CSEC Chemistry common mistakes
CSEC Chemistry · Understand the traps and how to avoid them.
Writing 'neutral' whenever a salt is formed from a strong acid and a strong base, without first identifying the actual ions present in solution and checking whether either ion hydrolyses.
Candidates memorise the rule 'strong acid + strong base = neutral' and then apply it to salts such as sodium ethanoate or ammonium chloride, because they classify the parent acid and base by name rather than by the ions that remain in solution.
What to do instead
For every salt, write the formula, split it into its ions, and ask two questions: is the cation the conjugate acid of a weak base (then it hydrolyses, giving an acidic solution), and is the anion the conjugate base of a weak acid (then it hydrolyses, giving an alkaline solution). Practise with NaCl, NH4Cl, CH3COONa, Na2CO3 and KNO3 until the ion-by-ion check is automatic.
Balancing an equation by changing subscripts in a formula (for example writing H2O2 as H2O to make oxygen atoms match) instead of placing coefficients in front of the species.
Candidates treat balancing as an arithmetic puzzle and adjust the nearest number they see, forgetting that a subscript is part of the chemical identity of the substance and that changing it produces a different compound.
What to do instead
Adopt a fixed procedure: list each element and count atoms on both sides, then adjust only the large numbers in front of formulae; never touch a subscript. Finish by re-counting every element, and check charge balance for ionic half-equations.
Calculating moles with the wrong molar mass, typically by using relative atomic mass of a single element for a compound, or by ignoring water of crystallisation in a hydrated salt.
Candidates rush to n = m/M and pick the first number they recall, and they do not systematically add up every atom in the formula, so hydrates such as CuSO4·5H2O are treated as anhydrous.
What to do instead
Before any mole calculation, write the formula out and add the relative atomic masses of every atom, including the water of crystallisation, showing the arithmetic. Keep a habit of stating M with units (g mol-1) before substituting into n = m/M.
Reporting a titre or a calculated result to more decimal places than the burette or the given data justify, for example quoting 23.456 cm3 from a burette read to 0.05 cm3.
Candidates equate a longer answer with a better answer, and they do not connect the precision of an instrument to the number of significant figures the result can carry.
What to do instead
Decide the precision of the apparatus before recording data: a burette is read to 0.05 cm3 and a measuring cylinder to 1 cm3. Carry the final answer to the same number of significant figures as the least precise measurement used, and show the rounding step explicitly.
Describing an ionic bond as a shared pair of electrons, or describing covalent bonding as transfer of electrons, when asked to explain bonding in a named compound.
The definitions of ionic, covalent and metallic bonding are learned as isolated sentences, so candidates retrieve the wrong one under exam pressure and do not test the description against the actual elements involved.
What to do instead
Before writing, classify the elements: metal plus non-metal usually means electron transfer and electrostatic attraction between ions; non-metal plus non-metal means shared pairs. Then name the specific ions or the specific shared pair in the compound given, rather than reciting a general definition.
Explaining a trend such as increasing boiling point down Group 17, or increasing reactivity down Group 1, by saying 'the atoms get bigger' without linking size to the force or energy that actually changes.
Candidates learn the direction of trends but not the causal chain, so they stop at the observable change in atomic radius instead of connecting it to intermolecular forces, ionisation energy or shielding.
What to do instead
Learn each trend as a three-step chain: what changes (size, shielding, nuclear charge), what that does to the force or energy in question, and what the observable consequence is. Write the chain out for Group 1, Group 17, Group 2 and the noble gases, and check that each explanation names the force being overcome.
Using qualitative tests loosely, for example reporting 'a white precipitate' for a sulphate test without stating the reagent (acidified barium chloride) and the observation that distinguishes it from a carbonate or sulphite.
Candidates remember the colour of a precipitate but not the reagent, the acidification step or the competing ions, so their answer would not distinguish two unknown solutions in a practical.
What to do instead
For each ion, learn the reagent, the condition (acidified or not, warmed or not) and the exact observation. State all three in the answer, and where relevant say what would be seen for a likely interfering ion so the test is shown to be discriminating.
Drawing a structural formula with the wrong number of bonds on carbon, or omitting lone pairs and charges on ions in dot-and-cross diagrams.
Candidates draw diagrams freehand without counting outer electrons first, and they do not check each atom against the octet or duet rule before moving on.
What to do instead
Before drawing, count valence electrons for the whole species. After drawing, audit each atom: carbon must have four bonds, oxygen two, hydrogen one, and every ion must show its charge. For dot-and-cross diagrams, use a different symbol for each atom's electrons so sharing and transfer are visible.
Treating an equilibrium as having 'stopped' at equilibrium, or stating that the position of equilibrium shifts to the side with more moles when pressure is increased, regardless of which side that is.
Candidates confuse the constancy of concentrations at equilibrium with zero reaction rate, and they apply Le Chatelier's principle as a slogan without counting gas moles on each side.
What to do instead
State explicitly that at equilibrium the forward and reverse rates are equal and non-zero, so concentrations remain constant. For pressure changes, count gas moles on each side first and then say the equilibrium shifts towards the side with fewer gas moles, naming that side for the specific equation.
In organic chemistry, naming a product from the name of the reactant alone, for example writing 'ethene' as the product of ethanol with concentrated sulphuric acid without specifying the conditions, or giving an incorrect IUPAC name for a branched chain.
Candidates memorise reaction names without the reagents and conditions that determine the product, and they number a carbon chain from the wrong end when substituents are present.
What to do instead
Learn every organic reaction as a triple: reactant, reagent and conditions, product. For naming, find the longest chain containing the functional group, number from the end that gives the functional group the lowest number, then apply the lowest locants to substituents, and write the full name with hyphens and commas in the correct places.