IB Chemistry HL common mistakes
IB Chemistry HL · Understand the traps and how to avoid them.
When drawing Lewis structures or predicting molecular geometry, the candidate places the lone pairs on the central atom but then counts them as bonding regions when assigning VSEPR shape, or omits lone pairs entirely and reports a linear or trigonal planar shape for a species that is bent or trigonal pyramidal.
VSEPR is taught as a shape-naming exercise, so candidates memorise '4 electron domains = tetrahedral' and stop reading the structure. Lone-pair repulsion is treated as a vocabulary item rather than a structural fact, and under exam pressure the candidate jumps from formula to shape without drawing the electron-domain count first.
What to do instead
Adopt a fixed three-line working: (1) count total valence electrons, (2) draw the Lewis structure and mark each lone pair, (3) count bonding domains plus lone pairs on the central atom, then apply the lone-pair correction to the ideal shape. Practise on species where the two answers differ (H2O vs CH4, NH3 vs BF3, SO2 vs CO2) and always write the electron-domain geometry next to the molecular geometry so the distinction is visible.
The candidate calculates enthalpy changes of reaction using average bond enthalpies for a process involving a change of state or a substance whose bonds are not all in the gas phase, and reports an answer without noting that the value is an estimate.
Bond-enthalpy questions look like simple subtraction, so the candidate applies the 'bonds broken minus bonds formed' recipe mechanically. The textbook caveat that average bond enthalpies refer to gaseous species and ignore intermolecular forces is read once and forgotten, and the exam's instruction to comment on the accuracy of the value is skipped because the candidate is focused on the arithmetic.
What to do instead
Before using bond enthalpies, check every species is gaseous; if a liquid or solid appears, state that the calculation ignores the enthalpy of vaporisation or fusion. Then deliberately write one sentence comparing the bond-enthalpy result with a value from standard enthalpies of formation, explaining the direction and rough size of the discrepancy. Build this sentence into your standard answer template so it is never omitted.
In titration-based internal assessment work, the candidate reports a mean titre by averaging all concordant runs including an obvious outlier, or declares runs concordant when they differ by more than the stated tolerance, and calculates percentage uncertainty using the mean titre instead of the burette reading.
Candidates conflate 'concordant' with 'close enough' and want to use all their data to look thorough. The uncertainty rule is also misapplied because the candidate takes the smallest scale division rather than the reading uncertainty of a difference between two burette readings, and the internal assessment rubric rewards correct propagation, so this single error cascades into the evaluation criterion.
What to do instead
Set a written concordance rule before collecting data (for example, within 0.10 cm3 of each other) and apply it explicitly, listing which runs were rejected and why. For uncertainty, treat each burette reading as +/- half the smallest division and combine the two readings, then divide by the mean titre; show the formula and the substitution. Rehearse this on past titration data sets so the arithmetic and the justification become automatic.
When answering an Option question on a topic such as medicinal chemistry or energy, the candidate gives a generic answer about 'the drug binding to a receptor' or 'the fuel releasing energy' without naming the specific structural feature, functional group, or numerical quantity the question asked for.
Option content is often studied last and in less depth, so the candidate relies on general chemistry intuition. Mark schemes for Options are typically point-specific, rewarding named functional groups, named interactions (hydrogen bonding, ion-dipole, hydrophobic), or a calculated value with units, so a plausible-sounding but unnamed answer earns nothing.
What to do instead
Build a one-page summary per Option subtopic listing the exact terms the syllabus names (for example, the functional groups in a given drug class, the types of intermolecular interaction at the binding site, the definition of a specified energy quantity). When practising, mark your own answers against the command word and count how many named points you produced; rewrite until you hit the expected number of marking points.
The candidate writes a conclusion in the internal assessment that restates the hypothesis as confirmed, without comparing the experimental value to a literature value with an uncertainty, and without identifying a specific systematic error and its direction of effect.
School practical reports often reward a tidy conclusion, so candidates carry over the habit of declaring success. The IA criteria demand evaluation of the methodology against the research question, and the candidate has not been taught to distinguish random from systematic error or to reason about the sign of a systematic shift.
What to do instead
Use a fixed conclusion-evaluation structure: state the experimental result with its absolute and percentage uncertainty, compare with the literature or theoretical value, state whether the difference is within uncertainty, then name at least two specific sources of error, classify each as random or systematic, and state the direction in which each would shift the result. Practise on your own data before submission so the reasoning is grounded in real numbers.
When asked to explain a trend in reactivity or a mechanism, the candidate describes what happens ('the nucleophile attacks the carbon') without identifying the attacking species, the bond broken, the intermediate formed, and the stereochemical or electronic reason for the outcome.
Candidates learn mechanisms as pictures and reproduce the picture in words, losing the causal chain. The command word 'explain' is read as 'describe', and the candidate does not realise that marks are attached to the named species and the electronic justification rather than the sequence of arrows.
What to do instead
For every mechanism, write a four-part sentence: the nucleophile or electrophile with its charge or lone pair, the bond that breaks and the atom that keeps the electrons, the intermediate or transition state, and the stereochemical consequence with its reason. Do this for SN1 and SN2 on the same substrate so the contrast forces you to name the rate-determining step and the reason for inversion or racemisation.
The candidate calculates the pH of a weak acid or a buffer by using the strong-acid formula or by neglecting the approximation's validity, and reports a pH without checking whether the degree of dissociation justifies the assumption.
The strong-acid calculation is learned first and becomes the default. The weak-acid treatment requires setting up an equilibrium expression and then testing whether x is small compared with the initial concentration; candidates skip the test because it feels like extra work and because the syllabus often presents the simplified formula without emphasising its conditions.
What to do instead
Always write the Ka expression, define x as the extent of dissociation, and then explicitly test x as a percentage of the initial concentration before simplifying. If the percentage exceeds roughly five per cent, solve the quadratic or state that the approximation fails. Apply the same discipline to buffers using the Henderson-Hasselbalch form and check the ratio of conjugate base to acid is within the range where the buffer is effective.
When asked to interpret a graph or data set in a practical-skills question, the candidate reads off a value without stating the uncertainty, ignores the units on the axes, or draws a line of best fit through every point including outliers rather than a trend line.
Data-handling questions are treated as reading comprehension rather than measurement. The candidate has not internalised that a reported quantity without units or uncertainty is incomplete, and graph-drawing is practised for presentation rather than for extracting a gradient with its uncertainty.
What to do instead
Adopt a checklist for every graph or table: axes labelled with quantity and unit, uncertainty stated for each reported value, line of best fit drawn to show trend rather than to touch all points, and gradient calculated from two widely separated points on the line with the uncertainty estimated from the spread of acceptable lines. Practise on unfamiliar data so the checklist is applied under time pressure.
The candidate answers a 'deduce' or 'suggest' question by recalling a memorised fact instead of applying the data or structure given in the question, and so produces an answer that is true in general but wrong for the specific case presented.
Revision rewards recall, and candidates default to retrieval when they see a familiar-looking context. The command words 'deduce' and 'suggest' signal that the answer must follow from the information supplied, but the candidate does not distinguish them from 'state' or 'define', and so ignores the structure, spectrum, or data table in the stem.
What to do instead
Underline the command word and the supplied data before writing. For 'deduce', trace each step from the given information and write the reasoning line, not just the conclusion. For 'suggest', propose a chemically justified answer and name the evidence in the stem that supports it. Practise by covering the mark scheme and writing the deduction chain, then comparing the number of reasoning steps with the marks available.
The candidate answers a question about the periodic trend in ionisation energy or atomic radius by stating the trend across a period or down a group without addressing the named exception or the anomaly the question highlights, such as the dip between groups 2 and 13 or between 15 and 16.
The general trend is taught as a rule, and the exceptions are taught as trivia to be memorised separately. When a question asks specifically about an anomaly, the candidate recites the general trend and assumes it covers the case, missing the marks attached to the sub-shell and electron-repulsion explanation.
What to do instead
For each trend, prepare a two-column note: the general trend with its nuclear-charge and shielding explanation, and the named anomalies with their sub-shell or electron-pair-repulsion explanation. When answering, check whether the question names a specific element or pair of elements; if so, address the anomaly explicitly and explain it in terms of the electron configuration and the relevant repulsion or penetration effect.