HUKUM HESS

WIN'S CHEMISTRY CLASS

2 Sept 202014:36

Summary

TLDRThe video script discusses Hess's Law, a fundamental concept in thermochemistry, which states that the total enthalpy change of a reaction is independent of the path taken and depends only on the initial and final states. The script uses the example of carbon combustion to illustrate how a single-step reaction can be broken into multiple steps, with the total enthalpy change remaining constant. It also explains how to adjust coefficients and manipulate equations to calculate the enthalpy change for different reactions, providing insights into the calculation of enthalpy changes for complex reactions.

Takeaways

🔬 The Hess's Law is a fundamental principle in thermochemistry, introduced by Germain Hess in 1848.
🔥 Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or several steps.
🌡 The law is based on experimental observations that not all chemical reactions occur in a single step.
🔍 An example given is the combustion of carbon, which can occur in a single step to form carbon dioxide or in two steps, first forming carbon monoxide.
📐 The enthalpy change for the combustion of carbon to carbon dioxide is the same whether it occurs in one or two steps, totaling -394 kJ.
📉 For reactions occurring in multiple steps, the overall enthalpy change is the sum of the enthalpy changes for each individual step.
🧪 Coefficients of identical substances in a balanced equation are adjusted based on their positions; opposite sides reduce each other, while the same side sums up.
📚 The law is applied to calculate the enthalpy change for the formation of SO3, showing that the enthalpy change remains consistent across different pathways.
📝 Hess's Law can be used to solve problems by manipulating known reactions to match the desired reaction conditions.
🔋 An example calculation is provided for the production of acetylene (C2H2) from its elements, demonstrating the application of Hess's Law to find the enthalpy change for a reaction not directly measured.

Q & A

Who is the scientist credited with Hess's Law?
-Hess's Law is credited to the German scientist Germain Hess.
What is the main concept of Hess's Law?
-Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps the reaction is carried out in.
What is an example of a reaction that can occur in more than one step according to the transcript?
-The combustion of carbon to form carbon dioxide can occur in two steps: first, carbon reacts with oxygen to form carbon monoxide, and then carbon monoxide reacts with oxygen to form carbon dioxide.
What is the enthalpy change for the single-step reaction of carbon combustion as mentioned in the transcript?
-The enthalpy change for the single-step reaction of carbon combustion to form carbon dioxide is -394 kilojoules.
How does the enthalpy change for a reaction in multiple steps compare to the single-step reaction according to Hess's Law?
-According to Hess's Law, the total enthalpy change for a reaction that occurs in multiple steps is the sum of the enthalpy changes for each step, and it should be equal to the enthalpy change of the reaction if it occurred in a single step.
What does the transcript say about the coefficients of substances in a reaction?
-The transcript explains that if the same substance appears on both sides of a reaction, its coefficients are subtracted if they are on opposite sides and added if they are on the same side.
How is the enthalpy change for a reaction calculated when the reaction occurs in multiple steps?
-The enthalpy change for a reaction that occurs in multiple steps is calculated by summing the enthalpy changes of each individual step.
What is the significance of the diagram mentioned in the transcript for understanding Hess's Law?
-The diagram helps visualize how the enthalpy change is calculated for reactions that occur in a single step versus multiple steps, illustrating the principle that the total enthalpy change is the same regardless of the reaction pathway.
Can you provide an example of how to apply Hess's Law to calculate the enthalpy change for a reaction involving SO3 formation as described in the transcript?
-Yes, the transcript provides an example where the formation of SO3 can occur in a single step with a ΔH of -794 kJ, or in two steps with ΔH1 of -539.8 kJ for the first step and ΔH2 for the second step. According to Hess's Law, ΔH for the overall reaction is the sum of ΔH1 and ΔH2.
What is the final enthalpy change calculated for the two-step reaction of SO3 formation in the transcript?
-The final enthalpy change for the two-step reaction of SO3 formation is calculated to be -250 kJ, after applying Hess's Law and considering the enthalpy changes of both steps.
How does the transcript describe the process of calculating the enthalpy change for the reaction of acetylene (C2H2) formation?
-The transcript describes a process where multiple reactions are adjusted (reversed or multiplied by factors) to match the desired reaction for acetylene formation. The total enthalpy change is then calculated by summing the adjusted enthalpy changes of these reactions.

Outlines

00:00

🔬 Hess's Law Introduction

The paragraph introduces Hess's Law, a principle in thermochemistry that states the total enthalpy change of a reaction is the same whether the reaction occurs in one step or several steps. The law is demonstrated using the example of carbon combustion, showing how the reaction can be split into two stages. The first stage involves the reaction of carbon with oxygen to form carbon monoxide, and the second stage involves the reaction of carbon monoxide with oxygen to form carbon dioxide. The enthalpy changes for each stage are detailed, and it is explained how the total enthalpy change for the overall reaction is the sum of the enthalpy changes for the individual stages.

05:01

📊 Diagrammatic Representation of Hess's Law

This paragraph explains how to graphically represent reactions according to Hess's Law. It uses the example of sulfur trioxide (SO3) formation, which can occur in one or two steps. The paragraph describes how to draw the diagrams for both single-step and two-step reactions, and how to calculate the overall enthalpy change by summing the enthalpy changes of the individual steps. It also provides a method for solving problems using Hess's Law by adjusting coefficients and reversing reactions to match the desired reaction.

10:02

🔄 Application of Hess's Law in Reaction Calculations

The final paragraph applies Hess's Law to calculate the enthalpy change for the production of acetylene (C2H2) from its elements. It details how to manipulate existing reaction equations to fit the desired reaction, including reversing reactions and adjusting coefficients. The paragraph walks through the process of calculating the enthalpy change for the formation of C2H2 by combining and adjusting the enthalpy changes from three given reactions. The final calculation results in a positive enthalpy change, indicating the reaction is endothermic.

Mindmap

Keywords

💡Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps or pathways the reaction takes. It's a fundamental concept in thermochemistry. In the video, Hess's Law is used to explain why the overall enthalpy change for the combustion of carbon, whether it occurs in one step or two, remains the same.

💡Enthalpy

Enthalpy is a thermodynamic potential that measures the total energy of a thermodynamic system. It is used in the video to describe the energy changes in chemical reactions, such as the combustion of carbon to carbon dioxide. The script mentions enthalpy changes for different reactions, illustrating how they can be calculated using Hess's Law.

💡Thermochemistry

Thermochemistry is the study of the heat and energy changes during chemical reactions. The video script discusses various thermochemical concepts, such as enthalpy and Hess's Law, to explain how reactions can occur in multiple steps while still obeying the conservation of energy.

💡Reversible Reactions

Reversible reactions are those that can proceed in both the forward and reverse directions. The script uses the concept of reversible reactions to explain how certain chemical processes, like the formation of CO2 from carbon, can occur in multiple steps.

💡Exothermic Reaction

An exothermic reaction is one that releases energy, usually in the form of heat, to its surroundings. The video mentions exothermic reactions, such as the combustion of carbon, which releases energy and results in a negative enthalpy change.

💡Endothermic Reaction

An endothermic reaction absorbs energy from its surroundings. While the video primarily focuses on exothermic reactions, the concept of endothermic reactions is implicitly discussed when contrasting the energy changes in different reaction steps.

💡Phases of Matter

The script refers to different phases of matter, such as solid, liquid, and gas, in the context of chemical reactions. Understanding the phases of reactants and products is crucial for calculating enthalpy changes and applying Hess's Law.

💡Calorimetry

Calorimetry is the technique of measuring the heat of chemical reactions. Although not explicitly mentioned in the script, the concept is relevant to the discussion of enthalpy changes and the application of Hess's Law.

💡Carbon Monoxide

Carbon monoxide (CO) is a compound formed during the incomplete combustion of carbon. The video script uses CO as an intermediate product in the two-step combustion process of carbon, illustrating how Hess's Law can be applied to multi-step reactions.

💡Carbon Dioxide

Carbon dioxide (CO2) is a compound that results from the complete combustion of carbon. The script discusses the formation of CO2 in both one-step and two-step reactions, highlighting how the overall enthalpy change remains constant.

💡Enthalpy Change Calculation

The calculation of enthalpy change is a key part of the video's discussion. It involves determining the energy difference between the reactants and products in a chemical reaction. The script provides examples of how to calculate enthalpy changes for different reactions using Hess's Law.

Highlights

In 1848, a scientist named Germain Hess introduced a law related to thermochemistry, now known as Hess's Law.

Hess's Law states that the enthalpy change of a reaction is independent of the pathway but depends only on the initial and final states.

The combustion of carbon to form carbon dioxide can occur in a single step or in two stages.

In the two-step reaction, carbon first reacts with oxygen to form carbon monoxide, releasing -111 kJ of enthalpy.

In the second step, carbon monoxide reacts with oxygen to form carbon dioxide, releasing -283 kJ of enthalpy.

Regardless of whether the reaction happens in one step or two, the total enthalpy change is the same: -394 kJ.

Hess's Law is illustrated through reaction diagrams that show enthalpy changes between reactants and products.

The enthalpy change for the formation of sulfur trioxide (SO3) from sulfur and oxygen can also occur in one or two stages.

For the two-step SO3 formation, sulfur first reacts with oxygen to form sulfur dioxide (SO2), releasing -539.8 kJ of enthalpy.

In the second step, SO2 reacts with oxygen to form SO3, and the enthalpy change can be calculated using Hess's Law.

Hess's Law states that for multi-step reactions, the total enthalpy change equals the sum of individual steps' enthalpy changes.

Another example: acetylene (C2H2) can be produced through multiple reactions, and Hess’s Law is used to calculate its enthalpy change.

In such calculations, reactions may be reversed or multiplied by factors to align with the desired chemical equation.

For each chemical equation, enthalpy changes must be adjusted accordingly when reactions are reversed or scaled.

After adjusting for coefficients and reversing reactions as needed, the total enthalpy change for C2H2 formation is +227 kJ/mol.