What is the Difference Between Heat of Formation and Heat of Reaction?
🆚 Go to Comparative Table 🆚The key difference between heat of formation and heat of reaction lies in the processes they represent. Here's a breakdown of their differences:
- Heat of Formation: It is the change in enthalpy during the formation of a mole of a substance from pure elements under standard conditions (25°C and 1 atmosphere of pressure). The standard heat of formation, denoted as ΔHf°, is a measure of the energy absorbed or released when a compound is formed from its elements.
- Heat of Reaction: It is the change in enthalpy of a chemical reaction that occurs at a constant pressure. The heat of reaction is the net amount of energy that should be added or released during a chemical reaction. The standard heat of reaction, denoted as ΔH°, can be calculated using the equation: ΔH° = Σ(products) - Σ(reactants), where Σ represents the sum of all the standard heats of formation of the products and reactants, respectively.
In summary, heat of formation is related to the energy change during the formation of a compound from its elements, while heat of reaction is the energy change during a chemical reaction at a constant pressure. Both values are essential for understanding the thermochemistry of reactions and can be used to determine the overall energy balance of a reaction.
Comparative Table: Heat of Formation vs Heat of Reaction
The heat of formation and heat of reaction are related concepts in thermochemistry. Here is a table comparing the differences between them:
| Property | Heat of Formation | Heat of Reaction |
|---|---|---|
| Definition | The enthalpy change that accompanies the formation of a compound from its elements in their most stable forms under standard conditions. | The enthalpy change that occurs when a reaction takes place under standard conditions. |
| Calculation | Standard heats of formation are determined under standard conditions, with a pressure of 1 atm for gases and a concentration of 1 M for species in solution. | The standard heat of reaction is equal to the sum of all the standard heats of formation of the products minus the sum of all the standard heats of formation of the reactants. |
| Units | Kilojoules per mole (kJ/mol). | Kilojoules per mole (kJ/mol). |
| Examples | - Heat of formation for H2(g): 0 kJ/mol. - Heat of formation for Al2O3(s): -1669.8 kJ/mol. |
- Heat of reaction for the combustion of methane: -890 kJ/mol. - Heat of reaction for the formation of H2O from its elements: -285.8 kJ/mol. |
In summary, the heat of formation is the enthalpy change that accompanies the formation of a compound from its elements under standard conditions, while the heat of reaction is the enthalpy change that occurs when a reaction takes place under standard conditions. The heat of reaction can be calculated using the standard heats of formation of the reactants and products involved in the reaction.
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