BSEH SS Chemistry common mistakes
BSEH SS Chemistry · Understand the traps and how to avoid them.
Solid State and Solutions - substituting an edge length given in picometres directly into d = ZM/(N_A a^3) and reporting the result in g/cm3, often with the wrong Z.
The density formula is memorised without tracking units; pm is confused with cm, and Z is guessed instead of being linked to the unit-cell type.
What to do instead
Convert a from pm to cm first (1 pm = 10^-10 cm), cube it, and use Z = 1 for simple cubic, 2 for bcc and 4 for fcc. Check the order of magnitude: a typical ionic solid density is a few g/cm3, not 10^10.
Electrochemistry - writing the Nernst equation with the wrong sign or inverted concentration quotient, especially in concentration cells.
The candidate memorises E = E° - (0.0592/n) log Q but does not derive Q from the balanced overall cell reaction, or mixes oxidation and reduction potentials.
What to do instead
Write and balance the half-reactions, identify anode and cathode, and use reduction potentials throughout. For the overall reaction use Q = products/reactants; for a metal-ion concentration cell use E = (0.0592/n) log([cathode]/[anode]).
Chemical Kinetics - using stoichiometric coefficients as reaction orders and applying the first-order half-life t1/2 = 0.693/k to zero- or second-order reactions.
Order and molecularity are conflated; the candidate pattern-matches the reaction equation instead of using experimental rate data or the correct integrated rate law.
What to do instead
Determine order only from rate data or the given mechanism. Use t1/2 = [A]0/(2k) for zero order, 0.693/k for first order and 1/(k[A]0) for second order; check units of k to confirm the order.
Surface Chemistry - treating a larger flocculation value as a stronger coagulant and ignoring the charge of the coagulating ion in Hardy-Schulze reasoning.
The inverse relation between flocculation value and coagulating power is not internalised, so the candidate compares numbers in the wrong direction.
What to do instead
State that coagulating power increases with the charge of the oppositely charged ion (Al3+ > Ca2+ > Na+ for a negative sol) and that flocculation value is inversely proportional to coagulating power. Compare ions only for the same sol and same sign of charge.
General Principles and Processes of Isolation of Elements - using calcination for sulphide ores and roasting for carbonate ores.
Both are heating processes, so the candidate remembers the words but not the ore type or the gas produced.
What to do instead
Roasting: heating a sulphide ore in excess air to give an oxide, often with SO2, e.g. 2ZnS + 3O2 -> 2ZnO + 2SO2. Calcination: heating a carbonate or hydrated ore in limited air, e.g. CaCO3 -> CaO + CO2. Match the ore formula to the gas evolved.
p-Block Elements - drawing period-2 oxoacids such as HNO3 with an expanded octet or five bonds to nitrogen, as if N behaved like P.
The candidate transfers the expanded-octet pattern of phosphorus and sulphur to nitrogen, oxygen or fluorine without checking valence-shell capacity.
What to do instead
For N, O and F use only s and p orbitals and do not exceed an octet. Count valence electrons, assign formal charges, and draw HNO3 with one N=O and two N-O single bonds. For oxoacid strength, compare stability of the conjugate base and the number of oxygen atoms, not expanded octet.
d- and f-Block Elements and Coordination Compounds - calculating magnetic moment from the total number of d electrons instead of the number of unpaired electrons, or assuming every octahedral complex is high spin.
The formula μ = sqrt(n(n+2)) BM is applied with n = d-electron count; ligand-field splitting and geometry are ignored.
What to do instead
Find the metal oxidation state, count d electrons, decide geometry, then fill t2g/eg using the spectrochemical series. Count unpaired electrons n and use μ = sqrt(n(n+2)) BM; tetrahedral complexes are normally high spin, while octahedral complexes can be low spin with strong-field ligands.
Haloalkanes and Haloarenes - predicting fast SN2 for tert-butyl chloride or saying an SN1 reaction gives a single inverted product.
Substrate class, solvent and stereochemical consequence are not linked; the candidate remembers 'SN2 = inversion' but forgets that SN1 proceeds through a planar carbocation.
What to do instead
Classify the halide: methyl/primary favour SN2 with backside attack and inversion; tertiary, benzylic and allylic favour SN1 with racemisation. Use polar aprotic solvents for SN2 and polar protic solvents for SN1. Remember aryl and vinyl halides are inert to both SN1 and SN2 under normal conditions.
Alcohols, Phenols and Ethers; Aldehydes, Ketones and Carboxylic Acids - using Fehling's solution for benzaldehyde or Tollens' reagent as a ketone test.
Named tests are memorised as aldehyde/ketone labels without the substrate exceptions and functional-group specificity.
What to do instead
Tollens' reagent gives a silver mirror with aldehydes, including aromatic aldehydes; Fehling's solution is reduced by aliphatic aldehydes but not generally by aromatic aldehydes such as benzaldehyde. Ketones do not respond to either. Use iodoform only for CH3CO- groups or CH3CHOH- groups, and Lucas test for 1°, 2°, 3° alcohols.
Amines, Biomolecules, Polymers and Chemistry in Everyday Life - ranking amine basicity in water solely by the number of alkyl groups, so that trimethylamine is placed above methylamine and aniline above ammonia.
Gas-phase inductive effects are applied to aqueous solution without considering solvation, steric hindrance and resonance delocalisation of the lone pair.
What to do instead
For methylamines in water the order is Me2NH > MeNH2 > Me3N > NH3 because solvation and steric effects compete with induction. Aniline is much weaker than ammonia and cyclohexylamine because the nitrogen lone pair is delocalised into the ring. In biomolecule and polymer questions, classify by functional group and linkage rather than by name alone.