To compare enthalpy changes fairly, chemists define standard conditions.
Standard Conditions
- Pressure: 100 kPa
- Temperature: 298 K (25 °C)
- Concentration: 1 mol dm⁻³ (for solutions)
- Substances in their standard states (most stable form at the above conditions)
The symbol ⊖ (or °) denotes standard conditions, e.g. ΔH⊖.
Types of Standard Enthalpy Change
| Name | Symbol | Definition |
|---|---|---|
| Combustion | \(\Delta H_c^{\ominus}\) | Enthalpy change when 1 mole of a substance is completely burned in excess oxygen under standard conditions. |
| Formation | \(\Delta H_f^{\ominus}\) | Enthalpy change when 1 mole of a compound is formed from its elements in their standard states. |
| Neutralisation | \(\Delta H_{neut}^{\ominus}\) | Enthalpy change when an acid and base react to form 1 mole of water. |
| Atomisation | \(\Delta H_{at}^{\ominus}\) | Enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state. Always endothermic. |
Think About It
The standard enthalpy of formation of any element in its standard state is zero by definition. Why?
Because no change occurs — the element is already in its standard state, so ΔH = 0.