Defining Standard Enthalpy of Formation
The standard enthalpy of formation (ΔH°f) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements, with all substances in their standard states at 298.15 K (25°C) and 1 atmosphere of pressure. It's a specific type of enthalpy change used as a reference point for thermochemical calculations.
Key Principles of ΔH°f
For a reaction to represent a standard enthalpy of formation, several conditions must be met: exactly one mole of the product compound must be formed, all reactants must be pure elements, and both reactants and products must be in their most stable physical states under standard conditions. For example, elemental oxygen would be O₂(g), not O(g) or O₃(g).
A Practical Example
Consider the formation of liquid water: H₂(g) + ½O₂(g) → H₂O(l). The standard enthalpy of formation for liquid water, ΔH°f[H₂O(l)], represents the heat released or absorbed during this specific reaction. Its value is approximately -285.8 kJ/mol, indicating an exothermic process where energy is released when water is formed from gaseous hydrogen and oxygen in their standard states.
Importance and Applications
Standard enthalpies of formation are crucial for calculating the standard enthalpy change (ΔH°rxn) of any reaction using Hess's Law. By knowing the ΔH°f values for all reactants and products, chemists can predict whether a reaction will be exothermic or endothermic and determine the energy required or released, without needing to perform direct experimental measurements for every reaction.