GTLE Subject Specialism Chemistry common mistakes
GTLE Subject Specialism Chemistry · Understand the traps and how to avoid them.
Writing electron configurations for transition-metal cations by removing electrons from the 4s orbital last (e.g., giving Fe2+ as [Ar]3d4 4s2 instead of [Ar]3d6), and treating 4s as always lower in energy than 3d.
Candidates memorize the filling order (4s before 3d) from neutral-atom aufbau diagrams and assume that order persists on ionization. In fact, once 3d is occupied, the 4s electrons are higher in energy and are removed first; the misconception is reinforced by simplified textbook diagrams that never distinguish filling order from ionization order.
What to do instead
Adopt a two-step rule: build the neutral atom with the aufbau order, then ionize by removing from the highest-n principal quantum level first (4s before 3d). Practice with Fe, Co, Ni, Cu, and their 2+ and 3+ ions, and check each against the known anomalous cases (Cr [Ar]3d5 4s1, Cu [Ar]3d10 4s1).
Ranking bond angles and molecular polarity by counting electron domains but ignoring lone-pair repulsion, or assigning a bent molecule as nonpolar because its bonds are individually polar.
Candidates apply VSEPR as a simple counting exercise and stop at the electron geometry, then treat bond dipoles as if they always cancel. They also confuse the electron-domain geometry with the molecular geometry, so H2O is reported as tetrahedral with 109.5° angles.
What to do instead
For every structure, draw the Lewis structure, state the electron-domain geometry, then the molecular geometry, then apply lone-pair > lone-pair > bond-pair repulsion to adjust angles. Sum bond dipoles as vectors and check symmetry explicitly: H2O and NH3 are polar because the dipoles do not cancel, CO2 and BF3 are nonpolar because they do. Practice predicting angles for CH4, NH3, H2O, and SF4 in one sitting.
Treating the limiting reagent as the reactant with the smallest number of moles, and stopping the calculation at moles of product instead of converting to mass, concentration, or volume.
Candidates learn 'smallest moles = limiting' as a shortcut that only works when stoichiometric coefficients are all 1. They also lose track of the final requested unit under time pressure and report an intermediate quantity as the answer.
What to do instead
Always divide moles of each reactant by its stoichiometric coefficient and compare those quotients; the smallest quotient identifies the limiting reagent. Then write a unit chain from moles of limiting reagent to moles of product to the requested unit (mass via molar mass, concentration via volume, gas volume via molar volume or ideal gas law). Do three mixed limiting-reagent problems per practice session and underline the requested unit before starting.
Applying the ideal gas law without converting temperature to kelvin, or using the wrong gas constant R for the pressure and volume units given.
Candidates plug numbers into PV = nRT from memory and use Celsius values directly, or use R = 8.314 with pressure in atm. The error is silent because the arithmetic still produces a plausible-looking number.
What to do instead
Before substituting, write down the units of every quantity and convert: T in K, P in the same unit as R (0.08206 L·atm·mol⁻¹·K⁻¹ for atm and litres, 8.314 J·mol⁻¹·K⁻¹ for SI). Include units in the calculation line so a mismatch is visible, and sanity-check the answer against a rough estimate.
Confusing ΔH with ΔG when deciding whether a reaction is spontaneous, or assuming an exothermic reaction is always spontaneous.
Candidates conflate enthalpy with free energy because both are 'energy changes' and both appear in Hess's law problems. They forget the −TΔS term, so they miss entropy-driven spontaneity and the temperature dependence of ΔG.
What to do instead
Use ΔG = ΔH − TΔS explicitly for every spontaneity question, and classify the four sign combinations (ΔH negative/positive, ΔS negative/positive) including the crossover temperature T = ΔH/ΔS. Keep ΔH, ΔS, and ΔG in separate columns in your working so the terms are never merged.
Writing equilibrium expressions with concentrations of pure solids or liquids included, or assuming the equilibrium position shifts whenever a catalyst is added.
Candidates memorize K = [products]/[reactants] without the heterogeneous-phase rule, and they confuse the rate effect of a catalyst with a change in equilibrium position. The two errors are common because both involve 'changing the reaction'.
What to do instead
State the rule explicitly each time: pure solids and pure liquids have constant activity and are omitted from K; catalysts change only the rate of approach to equilibrium, not the position. For Le Chatelier questions, identify which species appear in the K expression and predict the shift only for changes to those species, temperature, or pressure.
Calculating the pH of a weak acid by taking the square root of Ka times concentration without checking the approximation, or ignoring the autoionization of water for very dilute or very weak acids.
Candidates learn the simplified formula pH = ½(pKa − log C) and apply it to every weak acid, including cases where the degree of ionization is large or the concentration is near 10⁻⁷ M. The approximation fails silently because the answer still looks reasonable.
What to do instead
Always test the approximation: if the calculated [H+] is more than about 5% of the initial acid concentration, or if the acid is extremely weak or very dilute, solve the quadratic (or use the full charge balance). For buffer problems, use the Henderson–Hasselbalch equation and verify that the buffer ratio is between roughly 0.1 and 10.
Balancing redox equations in acidic or basic solution without adding H2O and H+ or OH− to balance oxygen and hydrogen, or reversing the sign of the cell potential when the half-reaction is reversed.
Candidates balance atoms and charge separately and forget that the half-reaction method requires explicit water and proton or hydroxide terms. They also treat E° as if it changes sign with the direction of the reaction, which it does not for the standard electrode potential of a couple.
What to do instead
Use a fixed half-reaction procedure: balance all atoms except O and H, balance O with H2O, balance H with H+, balance charge with electrons, then in basic solution add OH− to both sides and cancel. For cells, use E°cell = E°cathode − E°anode with both values taken as reduction potentials, and remember that reversing a half-reaction changes the sign of ΔG but not the tabulated E°.
Assigning R/S or E/Z configuration by rotating the molecule mentally without ranking substituents by Cahn–Ingold–Prelog priority, or drawing a mechanism that violates the octet rule or ignores carbocation stability.
Candidates rely on visual intuition for stereochemistry and skip the priority rules (atomic number, then mass number for isotopes, then duplicate atoms). In mechanisms, they move arrows without checking that electron flow is from nucleophile to electrophile and that intermediates are chemically reasonable.
What to do instead
For every stereocentre, rank the four groups explicitly by CIP rules, then orient the lowest-priority group away and read the remaining three. For mechanisms, draw every arrow from an electron-rich site to an electron-poor site, label each intermediate, and check carbocation stability (tertiary > secondary > primary) and octet compliance before finalizing.
Predicting coordination-complex geometry and colour by memorizing a few examples rather than applying crystal-field splitting, and assuming all transition-metal complexes are octahedral and high-spin.
Candidates see coordination chemistry as a list of facts to memorize and do not connect ligand field strength, d-electron count, and spin state. They also confuse coordination number with oxidation state when naming complexes.
What to do instead
For each complex, determine the oxidation state of the metal, the d-electron count, the coordination number, and the geometry, then place electrons in the split d orbitals using the spectrochemical series to decide high- or low-spin. Practice naming and drawing [Fe(CN)6]4−, [Fe(H2O)6]2+, [Cu(NH3)4]2+, and [Ni(CO)4] to cover low-spin, high-spin, square planar, and tetrahedral cases.