Madrid PAU Chemistry common mistakes
Madrid PAU Chemistry · Understand the traps and how to avoid them.
When filling orbital diagrams or writing electron configurations of ions, the candidate removes electrons from the highest-energy occupied orbital of the neutral atom without re-checking the (n+l) filling order, e.g. writing Fe2+ as [Ar]4s1 3d5 by taking both electrons from 4s, or writing Cu+ as [Ar]4s1 3d9.
The candidate memorised the rule '4s fills before 3d' as a fixed sequence and never learned that once 3d is occupied, 4s lies above 3d in energy, so ionisation removes 4s electrons first and then 3d. The anomaly of Cr and Cu is also memorised as a slogan rather than as extra stability of half-filled and filled subshells, so it is misapplied to ions.
What to do instead
Write the neutral-atom configuration first, mark the orbital energies, then remove electrons from the outermost (highest n, then highest l) orbital. Practise the pairs Fe/Fe2+/Fe3+, Cu/Cu+/Cu2+, Cr/Cr3+ and check each against the (n+l) rule. For anomalies, justify with exchange energy and symmetry of d5/d10 rather than quoting a rule.
The candidate predicts periodic trends by a single factor: explaining why the first ionisation energy of O is lower than that of N purely by 'smaller radius', or why atomic radius decreases across a period by 'more protons attract the same electrons' without mentioning that electrons are added to the same shell so shielding barely changes.
Trends are learned as one-line slogans ('radius decreases to the right', 'ionisation energy increases to the right') and the candidate does not build the two-variable model (effective nuclear charge versus shielding and penetration) needed to explain the O/N and Be/B inversions. When a question asks for justification, the slogan is recycled instead of the mechanism.
What to do instead
For every trend question, write two sentences: what happens to Z_eff (protons added, shielding constant within a shell) and what happens to the orbital involved (n and l, penetration). Memorise the four classic inversions (Be>B, N>O, Mg>Al, P>S) with their electron-configuration reason. Practise explaining, not just stating, each trend in the group and period.
The candidate draws a Lewis structure and stops there, without using VSEPR to assign the molecular geometry, without computing the hybridisation of the central atom, and without distinguishing the geometry of the electron pairs from the shape of the molecule (e.g. calling H2O linear or NH3 trigonal planar, or saying SF6 is octahedral but SO2 is 'bent like water' without noting the lone pair).
Lewis structures are taught first and geometry later, and the candidate treats them as separate topics. The candidate also forgets that lone pairs occupy more space than bonding pairs, so bond angles are compressed below the ideal value, and does not connect hybridisation (sp, sp2, sp3) to the observed geometry.
What to do instead
Use a fixed four-step routine for every molecule: count valence electrons, draw the Lewis structure, count electron domains (bonding + lone pairs) to get the electron-pair geometry, then remove lone pairs to get the molecular shape and predict the angle deviation. State the hybridisation explicitly and check it against the geometry. Drill the set H2O, NH3, SO2, CO2, BF3, CH4, PCl5, SF6.
The candidate predicts physical properties (boiling point, solubility, conductivity) from bond polarity alone, e.g. concluding that HCl boils higher than H2O because the H–Cl bond is polar, or that a polar molecule is automatically water-soluble, or that a molecular solid conducts because it contains ions.
The candidate conflates intramolecular bonds with intermolecular forces. Bond polarity determines dipole moments, but boiling point depends on the strength of the intermolecular forces (dispersion, dipole–dipole, hydrogen bonding) and on molecular mass and shape; solubility depends on the balance of solute–solute, solvent–solvent and solute–solvent interactions, not on polarity alone.
What to do instead
For each comparison, first identify the type of solid and the intermolecular forces present, then rank their strengths (hydrogen bonding > dipole–dipole > dispersion, with dispersion growing with size). Check the dipole moment as the vector sum of bond dipoles (CO2 cancels, H2O does not). Explicitly state that conductivity requires mobile charge carriers, so ionic solids conduct only when molten or dissolved.
In stoichiometry problems the candidate balances the equation but then uses the mass of the limiting reactant directly in the yield calculation, or divides by the coefficient of the wrong species, or forgets to convert between mass, moles and volume (using grams where moles are needed, or using 22.4 L for a gas at a temperature and pressure other than STP).
The candidate works with formulas mechanically without a dimensional check. The limiting reactant is identified by comparing masses rather than mole ratios, and the molar relationship between reactants and products is read from the coefficients only when the coefficients happen to be 1:1. Gas volume is treated as a fixed conversion rather than as a function of conditions.
What to do instead
Adopt a fixed sequence: balance, convert every given quantity to moles, divide each by its coefficient to find the limiting reactant, use the limiting reactant's mole ratio to find product moles, convert to the requested unit. Always write the unit next to each number and cancel it. For gases, use n = PV/RT and only use 22.4 L/mol at 0 °C and 1 atm.
The candidate calculates a reaction enthalpy by adding bond enthalpies of the bonds broken and formed without regard to sign, or by adding formation enthalpies of the products and subtracting the reactants in the wrong order, or by using Hess's law with an equation that was not reversed when the direction was reversed.
The candidate memorises the formula but not its physical meaning: that bond breaking costs energy (positive) and bond formation releases it (negative), and that formation enthalpies are defined for forming one mole of compound from elements, so products minus reactants is the correct order. When an intermediate reaction is reversed, the sign of its enthalpy is not flipped.
What to do instead
Before calculating, write the target equation and the data equations with their ΔH values, and annotate each arrow (reverse? multiply by coefficient?). For bond enthalpies, compute Σ(bonds broken) − Σ(bonds formed) and state the sign convention aloud. For formation enthalpies, write ΣΔHf(products) − ΣΔHf(reactants) and check that elements in their standard state have ΔHf = 0.
The candidate writes the equilibrium constant expression with the wrong powers or includes pure solids and liquids, and when a change is imposed (concentration, pressure, temperature) predicts the shift by 'the reaction moves to the side with fewer moles' without checking which side that is or whether the change affects K at all.
Le Chatelier's principle is memorised as a set of phrases rather than as a consequence of Q versus K. The candidate does not distinguish changes that shift the position of equilibrium (concentration, pressure for gases) from changes that alter the value of K (temperature), and forgets that pure solids and liquids have unit activity.
What to do instead
Always write K with only gases and aqueous species, each raised to its stoichiometric coefficient. For any perturbation, compute Q after the change and compare with K to decide the direction. Classify the change explicitly: does it alter K (only temperature) or only Q (concentration, pressure, catalyst)? For pressure, count gas moles on each side before predicting.
When calculating the pH of a weak acid or a buffer, the candidate uses [H+] = c·Ka without taking the square root, or neglects the autoionisation of water when the acid is very dilute, or applies the Henderson–Hasselbalch equation to a solution that is not a buffer (e.g. a weak acid titrated past its equivalence point).
The candidate memorises formulas as recipes and does not derive them from the equilibrium table, so the approximation that [H+] ≈ √(Ka·c) (valid only when the degree of dissociation is small and water contributes negligibly) is applied outside its range. The buffer region is not identified on the titration curve.
What to do instead
Set up the ICE table for HA ⇌ H+ + A− every time, state the approximations used and check them (e.g. α < 5 %, and [H+] >> 10−7). Use Henderson–Hasselbalch only when both a weak acid and its conjugate base are present in comparable amounts. For titrations, locate the buffer region, the equivalence point and the excess-titrant region before choosing a formula.
The candidate balances a redox equation in acidic medium by inspection or by adding H2O and H+ without first separating the half-reactions, and then carries the resulting electron count into the Nernst equation or Faraday calculation with the wrong number of electrons transferred.
Balancing by inspection works for simple cases and fails as soon as the medium is acidic or basic. The candidate does not track electrons as a conserved quantity, so the number n in ΔG = −nFE and in the Nernst equation is guessed from the overall equation rather than from the balanced half-reactions.
What to do instead
Balance by the ion–electron method: write the two half-reactions, balance atoms other than O and H, balance O with H2O, balance H with H+, balance charge with electrons, multiply to equalise electrons, add and cancel. In basic medium, add OH− to both sides to neutralise H+. Read n from the number of electrons cancelled and state it explicitly before using it.
In organic chemistry the candidate names or draws a compound without checking the principal functional group priority, or treats two structures as isomers when they are the same molecule, or predicts a substitution versus elimination product without considering the substrate class (primary/secondary/tertiary), the base strength and the temperature.
IUPAC priority rules and isomerism definitions are learned as lists, not as a decision procedure, so the candidate picks the wrong parent chain or the wrong suffix. Reactivity is memorised as 'haloalkanes undergo substitution' without the SN1/SN2/E1/E2 competition, which depends on steric hindrance, nucleophile/base strength and heat.
What to do instead
For naming, apply the priority order (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne > alkane) and choose the longest chain containing the principal group. For isomerism, compare connectivity first, then spatial arrangement. For reactions, classify the substrate and reagent before predicting: primary + strong nucleophile → SN2; tertiary + weak nucleophile → SN1; strong bulky base or heat → elimination.