The key difference between heat of formation and heat of reaction is that the heat of formation is the change in enthalpy during the formation of a mole of a substance at standard conditions whereas the heat of reaction is the change in enthalpy during a chemical reaction that occurs at a constant pressure.
Heat of formation and heat of reaction are important enthalpy values regarding chemical reactions. We define these terms for standard conditions, i.e. standard pressure and standard temperature. Here, heat or enthalpy is the energy that either releases from a system or absorbed by a system during the chemical reaction.
CONTENTS
1. Overview and Key Difference
2. What is Heat of Formation
3. What is Heat of Reaction
4. Side by Side Comparison – Heat of Formation vs Heat of Reaction in Tabular Form
5. Summary
What is Heat of Formation?
Heat of formation is the change of enthalpy during the formation of a mole of a substance from pure elements under standard conditions. Here, the standard conditions are 1 atm pressure and 298.15 Kelvin temperature. Since we consider the formation of one mole, the unit for this energy is kJ/mol. This energy is either the energy that the formation reaction releases or the energy that the reaction consumes during the progression. The equation for this enthalpy is as follows;
Here, ∆ is a symbol, which indicates a change in enthalpy, H is the amount of energy and f indicates a formation reaction. in this reaction, all constituents are in standard conditions, otherwise, it is not the heat of formation. For example, the formation of carbon dioxide is as follows.
The pure elemental form of carbon is graphite and the source of oxygen is diatomic oxygen molecule. When we do this formation reaction under standard conditions and measure the change in enthalpy, we call it the heat of formation.
What is Heat of Reaction?
Heat of reaction is the change of enthalpy of a reaction that occurs at a constant pressure. We measure this energy difference in the unit of kJ/mol. It gives the energy that is either released or absorbed during a chemical reaction. The symbol for this enthalpy change is ∆H. If the value is a positive value, we call it an endothermic reaction. If the value is negative, we call it an exothermic reaction. the equation for this enthalpy change is as follows;
What is the Difference Between Heat of Formation and Heat of Reaction?
Heat of formation is the change of enthalpy during the formation of a mole of a substance from pure elements under standard conditions. The symbol for this enthalpy change is, ∆Hf. Heat of reaction is the change of enthalpy of a reaction that occurs at a constant pressure. The symbol for this enthalpy change is, ∆H.
Summary – Heat of Formation vs Heat of Reaction
Enthalpy is the energy content. The change of enthalpy indicates how much energy exchanges between reactants, products and the surrounding. The difference between heat of formation and heat of reaction is that the heat of formation is the change in enthalpy during the formation of a mole of a substance at standard conditions whereas the heat of reaction is the change in enthalpy during a chemical reaction that occurs at a constant pressure.
Reference:
1. Libretexts. “Standard Enthalpy of Formation.” Chemistry LibreTexts, Libretexts, 9 Feb. 2017. Available here
2. Britannica, The Editors of Encyclopaedia. “Heat of Reaction.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 4 Apr. 2016. Available here