# Difference Between Heat of Formation and Heat of Reaction

Main difference

The main difference between heat of formation and heat of reaction is that heat of formation is the aggregate of energy exchange that is released or absorbed during the development of one mole of the compound while heat of reaction is the aggregate of energy change. that is released or absorbed during the process of any chemical reaction.

## Heat of formation versus heat of reaction

The heat of formation is the change in energy values ​​during the formation of elements, while the heat of reaction is the energy added or released during reactions. The enthalpy change, which usually occurs during the development of an element, is given by the heat of formation, while the total amount of heat exhibited by a chemical reaction that is added or released is given by the heat of reaction.

The heat of formation is written chemically as Hreaction = Hf (reactants) – ⅀Hf (products); on the other hand, the heat of reaction is written chemically as ΔH⊖rxn = ∑ΔH⊖f {products} −∑ΔH⊖f {reactants}.

The standard symbol for the heat of formation is ∆H f ; instead, the symbol for the heat of reaction is ∆H r .

## Comparison chart

 heat of formation heat of reaction The change in enthalpy when one mole of an element is produced from its integral elements is known as the heat of formation. The total value of energy that is present in the chemical that is absorbed or released during the process is known as the heat of reaction. energy values The change of energy values ​​during the formation of elements. The energy added or released during reactions. Concept The enthalpy change that usually occurs during the development of an element. The total amount of heat present in a chemical reaction is added or released Equation Hreaction = Hf (reagents) – ⅀Hf (products) ΔH⊖rxn = ∑ΔH⊖f {products} −∑ΔH⊖f {reagents} Symbol ∆H f ∆Hr _
What is the heat of formation?

The concept of heat of formation is defined as the modification or enthalpy change when one mole of an element is produced from its integral elements and is known as the heat of formation. In other words, it is defined as the amount of heat released or absorbed at the temperature of 25°C and 77°F, and at atmospheric pressure when one mole of a compound is produced from its component elements and each substance is present. in his normal physical state.

When this heat is given for the production of various substances at their standard rate, then this heat is called the standard heat of formation whose conditions are 298.15 Kelvin temperature and 1 atm pressure.

The standard pressure, provided by the IUPAC, is 10 5 Pa (1 bar). Although there is no definite standard temperature for reactions, this is why constant pressures are available for the heat of formation. The unit for the heat of formation is KJ/mol because we are considering only one mole of formation.

The heat of formation is symbolized as ∆H f . This symbol can be calculated using an equation. The heat of formation is written chemically as Hreaction = Hf (reactants) – ⅀Hf (products). This equation means that the standard heat of formation is normally equal to the differences between the sum of the changed standard enthalpies of formation of the products and the changed standard enthalpies of formation of reactants.

Consider the example of the formation of carbon dioxide from graphite, which is a pure elemental form of carbon, and the source of oxygen is a specific diatomic molecule. When this formation reaction occurs under given standard conditions and the change in enthalpy is measured, then the reaction will be called the heat of formation.

What is the heat of reaction?

The idea of ​​heat of reaction is defined as the total value of energy, which is present in the chemical substance that is absorbed or released during the process which is known as heat of reaction. It is also the change in heat enthalpy during a chemical reaction that occurs at a given constant pressure, and the heat of reaction is absorbed or released into the environment to maintain the reactants at a given constant temperature during a chemical reaction.

The heat of reaction is usually measured per mole of a specific substance, so its unit will be KJ/mol. When the heat of reaction is given for the standard rate, this is called the standard heat of reaction or standard enthalpy change of a reaction. The pure standard rate is used at 1 bar pressure and applicable temperature to find the pure standard rate of substances.

The heat of reaction is written chemically as where the symbol Σ is the Greek note sigma and means “the sum of”. The standard heat of reaction is generally equal to the addition of all the standard heats of formation of the reactants minus the addition of all the standard heats of formation of the reactants.

If a positive value appears in the heat of reaction, it means that the heat is absorbed from the outside to the reaction (endothermic), but if a negative value occurs in the process of the heat of reaction, it means that the heat is given to the outside. by reaction to the environment (exothermic). ∆H r is the symbol for the heat of reaction.

The heat of reaction or known as enthalpy change, is not directly determined by the specific passage of a reaction, but only at the full energy level of the reactants and products.

## Key differences

1. The change in energy values ​​during the formation of one mole of a compound from its constituent elements is known as the heat of formation, while the net amount of energy that is added or released during the process of chemical reactions is known as reaction heat.
2. The heat of formation gives the idea of ​​the change in enthalpy, which generally occurs during the development of an element, while the heat of reaction gives the idea of ​​the total amount of heat present in a chemical reaction that is released or released. absorbs.
3. Hreaction = Hf (reactants) – ⅀Hf (products) is the general formula for the heat of formation; on the other hand, the general formula for heat of reaction is ΔH⊖rxn = ∑ΔH⊖f {products} −∑ΔH⊖f {reactants}.
4. ∆H f is the symbol for the heat of formation; instead, ∆H f is the symbol for the heat of reaction.

## Final Thoughts

The above discussion concludes that the heat of formation is the aggregate change in energy that is released or absorbed during the development of a compound and is represented by ∆H f , while the heat of reaction is generally the aggregate change in energy that is released or absorbed during the process of any chemical reaction and symbolized by ∆H r .