European Baccalaureate Chemistry common mistakes
European Baccalaureate Chemistry · Understand the traps and how to avoid them.
Calculating a pH or a buffer composition without first checking whether the acid is weak or strong, or whether the simplified approximation (e.g. neglecting water autoionisation, or assuming [HA] ≈ c) is valid at the given concentration and Ka.
Candidates learn one formula (pH = ½(pKa − log c) or the Henderson–Hasselbalch equation) and apply it mechanically. The European Baccalaureate often gives dilute solutions or acids with relatively large Ka where the approximation fails, and the mark scheme rewards recognising the limit of validity.
What to do instead
Before substituting numbers, write down the decision criteria explicitly: strong vs weak (Ka value, position in a data table), c/Ka > ~100 for the approximation, and whether [H⁺] from water matters (c < 10⁻⁶ mol/L). Practise a set of problems where you must justify the choice of equation in one sentence before calculating.
Balancing redox half-equations in acidic or basic medium without adding H₂O and H⁺/OH⁻ to balance O and H, then combining half-equations with mismatched electron numbers.
Candidates balance atoms and charge only in their head and lose track of oxygen and hydrogen, especially in basic medium where the standard trick of adding OH⁻ to both sides is forgotten. The resulting overall equation looks plausible but is wrong by one or two H₂O.
What to do instead
Adopt a fixed written procedure: balance the element that changes oxidation state, balance O with H₂O, balance H with H⁺ (acidic) or with H₂O/OH⁻ (basic), then balance charge with electrons. Multiply each half-equation by an integer so the electron counts match, then add. Always re-check both atom and charge balance on the final equation.
Drawing a Lewis structure or predicting molecular shape without counting lone pairs on the central atom, so that the VSEPR geometry and the bond angle are reported for the electron-pair geometry rather than the molecular geometry.
Candidates count bonding pairs but forget non-bonding pairs, or they conflate 'electron domain geometry' with 'shape'. This is common with molecules such as H₂O, NH₃, SF₄ and ClF₃ where the two differ.
What to do instead
Use a fixed sequence: total valence electrons, then place bonds, then complete octets, then place remaining electrons as lone pairs on the central atom. For shape, count σ bonds + lone pairs to get the electron-domain geometry, then remove lone pairs to name the molecular geometry and adjust the angle (e.g. 104.5° for H₂O, not 109.5°).
Calculating enthalpy changes from bond energies or formation enthalpies without checking the sign convention, or mixing up 'bonds broken minus bonds formed' with 'products minus reactants'.
Two conventions coexist: ΔH = Σ(bonds broken) − Σ(bonds formed) for bond energies, but ΔH = ΣΔHf(products) − ΣΔHf(reactants) for formation enthalpies. Candidates memorise one and apply it to the other, or forget that bond breaking is endothermic.
What to do instead
Write the definition next to each calculation: for bond energies, ΔH = energy in (broken) − energy out (formed); for formation enthalpies, ΔH = Σ products − Σ reactants. Do a quick sanity check: combustion and bond formation should give negative ΔH. Keep a one-page sign-convention card and use it in every practice problem.
Reporting a rate or equilibrium result without specifying units and without distinguishing rate of reaction from rate of consumption of a specific reactant, including the stoichiometric factor.
Candidates compute Δ[concentration]/Δt for one species and call it 'the rate', ignoring that for aA → bB the rate is −(1/a)d[A]/dt = (1/b)d[B]/dt. Units (mol L⁻¹ s⁻¹) are often omitted, and orders of reaction are confused with stoichiometric coefficients.
What to do instead
Always write the general rate expression with stoichiometric coefficients before substituting. State units explicitly. Distinguish order (from experiment, not from the equation) from stoichiometry. Practise problems where the same reaction is monitored via two different species and the two rates must be related.
Treating Le Chatelier predictions as a memorised list ('pressure increases → shift to fewer moles') without checking whether the species involved are gases and whether the change actually affects the reaction quotient Q.
The qualitative rules are taught as slogans. Candidates misapply them to changes that do not shift equilibrium (adding a catalyst, adding an inert gas at constant volume) or apply the pressure rule to reactions with equal gas moles on both sides.
What to do instead
For every perturbation, write Q before and after the change and compare with K. Only if Q ≠ K does the position shift. Explicitly classify the change: concentration, partial pressure, total pressure at constant volume vs constant pressure, temperature (which changes K itself), catalyst (changes rate only).
Naming and drawing organic compounds without applying IUPAC priority rules for the principal functional group, and ignoring stereochemistry (E/Z, R/S) when the structure requires it.
Candidates pick the longest chain but choose the wrong suffix (e.g. naming an ester as an acid, or a ketone as an alcohol), number from the wrong end, or omit cis/trans or E/Z descriptors. Stereocentres are often left unmarked.
What to do instead
Use a checklist: identify the principal group by priority order (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane), choose the longest chain containing it, number to give it the lowest locant, then list substituents alphabetically. For alkenes, assign priorities by CIP rules and label E/Z; for chiral centres, assign R/S explicitly.
Describing an organic mechanism with curly arrows that start from an atom rather than a bond or lone pair, or that show a bond forming before the bond breaking is shown, producing an intermediate with the wrong charge or an impossible valence.
Candidates learn mechanisms as pictures and reproduce the overall transformation without tracking electron flow. Common errors are attacking a nucleophile at a saturated carbon without showing the leaving group departure, or drawing a carbocation that violates octet rules.
What to do instead
For each mechanism, draw every arrow from a lone pair or a bond (never from an atom), and check that each arrow ends at an atom or a bond. After each step, verify formal charges and octets. Practise the four core mechanisms (SN1, SN2, E1, E2) side by side, noting how the arrows differ and what determines the outcome (substrate, solvent, temperature, base strength).
Interpreting an IR, NMR or mass spectrum by reading off one peak in isolation, without using the molecular ion, the integration, or the splitting pattern together to constrain the structure.
Candidates match a single absorption (e.g. a C=O stretch) to a functional group and stop, or they misread the integration in ¹H NMR, or they forget the n+1 rule and the chemical-shift ranges. The result is a structure that fits one piece of data but contradicts the rest.
What to do instead
Work systematically: from MS get the molecular ion and hence the molecular formula; from IR identify key functional groups and note the absence of others (e.g. no O–H, no C=O); from ¹H NMR use chemical shift to assign environments, integration to count protons, and splitting to count neighbours. Only accept a structure that satisfies all three simultaneously, and check the degree of unsaturation.
Quantitative titration or gravimetric calculations that ignore the reaction stoichiometry, the dilution factor, or the significant figures implied by the apparatus (burette, pipette, balance).
Candidates plug concentrations into n = cV without checking the mole ratio in the balanced equation, forget that a pipetted aliquot was diluted, or report an answer to six decimal places from a two-decimal balance. This is especially common when the titration involves a polyprotic acid or a redox reaction with a non-1:1 ratio.
What to do instead
Write the balanced equation first and extract the mole ratio explicitly. Track the aliquot: moles in the aliquot → moles in the flask → moles in the original solution. Carry units through every step. At the end, round to the precision justified by the least precise measurement and state the uncertainty. Do a quick order-of-magnitude check before writing the final answer.