Standard Enthalpy of Formation

The enthalpy change of a reaction can be measured using a calorimeter, or it can be found by calculating the difference in enthalpy between the reactants and the products.

However, the absolute enthalpies of the reactants and products cannot be measured directly; therefore, chemists generally use the change in enthalpy, or ΔH, relative to a reference substance in an agreed-upon standard state.

The standard state is defined by a specific set of conditions. This includes a set temperature, usually 25 °C or 298 K, and a constant pressure of 1 bar for gases.

For solutions, the standard state is a 1 molar concentration of a pure solute in a solvent.

The standard state for a substance also includes the physical state of matter in which the substance exists under these conditions. For example, sodium chloride as a solid, mercury as a liquid, or helium as a gas.

If an element exists in more than one form under these conditions, the most stable form of the element is defined as the standard state. For example, carbon can exist as graphite crystals or as a diamond, but graphite is the most stable form and therefore, the standard state of carbon.

When elements in their standard states combine to form 1 mole of a pure compound, the enthalpy of the reaction is called standard enthalpy, or standard heat, of formation. This is denoted by ΔH_f°, where naught indicates the standard states of the constituent elements, while f indicates formation.

The standard enthalpy of formation for pure elements under standard state conditions is always zero because there is no reaction, and therefore no change in enthalpy, when the element is already in its standard state.

The values of the standard enthalpy of formation in kilojoules per mole for a compound can be found in reference tables. These substances include elements in non-standard states, such as gaseous sodium, and compounds such as sodium chloride.

The standard enthalpy change of a reaction can be calculated from the difference of products and reactants, which in turn, can be calculated by using the enthalpy values from the reference table.