Basics of Enthalpy of Formation

Enthalpy of Formation, or heat of formation, is the change in enthalpy during the formation of one mole of a substance from its constituent elements in their standard states.
It is typically expressed in kJ/mol, kilojoules per mole.
The formation of compounds from elements can either absorb heat (endothermic reaction) or release heat (exothermic reaction).
The standard enthalpy change of formation, denoted as ∆Hf°, takes place at a standard pressure of 1 bar and a specified temperature, usually 298K.

Standard Enthalpy of Formation

The standard enthalpy of formation of an element in its most stable form (standard state) is zero because there is no formation process to occur.
For compounds, the standard enthalpy of formation is calculated by: ∆Hf° = Σ ∆Hf° (products) - Σ ∆Hf° (reactants).
A negative ∆Hf° means the formation of the compound is exothermic, and a positive ∆Hf° means the formation is endothermic.

The type of bonds present in the substance can greatly affect its enthalpy of formation. Stronger bonds result in substances with more negative enthalpies of formation.
The geometrical arrangement of atoms and the effect this has on bond angles and distances can also affect the enthalpy of formation.

Enthalpy of formation plays a crucial role in determining the energy changes in reactions and therefore the feasibility of a reaction.
It is used to calculate the enthalpy change for any chemical reaction using Hess’s law, even for those which may be difficult to measure directly.
Negative enthalpy changes tend to make reactions more likely to occur spontaneously because they release energy, while positive changes make them less likely as they require energy.
The concept of enthalpy of formation is also significant in designing new material and energy-efficient industrial processes.