AQA A-level Chemistry Thermodynamics definitions

Mean Bond Enthalpy

The enthalpy change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds.

Enthalpy of formation

"Heat change when 1 mol compound forms from elements in standard states (298 K

Enthalpy of atomisation

"Heat needed to form 1 mol gaseous atoms from an element in its standard state"

First ionisation enthalpy

"Energy required to remove 1 mol electrons from 1 mol gaseous atoms"

Electron affinity

"Energy change when 1 mol gaseous atoms gains 1 mol electrons to form 1 mol gaseous –1 ions"

Bond enthalpy

"Energy needed to break 1 mol of a specific bond in gaseous molecules"

Enthalpy of hydration

"Enthalpy change when 1 mol gaseous ions dissolves in water to form aqueous ions"

Lattice enthalpy

"Enthalpy change when 1 mol solid ionic compound forms from its gaseous ions"

Enthalpy of solution

"Enthalpy change when 1 mol ionic solid dissolves in enough solvent to give an infinitely dilute solution"

Entropy

"Measure of disorder; more possible arrangements = higher entropy (J K⁻¹ mol⁻¹)"

Gibbs free energy change

"ΔG = ΔH − TΔS; if ΔG ≤ 0 reaction is feasible at constant temp/pressure"

Spontaneous (feasible) process

"Process that occurs without external influence; ΔG ≤ 0"

Standard entropy

"Absolute entropy of substance in its standard state at 298 K