Heat is a form of energy that is transferred between two systems in thermal contact. It is a physical quantity that is defined as the amount of energy that is transferred between two bodies due to a difference in temperature. In chemistry, heat is used to measure the energy changes that occur in chemical reactions.

Formation Heat

Heat of formation is the amount of heat that is released or absorbed when a molecule is formed from its constituent elements. It is measured at a constant pressure of 1 atmosphere and a temperature of 25°C. It can also be defined as the heat released or absorbed by the formation of a molecule of a substance from its constituent elements in a standard state, that is, at a pressure of 1 atm, a temperature of 298 K and in its most stable in solid state.

Example of heat of formation calculation:

The methane molecule (CH₄) is formed from hydrogen (H₂) and carbon (C). The heat of formation of methane is -74,8 KJ/mol. This means that 74,8 KJ of energy are released in the formation of one methane molecule.

Heat of Reaction

Heat of reaction is the amount of heat released or absorbed during a chemical reaction. It is measured at a constant pressure of 1 atmosphere and a temperature of 25°C. To calculate the heat of reaction, it is necessary to know the heat of formation of the substances involved in the reaction.

Heat of reaction calculation example:

The combustion of methane can be represented with the following chemical equation:

CH₄ + 2O₂ →CO₂ + 2H₂O

The equation tells us that 1 mole of methane (CH₄) and 2 moles of oxygen (O₂) to produce 1 mole of carbon dioxide (CO₂) and 2 moles of water (H₂EITHER). The heat of formation of carbon dioxide is -393,5 KJ/mol and the heat of formation of water is -285,8 KJ/mol.

Therefore, the heat of reaction for the combustion of methane is:

(-393,5 KJ/mol ) + 2(-285,8 KJ/mol) – (-74,8 KJ/mol) = -890,8 KJ/mol

This means that 890,8 KJ of energy are released in the combustion of one mole of methane.

Conclusions

Both the heat of formation and the heat of reaction are important in chemistry since they allow us to know the amount of energy released or absorbed in chemical reactions. The heat of formation is used to calculate the heat of reaction of a chemical reaction. Knowing these values ​​is essential to design industrial processes and optimize their energy efficiency.

References

• Chang, R. (2005). Chemistry. Mexico City: McGraw-Hill.
• Atkins, P., Jones, L. (2008). Principles of chemistry: the paths of discovery. Barcelona: Omega.

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