decimal places. Practise reading tables where one titre is deliberately an outlier so the selection step becomes habitual.
Candidates give an ionic equation with spectator ions left in, or with a formula that is not charge-balanced, and lose the equation mark even when the chemistry is right.
They write the molecular equation first and then delete symbols without recounting atoms and charges, and they are unsure which species actually dissociate.
What to do instead
Build the equation from the spectator-free half-lines: write the reacting ions, balance atoms, then balance charge with electrons or by adjusting coefficients, and finally cross-check total charge on each side (e.g. Ag+ + Cl- -> AgCl has zero net charge both sides). Only split strong acids, strong bases and soluble salts; keep weak acids, precipitates, gases and water intact.
For electron configuration of ions, candidates remove electrons from the 3d subshell before the 4s subshell, or misorder the 4s/3d filling for transition metals.
They apply the 'fill 4s before 3d' rule in reverse without realising that once 3d is occupied, 4s electrons are lost first, and they do not check the resulting configuration against the stability of half-filled or full d subshells.
What to do instead
Learn the two rules together: filling order is 4s then 3d, but ionisation order removes 4s before 3d. Work through Fe, Fe2+ and Fe3+ explicitly (Fe3+ is 3d5, a half-filled set), and for Cr and Cu note the 3d5 4s1 and 3d10 4s1 exceptions, then explain them via exchange energy and electron-electron repulsion in the 3d set.
Candidates describe the trend in ionisation energy across a period as a smooth increase and then cannot explain the two dips at Group 3 and Group 6.
They memorise 'nuclear charge increases' but do not connect the dips to the sub-shell structure (removing a 3p electron versus a 3s electron) and to electron pairing repulsion.
What to do instead
For every anomaly, answer in three linked steps: identify the electron being removed (subshell), state the shielding/penetration difference or the paired-electron repulsion, then state the effect on the energy required. Rehearse Al versus Mg and S versus P as the canonical examples, and always quote the actual configurations (e.g. Mg ends 3s2, Al ends 3p1).
In equilibrium and buffer questions, candidates calculate a pH from initial concentrations without allowing for the extent of dissociation, or apply Le Chatelier qualitatively when a numerical Kc calculation is required.
They conflate 'weak acid' with 'no dissociation' and never set up an ICE (initial-change-equilibrium) table, so they use the initial acid concentration in Ka expressions.
What to do instead
Always write the equilibrium expression, set up an ICE table, and substitute equilibrium (not initial) concentrations. For weak acids, state and justify the approximation that [HA]eq is approximately [HA]initial only when Ka is small and the degree of dissociation is under about 5 percent, and check that assumption numerically. For buffers, use the ratio of conjugate base to acid from the Henderson-Hasselbalch form and verify the ratio is within a sensible range.
Candidates draw a mechanism with curly arrows starting from atoms rather than from bonds or lone pairs, or show a nucleophile attacking a saturated carbon that has no leaving group.
They copy the overall shape of a mechanism from memory without tracking where the electron pair physically comes from and where it goes, and they do not check the substrate's substitution pattern.
What to do instead
Annotate every arrow with its source: a lone pair on a heteroatom, a pi bond, or a sigma bond. Before drawing, classify the substrate (primary, secondary, tertiary, aryl) and the reagent (nucleophile or base), then choose SN1, SN2 or elimination and state the evidence (e.g. tertiary plus polar protic solvent favours SN1). Redraw each mechanism from the electron bookkeeping rather than from a memorised picture.
Candidates identify an unknown organic compound from spectra by naming a functional group but never assemble the full structure, or they ignore the molecular ion and fragmentation pattern in the mass spectrum.
They treat IR, NMR and mass spectra as three separate questions rather than convergent evidence, and they do not use the molecular ion to fix the molecular formula before assigning fragments.
What to do instead
Use a fixed sequence: molecular ion gives Mr, deduce the formula from Mr and the degree of unsaturation, IR identifies key bonds (C=O near 1700, O-H broad near 3300), then the number of proton environments and their integration in the proton NMR fixes the skeleton, and splitting patterns confirm neighbouring hydrogens. Only name the compound after all four pieces agree, and check that the proposed structure reproduces the observed fragment ions.
In organic synthesis routes, candidates propose a reagent that gives the wrong regiochemistry or forget to protect a group, and they write 'reflux' or 'heat' as if it were a reagent.
They recall reaction names but not the conditions and selectivity rules, and they do not check whether other functional groups in the molecule would also react.
What to do instead
Write each step as substrate, reagent with conditions, and product, then audit the whole molecule for competing functional groups before moving on. Apply Markovnikov's rule and the peroxide effect explicitly for alkene additions, use Friedel-Crafts only on activated or unsubstituted rings, and state protection and deprotection steps when a group would otherwise react.
Candidates describe observations in qualitative analysis vaguely ('a precipitate formed', 'a gas was produced') without colour, solubility in excess reagent, or the gas test used.
They learn the cations and anions as a list without the diagnostic detail that distinguishes them, so they cannot separate, for example, a white precipitate that dissolves in excess from one that does not.
What to do instead
Build a decision table for each ion with three columns: reagent added, observation (colour and form), and behaviour in excess reagent. Add the confirmatory gas tests (limewater for CO2, damp red litmus for NH3, glowing splint for O2) and the flame-test colours. Practise by predicting the full sequence of observations for a mixture before checking the answer.