Standard Molar Enthalpy of Formation
Definition and meaning of Standard Molar Enthalpy of Formation in chemistry.
Standard molar enthalpy of formation is the specific heat change that results when exactly one mole of a chemical compound is formed directly from its constituent elements in their standard states.
In more detail
This fundamental thermodynamic value is always measured under standard conditions, typically at an absolute pressure of 1 bar and a specified constant temperature, most commonly 298.15 K. By rigorous chemical definition, the standard enthalpy of formation for any pure element in its most stable physical form at these conditions is exactly zero. These tabulated values are incredibly useful because they allow physical chemists to calculate the overall enthalpy change of almost any chemical reaction by employing Hess's Law. A negative reported value strongly indicates an exothermic formation process, while a positive value indicates an endothermic formation process.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | ΔH°f |
| Value for Pure Elements | 0 kJ/mol |
| Standard Pressure | 1 bar (or 1 atm) |
The standard molar enthalpy of formation for liquid water (H2O) is -285.8 kJ/mol, meaning significant energy is actively released when hydrogen and oxygen gases chemically combine.
Frequently asked questions
Why is the standard enthalpy of formation for oxygen gas exactly zero?
Because oxygen gas (O2) is already the most stable, naturally occurring elemental form of oxygen at standard conditions, so absolutely no energetic change is required to form it.