Hess Law Chemistry Problems - Enthalpy Change - Constant Heat of Summation

The Organic Chemistry Tutor

26 Sept 201620:42

EducationalLearning

32 Likes 10 Comments

TLDRThis video script explores Hess's Law in thermochemistry, illustrating how to calculate the enthalpy change for a reaction by combining and manipulating known reactions. It demonstrates the process through examples, including adjusting coefficients and reversing reactions to match the desired outcome, and emphasizes the importance of focusing on unique substances in each equation to predict enthalpy changes accurately.

Takeaways

🔍 Hess's Law allows the prediction of the enthalpy change for a reaction by combining the enthalpy changes of other known reactions.
⚖️ When adding reactions, substances that appear on both sides of the equation cancel out, simplifying to the net reaction.
📈 The enthalpy change of the resulting equation is the sum of the enthalpy changes of the individual reactions used to derive it.
🔄 Multiplying a reaction by a factor affects the enthalpy change proportionally; for example, doubling a reaction doubles its enthalpy change.
🔄 Conversely, if a reaction is halved, its enthalpy change is also halved.
🔀 Reversing a reaction changes the sign of its enthalpy change from positive to negative or vice versa.
🎯 Focus on substances that appear in only one of the reactions when combining them to form a new reaction equation.
✂️ Adjust the coefficients of the reactions to match the desired net reaction, which may involve reversing or multiplying the reactions.
📚 Practice problems demonstrate the application of Hess's Law in calculating enthalpy changes for complex reactions.
🧩 In practice problems, identify substances that cancel out and those that remain to construct the desired net reaction.
📉 The final enthalpy change is calculated by summing the adjusted enthalpy changes of the individual reactions.

Q & A

What is Hess's Law in the context of the video?
-Hess's Law states that if a reaction can be expressed as a sum of two or more other reactions, the overall enthalpy change of the reaction is the sum of the enthalpy changes of the individual reactions.
How does the enthalpy change of a reaction change if the reaction is multiplied by a certain factor?
-If a reaction is multiplied by a factor, the enthalpy change of that reaction is also multiplied by the same factor.
What happens to the enthalpy change when a reaction is reversed?
-When a reaction is reversed, the sign of the enthalpy change also changes, meaning it becomes its negative value.
In the practice problem involving reactions A+B, C+D, and A+B+D to E, how is the enthalpy change of the final reaction calculated?
-The enthalpy change for the final reaction is calculated by adjusting and adding the enthalpy changes of the initial reactions, ensuring that the stoichiometry matches the desired final reaction.
What is the significance of focusing on substances found in only one of the initial reactions when applying Hess's Law?
-Focusing on substances that are found in only one of the initial reactions helps in determining how to adjust (multiply or reverse) the reactions to achieve the desired final reaction.
How does the video script illustrate the application of Hess's Law with the decomposition of water and hydrofluoric acid?
-The script uses the decomposition reactions of water and hydrofluoric acid to demonstrate how to manipulate and combine these reactions to predict the enthalpy change for the reaction of fluorine with water to produce hydrofluoric acid and oxygen.
What is the enthalpy change for the reaction of fluorine with water to produce hydrofluoric acid and oxygen gas, as calculated in the script?
-The enthalpy change for the reaction of fluorine with water to produce hydrofluoric acid and oxygen gas is calculated to be -512 Joules.
In the script, why is it recommended not to focus on substances found in multiple reactions when applying Hess's Law?
-Focusing on substances found in multiple reactions can lead to confusion and incorrect manipulation of the reactions, as it may not be clear how to adjust these substances to achieve the desired final reaction.
What is the final enthalpy change calculated for the reaction of ammonia with oxygen gas to produce nitrogen monoxide and water, according to the script?
-The final enthalpy change for the reaction of ammonia with oxygen gas to produce nitrogen monoxide and water is calculated to be -890 Joules.
How does the script use the decomposition of water, carbon dioxide, and the reaction of acetylene with oxygen to calculate the enthalpy change for the decomposition of acetylene?
-The script manipulates the given reactions by adjusting their stoichiometry and enthalpy changes to match the desired reaction of acetylene decomposing into elemental carbon and hydrogen gas, resulting in a calculated enthalpy change of -226 Joules.

Outlines

00:00

🔍 Hess's Law and Enthalpy Change Calculation

This paragraph introduces Hess's Law, which states that the total enthalpy change of a chemical reaction can be calculated by summing the enthalpy changes of individual steps that lead to the overall reaction. The example given involves two reactions, the first converting A + B to C with an enthalpy change of 100 J, and the second converting C + D to E with an enthalpy change of 200 J. By adding these reactions, C cancels out, leaving A + B + D to E, with a total enthalpy change of 300 J. The paragraph also explains the effects of multiplying or reversing reactions on their enthalpy changes and provides a practice problem involving reactions with A, B, C, D, and E to demonstrate the application of Hess's Law.

05:02

🧪 Applying Hess's Law to Decomposition Reactions

This paragraph presents a practical application of Hess's Law using the decomposition of water and hydrofluoric acid into their constituent gases. The enthalpy changes for these decompositions are given as +572 J and +542 J, respectively. The goal is to find the enthalpy change for the reaction where fluorine reacts with water to form hydrofluoric acid and oxygen. The paragraph explains how to adjust and combine the given reactions to match the desired net reaction, emphasizing the importance of focusing on substances that appear in only one of the initial reactions to avoid confusion. The final enthalpy change for the reaction is calculated to be -512 J.

10:05

📚 More Practice with Hess's Law and Reaction Adjustments

The third paragraph continues the practice of using Hess's Law with additional reactions involving the decomposition of ammonia, water, and the formation of nitrogen monoxide. The enthalpy changes for these reactions are +92 J, +572 J, and -180 J, respectively. The task is to calculate the enthalpy change for the reaction where ammonia and oxygen produce nitrogen monoxide and water. The paragraph details the process of identifying substances to focus on, adjusting the given reactions by reversing or multiplying as necessary, and summing the adjusted enthalpy changes to find the final answer, which is -890 J.

15:06

🌐 Advanced Hess's Law Calculations with Multiple Reactions

This paragraph presents a more complex example involving three reactions: the decomposition of water, carbon dioxide, and the reaction of acetylene with oxygen to produce carbon dioxide and water. The enthalpy changes are given as +572 J, +394 J, and -2600 J, respectively. The challenge is to calculate the enthalpy change for the decomposition of acetylene into elemental carbon and hydrogen gas. The paragraph outlines the process of identifying substances to focus on, adjusting the reactions by reversing or multiplying, and summing the enthalpy changes. The final enthalpy change for the decomposition of acetylene is calculated to be -226 J.

20:07

🔬 Final Practice with Hess's Law for Decomposition

The final paragraph provides a last practice example using the decomposition of water, carbon dioxide, and the reaction of acetylene with oxygen. The enthalpy changes are +572 J, +394 J, and -2600 J, respectively. The goal is to calculate the enthalpy change for the decomposition of acetylene into elemental carbon and hydrogen gas. The paragraph explains the process of adjusting the given reactions, focusing on substances unique to each reaction, and summing the adjusted enthalpy changes. The final enthalpy change for the decomposition of acetylene is determined to be -226 J.

Mindmap

Keywords

💡Hess's Law

Hess's Law is a principle in thermodynamics that states the total enthalpy change for a chemical reaction is the same, no matter how many steps or what path the reaction takes to reach the final products. In the video, Hess's Law is the central theme, as it is used to predict the enthalpy change for a reaction by adding or modifying other known reactions, as illustrated in the examples provided.

💡Enthalpy Change

Enthalpy change, often denoted as ΔH, is the amount of heat absorbed or released in a chemical reaction at constant pressure. It is a key concept in the video, where the enthalpy changes of individual reactions are added or subtracted to find the enthalpy change for a new, combined reaction, as per Hess's Law.

💡Reaction

A reaction in chemistry refers to a process where one or more substances are transformed into others. In the video, reactions are the building blocks used to demonstrate Hess's Law, with each reaction having its own enthalpy change that contributes to the overall calculation.

💡Canceling

In the context of the video, 'canceling' refers to the process of eliminating a substance from both sides of a combined reaction equation when it appears as a reactant and a product in different reactions being added together. This simplifies the equation to the net reaction of interest, as shown in the script examples.

💡Multiplying Reactions

Multiplying reactions means increasing the coefficients of the reactants and products in a chemical equation by a certain factor. In the video, this concept is used to adjust the amounts of substances in reactions to match the desired net reaction, and the enthalpy change is also multiplied by the same factor.

💡Reversing Reactions

Reversing a reaction involves flipping the direction of the reaction, changing the products to reactants and vice versa. In the video, reversing reactions is a strategy used to align the substances in a way that they can be combined according to Hess's Law, with the enthalpy change sign also being reversed.

💡Practice Problem

A practice problem in the video is a specific example given to the viewer to apply the concepts of Hess's Law and enthalpy changes. It serves as an interactive way for the viewer to engage with the material and understand how to calculate the enthalpy change for a new reaction using known reactions.

💡Decomposition Reaction

A decomposition reaction is a type of chemical reaction where one substance breaks down into two or more simpler substances. In the video, decomposition reactions are used as part of the examples to demonstrate how to calculate enthalpy changes using Hess's Law.

💡Combination Reaction

A combination reaction is a chemical reaction where two or more substances combine to form a single product. While not explicitly mentioned in the script, the concept is implied in the discussion of reactions that form new compounds, such as the formation of hydrofluoric acid from fluorine and water.

💡Thermodynamics

Thermodynamics is the branch of physics that deals with heat and temperature and their relation to energy and work. In the video, thermodynamics is the scientific field within which Hess's Law and enthalpy changes are studied and applied to predict and understand chemical reactions.

Highlights

Introduction to Hess's Law and its application in solving thermochemical problems.

Demonstration of how to combine two reactions to form a third, using the concept of Hess's Law.

Explanation of how to calculate the enthalpy change for a new reaction by summing the enthalpy changes of the original reactions.

Rule of multiplying a reaction by a factor and its effect on the enthalpy change.

Rule of reversing a reaction and its impact on the sign of the enthalpy change.

Practice problem involving the calculation of enthalpy change for a reaction using given reactions and their enthalpy changes.

Strategy for adjusting equations by focusing on substances unique to one or the other of the original equations.

Method for reversing and multiplying reactions to match the desired net reaction.

Example of calculating the enthalpy change for the reaction of fluorine with water to produce hydrofluoric acid and oxygen gas.

Approach to modifying reactions by focusing on substances not found in multiple reactions to simplify the process.

Calculation of the enthalpy change for the reaction of ammonia with oxygen gas to produce nitrogen monoxide and water.

Use of the enthalpy values of decomposition reactions to predict the enthalpy change for a synthesis reaction.

Example of calculating the enthalpy change for the decomposition of acetylene into elemental carbon and hydrogen gas.

Technique for identifying substances to focus on by excluding those found in multiple reactions to avoid confusion.

Process of canceling out substances on both sides of the reaction to simplify the net reaction.

Final calculation of the enthalpy change for the desired reaction by summing the adjusted enthalpy values.

Transcripts

Browse More Related Video

Hess's Law Problems & Enthalpy Change - Chemistry

Hess's Law Common Test Question

Hess's Law and Heats of Formation

Enthalpy Change of Reaction & Formation - Thermochemistry & Calorimetry Practice Problems

Hess's law example | Thermodynamics | Chemistry | Khan Academy

Enthalpy: Crash Course Chemistry #18

Related Tags

Hess's Law Chemistry Enthalpy Change Reaction Prediction Thermochemistry Chemical Reactions Educational Video Science Tutorial Problem Solving Chemical Education