Given Reaction and Enthalpy of Formation

Oct 13, 2025 by miodeya.com 93 views

Introduction

In the realm of chemistry, understanding the thermodynamics of reactions is crucial for predicting their feasibility and spontaneity. One of the key thermodynamic properties that help us determine the nature of a reaction is the enthalpy of formation. In this article, we will delve into the concept of enthalpy of formation and use it to determine whether a given reaction is endothermic or exothermic.

What is Enthalpy of Formation?

Enthalpy of formation, denoted by the symbol ΔHf, is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. It is a measure of the energy change that occurs during the formation of a compound. The enthalpy of formation is typically expressed in units of kJ/mol (kilojoules per mole).

Given Reaction and Enthalpy of Formation

The given reaction is:

${6 \, C(s, \text{graphite}) + 3 \, H_2(g) \rightarrow C_6H_6(l)}$

The enthalpy of formation for benzene (C6H6) is given as 49.0 kJ/mol. This value represents the energy change that occurs when one mole of benzene is formed from its constituent elements in their standard states.

Determining the Reaction Thermodynamics

To determine whether the reaction is endothermic or exothermic, we need to calculate the enthalpy change (ΔH) for the reaction. The enthalpy change is the sum of the enthalpies of formation of the products and the enthalpies of formation of the reactants.

Calculating the Enthalpy Change

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

ΔH = Σ(ΔHf(products)) - Σ(ΔHf(reactants))

For the given reaction, the products are C6H6(l) and the reactants are C(s, graphite) and H2(g). The enthalpy of formation for C6H6(l) is given as 49.0 kJ/mol. The enthalpy of formation for C(s, graphite) is zero, since graphite is the standard state of carbon. The enthalpy of formation for H2(g) is also zero, since hydrogen gas is the standard state of hydrogen.

Calculating the Enthalpy Change (continued)

Substituting the values into the equation, we get:

ΔH = ΔHf(C6H6(l)) - (6 x ΔHf(C(s, graphite)) + 3 x ΔHf(H2(g)))

ΔH = 49.0 kJ/mol - (6 x 0 + 3 x 0)

ΔH = 49.0 kJ/mol

Conclusion

The enthalpy change for the reaction is 49.0 kJ/mol, which is positive. This indicates that the reaction is endothermic, meaning that it absorbs energy from the surroundings to proceed.

Implications of the Reaction Thermodynamics

The fact that the reaction is endothermic has important implications for its feasibility and spontaneity. An endothermic reaction requires energy input to proceed, which can be a limiting factor in its occurrence. Additionally, the reaction may not be spontaneous, meaning that it will not occur on its own without external energy input.

Factors Affecting the Reaction Thermodynamics

Several factors can affect the reaction thermodynamics, including the temperature, pressure, and concentration of the reactants. These factors can influence the enthalpy change and the spontaneity of the reaction.

Temperature and Reaction Thermodynamics

Temperature can have a significant impact on the reaction thermodynamics. As the temperature increases, the reaction may become more endothermic, requiring more energy input to proceed. Conversely, as the temperature decreases, the reaction may become less endothermic, requiring less energy input to proceed.

Pressure and Reaction Thermodynamics

Pressure can also affect the reaction thermodynamics. As the pressure increases, the reaction may become more endothermic, requiring more energy input to proceed. Conversely, as the pressure decreases, the reaction may become less endothermic, requiring less energy input to proceed.

Concentration and Reaction Thermodynamics

Concentration can also affect the reaction thermodynamics. As the concentration of the reactants increases, the reaction may become more endothermic, requiring more energy input to proceed. Conversely, as the concentration of the reactants decreases, the reaction may become less endothermic, requiring less energy input to proceed.

Conclusion

In conclusion, the enthalpy of formation for benzene (C6H6) is 49.0 kJ/mol. Using this value, we calculated the enthalpy change for the given reaction and determined that it is endothermic. The reaction thermodynamics are affected by several factors, including temperature, pressure, and concentration. Understanding these factors is crucial for predicting the feasibility and spontaneity of the reaction.

References

Atkins, P. W., & de Paula, J. (2010). Physical chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Physical chemistry for the life sciences (2nd ed.). Pearson Education.
Levine, I. N. (2014). Physical chemistry (6th ed.). McGraw-Hill Education.

Introduction

In our previous article, we explored the concept of enthalpy of formation and its application in determining the thermodynamics of a reaction. In this article, we will address some of the most frequently asked questions related to enthalpy of formation and reaction thermodynamics.

Q: What is the difference between enthalpy of formation and enthalpy of reaction?

A: The enthalpy of formation is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. The enthalpy of reaction, on the other hand, is the change in enthalpy that occurs during a chemical reaction. The enthalpy of reaction is calculated by subtracting the enthalpy of the reactants from the enthalpy of the products.

Q: How do I calculate the enthalpy of reaction?

A: To calculate the enthalpy of reaction, you need to know the enthalpy of formation of the products and the enthalpy of formation of the reactants. The enthalpy of reaction is calculated using the following equation:

ΔH = Σ(ΔHf(products)) - Σ(ΔHf(reactants))

Q: What is the standard state of an element?

A: The standard state of an element is the state in which the element exists under standard conditions of temperature and pressure. For example, the standard state of carbon is graphite, and the standard state of hydrogen is hydrogen gas.

Q: How do I determine the standard state of an element?

A: The standard state of an element is typically determined by the International Union of Pure and Applied Chemistry (IUPAC). You can consult the IUPAC website or a reliable chemistry textbook for more information.

Q: What is the difference between an endothermic and an exothermic reaction?

A: An endothermic reaction is a reaction that absorbs energy from the surroundings, while an exothermic reaction is a reaction that releases energy to the surroundings. The enthalpy change of a reaction is a measure of its thermodynamics, and a positive enthalpy change indicates an endothermic reaction, while a negative enthalpy change indicates an exothermic reaction.

Q: How do I determine whether a reaction is endothermic or exothermic?

A: To determine whether a reaction is endothermic or exothermic, you need to calculate the enthalpy change of the reaction using the equation:

ΔH = Σ(ΔHf(products)) - Σ(ΔHf(reactants))

If the enthalpy change is positive, the reaction is endothermic. If the enthalpy change is negative, the reaction is exothermic.

Q: What are some common applications of enthalpy of formation?

A: Enthalpy of formation has many applications in chemistry, including:

Predicting the thermodynamics of chemical reactions
Determining the spontaneity of chemical reactions
Calculating the energy change of chemical reactions
Understanding the stability of chemical compounds

Q: What are some common mistakes to avoid when calculating enthalpy of formation?

A: Some common mistakes to avoid when calculating enthalpy of formation include:

Failing to use the correct values for the enthalpy of formation of the products and reactants
Failing to account for the standard state of the elements
Failing to use the correct units for the enthalpy of formation
Failing to calculate the enthalpy change correctly

Conclusion

In conclusion, enthalpy of formation is a fundamental concept in chemistry that is used to predict the thermodynamics of chemical reactions. By understanding the enthalpy of formation and how to calculate it, you can gain a deeper understanding of the thermodynamics of chemical reactions and make more accurate predictions about their spontaneity and energy change.

References

Atkins, P. W., & de Paula, J. (2010). Physical chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Physical chemistry for the life sciences (2nd ed.). Pearson Education.
Levine, I. N. (2014). Physical chemistry (6th ed.). McGraw-Hill Education.