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.
Key points:
- Standard conditions:
- Temperature: 25°C (298 K)
- Pressure: 1 atmosphere (1 atm)
- Elements in standard state:
- The most stable form of an element at 25°C and 1 atm is considered its standard state.
- Example: O₂ gas, C as graphite, H₂ gas.
- Significance of the value:
- If ΔHf° is negative, the formation of the compound releases heat (exothermic).
- If ΔHf° is positive, the formation of the compound absorbs heat (endothermic).
- Applications:
- Used to calculate enthalpy changes of reactions using Hess’s Law.
- Helps compare the stability of different compounds.
Simple example:
- Formation of water from hydrogen and oxygen:
Hydrogen gas + Oxygen gas → 1 mole of H₂O (liquid)
This reaction releases heat, so the standard enthalpy of formation of water is negative.
In short, the standard enthalpy of formation tells us how much energy is involved in forming a compound from its basic elements.