Understanding Standard Enthalpy of Formation

Oct 13, 2025 by pynkocean.com 105 views

Introduction

In chemistry, enthalpy is a measure of the total energy of a system, including both internal energy and the energy associated with the pressure and volume of a system. The standard enthalpy of formation ($$\Delta H_f^{\circ}$$) is a specific type of enthalpy that is used to calculate the enthalpy change of a reaction. In this article, we will calculate the enthalpy in $$\frac{kJ}{mol}$$ of the following reaction using the standard enthalpies of formation provided:

${ CH_3CH_2OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) }$

Understanding Standard Enthalpy of Formation

The standard enthalpy of formation ($$\Delta H_f^{\circ}$$) is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. The standard state of an element is its most stable form at a pressure of 1 bar and a temperature of 25°C. The standard enthalpy of formation is typically denoted by the symbol $$\Delta H_f^{\circ}$$ and is expressed in units of kJ/mol.

Given Reaction and Standard Enthalpies of Formation

The given reaction is:

${ CH_3CH_2OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) }$

The standard enthalpies of formation for the reactants and products are:

Compound	$$\Delta H_f^{\circ}$$ (kJ/mol)
CH_3CH_2OH(g)	-276.8
O_2(g)	0
CO_2(g)	-393.5
H_2O(g)	-241.8

Calculating Enthalpy Change

To calculate the enthalpy change of the reaction, we need to multiply the standard enthalpy of formation of each reactant by its stoichiometric coefficient and subtract the product from the sum of the standard enthalpies of formation of each product multiplied by its stoichiometric coefficient.

The balanced equation for the reaction is:

${ CH_3CH_2OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) }$

The enthalpy change of the reaction can be calculated as follows:

\Delta H^{\circ} = \sum \Delta H_f^{\circ} (\text{products}) - \sum \Delta H_f^{\circ} (\text{reactants})

\Delta H^{\circ} = 2 \times (-393.5) + 3 \times (-241.8) - (-276.8 + 3 \times 0)

\Delta H^{\circ} = -787 + -725.4 + 276.8

\Delta H^{\circ} = -1235.6 \text{ kJ/mol}

Conclusion

In this article, we calculated the enthalpy in $$\frac{kJ}{mol}$$ of the following reaction using the standard enthalpies of formation provided:

${ CH_3CH_2OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g) }$

The enthalpy change of the reaction was calculated to be -1235.6 kJ/mol. This result indicates that the reaction is highly exothermic, releasing a significant amount of energy in the form of heat.

References

Atkins, P. W., & De Paula, J. (2010). Physical chemistry. Oxford University Press.
Chang, R. (2010). Chemistry. McGraw-Hill.
Levine, I. N. (2014). Physical chemistry. McGraw-Hill.

Introduction

In our previous article, we discussed the concept of enthalpy and standard enthalpy of formation, and calculated the enthalpy change of a reaction using the standard enthalpies of formation provided. In this article, we will address some of the most frequently asked questions related to enthalpy and standard enthalpy of formation.

Q: What is the difference between enthalpy and internal energy?

A: Enthalpy (H) is a measure of the total energy of a system, including both internal energy (U) and the energy associated with the pressure and volume of a system. Internal energy, on the other hand, is the total energy of a system, excluding the energy associated with the pressure and volume.

Q: What is the standard state of an element?

A: The standard state of an element is its most stable form at a pressure of 1 bar and a temperature of 25°C.

Q: How is the standard enthalpy of formation calculated?

A: The standard enthalpy of formation is calculated by measuring the enthalpy change of a reaction in which one mole of a compound is formed from its constituent elements in their standard states.

Q: What is the unit of standard enthalpy of formation?

A: The unit of standard enthalpy of formation is kJ/mol.

Q: How is the enthalpy change of a reaction calculated?

A: The enthalpy change of a reaction is calculated by multiplying the standard enthalpy of formation of each reactant by its stoichiometric coefficient and subtracting the product from the sum of the standard enthalpies of formation of each product multiplied by its stoichiometric coefficient.

Q: What is the difference between a reaction that is exothermic and one that is endothermic?

A: A reaction that is exothermic releases energy in the form of heat, while a reaction that is endothermic absorbs energy from the surroundings.

Q: How is the standard enthalpy of formation used in real-world applications?

A: The standard enthalpy of formation is used in a variety of real-world applications, including the design of chemical processes, the development of new materials, and the prediction of the behavior of complex systems.

Q: What are some common mistakes to avoid when working with enthalpy and standard enthalpy of formation?

A: Some common mistakes to avoid when working with enthalpy and standard enthalpy of formation include:

Failing to account for the standard state of an element
Using the wrong unit of measurement
Failing to consider the stoichiometric coefficients of the reactants and products
Failing to account for the enthalpy change of the reaction

Conclusion

In this article, we addressed some of the most frequently asked questions related to enthalpy and standard enthalpy of formation. We hope that this article has provided you with a better understanding of these important concepts and has helped you to avoid some common mistakes.

References

Atkins, P. W., & De Paula, J. (2010). Physical chemistry. Oxford University Press.
Chang, R. (2010). Chemistry. McGraw-Hill.
Levine, I. N. (2014). Physical chemistry. McGraw-Hill.