5.1.3—Terms

A standard enthalpy change is measured for substances in their standard states under the syllabus standard conditions: 298 K and 101 kPa. The standard-state symbol is written as ⦵. Always state the physical state and the amount of reaction defined by the balanced equation.

The standard enthalpy change of reaction, ΔHᵣ⦵, is the enthalpy change for the reaction as written under standard conditions. Reversing the equation reverses the sign; multiplying the equation changes the enthalpy change by the same factor.

Use the label that matches the process: standard enthalpy of formation, ΔHf⦵, forms one mole of a compound from its elements in their standard states; standard enthalpy of combustion, ΔHc⦵, completely burns one mole of a substance in oxygen; standard enthalpy of neutralisation, ΔHneut⦵, forms one mole of water when an acid and an alkali react under the stated conditions.

Before using a named value, check the equation, stoichiometric amount, physical states and standard conditions. Do not omit state symbols or silently change the amount of substance represented by the definition.