NSSCAS Chemistry (8224) common mistakes
NSSCAS Chemistry (8224) · Understand the traps and how to avoid them.
When drawing dot-and-cross diagrams for ionic compounds such as MgCl2, the candidate draws one covalent molecule with shared electron pairs between Mg and each Cl instead of showing a Mg2+ ion with an empty outer shell and two separate Cl- ions each with a full octet and a transferred pair.
Dot-and-cross notation is first learned for covalent bonding, so candidates apply the same 'pairs between atoms' template to every compound and never check whether the electronegativity difference or the metal/non-metal identity means electrons are transferred rather than shared. The visual habit overrides the underlying principle.
What to do instead
Before drawing, classify the compound: metal plus non-metal means ionic, so transfer electrons and draw square brackets with charges on each ion. Practise a fixed routine: count valence electrons, decide transfer or sharing, then draw. Draw MgCl2, Na2O and CaO side by side and label the charge on every ion, including the 2+ on the cation.
When asked to explain a trend such as why the first ionisation energy of aluminium is lower than that of magnesium, the candidate writes only 'because Al is further to the right' or 'because the nuclear charge is higher', without identifying the specific electron being removed and the subshell it occupies.
Candidates memorise the general left-to-right increase in nuclear charge and apply it as a slogan, missing that the trend reverses when the electron removed comes from a higher-energy p subshell rather than a filled s subshell. They also confuse shielding by inner shells with the extra stability of a full subshell.
What to do instead
Answer every ionisation-energy question in three named steps: state the electron configuration of the atom and which electron is lost, state the subshell and whether it is full, half-full or singly occupied, then state the balance of nuclear charge against shielding and distance. For Mg versus Al, write that Al loses a 3p electron which is higher in energy and better shielded than the 3s electron of Mg.
In mole calculations involving solutions and gases, the candidate substitutes numbers into a remembered formula without converting units, for example using a volume in cm3 directly in n = cV (where V must be in dm3) or using temperature in degrees Celsius in pV = nRT.
Formulas are recalled as strings of symbols rather than as relationships with defined units. Under time pressure the candidate grabs the numbers in the order they appear in the question and never writes the units alongside each quantity, so a factor-of-1000 error goes undetected.
What to do instead
Adopt a habit of writing the unit after every number in the working, then converting before substitution: divide cm3 by 1000 to get dm3, add 273 to get kelvin. Always estimate the answer first (for example, 25 cm3 of 0.1 mol/dm3 is about 2.5 x 10-3 mol) so a wrong power of ten is obvious.
When calculating an enthalpy change from bond energies or from formation data, the candidate mixes up the sign convention, adding bond-breaking energies when they should subtract, or reversing the direction of formation enthalpies for reactants and products.
The candidate treats enthalpy as bookkeeping of numbers rather than as energy flow. Bond breaking is endothermic and bond making is exothermic, but this is learned as a phrase, not as a rule applied to the direction of the arrows in the equation, so signs are assigned by guesswork.
What to do instead
Draw an energy cycle or arrow diagram every time before calculating. For bond energies write: energy in to break bonds of reactants minus energy out to form bonds of products. For formation data write: sum of formation enthalpies of products minus sum of formation enthalpies of reactants. Check the sign against whether the reaction is expected to be exothermic or endothermic.
When describing how a change in conditions shifts an equilibrium, the candidate writes that the position of equilibrium moves but then claims the value of the equilibrium constant changes with pressure or with a catalyst, or states that a catalyst shifts the equilibrium to the right.
The candidate conflates rate with position. Catalysts and pressure change how fast equilibrium is reached or how concentrations are distributed, but only temperature changes the value of K. The distinction between the equilibrium constant and the equilibrium position is not firmly held.
What to do instead
Learn one sentence and apply it: only temperature changes the numerical value of K; concentration, pressure and catalysts change the position but not K. For each perturbation, state which side the equilibrium favours and why, then explicitly state whether K changes. Practise explaining why a catalyst increases the rate of both forward and reverse reactions equally.
When asked to explain why a reaction rate increases with temperature, the candidate writes only 'the particles move faster and collide more often', omitting that a greater proportion of collisions now exceeds the activation energy.
The collision-frequency explanation is the intuitive one and is repeated in everyday descriptions. Candidates do not realise that for many reactions the increase in collision frequency alone is far too small to account for the observed rate increase, so the mark scheme requires the activation-energy argument.
What to do instead
Write the answer in two parts every time: more frequent collisions, and more importantly a greater fraction of molecules with energy greater than the activation energy, shown on a Maxwell-Boltzmann sketch with the area beyond Ea shaded. Practise sketching the curve at two temperatures and labelling the shaded areas.
In titrations and salt-preparation questions, the candidate chooses an indicator or a method without checking the strength of the acid and base, for example using methyl orange for a weak acid-weak base titration or using a precipitation route for a soluble salt.
Indicator choice is memorised as 'methyl orange for acid, phenolphthalein for base' without linking it to the pH at the equivalence point. Similarly, salt-preparation methods are learned as a list rather than as a decision tree based on solubility.
What to do instead
Build a decision table: strong acid-strong base gives equivalence near pH 7 so either indicator works; weak acid-strong base gives a basic equivalence point so use phenolphthalein; strong acid-weak base gives an acidic equivalence point so use methyl orange; weak acid-weak base has no sharp end point so no simple indicator works. For salts, first check solubility: soluble salts by titration or excess solid plus filtration, insoluble salts by precipitation, then washing and drying.
When writing half-equations and combining them into a full ionic equation, the candidate balances atoms but not charge, or fails to multiply one half-equation so that the electrons cancel, leaving electrons in the final equation.
The candidate treats balancing as counting atoms only, because earlier equation-balancing practice never involved charge. The electron count is also treated as a separate step rather than as the thing that determines the multipliers.
What to do instead
Follow a fixed sequence: balance the main element, balance oxygen with water, balance hydrogen with H+, balance charge with electrons, then multiply each half-equation so the electron numbers match before adding. Finish by checking that charge balances on both sides and that no electrons appear in the final equation. Practise with dichromate and iron(II) and with manganate(VII) and ethanedioate.
In organic reaction questions, the candidate names a product or reagent without specifying the essential condition, for example writing 'ethanol plus acidified dichromate gives ethanoic acid' without reflux, or writing 'chlorine with ethane gives chloroethane' without ultraviolet light.
Candidates learn reactions as reagent-product pairs and treat conditions as decoration. They do not appreciate that the same reagents under different conditions (distil versus reflux, heat with dilute versus concentrated acid) give different products.
What to do instead
For every organic reaction, write a three-column table: reagent, condition, product, and include the contrasting case. For oxidation of a primary alcohol, distil for the aldehyde and reflux for the carboxylic acid. For halogenation of alkanes, write UV light; for alkenes, write room temperature. Test yourself by covering the condition column and reproducing it.
When recording and processing practical data, the candidate reports a burette reading or thermometer reading to more decimal places than the apparatus can deliver, or calculates a mean from titre values that include a rough titration, without excluding it.
Candidates equate more decimal places with more precision and have not been taught the resolution of common apparatus. The rough titration is seen as just another data point rather than as a trial run to be discarded.
What to do instead
Learn the resolution of standard apparatus: burette to 0.05 cm3 (two decimal places), pipette to 0.1 cm3, thermometer typically to 0.5 or 1 degree, balance to 0.01 g. Record all readings to the apparatus resolution and quote the mean to the same precision. Exclude the rough titre and any obvious outlier, and state that you have done so.