Energetics is a core part of IB Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Hess's lawΔH path-independent (state function)
Energy cycleDifferent routes to same state
Bond enthalpyEnergy to break 1 mol of bonds (g)
Mean bond enthalpyAverage over molecules
Bonds brokenReactant side; +
Bonds madeProduct side; -
Lattice energyEnergy to break ionic lattice (g)
Born-Haber cycleHess cycle for ionic compounds
Entropy SMeasure of disorder (J/K/mol)
ΔSIncreases for gas formation, dissolution
ΔG = ΔH − TΔSSpontaneous if ΔG < 0
Standard ΔS°Tabulated entropy values
Enthalpy HHeat content at constant p
ΔHEnthalpy change
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
When you reverse a reaction in a Hess cycle, what happens to ΔH?✗ ΔH stays the same because it's the same reaction. ✓ Its sign reverses — if the forward reaction is +100 kJ/mol, the reverse is -100 kJ/mol.
What kind of bond forms between a ligand and a central metal ion in a complex?✗ The ligand forms an ionic bond with the metal because the metal is positively charged. ✓ A dative (coordinate) covalent bond — the ligand donates a lone pair into an empty orbital on the metal.
Why can an endothermic reaction still be spontaneous?✗ Endothermic reactions are never spontaneous because they require energy input. ✓ If the entropy increase (TΔS) is large enough to outweigh the positive ΔH, ΔG can still be negative.
What state are the ions in when defining lattice enthalpy of formation?✗ The ions are in aqueous solution before forming the lattice. ✓ Gaseous ions — lattice enthalpy is the energy change when one mole of solid ionic compound forms from its constituent ions in the GAS phase.
For a process to be spontaneous, what must happen to the total entropy of the universe?✗ The entropy of the system must increase for the process to be spontaneous. ✓ It must increase (ΔS_total > 0) — the system's entropy alone can decrease as long as the surroundings' entropy increases more.
Practise Energetics — free games
Test yourself with these quick revision games for this topic:
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