IB Chemistry Study Guide: HL and SL, Internal Assessment, and the Hardest Topics

IB Chemistry combines the breadth of a comprehensive chemistry curriculum with the analytical depth of a research-oriented course. The subject rewards students who understand chemistry as a connected discipline rather than a set of isolated topics — thermodynamics, kinetics, equilibrium, and organic chemistry all draw on the same foundational concepts of electron density, bond polarity, and energy change.

The IB's emphasis on data analysis and experimental reasoning means that even at Grade 7, marks are earned more through applied reasoning than memorisation. Every exam paper contains questions about unfamiliar data that require chemical thinking rather than content recall.

Structure and bonding

Electron configuration and periodic trends: Electron configuration (s, p, d, f orbitals; Aufbau principle, Pauli exclusion, Hund's rule). The periodic trends follow from electron configuration and effective nuclear charge (Zeff): atomic radius decreases across a period (increasing Zeff, same shell), increases down a group (additional shells). Ionisation energy increases across a period (greater Zeff for each successive electron), decreases down a group (outer electrons further from nucleus, more shielding).

Bonding types and their properties: Ionic bonding (electrostatic attraction between oppositely charged ions — high melting point due to lattice energy, conducts when molten or dissolved). Covalent bonding (shared electron pairs — properties vary with molecular structure and intermolecular forces). Metallic bonding (delocalised electrons in a lattice — high conductivity, malleability, luster).

VSEPR and molecular shape: Electron pairs (bonding and lone pairs) repel each other. Lone pairs exert more repulsion than bonding pairs. Know the shapes and bond angles for 2–6 electron pairs around central atoms: linear (180°), trigonal planar (120°), tetrahedral (109.5°), trigonal bipyramidal (120°/90°), octahedral (90°). Lone pairs reduce bond angles: water (2 bond pairs, 2 lone pairs → bent, 104.5°), ammonia (3 bond pairs, 1 lone pair → trigonal pyramidal, 107°).

Hybridisation (HL): sp³ = 4 sigma bonds (tetrahedral, 109.5°); sp² = 3 sigma bonds + 1 pi bond (trigonal planar, 120°, double bond); sp = 2 sigma bonds + 2 pi bonds (linear, 180°, triple bond). Sigma bonds allow rotation; pi bonds do not (hence cis-trans isomerism in alkenes).

Energetics and thermodynamics

Enthalpy: ΔH°reaction = Σ[ΔH°f(products)] − Σ[ΔH°f(reactants)]. Hess's Law: path-independent enthalpy change.

Entropy (ΔS): A measure of disorder. ΔS > 0 when: more gas moles are produced, a solid dissolves, a liquid vaporises. ΔS < 0 when: fewer gas moles, a gas is absorbed, order increases.

Gibbs free energy: ΔG = ΔH − TΔS. Spontaneous when ΔG < 0. A reaction can be: spontaneous at all T (ΔH < 0, ΔS > 0), non-spontaneous at all T (ΔH > 0, ΔS < 0), spontaneous only at high T (ΔH > 0, ΔS > 0 — entropy term TΔS dominates), or spontaneous only at low T (ΔH < 0, ΔS < 0 — enthalpy term dominates).

At equilibrium: ΔG = 0 → ΔH = TΔS. This gives the temperature at which equilibrium is reached for reactions with known ΔH and ΔS.

Kinetics and equilibrium

Rate laws and orders: Rate = k[A]^m[B]^n. Determine orders from experimental data: if doubling [A] doubles rate, reaction is first order in A. Half-life for first-order reactions: t½ = ln2/k = 0.693/k. Arrhenius equation: k = Ae^(−Ea/RT), or ln k = −Ea/R × (1/T) + ln A. Plot ln k vs 1/T: slope = −Ea/R.

Equilibrium: Kc = [products]^stoich / [reactants]^stoich (excluding pure solids and liquids). Relationship between Kc and Kp: Kp = Kc(RT)^Δn. Solubility product Ksp. Acid-base equilibrium: Ka × Kb = Kw = 10^(−14) at 25°C.

ICE tables: As covered in other guides — systematic approach to finding equilibrium concentrations from initial conditions and the equilibrium constant.

Organic chemistry: functional groups and mechanisms

The organic reaction types you must know:

• Nucleophilic substitution (SN1: favoured by tertiary carbons, polar protic solvents; SN2: favoured by primary carbons, polar aprotic solvents, gives inversion of configuration — Walden inversion)
• Electrophilic addition to alkenes (addition of HBr — Markovnikov; addition of Br₂ — bromonium ion mechanism)
• Elimination (E1 and E2 — produce alkenes from haloalkanes or alcohols)
• Electrophilic aromatic substitution (nitration of benzene — nitronium ion NO₂⁺ as electrophile)
• Condensation: esterification, amide formation, polymerisation (polyesters — PET, polyamides — nylon)
• Hydrolysis: reverse of condensation

Spectroscopy: Mass spectrometry (molecular ion peak = M⁺ = relative molecular mass; fragmentation pattern — identify common fragments); IR spectroscopy (identify functional groups from absorption frequencies: O-H broad ~3200–3500, C=O sharp ~1700, N-H ~3300); NMR spectroscopy (¹H NMR: chemical shift indicates electronic environment; splitting pattern follows n+1 rule; integration indicates number of H atoms).

Internal Assessment strategy

The IA constitutes 20% of the final grade and is entirely within your control. The highest-scoring IAs combine a clear, specific research question with rigorous quantitative data collection, appropriate statistical analysis, and a critical evaluation of methodology.

Data collection quality: Collect enough data for meaningful statistics (minimum 5 data points per condition, 3 trials per data point for averaging). Record all raw data with appropriate significant figures and units. Calculate uncertainties for all measurements (absolute uncertainty = ± half the smallest division for analogue instruments; ± the calibration uncertainty for digital instruments).

Graphing: Use appropriate graph types (scatter plots for continuous variables, bar graphs for discrete categories). Include error bars (± one standard deviation or ± uncertainty). Draw line of best fit only when appropriate. Calculate the gradient of a linear graph with uncertainty (draw max and min gradients through error bars).

Use the Cornell Notes Tool for organic reaction mechanism summaries and the Spaced Repetition Flashcard Tool for periodic trends, formula definitions, and equilibrium expressions. See the IB Biology study guide for the equivalent strategy on the other major IB science.