BSSE Chemistry 1429 common mistakes
BSSE Chemistry 1429 · Understand the traps and how to avoid them.
Assigning electron configurations by filling orbitals in strict left-to-right order without applying the n+l (Aufbau) rule, so 4s is filled before 3d in some cases but 4f/5d ordering is scrambled, and Hund's rule is ignored within the p and d subshells.
Students memorize a single diagonal-arrow diagram and never internalize that subshell energies are determined by n+l and that degenerate orbitals fill singly before pairing; exam items that reorder the subshells or ask about exceptions expose the rote pattern.
What to do instead
For every configuration, write the subshells with their n+l values beside them, sort by n+l (lower n first on ties), then apply Hund's rule by drawing orbital boxes. Practice the Cr and Cu exceptions explicitly and state the reason (extra exchange energy and half-filled/fully filled stability).
Drawing a Lewis structure, then counting electron domains from the drawn bonds and lone pairs on the central atom only, ignoring lone pairs that change the geometry, so a molecule like a tetrahedral electron-domain species is reported as tetrahedral when the shape is actually bent or trigonal pyramidal.
Students conflate electron-domain geometry with molecular shape and forget that lone pairs occupy space and repel more strongly than bonding pairs, compressing bond angles.
What to do instead
Use a fixed two-step ritual: count all electron domains (bonds + lone pairs) for the electron-domain geometry, then subtract lone pairs to name the molecular shape. Always state predicted bond angles and note the lone-pair compression (for example, less than 109.5 degrees).
Ranking boiling points by molecular weight alone, ignoring hydrogen bonding and dispersion effects, and treating all hydrogen bonds as equal regardless of N-H, O-H, or F-H donor strength.
Students memorize 'bigger means higher boiling point' and do not connect intermolecular force type to the specific functional groups or geometry (for example, why a branched isomer boils lower than its straight-chain isomer).
What to do instead
For each comparison, list the force types present (dispersion, dipole-dipole, hydrogen bonding) and identify the donor and acceptor atoms. Then weigh them in order: hydrogen bonding dominates, then dipole-dipole, then dispersion scaled by surface area and polarizability. Justify branching and shape effects explicitly.
Treating the limiting reactant as the species with the smallest number of moles rather than the one that runs out first per stoichiometric ratio, and skipping unit conversion between grams, moles, and solution volume.
Students shortcut the stoichiometric comparison and forget that coefficients define the ratio, so a reactant with fewer moles can still be in excess. Dimensional analysis is often done loosely, hiding unit errors.
What to do instead
Convert every quantity to moles first, divide each by its stoichiometric coefficient, and compare those quotients; the smallest is limiting. Always carry units through every line and cancel them explicitly, ending with a percent-yield or mass check.
Computing enthalpy changes without tracking sign conventions, adding bond-formation and bond-breaking energies with the wrong sign, and ignoring that Hess's law requires reversing the sign when a reaction is reversed.
Students memorize 'bonds broken minus bonds formed' without understanding it is the reverse of the standard formation convention, and they treat enthalpy as an unsigned quantity.
What to do instead
Adopt one consistent convention and state it at the top of each solution: for formation enthalpies, products minus reactants; for bond energies, broken minus formed. When reversing an equation, flip the sign of its enthalpy on the same line. Sanity-check whether the sign matches whether the reaction feels exothermic or endothermic.
Writing equilibrium expressions that include pure solids or liquids, or plugging initial concentrations into K instead of equilibrium concentrations, and applying the acid dissociation constant to a strong acid.
Students copy the balanced equation mechanically without recalling that constant-activity phases are omitted, and they do not distinguish the ICE table's initial row from its equilibrium row.
What to do instead
Before writing K, cross out any species that is a pure solid or pure liquid. Build a full ICE table and only substitute the equilibrium row into K. For acids and bases, first classify strong versus weak; strong species dissociate completely and need no Ka.
Reading a rate from a concentration-time graph by taking the slope between two arbitrary points rather than the tangent at the specified time, or confusing average and instantaneous rate, and misreading reaction order from units of k.
Students treat all slopes as equivalent and do not link the units of the rate constant to the overall order, so they misidentify first- versus second-order behavior.
What to do instead
For instantaneous rate, draw a tangent at the stated time and take its slope; for average rate, use the two given points. Memorize the k-unit signature (per second for first order, per molar per second for second order) and verify the order by checking which integrated plot is linear.
Balancing redox half-reactions in acidic or basic solution without adding water and hydroxide to neutralize, and computing cell potential without reversing the sign convention for the anode or accounting for n in the free-energy relation.
Students balance atoms and charge in acid but forget the basic-medium conversion step, and they mix up oxidation potential with reduction potential signs.
What to do instead
Balance each half-reaction in acid first, then for basic conditions add hydroxide to both sides to neutralize hydrogen ions and cancel waters. Write E-cell as E-cathode minus E-anode using reduction potentials only, and use delta-G equals negative nFE with n taken from the balanced electron count.
Naming or drawing organic structures without identifying the principal functional group and the longest carbon chain containing it, and ignoring stereochemistry when a chiral center is present.
Students apply IUPAC rules in arbitrary order and default to the longest chain regardless of whether it contains the functional group, and they treat wedges and dashes as decoration.
What to do instead
Follow a fixed priority: identify the principal functional group, choose the longest chain containing it, number to give it the lowest locant, then list substituents alphabetically. For any carbon with four different groups, assign R/S explicitly using the priority rules and note whether the drawing is a specific stereoisomer.
Reporting a measured result with more significant figures than the instrument supports, or omitting the uncertainty and the correct treatment of outliers when averaging repeated trials.
Students copy calculator displays verbatim and treat precision as a cosmetic afterthought, so they cannot judge whether a discrepancy is real or within instrument error.
What to do instead
Record the instrument's smallest division and report results to that precision, propagating uncertainty through calculations. Average replicate trials, compute the spread, and state whether a suspected outlier falls outside the expected range before discarding it. Report percent error against the accepted value.