JCE Chemistry common mistakes
JCE Chemistry · Understand the traps and how to avoid them.
Writing ionic equations that include spectator ions, e.g. giving Zn + CuSO4 → ZnSO4 + Cu as the answer when an ionic equation is demanded, instead of Zn + Cu²⁺ → Zn²⁺ + Cu.
Candidates learn the full symbol equation first and stop there; they do not reliably identify which ions actually change, and often think sulfate must appear because it was in the formula.
What to do instead
Before writing, list every ion present on each side, then delete ions that are identical and unchanged. Practise a five-step drill: full equation, split all soluble ionic species, cancel spectators, check charge balance, check atom balance. Do ten precipitation and ten displacement examples until cancelling becomes automatic.
Calculating moles with the wrong molar mass, typically by using relative atomic mass of a molecule (e.g. 16 for O2, 14 for N2) or by forgetting waters of crystallisation in a hydrated salt.
The periodic table value is memorised as 'the mass of oxygen', and the subscript in the formula is not treated as part of the mass; hydrated salts are read as if the dot were punctuation.
What to do instead
Always rewrite the formula with subscripts expanded before adding masses: O2 = 2 × 16, CuSO4·5H2O = 64 + 32 + 64 + 5 × 18. Write the arithmetic out in full on the page so a marker can see where each number came from, and sanity-check that a gas mole value is plausible for the mass given.
Describing bonding with phrases like 'sharing electrons' or 'transferring electrons' without stating the species involved, the number of electrons, and the resulting charge or attraction.
Textbook summaries are memorised as slogans; candidates do not realise that mark schemes award points for named particles (outer shell electrons, ions, electrostatic attraction) rather than the general idea.
What to do instead
Use a fixed sentence template for each bond type: ionic = 'outer electron transferred from metal atom to non-metal atom, forming oppositely charged ions held by strong electrostatic attraction in a giant lattice'; covalent = 'pair of outer electrons shared between two nuclei, attracted to both'. Practise converting each template into a labelled dot-and-cross diagram.
Confusing rate of reaction with yield or with equilibrium position, e.g. stating that adding a catalyst increases the amount of product, or that increasing pressure shifts equilibrium and also speeds up the reaction without distinguishing the two effects.
Both topics are taught with similar graphs and both use the word 'increase', so the ideas merge; candidates do not separate 'how fast' from 'how much' or 'which side'.
What to do instead
Keep two columns on one page: 'rate' (collision frequency, activation energy, catalyst, surface area, concentration, temperature) and 'position of equilibrium' (only changes that alter the balance of forwards and backwards rates). For every past question, underline whether it asks about speed, amount, or position before answering.
Drawing dot-and-cross diagrams or electron shell diagrams that violate the octet for the wrong atoms, e.g. giving chlorine seven bonding electrons in HCl, or placing the shared pair on only one atom.
Candidates count total outer electrons but do not redistribute them, and they forget that hydrogen fills a duplet rather than an octet.
What to do instead
Use a strict procedure: count total outer electrons from both atoms, place a bonding pair between the nuclei, then fill outer shells to duplet (H) or octet (others), and finally check that each atom's count matches its group number requirement. Apply the same procedure to H2O, NH3, CH4, CO2 and HCl as a set.
Naming or writing formulas for salts incorrectly, e.g. writing sodium chloride as NaCl2, iron(II) sulfate as FeSO4 when iron(III) is meant, or naming CuO as copper(I) oxide.
Candidates balance charges by intuition rather than by using the ion charges, and they ignore Roman numerals in names because the convention is unfamiliar.
What to do instead
Memorise the common ion table (Na⁺, Mg²⁺, Al³⁺, Cl⁻, O²⁻, OH⁻, SO4²⁻, CO3²⁻, NO3⁻) and always cross-multiply charges to get subscripts. Treat the Roman numeral as a direct instruction for the metal's charge, and check that the total positive charge equals the total negative charge before moving on.
Writing half-equations or redox answers without balancing electrons and charge, e.g. Fe²⁺ → Fe³⁺ + e⁻ written as Fe²⁺ → Fe³⁺, or an overall equation where the electrons do not cancel.
Candidates focus on balancing atoms and overlook charge, because earlier equation work only required atom balance.
What to do instead
Adopt a fixed order: balance atoms other than O and H, balance O with H2O, balance H with H⁺ (or use OH⁻ in alkaline conditions), then balance charge with electrons. Add a final check that the sum of charges is equal on both sides and that electrons cancel when the two half-equations are combined.
Describing electrolysis products by naming the electrode material or the electrolyte without applying the reactivity/priority rules, e.g. predicting hydrogen at the cathode when a reactive metal such as sodium is in solution, or oxygen at the anode when a halide is concentrated.
The rules for preferential discharge are stated as lists and are not linked to a mental model of competition between ions, so candidates guess from the formula.
What to do instead
Learn two short priority lists: at the cathode, the less reactive metal or hydrogen is discharged (so Cu²⁺ before H⁺ before Na⁺); at the anode, halides are discharged before OH⁻ when concentrated, otherwise OH⁻ gives oxygen, and a reactive anode itself dissolves. Apply the list to molten and aqueous cases separately, and state the reason ('lower in the reactivity series') in every answer.
Giving a single-word or single-number answer to a 'describe and explain' or 'suggest why' question, e.g. answering 'because it is exothermic' or 'the rate increases' with no mechanism.
Candidates read the command word as 'state', and mark schemes award explanation marks for linked cause-and-effect statements that are not obvious from the data alone.
What to do instead
Train a command-word response: state, describe, explain. For 'explain', force a because-clause that names particles and energy, e.g. 'particles have more kinetic energy, so more collisions exceed the activation energy, so the rate increases'. Practise rewriting your own one-line answers into two linked sentences and check each against the number of marks available.
Reading a graph or table of results and quoting a value without unit, without the correct significant figures, or ignoring an anomalous point when calculating a mean.
Candidates treat graph-reading as transcription and do not distinguish the precision of the instrument from the precision of the answer; anomalous results are averaged in because they look like data.
What to do instead
Before quoting, identify the axis unit and the scale division, then give the value to the same precision as the smallest division. When averaging repeat results, circle and exclude any point that is clearly inconsistent, state that it is anomalous, and say whether it should be repeated. Check that final answers carry units and match the precision of the given data.