Standard Conditions vs STP
Standard conditions (⦵) for thermodynamic data:
- Pressure: 100 kPa (1 bar)
- Concentration: 1.00 mol dm⁻³ (for solutions)
- Temperature: 298 K (25 °C). Conventional, not part of the formal definition
- All substances in their standard state (most stable form at 100 kPa)
Standard states: C = graphite (not diamond), Br₂ = liquid, H₂O = liquid
The Four Definitions
| Type | Symbol | Exact IB Definition | Sign |
|---|---|---|---|
| Reaction | ΔHr⦵ | Enthalpy change when molar amounts as written in the balanced equation react completely under standard conditions | ± either |
| Formation | ΔHf⦵ | Enthalpy change when one mole of a compound is formed from its elements in their standard states | ± either |
| Combustion | ΔHc⦵ | Enthalpy change when one mole of a substance undergoes complete combustion in excess O₂ | Always − |
| Neutralisation | ΔHneut⦵ | Enthalpy change when an acid reacts with a base to form one mole of water | Always − |
⚠️ Examiner Trap. "One Mole"
Formation, combustion, and neutralisation all specify "one mole". The Haber process equation N₂ + 3H₂ → 2NH₃ gives ΔHr⦵. For ΔHf⦵ of NH₃, you must halve it: ½N₂ + ³⁄₂H₂ → NH₃.
📘 Key Fact
ΔHf⦵ of any element in its standard state = 0 by definition (e.g. O₂(g), Fe(s), Br₂(l)).