MATSEC AM Chemistry common mistakes
MATSEC AM Chemistry · Understand the traps and how to avoid them.
Writing a mechanism (e.g. electrophilic addition to an alkene, or nucleophilic substitution of a halogenoalkane) using only the overall transformation and then adding curly arrows that start from a neutral atom or point from a product to a reagent, rather than drawing every intermediate with its charge and lone pair and showing arrows from electron-rich site to electron-poor site.
Candidates memorise the summary equation and treat the mechanism as decoration; they have not practised drawing the carbocation or the transition-state geometry, so they cannot place the arrow tail correctly and lose the mechanism marks even when the product is right.
What to do instead
For each named mechanism, write a fixed template: draw all species with full structural formulae, mark every lone pair and formal charge, and draw one arrow per bond made or broken, always from a lone pair or bond to an atom or bond. Practise three examples per mechanism (primary, secondary, tertiary where relevant) and self-mark against the rule 'tail on electrons, head on the atom or bond receiving them'.
Balancing redox equations, especially in acidic or alkaline solution, by inspection only, so that H, O and charge are not conserved, or by adding H2O and H+ on the wrong side; half-equations are not separated and electrons are not cancelled explicitly.
Candidates try to balance the full equation in one step and skip the half-equation method, and they do not check charge balance, which is the step most often omitted.
What to do instead
Always split into oxidation and reduction half-equations, balance atoms other than O and H, add H2O to balance O, add H+ to balance H, add electrons to balance charge, then equalise electrons and add. Finish with a written check: total atoms and total charge on each side must match. Do this for at least one permanganate and one dichromate example in both acid and alkali.
Calculating enthalpy changes of reaction from bond enthalpies or from standard enthalpies of formation without checking which quantities are defined for which species, and without handling the sign convention; for example adding bond-breaking and bond-forming terms with the same sign, or using formation data for elements as non-zero.
The two routes look similar on paper, and candidates confuse 'bonds broken minus bonds formed' with 'products minus reactants'; they also forget that bond enthalpies are average values for gases, so the answer is an estimate.
What to do instead
Write the defining equation before substituting: ΔH = Σ(bond enthalpies broken) − Σ(bond enthalpies formed), or ΔH = ΣΔHf(products) − ΣΔHf(reactants) with ΔHf of an element in its standard state = 0. State units and sign explicitly, and add one sentence saying whether the value is exact or an average-based estimate.
Describing trends in periodic properties (atomic radius, first ionisation energy, electronegativity) by quoting the direction of the trend without giving the underlying reason in terms of nuclear charge, shielding and distance, or by mixing up the group and period directions.
Candidates learn the trend as a fact to recite and never link it to effective nuclear charge, so under exam pressure they invert the period trend or give a reason that contradicts the direction they stated.
What to do instead
For every trend answer, use a three-part sentence: state the trend, state the change in nuclear charge and shielding, state the effect on attraction for the outer electron. Draw the period and group arrows on a blank periodic table once per revision session and explain one anomaly (e.g. the drop between Group 2 and Group 13) in the same three-part form.
In organic synthesis and reaction-scheme questions, giving a reagent without the essential condition (e.g. 'NaOH' for hydrolysis of a halogenoalkane without 'aqueous, heat under reflux', or 'H2' for reduction without 'Ni catalyst'), or giving a condition that would give the wrong product (ethanolic NaOH instead of aqueous).
Candidates treat the reagent name as the whole answer and do not realise that the same reagent under different conditions gives substitution or elimination, oxidation or no reaction.
What to do instead
Build a two-column reaction map: reagent plus conditions on the left, product and reaction type on the right. For each entry, write the contrasting condition beside it (aqueous vs ethanolic KOH; acidified vs alkaline KMnO4; warm vs strong heat). In answers, always write reagent, solvent or medium, temperature, and catalyst where relevant.
Drawing or naming organic structures with incorrect skeletal formulae: missing hydrogens on heteroatoms, carbon shown with five bonds, a curly arrow or charge implying the wrong valency, or naming a compound with the longest chain chosen wrongly or the numbering started from the wrong end.
Candidates rush the structure to get to the chemistry, and IUPAC numbering rules (lowest locants, principal functional group priority) are applied from memory rather than systematically.
What to do instead
Adopt a fixed drawing checklist: count bonds at every carbon and heteroatom, show all O–H and N–H hydrogens, and check formal charges. For naming, apply the priority order (acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > halogeno/alkyl) and choose the longest chain containing the principal group before numbering.
In volumetric analysis calculations, using the wrong mole ratio from the equation, forgetting to convert cm3 to dm3, or treating a back-titration or a dilution as a single-step calculation and so using the wrong volume for the concentration.
Candidates chain numbers without tracking what each volume represents (aliquot vs total vs titre) and do not write the balanced equation with the stoichiometric ratio in front of them.
What to do instead
Use a fixed layout: balanced equation with the mole ratio stated; moles of titrant = c × V(dm3); moles of analyte = moles of titrant × ratio; then concentration or mass. For back-titrations, compute moles of excess reagent first and subtract from the initial moles. For dilutions, carry the moles through each step explicitly and label every volume with what it is.
Confusing the language of equilibrium and kinetics: stating that a catalyst increases the equilibrium yield, that a reaction 'shifts' faster, or that Kc changes when concentration changes; and using Le Chatelier qualitatively without linking it to the expression for K.
The two topics are taught close together and candidates reuse phrases such as 'shifts to the right' without distinguishing rate from position of equilibrium or from the value of the equilibrium constant.
What to do instead
Write three separate statements for every disturbance: effect on rate, effect on position of equilibrium, effect on the value of K. Memorise that only temperature changes K; a catalyst changes rate and time to equilibrium only. For each Le Chatelier question, write the K expression and show which term the change alters before predicting the direction.
Describing observations in qualitative analysis and organic tests imprecisely, e.g. 'a precipitate forms' without the colour, 'gas evolved' without identifying it by a named test, or claiming a positive iodoform or Fehling's result for a compound that cannot give it.
Candidates learn the test names but not the visual outcome or the structural requirement (methyl ketone or CH3CH(OH)– for iodoform; aldehyde for Fehling's/Tollens'), so they cannot match observation to structure.
What to do instead
Make a table with columns: test, reagent and conditions, observation for a positive result, functional group or ion responsible, and one negative control. Learn the structural requirement next to each test, and practise deducing a structure from a set of observations rather than reciting tests.
Answering structured questions on bonding and structure (e.g. why MgO has a higher melting point than NaCl, or why ice is less dense than water) with a single vague phrase such as 'stronger bonds' instead of comparing the type and number of interactions, the charges involved, and the energy needed to overcome them.
Candidates know the qualitative idea but not the comparative structure of an explanation, and they do not distinguish intramolecular bonds from intermolecular forces.
What to do instead
Use a comparison frame: identify the particles present and the forces between them in each substance; compare magnitude using charge and distance (e.g. 2+/2− vs 1+/1−, or more hydrogen bonds per molecule); state that melting or boiling requires overcoming those forces, not breaking covalent bonds. Write two full model answers (ionic vs ionic; hydrogen-bonded vs non-polar molecular) and reuse the frame.