The standard enthalpy of formation is the heat change that occurs when one mole of a compound is formed from its elements in their standard states under standard conditions (usually 25 °C and 1 atm pressure).
Key points:
- Standard conditions:
- Temperature: 25 °C (298 K)
- Pressure: 1 atm
- Elements in standard states:
- Pure elements in their most stable form at 25 °C and 1 atm (e.g., O₂ gas, C as graphite, H₂ gas).
- Significance:
- Indicates whether forming the compound releases energy (exothermic) or requires energy (endothermic).
- Used to calculate the enthalpy change of chemical reactions.
- Examples:
- Formation of water: Hydrogen gas reacts with oxygen gas to form liquid water.
- Formation of carbon dioxide: Carbon (graphite) reacts with oxygen gas to form CO₂ gas.
In short:
It is the heat change when 1 mole of a compound is formed from its elements in their natural states under standard conditions.