ORT Chemistry common mistakes
ORT Chemistry · Understand the traps and how to avoid them.
When writing electron configurations for transition metals, the candidate fills the 4s subshell before 3d but then fails to remove 4s electrons first when forming cations, producing configurations such as Fe2+ = [Ar]3d4 4s2 instead of [Ar]3d6.
The aufbau order (4s below 3d) is learned as a filling rule and then misapplied as an ionization rule; candidates do not realize that once 3d is occupied, 4s rises above 3d in energy, so 4s electrons leave first. The 4s/3d crossover is counterintuitive and rarely drilled.
What to do instead
Memorize the rule 'fill 4s first, empty 4s first' and practise ten cation configurations (Fe2+, Fe3+, Cu+, Cr3+, Mn2+, Co2+, Ni2+, Zn2+, Ti3+, V2+). For each, write the neutral atom, then delete 4s electrons before touching 3d, and check the result against the anomalous neutral cases Cr ([Ar]3d5 4s1) and Cu ([Ar]3d10 4s1).
The candidate draws a Lewis structure that satisfies the octet on every atom but ignores formal charge and the total valence-electron count, so they accept structures like a singly bonded O-O in O2 or an expanded octet on nitrogen.
Octet satisfaction is taught as the sole success criterion, so candidates stop checking electron bookkeeping and do not know which period-2 elements can never exceed eight electrons. Formal charge is treated as an optional decoration rather than a diagnostic.
What to do instead
Adopt a fixed three-step audit: (1) sum valence electrons from group numbers and match the structure's electron count; (2) compute formal charge on each atom and prefer the structure with charges closest to zero and negative charge on the more electronegative atom; (3) apply the hard rule that N, O, F, and Ne never exceed an octet, while period 3 and below may expand. Rehearse on O2, NO, CO, SO2, and NO3-.
When asked to compare acidity, solubility, or thermal stability across a series (for example the hydroxides of group 2, or the hydrides of group 17), the candidate answers from memory of isolated facts rather than deriving the trend from periodic position and bond strength, and reverses the direction.
s- and p-block trends are numerous and similar-sounding, and candidates memorize single examples instead of the underlying drivers (lattice energy, hydration energy, bond enthalpy, polarizability). Under time pressure they guess the direction.
What to do instead
For each trend, write the controlling quantity next to it: group 2 hydroxide solubility rises down the group because lattice energy falls faster than hydration energy; group 2 sulfate solubility falls down the group because the larger cation stabilizes the large sulfate less; hydrogen halide thermal stability falls down the group because H-X bond enthalpy decreases. Re-derive each trend from the driver before answering, and test yourself by predicting an unfamiliar member of the series.
In limiting-reagent problems the candidate computes moles of each reactant, divides by the stoichiometric coefficient, and then proceeds with the larger quotient or with the reactant of larger mass, instead of the smallest mole-to-coefficient ratio.
The procedure is remembered as 'compare the amounts' without the coefficient normalization, and mass is confused with amount. Candidates also skip writing the balanced equation first, so the coefficients they divide by are wrong.
What to do instead
Use a rigid template: balance the equation; convert every given mass or volume to moles; divide each reactant's moles by its coefficient; the smallest quotient is the limiting reagent and sets the theoretical yield. Then compute the answer from that reagent only. Practise with a case where the limiting reagent has the larger mass, and always finish by converting theoretical to actual yield with the percent-yield formula.
When calculating the pH of a weak acid or a buffer, the candidate substitutes the analytical concentration directly into pH = -log[H+] without solving the equilibrium, or applies the Henderson-Hasselbalch equation to a buffer whose components react with added strong acid or base without recomputing the new ratio.
Strong-acid habits are carried over to weak systems, and the small-extent approximation is used without checking that the degree of dissociation is negligible. Buffer problems are treated as plug-in formulas rather than as stoichiometry followed by equilibrium.
What to do instead
For every weak-acid problem, first write Ka = x^2/(C - x), state the assumption C >> x, solve, then verify the assumption (x/C < about 5 percent); if it fails, solve the quadratic. For buffers with added strong acid or base, do a stoichiometric step first (neutralize, find new amounts of conjugate pair), then apply Henderson-Hasselbalch to the new ratio. Check the final pH lies in a physically sensible range.
The candidate balances a redox equation in acidic or basic medium by inspection and ends up with unequal charge on the two sides, or forgets to add OH- and water when converting from acidic to basic conditions.
Atom balance is checked but charge balance is not, and the acid-to-base conversion step is a procedural add-on that is easily dropped. Inspection balancing works for simple cases and then silently fails for permanganate or dichromate systems.
What to do instead
Use the half-reaction method mechanically: split, balance atoms other than O and H, balance O with H2O, balance H with H+, balance charge with electrons, equalize electron counts, add the halves, then for basic medium add OH- to both sides equal to the H+ count and cancel water. Always finish by verifying that total charge is identical on both sides and that every element balances.
In predicting the major product of an electrophilic addition or a substitution, the candidate names a product without specifying regiochemistry and stereochemistry, or applies Markovnikov's rule to a reaction that proceeds through a free-radical or anti-Markovnikov pathway.
The candidate recognizes the functional group but not the mechanism, so the intermediate (carbocation versus radical) that determines orientation is never identified. Stereochemical consequences of a planar carbocation or an SN2 backside attack are not considered at all.
What to do instead
Before naming a product, state in one line the mechanism and the intermediate: carbocation (Markovnikov, racemization possible), radical (anti-Markovnikov with HBr/peroxide), concerted SN2 (inversion), or bromonium ion (anti addition). Then derive the product from that intermediate. Practise paired contrasts: HBr with and without peroxide on propene; SN1 versus SN2 on a secondary halide; addition of Br2 versus HBr to cyclohexene.
When planning a multi-step synthesis, the candidate chooses reagents that give the right functional group but ignore protecting-group needs and competing reactivity, for example oxidizing a primary alcohol to a carboxylic acid when an aldehyde was wanted, or using a Grignard reagent on a substrate that still bears an acidic proton.
Reactions are learned one at a time as isolated transformations, without a sense of which other groups in the molecule will react under the same conditions. Selectivity and functional-group compatibility are not practised.
What to do instead
For each target, list every functional group present and mark which ones the chosen reagent would also attack; then select a milder or more selective reagent (PCC or controlled oxidation for aldehyde, protecting groups for acidic protons before Grignard). Work backward from the target one bond at a time and verify each step against the compatibility list. Do at least five three-step syntheses end to end.
When identifying an unknown from qualitative tests, the candidate reports a conclusion from a single observation (for example 'white precipitate with AgNO3, therefore Cl-') without confirming solubility in acid, without a flame test, and without excluding carbonate or sulfate interference.
Each test is memorized independently, and the candidate does not build a decision sequence. Interfering ions that give similar precipitates (AgCl, AgBr, Ag2CO3, Ag2SO4) are not considered, and confirmatory tests are skipped.
What to do instead
Learn a fixed scheme: preliminary tests (state, colour, flame), then group separation by reagent, then confirmatory tests on the isolated fraction. For halides, acidify with dilute HNO3 before adding AgNO3 to destroy carbonate, then confirm with ammonia solubility (AgCl dissolves, AgBr partly, AgI not). Always state the observation, the inference, and the confirmation as three separate lines.
In the laboratory, the candidate heats a closed system or a flammable liquid over an open flame, pipettes by mouth, or disposes of waste down the sink, treating safety rules as trivia to be memorized rather than as operating constraints.
Safety content is studied as a list of prohibitions detached from the practical procedures the candidate will perform, so the rule is recalled but not applied at the moment of the operation. Time pressure in practical work encourages shortcuts.
What to do instead
Attach each safety rule to the specific operation it governs: heating a closed system is prohibited because pressure builds and glass bursts; flammable liquids are heated by water bath or heating mantle, never open flame, because vapour ignites; volatile or toxic liquids are pipetted with a bulb or dispenser; waste is segregated by halogenated, heavy-metal, and organic categories. Rehearse by walking through a procedure and naming the hazard and control at each step before starting.