Standard enthalpy of formation
Standard enthalpy of formation, , is the enthalpy change for producing exactly one mole of a compound directly from its constituent elements, with all substances in their standard states under standard conditions.
For this course, standard conditions are 298 K and 100 kPa. An equation representing an enthalpy of formation must therefore produce exactly one mole of the compound.
By convention, an element in its standard state has . Formation enthalpies of compounds may be positive or negative.
Standard enthalpy of atomisation
Standard enthalpy of atomisation, , is the enthalpy change for converting an element in its standard state into exactly one mole of gaseous atoms under standard conditions.
Atomisation requires particles in an element to be separated into individual gaseous atoms, so it is an endothermic process and its enthalpy change is positive.
| Element in its standard state | Atomisation equation | Key requirement |
|---|---|---|
| Potassium, solid | Forms 1 mol of atoms | |
| Mercury, liquid | Starts from mercury's standard state | |
| Chlorine, gas | The coefficient gives 1 mol of atoms |
Worked example: writing atomisation equations
Question: Write equations representing the standard enthalpy of atomisation of potassium and mercury.
Potassium: its standard state is solid, and one mole of gaseous atoms must be produced:
Mercury: its standard state is liquid, so the starting state must be :
Exam Tip: In a formation equation, check that exactly one mole of compound is formed. In an atomisation equation, check that exactly one mole of gaseous atoms is formed and that the element starts in its standard state.