Chapter summary
Chemical Energetics, at a glance
Track energy through bonds and cycles, then combine enthalpy and entropy to assess thermodynamic feasibility.
A-Level 9476 (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Standard enthalpy change of formation, ΔHf° | The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions (298 K, 100 kPa). |
| Standard enthalpy change of combustion, ΔHc° | The enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions (298 K, 100 kPa). |
| Standard enthalpy change of neutralisation | The enthalpy change when one mole of water is formed by the reaction of an acid with a base in dilute aqueous solution under standard conditions (298 K, 100 kPa). |
| Standard enthalpy change of atomisation | The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions (298 K, 100 kPa). |
| Bond energy | The average energy required to break one mole of a particular covalent bond in gaseous molecules. |
| Lattice energy | The enthalpy change when one mole of a solid ionic compound is formed from its constituent gaseous ions under standard conditions (298 K, 100 kPa). It is always negative. |
| Standard enthalpy change of hydration | The enthalpy change when one mole of gaseous ions is dissolved in a large excess of water under standard conditions (298 K, 100 kPa). |
| Standard enthalpy change of solution | The enthalpy change when one mole of a solute dissolves in a large excess of solvent, so that further dilution gives no further enthalpy change, under standard conditions (298 K, 100 kPa). |
| Hess's law | The enthalpy change of a reaction depends only on the initial and final states, and not on the route taken. |
| Entropy, S | A measure of the number of ways the particles of a system, and their energy, can be arranged. More ways means a greater entropy (more disorder). |
Heat gained by the solution is heat lost by the reaction. Hess cycles require identical endpoint states. Bond-energy estimates use gaseous bonds broken minus gaseous bonds formed. The syllabus lattice energy is negative for lattice formation; dissolution uses its reverse plus hydration.
ΔG° = ΔH° - TΔS° with T in K and matching units. Heating favours processes with positive ΔS. A negative standard free-energy change indicates a thermodynamic tendency under standard conditions, not a guaranteed fast or complete reaction.
Quick recall
Define standard enthalpy change of formation.