Energetics & Thermodynamics is a core part of A-Level Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Enthalpy change ΔHHeat energy change at constant pressure; negative for exothermic, positive for endothermic.
Standard enthalpy of formationΔH_f°, enthalpy change when 1 mole of compound forms from its elements in standard states.
Standard enthalpy of combustionΔH_c°, enthalpy change when 1 mole of substance is completely burned in oxygen under standard conditions.
Bond enthalpyAverage energy required to break 1 mole of a particular covalent bond in the gas phase.
CalorimetryExperimental measurement of heat change using q = mcΔT, often with a polystyrene cup or bomb calorimeter.
Standard conditions100 kPa pressure, a stated temperature (usually 298 K) and 1 mol dm⁻³ for solutions.
Enthalpy ΔHHeat change at constant pressure (kJ/mol).
ExothermicReleases heat; ΔH negative.
EndothermicAbsorbs heat; ΔH positive.
Hess's lawΔH independent of route taken.
Standard state298 K, 100 kPa, pure substance.
ΔHfEnthalpy of formation from elements.
Gibbs energy ΔGΔH - TΔS; <0 means feasible.
FeasibilityThermodynamic ability for reaction to occur.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
In an exothermic reaction, what happens energetically with the bonds?✗ Bond breaking releases energy and bond making absorbs energy. ✓ Energy released forming new bonds exceeds energy absorbed breaking existing bonds.
At room temperature and pressure, what volume does 1 mole of any gas occupy?✗ Heavier gases occupy a smaller volume per mole because their particles are denser. ✓ Approximately 24 dm^3 (24,000 cm^3), regardless of the identity of the gas.
Why does Hess's Law work?✗ Hess's Law works because energy always cancels out around any cycle. ✓ Enthalpy is a state function — change depends only on initial and final states, not the route taken.
How do alkenes typically react with bromine?✗ Bromine substitutes for a hydrogen on the alkene, releasing HBr. ✓ By electrophilic addition — bromine adds across the double bond to form a dibromoalkane.
What are the units of Kc for a reaction?✗ Kc always has units of mol dm^-3. ✓ They depend on Δn (the difference between moles of gaseous/aqueous products and reactants) — Kc can be dimensionless or have units like mol dm^-3.
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