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Understanding the Standard Enthalpy of Formation

Key Takeaways

  • The value of reaction enthalpy, when 1 mole of a substance is formed at standard condition, is the standard enthalpy of formation.

  • Enthalpy of formation can predict the relative stability of a product.

  • Enthalpy of formation is associated with the enthalpy of combustion.

Enthalpy of formation

The concept of enthalpy can be observed in our daily lives, for instance when using chemical heat or a cold pack, where various reactions lead to the absorption or release of heat, bringing about a change in enthalpy. In the chemical and thermodynamic world, enthalpy emphasizes the concept of heating/cooling and energy transfer, the understanding of which is important in the production of any type of substance. Different enthalpies can be associated with the reaction; however, this article will aim to discuss the standard enthalpy of formation. 

Standard Enthalpy of Formation

Ever wonder how much energy must be involved in the formation of water? That energy is the enthalpy of formation. It is the measurement of the heat absorbed or released during a reaction when 1 mole of a compound is formed from its constituent elements. For instance, when water is formed through the reaction between water and hydrogen parts, the enthalpy of formation is measured at -285.7kJ/mol.

The standard condition in a thermodynamic reaction occurs when 1 mole of a substance is formed at the most stable state of 1 atm. pressure and 298.15 K temperature. The value of reaction enthalpy measured in this condition is the standard enthalpy of formation. In this stable state, the standard enthalpy of formation is the enthalpy change within a reaction. Mathematically, the enthalpy of formation can be expressed as:

 Formula for enthalpy of formation


Δ represents the change in enthalpy

H° is the standard enthalpy change 

H°(f) is the standard enthalpy of formation

The above equation indicates that the enthalpy change required for creating a mole of a compound is the difference between the sum of the value of enthalpy of formation of products (compound) and its reactants (elements). 

Summarizing the Standard Enthalpy of Formation Equation

The general takeaway from the above equation is:

  • ➔ For a pure element, the standard enthalpy of formation is zero.
  • ➔ The negative value means the product is an exothermic compound.
  • ➔ The positive value indicates the formation of an endothermic compound.

The study of the standard enthalpy of formation is ideal for analyzing the relative stability of a product. In automotive or aerospace applications, the value for the standard enthalpy of formation is important in studying the intermetallic properties of the material used at different temperature ranges. The use of phase stability data measured from the measurement of standard enthalpy of formation for different compounds can be correlated with other thermodynamic data to form a phase diagram. This phase diagram can then be used for predicting the physical, chemical, and mechanical properties of the alloy or polymers used so as to improve them, minimize defects, and achieve the desired solution. 

Relation With the Enthalpy of Combustion

The enthalpy of formation often finds itself associated with the enthalpy of combustion. The enthalpy of combustion describes the enthalpy change when one mole of a substance is completely burned. For certain compounds, the enthalpy of formation only occurs when certain elements undergo enthalpy of combustion. For example, when hydrogen reacts with oxygen to form water, the reaction represents the enthalpy of combustion of hydrogen and also the enthalpy of formation of water.

H2+1/2 O2 ➔H2O

In a thermodynamic system, the evaluation of these enthalpies facilitates the calculation of the air-fuel ratio requirement of the combustible fuel as well as the calorific value of the fuel.

Hess’s law can be applied to calculate the enthalpy change of a reaction by analyzing the enthalpy of formation of associated elements. According to Hess’s law, this can be done in several steps. The final enthalpy change remains the same even though the calculation is broken down. 

Simplifying the Calculation of the Enthalpy of Formation

The energy involved in running a thermodynamic system can be analyzed accurately only when all the enthalpies associated with the reaction can be calculated accurately. Modeling and simulating a thermochemical system is one way the accuracy can be maintained in the design. Systems utilizing solid-liquid, solid-gas, or liquid-gas interfaces can make use of CFD tools that can help model the surface reaction and heat transfer during the phase transition. Solving energy equations, including the standard enthalpy of formation, can help in the analysis of the heat of reaction. With CFD simulation, the heat transfer system can be analyzed for different temperature profiles while flow mixes or material uses can be easily evaluated.

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