Standard Conditions vs STP
Standard State
Standard Conditions (⦵) for Thermodynamic Data
Standard conditions for thermodynamic measurements are defined as:
- Pressure: 100 kPa (1 bar)
- Concentration: 1.00 mol dm⁻³ (for aqueous solutions)
- Temperature: 298 K (25 °C) - standard reference temperature
- All substances in their standard state (most stable physical form at 100 kPa and 298 K)
Key standard states: Carbon = graphite (s), Bromine = Br₂(l), Mercury = Hg(l), Iodine = I₂(s), Water = H₂O(l).
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
The "One Mole" Requirement in Standard Definitions
Formation (ΔHf⦵), combustion (ΔHc⦵), and neutralisation (ΔHneut⦵) all strictly specify "one mole" in their definitions:
- The Haber equation
N₂ + 3H₂ → 2NH₃gives ΔHr⦵ for 2 moles of product. - To calculate ΔHf⦵ of NH₃, you must halve the entire equation:
½N₂(g) + 1½H₂(g) → NH₃(g).
Core Principle
ΔHf⦵ of Elements = 0
ΔHf⦵ of any element in its standard state is zero by definition (e.g. O₂(g), Fe(s), Br₂(l), C(graphite) all have ΔHf⦵ = 0 kJ mol⁻¹).
Notation
The Standard State Symbol (⦵)
The superscript standard symbol ⊖ or ⦵ (e.g. ΔH⊖) indicates all reactants and products are in their standard states at 100 kPa and 1.00 mol dm⁻³.
AQA GCSE & IB Chemistry
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