CAPE Chemistry Unit 1 common mistakes
CAPE Chemistry Unit 1 · Understand the traps and how to avoid them.
Writing electron configurations such as 1s2 2s2 2p6 3s2 3p6 3d2 for calcium instead of 4s2, filling the 3d subshell before the 4s subshell or misordering the two after the fact.
Candidates memorise the 'aufbau' arrow order but do not understand that 4s has lower energy than 3d for neutral atoms, and they then write configurations in energy-filling order rather than in principal-quantum-number order. The same gap produces wrong configurations for transition-metal ions, where 4s electrons are removed before 3d.
What to do instead
Learn the (n+l) rule and apply it explicitly: for 4s, n+l = 4; for 3d, n+l = 5, so 4s fills first. Then practise writing final configurations in n order (…3p6 3d2 4s2) and derive ion configurations by removing from the highest n shell first. Check every answer against the periodic table block position.
Balancing a redox equation in acidic or alkaline solution by inspection, leaving H and O unbalanced, or adding H2O to the wrong side and forgetting to add OH- at the end for alkaline conditions.
Candidates can balance mass but lose track of charge, and they treat half-equations as simple atom counts rather than as charge-balanced entities. They also forget that alkaline conditions require converting H+ to H2O by adding OH- to both sides.
What to do instead
Use the fixed half-equation procedure: balance all atoms except H and O, add H2O to balance O, add H+ to balance H, then add electrons to balance charge; multiply each half-equation by an integer so electrons cancel; add the halves and cancel. For alkaline, add OH- to both sides equal to the H+ count and combine H+ + OH- into H2O. Always verify total charge on each side as a final check.
Treating the mole and concentration calculations as plugging numbers into any formula that looks familiar, for example using c = n/V with V in cm3, or confusing the molar mass of a hydrate with that of the anhydrous salt.
Formula recall is fragile and candidates do not track units. Hydrate problems in particular trip candidates because they subtract the wrong mass or forget to include water of crystallisation in the molar mass.
What to do instead
Write the unit next to every quantity before substituting, and convert cm3 to dm3 by dividing by 1000 at the start. For hydrates, compute M of the hydrated formula including the water, and set up n(water) from the mass lost on heating. After calculating, sanity-check the magnitude (e.g. a titration titre near 25 cm3 should give a sensible concentration).
Describing a covalent bond as 'sharing electrons' without specifying the number of electrons shared, and confusing the number of bonding pairs with the number of lone pairs when predicting shapes.
Candidates learn shapes by memorising examples rather than applying VSEPR counting. They then misassign electron-pair geometry to molecular geometry, for example calling water linear or ammonia trigonal planar.
What to do instead
For every molecule, first draw the Lewis structure and count bonding pairs and lone pairs on the central atom. State the total electron domains to get the electron-pair geometry, then remove lone-pair influence to get the molecular shape. Learn the lone-pair repulsion order (lone-lone > lone-bond > bond-bond) and quote approximate bond angles, e.g. 104.5 degrees in water.
Predicting the effect of a change in conditions on an equilibrium position by intuition, e.g. saying that adding a catalyst shifts the equilibrium to the right or that increasing pressure always shifts the equilibrium towards fewer moles regardless of the phases involved.
Candidates conflate rate with position and do not apply Le Chatelier's principle systematically. They also forget that a catalyst affects both forward and reverse rates equally and therefore does not change the equilibrium constant or position.
What to do instead
Use a step-by-step script: identify the change, decide which side is favoured to oppose it, state the shift direction, and state the consequence for the specified quantity. Explicitly note that a catalyst changes only the time to reach equilibrium. For pressure changes, count only gaseous moles.
Calculating enthalpy changes from bond enthalpies or calorimetry data without checking signs, so that an exothermic reaction is reported as positive or a temperature rise is treated as a fall.
Candidates memorise q = mcΔT but do not connect the sign of q to the sign of ΔH, and they forget to divide by the number of moles actually reacting or to convert J to kJ.
What to do instead
Write the sign convention at the top of every calculation: heat released means q negative and ΔH negative for the system. Compute q = mcΔT in joules, convert to kJ, divide by moles reacted, and attach the sign. For bond enthalpies, use ΔH = sum(bonds broken) - sum(bonds formed) and check that bond breaking is endothermic.
Drawing organic mechanisms with curved arrows that start from an atom rather than from a lone pair or a bond, or that show a bond forming before the bond breaking that supplies the electrons.
Candidates copy mechanisms from memory without understanding that arrows track electron pairs. They also omit charges and lone pairs, so the arrow has no valid origin.
What to do instead
Before drawing any arrow, mark all lone pairs and formal charges. Start each arrow at a lone pair or at the middle of a bond, and end it at the atom or bond being formed. Check that every arrow moves two electrons and that the total charge is conserved across the step. Practise nucleophilic substitution and addition mechanisms from first principles.
Naming organic compounds by listing substituents in the order they appear in the structure, or choosing the wrong principal functional group as the suffix.
Candidates do not apply the IUPAC priority rules and do not number the parent chain to give the lowest locant set. They also confuse prefixes and suffixes, e.g. naming an alcohol as a hydroxyalkane when the alcohol should be the suffix.
What to do instead
Follow the fixed sequence: identify the principal functional group from the priority table, choose the longest chain containing it, number to give that group the lowest locant, then list substituents alphabetically with locants. Write the full name, then re-derive the structure from your name to verify it matches.
Describing intermolecular forces as 'van der Waals' for every substance, or claiming hydrogen bonding occurs whenever hydrogen is bonded to any atom.
Candidates use 'van der Waals' as a catch-all and do not distinguish London dispersion, permanent dipole-dipole, and hydrogen bonding. Hydrogen bonding is over-applied because the N, O, F requirement is not checked.
What to do instead
Classify in order: is H bonded to N, O or F (hydrogen bonding)? Is the molecule polar (permanent dipole-dipole)? Otherwise London dispersion only. Link the classification to the observed boiling point trend and explain the anomaly, e.g. water's high boiling point from hydrogen bonding.
Reporting a titration result using a single rough titre, or averaging titres that differ by more than the acceptable range, and quoting the answer to too many significant figures.
Candidates rush the practical and do not understand concordance. They also carry calculator digits through to the final answer without rounding to the precision of the burette readings.
What to do instead
Record all titres to two decimal places, identify concordant titres (within 0.10 cm3), average only those, and use the average in the calculation. Round the final answer to the appropriate number of significant figures, usually three, and state the uncertainty in the burette reading.