AEPA Chemistry NT306 common mistakes
AEPA Chemistry NT306 · Understand the traps and how to avoid them.
Treating a calculation result as the answer without propagating units or checking significant figures, so the final numeric choice is off by a factor tied to a unit conversion (e.g., mL vs L, kJ vs J).
Candidates rush to match a number to an answer choice and stop; they select the option whose digits match their raw calculation even when the magnitude or units are wrong.
What to do instead
Carry units through every step and cancel them explicitly; before choosing, confirm the answer's units and magnitude are physically sensible (e.g., molarity in mol/L, not mol/mL).
Miscounting significant figures when multiplying/dividing versus adding/subtracting, or ignoring sig-fig rules when reporting a computed value.
The two rule sets are often conflated, and candidates apply the addition rule (decimal places) to multiplication problems or vice versa.
What to do instead
State the rule aloud before computing: multiplication/division uses the fewest sig figs; addition/subtraction uses the fewest decimal places. Apply the rule at the final step, not at intermediate steps.
Balancing a chemical equation by changing subscripts in a formula rather than coefficients, producing a 'balanced' equation with the wrong species.
Under time pressure, adjusting a subscript is faster and superficially makes atom counts match, and candidates don't re-verify that formulas stayed intact.
What to do instead
Balance only with coefficients; after balancing, re-read every formula and confirm no subscript changed. Then verify atom counts on both sides including polyatomic ions as units.
Identifying the limiting reactant by comparing given masses or moles directly instead of converting to moles and dividing by stoichiometric coefficients.
The larger mass or larger mole amount feels intuitively 'in excess', and candidates skip the coefficient division step.
What to do instead
Convert each reactant to moles, divide by its coefficient, and pick the smallest quotient as limiting. Always state the quotient for each reactant before concluding.
Confusing the sign convention for enthalpy and predicting exothermic/endothermic from the sign of ΔH incorrectly, or reversing the sign when a reaction is reversed.
Everyday language ('gives off heat') competes with the formal convention (ΔH negative for exothermic), and reversal rules are easy to invert.
What to do instead
Anchor on one rule: heat released to surroundings means ΔH < 0 (exothermic). When reversing a reaction, flip the sign of ΔH; when multiplying, scale ΔH by the same factor.
Assuming a reaction is spontaneous based only on the sign of ΔH, ignoring the entropy term and temperature dependence in ΔG = ΔH − TΔS.
Exothermic reactions are intuitively 'favorable', so candidates equate negative ΔH with spontaneity and overlook entropy-driven cases.
What to do instead
Build the four-case table (ΔH +/−, ΔS +/−) and use ΔG = ΔH − TΔS; explicitly note when temperature determines the sign and when the reaction is spontaneous only at high or low T.
Predicting reaction rate changes by confusing kinetic factors (temperature, catalyst, surface area, concentration) with thermodynamic favorability, e.g., claiming a catalyst increases yield or that a spontaneous reaction must be fast.
Rate and equilibrium are taught together, and candidates conflate 'how fast' with 'how far' or 'how favorable'.
What to do instead
For each factor, ask whether it changes the activation energy/pathway (kinetics) or the energy difference between reactants and products (thermodynamics). A catalyst lowers Ea and speeds both directions equally without shifting equilibrium.
Applying Le Châtelier's principle in the wrong direction, e.g., saying adding more reactant decreases product or that increasing pressure shifts toward the side with more gas moles.
The stress-response direction is easy to invert, and pressure/volume effects on gas equilibria are the least practiced.
What to do instead
State the stress and the system's response in one sentence before choosing: adding reactant shifts right; increasing pressure shifts toward fewer gas moles; temperature changes alter K itself. Check the balanced equation's gas-mole counts each time.
Writing electron configurations that violate the Aufbau order or Hund's rule, or placing the wrong number of electrons in d and f subshells for transition metals and ions.
The (n+l) filling order is memorized imperfectly, and ion configurations (removing from the highest n shell first) are commonly mishandled.
What to do instead
Use the diagonal filling diagram, then apply Hund's rule to degenerate orbitals. For cations, remove electrons from the highest principal quantum number shell first, and verify total electron count equals the charge-adjusted atomic number.
Misidentifying intermolecular forces and using them to predict boiling point or solubility without ranking force strength correctly (e.g., treating dipole–dipole as stronger than hydrogen bonding, or ignoring London dispersion for larger molecules).
Candidates match on polarity alone and forget that dispersion grows with polarizability/molecular size, which can dominate for large nonpolar molecules.
What to do instead
Rank forces: ion–dipole > hydrogen bonding > dipole–dipole > London dispersion. Then weigh molar mass/surface area for dispersion. Predict higher boiling point for stronger net forces and 'like dissolves like' for solubility.