Calculating Enthalpy Changes

Oct 13, 2025 by wbrum.org 121 views

Introduction

In chemistry, thermochemical equations are used to describe the relationship between the energy changes that occur during a chemical reaction and the reactants and products involved. These equations are essential in understanding the thermodynamics of chemical reactions and are used to calculate the enthalpy changes associated with a reaction. In this article, we will discuss the given data and use it to calculate the enthalpy change for a specific reaction.

Given Data

The given data consists of three thermochemical equations:

$2 \, O_3(g) \rightarrow 3 \, O_2(g)$ with $\Delta H = -427 \, \text{kJ}$
$O_2(g) \rightarrow 2 \, O(g)$ with $\Delta H = +495 \, \text{kJ}$
$NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$ with $\Delta H = -199 \, \text{kJ}$

Calculating Enthalpy Changes

To calculate the enthalpy change for a specific reaction, we need to use the given data and manipulate it to obtain the desired reaction. Let's consider the reaction $2 \, O(g) \rightarrow O_2(g)$ , which is not directly given in the data. However, we can use the given equations to obtain this reaction.

First, let's reverse the second equation to obtain $2 \, O(g) \rightarrow O_2(g)$ with $\Delta H = -495 \, \text{kJ}$ . This is done by multiplying the original equation by $-1$ and changing the sign of the enthalpy change.

Next, let's use the first equation to obtain $2 \, O_3(g) \rightarrow 3 \, O_2(g)$ with $\Delta H = -427 \, \text{kJ}$ . We can then multiply this equation by $1/3$ to obtain $2/3 \, O_3(g) \rightarrow O_2(g)$ with $\Delta H = -141.67 \, \text{kJ}$ .

Now, let's add the two equations to obtain the desired reaction $2 \, O(g) \rightarrow O_2(g)$ with $\Delta H = -636.67 \, \text{kJ}$ .

Calculating the Enthalpy Change for the Desired Reaction

The desired reaction is $2 \, O(g) \rightarrow O_2(g)$ . To calculate the enthalpy change for this reaction, we need to use the given data and manipulate it to obtain the desired reaction.

Let's start by using the first equation to obtain $2 \, O_3(g) \rightarrow 3 \, O_2(g)$ with $\Delta H = -427 \, \text{kJ}$ . We can then multiply this equation by $1/3$ to obtain $2/3 \, O_3(g) \rightarrow O_2(g)$ with $\Delta H = -141.67 \, \text{kJ}$ .

Next, let's use the second equation to obtain $O_2(g) \rightarrow 2 \, O(g)$ with $\Delta H = +495 \, \text{kJ}$ . We can then multiply this equation by $1/2$ to obtain $O_2(g) \rightarrow O(g)$ with $\Delta H = +247.5 \, \text{kJ}$ .

Now, let's add the two equations to obtain the desired reaction $2 \, O(g) \rightarrow O_2(g)$ with $\Delta H = -389.17 \, \text{kJ}$ .

Conclusion

In this article, we have discussed the given data and used it to calculate the enthalpy change for a specific reaction. We have shown that by manipulating the given equations, we can obtain the desired reaction and calculate the enthalpy change associated with it. This demonstrates the importance of thermochemical equations in understanding the thermodynamics of chemical reactions.

References

Atkins, P. W., & de Paula, J. (2010). Physical Chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Chemistry: The Central Science (11th ed.). McGraw-Hill.

Calculating the Enthalpy Change for the Desired Reaction

The desired reaction is $2 \, O(g) \rightarrow O_2(g)$ . To calculate the enthalpy change for this reaction, we need to use the given data and manipulate it to obtain the desired reaction.

Now, let's add the two equations to obtain the desired reaction $2 \, O(g) \rightarrow O_2(g)$ with $\Delta H = -389.17 \, \text{kJ}$ .

Calculating the Enthalpy Change for the Desired Reaction

The desired reaction is $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$ . To calculate the enthalpy change for this reaction, we need to use the given data and manipulate it to obtain the desired reaction.

Next, let's use the third equation to obtain $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$ with $\Delta H = -199 \, \text{kJ}$ .

Now, let's add the two equations to obtain the desired reaction $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$ with $\Delta H = -341.67 \, \text{kJ}$ .

Conclusion

References

Atkins, P. W., & de Paula, J. (2010). Physical Chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Chemistry: The Central Science (11th ed.). McGraw-Hill.

Introduction

In our previous article, we discussed the importance of thermochemical equations in understanding the thermodynamics of chemical reactions. We also showed how to calculate the enthalpy change for a specific reaction using the given data. In this article, we will answer some frequently asked questions about thermochemical equations and enthalpy changes.

Q: What is a thermochemical equation?

A: A thermochemical equation is a chemical equation that includes the enthalpy change associated with the reaction. It is used to describe the relationship between the energy changes that occur during a chemical reaction and the reactants and products involved.

Q: How do I calculate the enthalpy change for a reaction?

A: To calculate the enthalpy change for a reaction, you need to use the given data and manipulate it to obtain the desired reaction. This involves reversing equations, multiplying them by fractions, and adding them together to obtain the desired reaction.

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

A: Enthalpy (H) is a measure of the total energy of a system, including the energy associated with the motion of its particles. Internal energy (U) is a measure of the energy associated with the motion of the particles in a system. The enthalpy change is the change in the total energy of a system, while the internal energy change is the change in the energy associated with the motion of the particles.

Q: How do I determine the sign of the enthalpy change for a reaction?

A: The sign of the enthalpy change for a reaction is determined by the direction of the reaction. If the reaction is exothermic (releases energy), the enthalpy change is negative. If the reaction is endothermic (absorbs energy), the enthalpy change is positive.

Q: What is the significance of the enthalpy change for a reaction?

A: The enthalpy change for a reaction is a measure of the energy associated with the reaction. It can be used to predict the spontaneity of a reaction, the amount of energy released or absorbed during the reaction, and the feasibility of a reaction.

Q: How do I use thermochemical equations to predict the spontaneity of a reaction?

A: To predict the spontaneity of a reaction using thermochemical equations, you need to calculate the enthalpy change for the reaction. If the enthalpy change is negative, the reaction is spontaneous. If the enthalpy change is positive, the reaction is non-spontaneous.

Q: What are some common mistakes to avoid when working with thermochemical equations?

A: Some common mistakes to avoid when working with thermochemical equations include:

Not reversing equations correctly
Not multiplying equations by fractions correctly
Not adding equations together correctly
Not considering the direction of the reaction when determining the sign of the enthalpy change

Conclusion

In this article, we have answered some frequently asked questions about thermochemical equations and enthalpy changes. We have shown how to calculate the enthalpy change for a reaction using the given data and how to use thermochemical equations to predict the spontaneity of a reaction. We hope that this article has been helpful in understanding the importance of thermochemical equations in understanding the thermodynamics of chemical reactions.

References

Atkins, P. W., & de Paula, J. (2010). Physical Chemistry (9th ed.). Oxford University Press.
Chang, R. (2010). Chemistry: The Central Science (11th ed.). McGraw-Hill.