Master Hess's Law and Enthalpy Change in Chemistry

Master Hess's Law and Enthalpy Change in Chemistry

Table of Contents:

1. Introduction
2. What is Hess's Law?
3. Calculating Enthalpy Change using Hess's Law
4. Adjusting Reactions in Hess's Law 4.1 Adjusting Reaction Coefficients 4.2 Reversing Reactions
5. Focusing on the Right Species
6. Adding Reactions
7. Example 1: Calculating Enthalpy Change of Ammonia Reaction
8. Example 2: Calculating Enthalpy Change of Hydrogen and Oxygen Reaction
9. Example 3: Calculating Enthalpy Change of Nitrogen Monoxide and Atomic Oxygen Reaction
10. Conclusion

Introduction

In chemistry, calculating the enthalpy change of a reaction is crucial in understanding the energy involved in the reaction. One method to determine the enthalpy change is through Hess's Law. This law allows us to use the enthalpy changes of other reactions to calculate the enthalpy change of a target reaction. In this article, we will explore the concept of Hess's Law and learn how to Apply it step by step to solve different examples.

What is Hess's Law?

Hess's Law states that the enthalpy change of a reaction is the same regardless of the route taken from reactants to products. This means that if a reaction can be expressed as a series of other reactions, the sum of the enthalpy changes of those reactions will be equal to the enthalpy change of the target reaction. By utilizing this law, we can calculate the enthalpy change of a reaction using known enthalpy changes of other reactions.

Calculating Enthalpy Change using Hess's Law

To calculate the enthalpy change of a reaction using Hess's Law, we need to follow a set of steps. First, we identify the reactions that can be used to construct the target reaction. Next, we adjust these reactions by changing coefficients or reversing them as necessary. Then, we focus on the specific species in the reactions and ensure they appear only once in the overall reaction. Finally, we add the adjusted reactions, taking into account the enthalpy changes, to obtain the enthalpy change of the target reaction.

Adjusting Reactions in Hess's Law

When using Hess's Law, we often need to adjust the reactions to ensure they match the target reaction. There are two main adjustments: changing reaction coefficients and reversing reactions.

• Adjusting Reaction Coefficients: We can change the coefficients in front of the reactants and products to balance the number of each species in the overall reaction. It is important to adjust these coefficients carefully, considering the stoichiometry of the reactions.

• Reversing Reactions: If a reaction needs to be reversed to match the target reaction, we must also change the sign of the enthalpy change associated with that reaction. This is because the enthalpy change is a state function and depends on the direction of the reaction.

Focusing on the Right Species

To simplify the application of Hess's Law, we need to focus on the species that appear only once in the reactions. By doing so, we avoid unnecessary complications and reduce the complexity of the calculations. For example, if a species is present in multiple reactions, it is better to choose another species that appears only in one reaction.

Adding Reactions

After adjusting the reactions, we add them together to obtain the net reaction. This net reaction represents the target reaction with the adjusted enthalpy change. By adding the enthalpy changes of the adjusted reactions, we can determine the enthalpy change of the target reaction.

Example 1: Calculating Enthalpy Change of Ammonia Reaction

Let's consider a reaction where nitrogen gas reacts with hydrogen gas to produce ammonia. The enthalpy change for this reaction is given as -92 kilojoules. We can calculate this enthalpy change using other known reactions and applying Hess's Law.

Example 2: Calculating Enthalpy Change of Hydrogen and Oxygen Reaction

Another example involves the decomposition of Water into hydrogen gas and oxygen gas. The enthalpy change for this reaction is positive 286 kilojoules. By utilizing Hess's Law and combining this reaction with others, we can determine the enthalpy change of the target reaction.

Example 3: Calculating Enthalpy Change of Nitrogen Monoxide and Atomic Oxygen Reaction

In this example, we will calculate the enthalpy change of the reaction between nitrogen monoxide and atomic oxygen to produce nitrogen dioxide and oxygen gas. By applying Hess's Law and considering the enthalpy changes of different reactions, we can solve for the enthalpy change of the target reaction.

Conclusion

Hess's Law provides a useful tool in calculating the enthalpy change of a reaction by utilizing known enthalpy changes of other reactions. By adjusting and combining these reactions, we can determine the enthalpy change of the target reaction accurately. Understanding the concepts and steps involved in applying Hess's Law allows us to unlock a deeper understanding of the energy changes in chemical reactions.

Highlights:

• Hess's Law enables the calculation of enthalpy change using known reactions.
• Adjusting reaction coefficients and reversing reactions are crucial in applying Hess's Law.
• Focusing on species that appear only once simplifies the calculations.
• Adding adjusted reactions determines the enthalpy change of the target reaction.
• Examples illustrate the step-by-step application of Hess's Law.

FAQ:

Q: What is Hess's Law? A: Hess's Law states that the enthalpy change of a reaction is independent of the route taken from reactants to products. It allows us to use known enthalpy changes of other reactions to calculate the enthalpy change of a target reaction.

Q: How do You adjust reactions in Hess's Law? A: Reactions can be adjusted by changing coefficients and reversing them if necessary. Adjusting coefficients ensures the stoichiometry matches the target reaction, while reversing reactions requires changing the sign of the enthalpy change.

Q: Why is it important to focus on the right species in Hess's Law? A: Focusing on the species that appear only once in the reactions simplifies the calculations and avoids unnecessary complications. It reduces the complexity and improves the accuracy of the calculations.

Q: How do you calculate the enthalpy change using Hess's Law? A: To calculate the enthalpy change, we adjust the reactions, focus on the right species, and add the adjusted reactions together. The sum of the enthalpy changes of the adjusted reactions gives the enthalpy change of the target reaction.