Chemistry

Difference Between Heat of Formation and Heat of Reaction

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Main Difference

The main difference between Heat of Formation and Heat of Reaction is that Heat of Formation is the aggregate of change in energy which is either released or absorbed during the development of one mole of the compound, whereas Heat of Reaction is the aggregate of change in energy which is either released or absorbed during the process of any chemical reaction.

Heat of Formation vs. Heat of Reaction

The heat of formation is the change of energy values during the formation of elements, while the heat of reaction is the energy add or released during the reactions. The change of enthalpy, which generally comes during the development of an element is given by the heat of formation, whereas the total amount of heat present in a chemical reaction is either added or released is given by heat of reaction.

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

The standard symbol of the heat of formation is ∆Hf; on the contrary, the symbol of the heat of reaction is ∆Hr.

Comparison Chart

Heat of FormationHeat of Reaction
The modification of 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 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 elementsThe energy add or released during the reactions
Concept
The change of enthalpy which generally comes during the development of an elementThe total amount of heat present in a chemical reaction is either added or released
Equation
Hreaction = Hf(reactants) – ⅀Hf(products)ΔH⊖rxn=∑ΔH⊖f{products}−∑ΔH⊖f{reactants}
Symbol
∆Hf∆Hr
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What is Heat of Formation?

The concept heat of formation is defined as the modification or change of enthalpy when one mole of an element is produced from its integral elements 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 one atmospheric pressure when one mole of a compound is produced from its component elements, and every substance is present in its normal physical state.

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

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The standard pressure, which is given by IUPAC, is 105 Pa (1bar). Though there is present no defined standard temperature given for the reactions, that’s why constant pressures are available for the heat of formation. The unit for the heat of formation is KJ/mole because we are considering only one mole of formation.

The heat of formation is symbolized as ∆Hf. This symbol can be calculated by using an equation. The heat of formation is chemically written 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 standard changed enthalpies of formation of the products and the standard changed enthalpies of formation of reactants.

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

What is Heat of Reaction?

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

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

The heat of reaction is chemically written 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 comes in the heat of reaction, then it means that heat is absorbed from the outside to the reaction (endothermic), but if a negative value occurs in the process of the heat of reaction, then it means that the heat is given to the outside from the reaction to the environment (exothermic). ∆Hr is the symbol of the heat of reaction.

The heat of reaction or known as the change in enthalpy, does not directly determine by the specific passageway of a reaction, but only on the complete energy level of the reactants and products.

Key Differences

  1. The change of 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 is added or released during the process of chemical reactions is known as the heat of reaction.
  2. The heat of formation gives the idea about the alteration of enthalpy, which generally comes during the development of an element, whereas the heat of reaction gives the idea of the total amount of heat present in a chemical reaction is either released or absorbed.
  3. Hreaction = Hf(reactants) – ⅀Hf(products) is the general formula of the heat of formation; on the other hand, is the general formula of the heat of reaction is ΔH⊖rxn=∑ΔH⊖f{products}−∑ΔH⊖f{reactants}.
  4. ∆Hf is the symbol of the heat of formation; on the contrary, ∆Hf is the symbol of the heat of reaction.

Conclusion

Above discussion concludes that the heat of formation is the aggregate of alteration in energy which is either released or absorbed during the development of a compound and represented by ∆Hf, whereas heat of reaction is generally the aggregate of change in energy which is either released or absorbed during the process of any chemical reaction and symbolized by ∆Hr.

Harlon Moss

Harlon currently works as a quality moderator and content writer for Difference Wiki. He graduated from the University of California in 2010 with a degree in Computer Science. Follow him on Twitter @HarlonMoss

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