NYSTCE Chemistry common mistakes
NYSTCE Chemistry · Understand the traps and how to avoid them.
When a problem gives mass of one reactant and asks for mass of product, the candidate divides or multiplies by the molar mass of the wrong species, or skips the mole-ratio step entirely and treats the balanced equation's coefficients as a mass ratio.
Under time pressure, candidates collapse the three-step mass-mass path (mass to moles, mole ratio, moles to mass) into one operation, and they grab whichever molar mass appears first in the prompt. The habit is reinforced by single-step percent-composition drills where no ratio is needed.
What to do instead
Adopt a fixed written template for every stoichiometry item: grams A, moles A (divide by M_A), moles B (multiply by coefficient ratio), grams B (multiply by M_B). Annotate each arrow with the unit conversion so the ratio step is physically visible on the page. Practise ten mass-mass problems where the two molar masses are numerically close, forcing reliance on the coefficient ratio rather than on arithmetic intuition.
The candidate predicts a molecule's geometry or polarity from the number of electron groups around the central atom without first subtracting lone pairs, so water is called linear and ammonia trigonal planar; or the candidate calls a molecule with polar bonds nonpolar without checking whether the bond dipoles cancel by symmetry.
VSEPR is often memorised as a lookup table mapping steric number to shape, and the lone-pair subtraction is the step most easily dropped. Polarity is conflated with bond polarity because the vector-cancellation idea is spatial and harder to hold mentally than an electronegativity difference.
What to do instead
For every structure, draw the Lewis dot structure first, then tabulate bonding pairs and lone pairs on the central atom separately, then apply the electron-domain geometry, then the molecular geometry. For polarity, sketch the bond dipoles as arrows on the drawing and ask whether they sum to zero. Drill a paired set such as CO2 versus SO2 and CCl4 versus CHCl3 to make the symmetry test explicit.
The candidate balances a redox half-reaction in acidic solution but forgets to add hydroxide and cancel water when the prompt specifies basic conditions, or balances oxygen by adding water to the wrong side; in cell problems the candidate also assigns oxidation to the cathode.
The acidic procedure is taught first and practised most, so it becomes the default script. The basic-solution adjustment is a short add-on step that is dropped under pressure. Electrode naming is confusing because reduction at the cathode must be recalled by a mnemonic rather than by the definition of reduction as gain of electrons.
What to do instead
Write the basic-solution correction as a literal final line of the procedure: after balancing in acid, add OH- to both sides equal to the H+ count, combine H+ and OH- into water, and cancel. For electrodes, derive rather than recall: oxidation loses electrons, electrons flow from anode to cathode in the external circuit, so reduction occurs where electrons arrive, at the cathode. Redo two full basic-solution balances each study session until the correction step is automatic.
The candidate treats a weak acid as if it dissociated completely, computing pH as the negative log of the formal concentration, or applies the Henderson-Hasselbalch equation to a buffer whose acid and base concentrations differ by more than roughly tenfold without checking the assumption.
Strong-acid pH problems dominate early practice, and the shortcut pH equals negative log of concentration becomes a reflex. The weak-acid ICE table is longer and error-prone, so candidates avoid it. Buffer problems look like plug-in items, hiding the assumption that the buffer components are comparable in concentration.
What to do instead
Before any pH calculation, classify the solute explicitly as strong or weak and write that classification at the top of the work. For weak acids, set up the equilibrium expression with an unknown extent of dissociation and state the small-x approximation only after confirming the ratio of concentration to Ka exceeds roughly 400. For buffers, verify the ratio of conjugate base to acid lies between about 0.1 and 10 before using the log form, and otherwise solve the equilibrium expression directly.
The candidate confuses the signs and definitions of heat, work, and internal energy, asserting that an exothermic reaction has positive enthalpy change, or that bond breaking releases energy; the candidate also cannot distinguish a state function from a path function when asked which quantities depend on route.
Everyday language treats heat as a substance that flows out and is therefore negative, which collides with the convention that enthalpy change is negative for exothermic processes. Bond energy is taught as energy stored in bonds, so candidates infer that forming bonds costs energy rather than releases it. Path versus state is abstract and often memorised as a list rather than understood.
What to do instead
Anchor the conventions in a single written statement: system enthalpy change is negative when the system loses heat, and bond breaking always requires input of energy while bond formation always releases it. Test understanding by computing enthalpy change from bond energies for a simple reaction and checking the sign against intuition. For state functions, classify by asking whether the quantity can be computed from initial and final states alone, and verify with a two-step versus one-step path to the same endpoints.
The candidate answers periodic-trend questions using atomic number as the organising principle rather than effective nuclear charge and shielding, so atomic radius is predicted to increase across a period, or ionisation energy is predicted to rise down a group; the candidate also ignores the half-filled and fully filled subshell exceptions when ranking ionisation energies.
Left-to-right reading order creates an implicit assumption that size grows with atomic number. The trend rules are memorised as directional slogans without the underlying Coulombic reasoning, so they collapse when a question reverses the direction or asks for a comparison across both axes.
What to do instead
For every trend question, state in one sentence the competing factors: nuclear charge pulling electrons inward versus shielding and distance pushing them outward. Then reason to the answer rather than recalling a slogan. Build a small ranking exercise that includes the exceptions, such as comparing the first ionisation energies of nitrogen and oxygen, and explain the half-filled stability in writing.
The candidate misreads a potential energy diagram or a rate-versus-temperature question, claiming that a catalyst increases the activation energy, that a catalyst shifts the equilibrium position, or that raising temperature increases the rate constant by increasing collision frequency alone rather than by increasing the fraction of collisions with sufficient energy.
Catalysts and temperature are both described loosely as speeding up reactions, and the distinct mechanisms are not separated. The equilibrium-versus-kinetics distinction is a classic conflation, and the exponential Boltzmann factor is harder to articulate than a vague statement about more collisions.
What to do instead
Maintain a two-column note contrasting kinetics and thermodynamics: what changes the rate, what changes the position of equilibrium, and what changes neither. For catalysts, state explicitly that the activation energy is lowered for both forward and reverse paths equally, so the equilibrium constant is unchanged. For temperature, explain the Maxwell-Boltzmann distribution shift and the exponential dependence of the rate constant, and practise interpreting a diagram with two curves at different temperatures.
The candidate answers nuclear-decay items by balancing mass number only and ignoring atomic number, or misidentifies which particle is emitted in alpha versus beta decay, and confuses half-life problems by applying the decay constant as a simple division rather than an exponential relationship.
Mass-number conservation is the more visually obvious check, so charge conservation is skipped. The notation for alpha and beta particles is similar and the transformation of the nucleus is not visualised. Half-life is often taught with simple halving examples that fail once the elapsed time is not an exact multiple.
What to do instead
For every nuclear equation, write two conservation checks side by side, one for mass number and one for atomic number, and refuse to move on until both are satisfied. Convert each decay type into a before-and-after nuclear statement so the change in element is explicit. For half-life, practise with elapsed times that are not whole multiples, using the exponential form and confirming the answer lies between the bracketing whole-multiple values.
The candidate names an organic structure without applying the priority rules, choosing the longest chain that happens to be drawn horizontally rather than the true parent chain, or numbering from the end nearest a substituent of low priority instead of the end that gives the lowest locants to the principal functional group.
The drawn orientation of a structure biases chain selection toward the horizontal path. Priority rules are numerous and candidates learn them as a list without a consistent order of application, so the first rule that comes to mind wins. Stereochemistry is then omitted because the naming task feels complete once the parent is chosen.
What to do instead
Follow a written decision sequence for every name: identify the principal functional group, then select the parent chain containing it, then number to give that group the lowest locant, then list substituents alphabetically, then assign stereochemistry. Redraw any structure in a different orientation before naming to break the horizontal bias. Practise with branched structures whose longest chain runs diagonally.
In laboratory-safety and instruction items, the candidate selects the most cautious-sounding option rather than the procedurally correct one, for example choosing to neutralise an acid spill with a strong base, to return unused reagent to its stock bottle, or to dilute a concentrated acid by adding water to the acid.
Safety questions reward a general disposition toward caution, and candidates transfer that disposition to specific procedures where the cautious-sounding action is actually unsafe. The underlying chemistry of the hazard, such as the exothermic heat of neutralisation or the spattering risk in acid dilution, is not connected to the procedural rule.
What to do instead
Learn each safety rule together with the chemical reason it exists, and rehearse the reason when answering. For spills, identify the correct absorbent or dilute-and-flush sequence rather than a neutralising reagent. For dilution, state the rule as acid into water, never water into acid, and explain the density and heat-release basis. Review the standard handling rules for waste, glassware, and personal protective equipment as procedures with rationales rather than as slogans.