AP Intermediate Chemistry (BIEAP) common mistakes
AP Intermediate Chemistry (BIEAP) · Understand the traps and how to avoid them.
Writing the electron configuration of transition-metal cations by removing electrons from the 4s orbital last (e.g. Fe2+ as [Ar]3d4 4s2) instead of removing 4s before 3d.
Students memorise 'fill 4s before 3d' and apply the same order to ionisation, but 4s rises above 3d in energy once the 3d shell is occupied, so the 4s electrons are lost first. The aufbau filling rule is confused with the removal rule.
What to do instead
Derive the configuration of the neutral atom, then remove electrons from the highest principal quantum number shell first. Practise Fe, Fe2+, Fe3+, Cu, Cu+, Cr3+ and Mn2+ until the pattern is automatic, and always write the d count with the orbital diagram to check for half-filled stability.
Predicting ionic radius, ionisation enthalpy and electronegativity trends across a period or down a group using a single rule, then being surprised by the anomalies (e.g. claiming ionisation enthalpy always increases from left to right, so O > N).
The trend is learned as a slogan; the underlying reason (effective nuclear charge, shielding, and for N the extra stability of a half-filled 2p3) is not connected to the trend, so exceptions are treated as errors to memorise rather than consequences to derive.
What to do instead
For every anomaly, write the valence configuration of the species involved and state the electron being removed or added. Explain N vs O, Be vs B, Mg vs Al, and the Ga/In/Tl inert-pair effect from configuration and shielding. Practise ranking sets of species by radius and ionisation enthalpy with a one-line justification each.
Drawing a Lewis structure, stopping at the first structure that balances atoms, and never computing formal charge or checking for expanded octet, resonance and lone-pair placement; then predicting wrong geometry or bond order.
Students equate 'all atoms joined and octets filled' with 'correct structure' and skip the validation step. Geometry is then assigned from the wrong number of electron domains, and resonance is ignored when comparing bond lengths.
What to do instead
Adopt a fixed procedure: count valence electrons, build the skeleton, distribute, compute formal charge on every atom, choose the structure with charges closest to zero and negative charge on the more electronegative atom, then count sigma bonds and lone pairs to get VSEPR geometry and hybridisation. Apply it to CO2, SO2, SO4 2-, NO3-, PCl5 and XeF4 and state the bond order from resonance.
Substituting numbers into the ideal gas law without converting to SI (using °C instead of K, mL instead of L, or g instead of mol), or applying the ideal equation to a gas whose behaviour is clearly non-ideal without comment.
Gas problems are treated as formula recall rather than unit bookkeeping; the 273.15 conversion and the molar-mass step are skipped under time pressure, and the conditions under which compressibility factor Z deviates from 1 are never linked to the equation.
What to do instead
Write a standard data block (P in Pa or atm, V in L, T in K, n in mol) before any calculation, and convert at the start. For non-ideal questions, use Z = PV/nRT and the van der Waals equation, and state whether the deviation arises from molecular volume (a) or intermolecular attraction (b). Practise one Boyle, one Charles, one combined and one Dalton partial-pressure problem with explicit unit conversion each.
Confusing ΔH with ΔU and applying Hess's law or bond-enthalpy arithmetic without accounting for stoichiometry and phase, e.g. using ΔH = ΔU + ΔnRT with the wrong sign or omitting the enthalpy of vaporisation when water changes state.
The relationship between heat at constant pressure and constant volume is memorised as a formula without the sign convention for work done by the system; bond-energy sums are done per bond rather than multiplied by the number of bonds, and phase changes are ignored.
What to do instead
For every thermochemistry problem, list the phases of all reactants and products and include Δvap or Δfus where a state changes. Derive ΔH = ΔU + Δn(g)RT and check the sign against whether gas moles increase. Compute bond-enthalpy problems by writing bonds broken minus bonds formed, each multiplied by its count. Verify with a worked example such as combustion of methane.
Writing an ICE table for a weak acid or buffer but then applying the full initial concentration in the Ka expression, or treating a sparingly soluble salt as fully dissociated when computing Ksp.
The small-x approximation is applied without checking whether x is truly negligible relative to the initial concentration, and the definition of Ksp as the product of ion concentrations at saturation is confused with complete dissolution.
What to do instead
Always write the equilibrium expression with the equilibrium (not initial) concentrations, then test the approximation by comparing x to the initial concentration; if x exceeds about 5 percent of it, solve the quadratic. For Ksp, set up the ion ratio from the formula, and remember common-ion effects shift solubility. Work through acetic acid, an ammonia buffer, and AgCl in water versus in NaCl solution.
Balancing redox equations in acidic or basic medium by inspection and forgetting to add H+ or OH- and water, or mis-assigning oxidation numbers in species such as H2O2, Cr2O7 2- and S2O3 2-.
The half-reaction method is known in outline but the atom and charge balance checks are skipped; oxidation numbers are assigned by position in a formula rather than by electronegativity and rules for oxygen and hydrogen in peroxides and hydrides.
What to do instead
Use the half-reaction method in a fixed sequence: assign oxidation numbers, split into oxidation and reduction halves, balance atoms other than O and H, balance O with H2O, balance H with H+, balance charge with electrons, equalise electrons, add, then convert to basic medium by adding OH- to both sides and cancelling water. Verify that both atoms and charge balance. Practise MnO4- with Fe2+ and Cr2O7 2- with I-.
Treating reaction order as equal to the stoichiometric coefficients, or using the integrated rate law for the wrong order when a half-life or concentration-time data set is given.
The difference between molecularity of an elementary step and experimentally determined order is not internalised, so the balanced equation is read as the rate law; the distinctive signatures of zero-, first- and second-order kinetics (constant rate, constant half-life, half-life proportional to initial concentration) are not linked to the data.
What to do instead
For any data set, test the orders: plot or compare rate versus concentration, check whether half-life is constant (first order) or doubles as concentration halves (second order). State explicitly that order comes from experiment and only matches coefficients for a single elementary step. Practise one zero-, one first- and one second-order problem plus an Arrhenius calculation using ln(k2/k1) = (Ea/R)(1/T1 - 1/T2).
Writing the formula of a coordination compound from its name without checking the charge balance of the complex ion, or predicting the number of ions and the magnetic behaviour without counting d electrons and identifying the ligand field.
The naming rules (ligands alphabetical, oxidation state in Roman numerals) are memorised separately from the charge arithmetic, so the complex ion charge is never verified; magnetic moment is guessed from the metal rather than from the d-electron count and whether the ligand is strong or weak field.
What to do instead
After writing any complex, sum the charges of metal and ligands and confirm it equals the stated ion charge. For magnetism, write the d configuration of the metal ion, decide low-spin or high-spin from the ligand, count unpaired electrons, and compute the spin-only moment with the square-root-of-n(n+2) formula. Practise [Fe(CN)6]4-, [Fe(H2O)6]2+, [Ni(CO)4] and [Co(NH3)6]Cl3.
Naming or drawing an organic structure without applying the priority rules: choosing the longest chain incorrectly because a substituent or functional group is attached, numbering from the wrong end, or ignoring E/Z and R/S when stereochemistry is specified.
IUPAC rules are applied piecemeal: the principal functional group, the longest carbon chain containing it, lowest locants, and alphabetical substituent order are not treated as a strict sequence; cis/trans and E/Z are used interchangeably without the Cahn-Ingold-Prelog priority comparison.
What to do instead
Follow a fixed naming checklist: identify the principal functional group, select the parent chain containing it, number to give the principal group the lowest locant, list substituents alphabetically with locants, then assign stereochemistry by CIP priority at each stereocentre or double bond. Reverse the checklist when drawing from a name. Drill with a substituted carboxylic acid, an alkene with two substituents, and a chiral haloalkane.