Hess's Law Chemistry Worksheet 16.5 Answers Explained
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Hess's Law is a fundamental principle in chemistry that relates to the enthalpy changes of chemical reactions. It states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction takes. In other words, if a reaction can be expressed as the sum of multiple reactions, the total enthalpy change will equal the sum of the enthalpy changes for those individual steps. This law is crucial for calculating enthalpy changes when direct measurement is difficult.
In this article, we will explore Hess's Law and provide a detailed explanation of the answers for Worksheet 16.5, making it easier for students to understand how to apply this concept effectively. We’ll also present some illustrative examples and helpful tips for mastering Hess's Law.
Understanding Hess's Law 🌡️
Hess's Law is based on the first law of thermodynamics, which states that energy cannot be created or destroyed in an isolated system. The energy of the products and reactants is independent of the path taken. This principle allows chemists to calculate enthalpy changes for reactions by using known enthalpy values of individual reactions.
The Importance of Hess's Law 🔍
- Calculating Enthalpy Changes: Many reactions are difficult to measure directly. Hess's Law provides a way to calculate enthalpy changes using simpler reactions whose enthalpy changes are known.
- Thermodynamic Predictions: It can be used to predict whether a reaction will be exothermic or endothermic based on the enthalpy changes of its components.
- Practical Applications: It is widely used in various fields, including engineering, environmental science, and industrial chemistry.
Worksheet 16.5 Overview ✏️
Worksheet 16.5 typically contains a series of problems that require students to apply Hess's Law to determine enthalpy changes. Below, we will outline the steps involved in solving such problems and provide answers with explanations.
Sample Problem Structure
Before we dive into the answers, let's look at a typical structure for a problem in Worksheet 16.5.
- Identify the Reactions: You will be given a series of chemical equations.
- Assign Enthalpy Values: Each reaction will have a corresponding enthalpy change (ΔH).
- Combine Reactions: Use Hess's Law to combine reactions to arrive at the target reaction.
- Calculate Total Enthalpy Change: Sum the enthalpy changes from the individual reactions.
Example Problems with Solutions
Let’s consider a simplified version of a problem you might encounter in Worksheet 16.5.
Problem 1: Formation of Water 💧
Given Reactions:
- ( \text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) ) ΔH = -286 kJ
- ( \text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g) ) ΔH = -394 kJ
Target Reaction:
- ( \text{H}_2(g) + \text{C}(s) + \frac{1}{2}\text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) + \text{CO}_2(g) )
Solution Steps:
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Reverse the first reaction (since it needs to be subtractive):
( \text{H}_2\text{O}(l) \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g) ) ΔH = +286 kJ
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Now add the reversed reaction to the second reaction:
[ \begin{align*} \text{H}_2\text{O}(l) & \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \quad (+286 \text{ kJ})\ \text{C}(s) + \text{O}_2(g) & \rightarrow \text{CO}_2(g) \quad (-394 \text{ kJ})\ \hline \text{H}_2(g) + \text{C}(s) + \frac{1}{2}\text{O}_2(g) & \rightarrow \text{H}_2\text{O}(l) + \text{CO}_2(g) \quad \Delta H = -108 \text{ kJ} \end{align*} ]
The total enthalpy change is therefore:
[ \Delta H = 286 \text{ kJ} - 394 \text{ kJ} = -108 \text{ kJ} ]
Important Notes 📌
- Reversing a Reaction: Remember that when you reverse a reaction, the sign of the enthalpy change also reverses.
- Stoichiometric Coefficients: When you multiply a reaction by a coefficient, you must also multiply the ΔH by that coefficient.
Sample Problems Continued
Let's explore some additional examples using a table to present data clearly.
<table> <tr> <th>Reaction</th> <th>ΔH (kJ)</th> </tr> <tr> <td>1. A + B → C</td> <td>-200</td> </tr> <tr> <td>2. C → D + E</td> <td>+150</td> </tr> <tr> <td>3. D + F → G</td> <td>-100</td> </tr> </table>
Target Reaction:
- ( A + B + F \rightarrow G + E )
Solution:
- Keep the first reaction as is.
- Reverse the second reaction:
- ( D + E \rightarrow C \quad ΔH = -150 )
- Keep the third reaction as is.
Using Hess's Law to combine these gives:
[ A + B + F \rightarrow G + E \quad \Delta H = -200 + (-150) + (-100) = -450 \text{ kJ} ]
Conclusion and Final Thoughts 🌟
Hess's Law is a powerful tool that simplifies the process of calculating the enthalpy changes in chemical reactions. Understanding how to apply it through practice worksheets, like Worksheet 16.5, can significantly enhance your ability to solve complex thermochemical problems. The examples provided illustrate the methodical approach necessary for using Hess's Law effectively.
As you continue your studies in chemistry, remember the importance of practicing problems and consulting various resources to solidify your understanding. The mastery of Hess's Law will not only benefit you in academics but also in practical applications within the field of chemistry!