PRC Chemist common mistakes
PRC Chemist · Understand the traps and how to avoid them.
Choosing an answer from a familiar formula (e.g., pH = -log[H+], Nernst equation, ideal gas law) without first checking whether the conditions of validity hold (strong vs. weak acid, standard vs. nonstandard state, ideal vs. real gas).
Exam prompts are built to look like textbook templates; under time pressure the candidate pattern-matches to the template and skips the assumption check that the question is actually testing.
What to do instead
Before selecting, write the two or three assumptions behind the formula (complete dissociation, unit activity, low pressure, dilute solution) and confirm each against the data given. If any fails, switch to the general treatment (equilibrium expression, activity, van der Waals or fugacity).
Reporting a numerical answer with more significant figures than the data support, or rounding intermediate values before the final step.
Calculators return long decimals and candidates transcribe them, or they round at each step to save time, producing answers that miss the option by a small margin.
What to do instead
Carry full precision through intermediate steps and round only at the end. Match the final significant figures to the least precise input. If the computed value falls between two options, recheck for a rounding or unit error rather than guessing.
Ignoring charge and mass balance when setting up equilibrium or titration problems, so the answer satisfies one equation but not the full system.
Candidates set up a single expression (e.g., Ka) and solve for one unknown, forgetting that in polyprotic or buffer systems the proton balance and electroneutrality constrain the result.
What to do instead
Write the full set: mass balance, charge balance, and all relevant equilibrium constants. For buffers, verify with the Henderson-Hasselbalch approximation and check that it is valid (ratio between 0.1 and 10, concentrations well above Ka).
Treating a titration endpoint as the equivalence point, or selecting an indicator by habit rather than by matching its transition range to the pH at equivalence.
The two terms are taught together and the visual color change is memorable, so candidates conflate the observed endpoint with the stoichiometric point and pick phenolphthalein for every titration.
What to do instead
Compute the pH at equivalence from the conjugate species formed, then choose an indicator whose transition range brackets that pH. For weak acid–strong base, expect a basic equivalence pH and a phenolphthalein-type indicator; for strong acid–strong base, any indicator spanning roughly pH 4–10 works.
Drawing a mechanism that violates valence or electron bookkeeping, such as a carbon with five bonds or a nucleophile attacking a saturated carbon without a leaving group.
Candidates memorize arrow-pushing patterns and apply them to substrates where the pattern does not fit, without checking formal charges and octet counts at each step.
What to do instead
After each arrow, count bonds and formal charges on every atom. Confirm the substrate has a leaving group or an accessible pi system, and confirm the nucleophile/electrophile labels match the actual electron flow. Redraw any step that breaks the octet rule.
Predicting reaction products without considering regiochemistry and stereochemistry (Markovnikov vs. anti-Markovnikov, syn vs. anti addition, SN1 vs. SN2 stereochemical outcome).
Candidates focus on whether a reaction occurs and stop there, missing that the exam often asks which isomer forms, and the options differ only in regiochemistry or configuration.
What to do instead
For every transformation, state the regiochemical rule and the stereochemical outcome explicitly before looking at options. Note whether the mechanism proceeds through a carbocation (racemization/rearrangement possible) or a concerted pathway (stereospecific), and check for rearrangements.
Applying a thermodynamic relation (e.g., ΔG = ΔH − TΔS, or Clausius-Clapeyron) outside its assumptions, such as assuming ΔH and ΔS are temperature-independent over a wide range.
These equations are compact and easy to recall, so candidates plug in values without checking whether constant heat capacity or ideal behavior is implied.
What to do instead
State the assumptions before substituting: constant ΔH/ΔS over the interval, ideal gas behavior, reversible path. If the range is wide or a phase change intervenes, integrate heat capacities or split the path. Check the sign of ΔG against spontaneity as a sanity test.
Confusing kinetics with thermodynamics, e.g., concluding a reaction is fast because it is thermodynamically favorable, or reading an activation energy off a ΔG diagram.
Both are plotted on energy diagrams and both use the word 'energy', so candidates misread the axes and treat the reaction coordinate diagram as a free-energy diagram.
What to do instead
Label every diagram: is the vertical axis Gibbs energy or potential energy along the reaction coordinate? Thermodynamics answers 'will it go'; kinetics answers 'how fast'. Rate depends on Ea and the Arrhenius prefactor, not on ΔG alone.
Treating enzyme kinetics as simple first-order chemistry, ignoring saturation, cooperativity, or the distinction between Km and substrate affinity under non-Michaelis-Menten conditions.
Introductory treatments emphasize the Michaelis-Menten equation, and candidates apply it to allosteric enzymes or inhibitor cases where the model does not hold.
What to do instead
First classify the system: Michaelis-Menten, allosteric (sigmoidal, use Hill), or inhibited (competitive raises apparent Km, uncompetitive lowers both, noncompetitive lowers Vmax). Confirm the plot shape matches the model before extracting constants.
Carrying out stoichiometric or yield calculations in industrial problems without accounting for recycle streams, limiting reagent, selectivity, or conversion, so the calculated reactor output does not match the process.
Academic problems usually give clean stoichiometry, while industrial problems embed conversion and selectivity; candidates compute theoretical yield and stop, missing that only converted feed proceeds and byproducts consume material.
What to do instead
Draw the process flow with recycle, define conversion (reacted/fed), selectivity (desired/product total), and yield (desired/fed), and do a balance around each unit. Use the limiting reagent from the actual feed composition, then verify the overall atom balance closes.