Heats of Formation and Reaction Reference Page
Germain Hess and the Enthalpy of Doom

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Intro to Heats of Formation and Reaction

Chemical reactions involve changes in the internal energy and enthalpy of the reacting species. Because these changes cannot usually be measured directly, thermodynamics uses reference states and tabulated data to calculate them.

One of the most useful tools for this purpose is the standard enthalpy of formation, which allows reaction enthalpies to be determined using Hess’s Law. The video on this page explains how heats of formation and reaction are defined and how they are used to compute energy changes for chemical reactions.

Germain Hess and the Enthalpy of Doom

This video explains heats of formation and reaction and shows how Hess’s Law allows reaction enthalpies to be calculated from tabulated thermodynamic data.

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Examples and Definitions

Definitions

Heat or Enthalpy of Formation
The enthalpy change associated with forming one mole of a chemical species from its constituent elements at their standard temperature, pressure, and state of aggregation.

By definition, the enthalpy of formation of an element in its standard state is zero.

Heat or Enthalpy of Reaction
The enthalpy change associated with one mole of reaction for a chemical reaction as written.

It can be calculated from the enthalpies of formation of the reactants and products using their stoichiometric coefficients, with the convention that reactant coefficients are negative and product coefficients are positive.

Hess’s Law
The enthalpy change of a reaction depends only on the initial and final states. Therefore, the enthalpy change is the same regardless of the sequence of intermediate reactions used to calculate it.