Matura Chemistry common mistakes
Matura Chemistry · Understand the traps and how to avoid them.
Writing the electron configuration of a cation by removing electrons from the outermost shell without checking the actual orbital order - e.g. giving Fe²⁺ as [Ar]4s²3d⁴ instead of [Ar]3d⁶, or Cu⁺ as [Ar]4s¹3d⁹.
Candidates memorise 'fill 4s before 3d' and apply the same rule backwards when ionising, forgetting that once 3d is occupied the 4s electrons are higher in energy and leave first. The 3d/4s crossover is taught as a filling rule, not as an energy-ordering rule.
What to do instead
Always build the neutral atom first, then remove electrons from the highest-n subshell (4s before 3d for transition metals). Practise the pairs Fe/Fe²⁺/Fe³⁺, Cu/Cu⁺/Cu²⁺, Cr/Cr³⁺ and check the result against the noble-gas core plus a filled or half-filled d count.
Predicting molecular shape and polarity from the number of bonded atoms only, ignoring lone pairs - e.g. calling H₂O linear, NH₃ trigonal planar, or SO₂ non-polar.
VSEPR is often reduced in revision to 'count the atoms', and lone pairs are treated as decoration. Candidates also confuse the electron-pair geometry with the molecular geometry, so they report the parent shape rather than the derived one.
What to do instead
Use a fixed four-step routine: total valence electrons → Lewis structure → count bonding pairs AND lone pairs on the central atom → apply the lone-pair penalty (each lone pair removes roughly 2.5° and changes the name). Then test polarity by drawing the bond dipoles as vectors and checking whether they cancel; SO₂ and H₂O are bent, so they are polar.
Treating a weak acid or weak base as if it dissociated completely, so pH is computed as −log C instead of via Ka, and buffer problems are solved without the Henderson–Hasselbalch relationship.
Strong-acid formulas are drilled first and become automatic; the approximation 'weak acid is almost undissociated' is stated in theory but not converted into a calculation habit. Candidates also skip checking whether the 5% approximation is valid.
What to do instead
Before substituting, classify the species: strong/weak acid, salt of weak acid, buffer. For a weak acid write Ka = x²/(C − x), assume x ≪ C only after checking C/Ka > 400, then verify by back-substitution. For buffers use pH = pKa + log([A⁻]/[HA]) and state explicitly which component is the acid.
Balancing redox equations in acidic or alkaline medium without adding H₂O and H⁺/OH⁻, so oxygen and hydrogen atoms do not balance even though the electrons do.
Candidates focus on the electron balance because that is the 'chemistry' of redox, and treat H and O as automatically correct. Alkaline conditions are especially missed because the half-reaction method is usually practised in acid.
What to do instead
Use the strict half-reaction sequence: balance all atoms other than H and O, add H₂O to balance O, add H⁺ to balance H, add electrons to balance charge - and only then combine. For alkaline medium, add OH⁻ to both sides to neutralise H⁺ and cancel the resulting water. Finish by checking that charge and every element balance in the final equation.
Calculating reaction enthalpy from bond energies or from ΔHf° without tracking signs and stoichiometric coefficients, e.g. using ΔH = Σ(bonds broken) − Σ(bonds formed) with the terms reversed, or forgetting to multiply by coefficients in Hess's law.
The two conventions (bonds broken minus bonds formed vs products minus reactants) look interchangeable but have opposite sign logic, and under exam pressure candidates grab whichever formula they last saw. Coefficients are dropped because the equation is treated as a list of substances rather than a molar balance.
What to do instead
Write the definition next to the calculation every time: ΔH = ΣΔHf°(products) − ΣΔHf°(reactants), with coefficients as multipliers; for bond enthalpies, ΔH = ΣE(broken) − ΣE(formed) (breaking costs energy, forming releases it). Sketch the energy-level diagram - it catches a reversed sign immediately.
Applying Le Chatelier's principle qualitatively but failing to distinguish rate from position of equilibrium - claiming a catalyst shifts the equilibrium, or that raising pressure always shifts an equilibrium to the side with fewer gas moles regardless of whether gas moles differ.
Catalysis and equilibrium are taught in adjacent lessons and both 'speed things up' in everyday language. Candidates also answer with a direction ('right') without stating the observable consequence, and forget that pressure has no effect when Δn(gas) = 0.
What to do instead
For every disturbance answer three things: which rate changes, which direction the position moves, and what is observed. Memorise that a catalyst changes only the time to reach equilibrium (K unchanged), and that pressure changes matter only when the number of gas moles differs between sides. Check with Q vs K reasoning, not just intuition.
Naming and drawing organic structures without applying the priority rules - choosing the wrong parent chain, numbering from the end that gives the lowest locant to a substituent instead of to the principal functional group, or writing '2-methylpropan-2-ol' style names with the suffix misplaced.
IUPAC rules are learned as a list, and the decisive step (principal functional group outranks multiple bonds outranks substituents) is skipped. Candidates also count the longest chain through a substituent rather than through the carbon skeleton.
What to do instead
Follow a fixed decision order: identify the principal functional group → choose the longest chain containing it → number to give it the lowest locant → name substituents alphabetically with lowest locants → assemble with the suffix on the parent. Redraw the structure from your own name to verify it matches the original.
Confusing the reactivity of alcohols, phenols and halogenoalkanes - writing that ethanol reacts with NaOH to give ethoxide, that phenol is easily oxidised like a secondary alcohol, or that a tertiary halogenoalkane undergoes SN2 readily.
The three classes share the −OH or −X group, so candidates transfer one reaction pattern across all of them. Acidity differences (phenol ≫ water > ethanol) and the SN1/SN2 dependence on substrate class are memorised as facts without a mechanistic picture.
What to do instead
Anchor each class to a mechanism: ethanol + Na gives ethoxide (a strong base is needed, not NaOH); phenol's acidity comes from resonance-stabilised phenoxide and it undergoes electrophilic substitution on the ring, not nucleophilic substitution; tertiary halogenoalkanes go SN1 (carbocation stability), primary go SN2. Draw the carbocation or the phenoxide to justify each claim.
Describing a laboratory test or experiment without the observable, the equation and the control - e.g. writing 'add AgNO₃ to detect chloride' without stating the cream precipitate, its solubility in ammonia, and the ionic equation.
Candidates learn the reagent but not the full observation–equation–conclusion chain, and treat the practical question as a recall question. The distinction between a positive test and a confirmatory test (e.g. bromide vs iodide) is blurred.
What to do instead
Answer every identification question in a fixed three-part format: reagent and conditions → observable (colour, precipitate, gas, smell) → ionic equation and conclusion. Learn the halide series as a table (AgCl white, soluble in dilute NH₃; AgBr cream, soluble in concentrated NH₃; AgI yellow, insoluble), and always include a blank/control where the question asks for a reliable method.
Reporting a calculated result with more significant figures than the data allow, or omitting units and the titration/gravimetric correction - e.g. quoting a concentration to five decimals from burette readings to 0.1 cm³, or forgetting to subtract the blank or to convert cm³ to dm³.
Calculators return full precision and candidates copy the display. Unit conversion (cm³ → dm³, g → mol via molar mass) is done silently and errors are not caught because no sanity check is performed.
What to do instead
Carry units through every step and cancel them explicitly; convert cm³ to dm³ by dividing by 1000 before using c = n/V. Round only at the end, to the precision of the least precise measurement (usually 3–4 significant figures for burette data), and sanity-check the answer against the expected order of magnitude. State the result with its unit and, where relevant, the uncertainty.