Hess's Law Problems & Enthalpy Change - Chemistry

The Organic Chemistry Tutor
22 Sept 201714:03
EducationalLearning
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TLDRThis video script delves into Hess's Law, a fundamental principle in thermodynamics for calculating the enthalpy change of chemical reactions. It guides viewers through the process of manipulating known reactions and their enthalpy changes to determine the enthalpy change of a target reaction. The script provides step-by-step examples, illustrating how to adjust coefficients, reverse reactions, and sum enthalpy changes to find the desired result. The method is demonstrated with various reactions involving nitrogen, hydrogen, oxygen, and water, showcasing the power of Hess's Law in predicting the energetics of chemical processes.

Takeaways
  • 🔍 Hess's Law is a principle used to calculate the enthalpy change of a reaction by combining the enthalpy changes of known reactions.
  • 🌐 The enthalpy change for the reaction of nitrogen gas and hydrogen gas to produce ammonia is -92 kilojoules.
  • 💧 The enthalpy change for the decomposition of water into hydrogen gas and oxygen gas is +286 kilojoules.
  • 🔄 To apply Hess's Law, adjust the given reactions so that when added together, they result in the desired reaction.
  • 📚 Focus on reactants and products that appear only once in the given reactions to simplify the process.
  • ⚖️ Reverse reactions and adjust coefficients as necessary to align with the target reaction, remembering to change the sign of the enthalpy change when reversing.
  • 📈 Multiply the enthalpy change by the coefficient used to adjust the reaction, ensuring the correct calculation of the final enthalpy change.
  • 🔗 In the example provided, reversing and adjusting the reactions for nitrogen gas and water results in the enthalpy change for the reaction of ammonia with oxygen gas producing nitrogen gas and water being -1532 kilojoules.
  • 🌡️ Another example involves using the enthalpy changes of water decomposition and oxygen breakdown to calculate the enthalpy change for the formation of liquid water from hydrogen gas and atomic oxygen, resulting in -533.5 kilojoules.
  • 🌀 A third example demonstrates how to use the enthalpy changes of ozone decomposition, oxygen gas breakdown, and the reaction of nitrogen monoxide with ozone to determine the enthalpy change for the reaction of nitrogen monoxide with atomic oxygen to form nitrogen dioxide, resulting in -233 kilojoules.
Q & A

  • What is Hess's Law and how does it relate to calculating the enthalpy change of a reaction?

    -Hess's Law states that the total enthalpy change for a reaction is the same, no matter how many steps or what pathway the reaction takes to reach the final products. It allows chemists to calculate the enthalpy change of a reaction by using known enthalpy changes of other reactions and adjusting them to match the desired reaction.

  • What is the enthalpy change for the reaction where nitrogen gas reacts with hydrogen gas to produce ammonia?

    -The enthalpy change for the reaction where nitrogen gas reacts with hydrogen gas to produce ammonia is negative 92 kilojoules.

  • In the given script, what is the enthalpy change for the decomposition of water into hydrogen and oxygen gases?

    -The enthalpy change for the decomposition of water into hydrogen and oxygen gases is positive 286 kilojoules.

  • How can you adjust the given reactions to calculate the enthalpy change for the reaction of ammonia with oxygen gas producing nitrogen gas and water?

    -You can adjust the given reactions by reversing and/or multiplying them to match the desired reaction. Then, you adjust the enthalpy changes accordingly and sum them up to find the enthalpy change for the desired reaction.

  • Why should you focus on reactants and products that appear only once in the given reactions when using Hess's Law?

    -Focusing on reactants and products that appear only once simplifies the process of adjusting the reactions. It avoids unnecessary complexity and ensures that the reactions are correctly manipulated to match the desired reaction.

  • What happens when you reverse a reaction in the context of Hess's Law?

    -When you reverse a reaction, the sign of the enthalpy change for that reaction also changes. For example, if the original reaction has a negative enthalpy change, reversing it will make it positive and vice versa.

  • How does multiplying a reaction by a certain coefficient affect the enthalpy change?

    -Multiplying a reaction by a certain coefficient means that the enthalpy change for that reaction is also multiplied by the same coefficient. This is because the energy change is proportional to the amount of reactants and products involved in the reaction.

  • What is the calculated enthalpy change for the reaction where ammonia reacts with oxygen gas to produce nitrogen gas and water, using the script's method?

    -The calculated enthalpy change for the reaction where ammonia reacts with oxygen gas to produce nitrogen gas and water is negative 1532 kilojoules (184 + (-1716)).

  • Can you provide an example of using Hess's Law with different reactions involving water, oxygen, and hydrogen?

    -Yes, the script provides an example where water decomposes into hydrogen and oxygen with an enthalpy change of positive 576 kilojoules, and oxygen breaks down into atomic oxygen with an input of positive 495 kilojoules. Using these, you can determine the enthalpy change for the reaction of hydrogen gas with atomic oxygen to produce liquid water.

  • What is the enthalpy change for the reaction of nitrogen monoxide with atomic oxygen to produce nitrogen dioxide, according to the script?

    -The enthalpy change for the reaction of nitrogen monoxide with atomic oxygen to produce nitrogen dioxide is negative 233 kilojoules, calculated by summing the adjusted enthalpy changes from the given reactions.

Outlines
00:00
🔍 Hess's Law and Enthalpy Change Calculation

This paragraph introduces Hess's Law, a principle used to calculate the enthalpy change of a chemical reaction by manipulating known reactions. It uses the example of nitrogen and hydrogen gases forming ammonia, with an enthalpy change of -92 kJ. Another example is the decomposition of water into hydrogen and oxygen gases with an enthalpy change of +286 kJ. The paragraph explains the process of adjusting these reactions to derive the enthalpy change for the reaction of ammonia with oxygen gas to produce nitrogen gas and water. It emphasizes focusing on substances that appear only once in the reactions and adjusting coefficients and enthalpy values accordingly.

05:03
📚 Applying Hess's Law to New Reactions

The second paragraph continues the discussion on Hess's Law, providing an example of calculating the enthalpy change for the reaction of hydrogen gas with atomic oxygen to form liquid water, given the decomposition of water and the breakdown of oxygen into atomic oxygen with specific enthalpy changes. The summary details the steps of identifying key species, reversing and adjusting reactions, and combining them to find the desired enthalpy change. It concludes with a worked example, resulting in an enthalpy change of -533.5 kJ for the formation of water from hydrogen and oxygen.

10:04
🧪 Advanced Examples of Hess's Law Utilization

The final paragraph presents more complex examples of using Hess's Law, involving the decomposition of ozone into oxygen, the breakdown of oxygen into atomic oxygen, and the reaction of nitrogen monoxide with ozone to form nitrogen dioxide and oxygen. It guides through the process of focusing on specific equations, reversing and adjusting them to cancel out unwanted substances and isolate the desired reaction. The summary explains how to combine these adjusted reactions to find the enthalpy change for the reaction of nitrogen monoxide with atomic oxygen to form nitrogen dioxide, resulting in an enthalpy change of -233 kJ.

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 pathway is taken to reach the final products from the initial reactants. In the video, Hess's Law is the central theme, used to calculate the enthalpy change of reactions by manipulating known reactions and their enthalpy changes to match the desired reaction.
💡Enthalpy Change
Enthalpy change refers to the amount of heat absorbed or released during a chemical reaction at constant pressure. It is a key concept in the video, where the enthalpy change is calculated for various reactions using Hess's Law. For example, the script mentions the enthalpy change for the formation of ammonia as negative 92 kilojoules.
💡Nitrogen Gas
Nitrogen gas (N2) is a reactant in the first reaction discussed in the video, where it reacts with hydrogen gas to form ammonia. The script uses nitrogen gas to illustrate the process of adjusting the coefficients in a reaction to match the desired reaction for enthalpy change calculation.
💡Hydrogen Gas
Hydrogen gas (H2) is another reactant in the formation of ammonia, as well as a product in the decomposition of water. The script emphasizes that hydrogen gas is present in both the formation of ammonia and the decomposition of water, and its role is crucial in the manipulation of reactions according to Hess's Law.
💡Ammonia
Ammonia (NH3) is the product of the reaction between nitrogen gas and hydrogen gas. In the video, the enthalpy change for the formation of ammonia is given as negative 92 kilojoules, and it is used as a starting point for calculating the enthalpy change of other reactions.
💡Water
Water (H2O) is both a reactant and product in different reactions within the script. It is decomposed into hydrogen and oxygen gases with an enthalpy change of positive 286 kilojoules and is also a product in the reaction of ammonia with oxygen gas.
💡Oxygen Gas
Oxygen gas (O2) is a product of the decomposition of water and a reactant in the reaction that produces nitrogen dioxide from nitrogen monoxide and ozone. The script uses oxygen gas to demonstrate the concept of reversing and adjusting reactions to calculate the desired enthalpy change.
💡Reversing Reactions
Reversing reactions is a technique used in the application of Hess's Law where the direction of a chemical reaction is flipped, and the sign of the enthalpy change is changed from positive to negative or vice versa. This is illustrated in the video when adjusting the reactions to match the desired net reaction.
💡Coefficients
In the context of chemical equations, coefficients represent the number of molecules or moles of each reactant and product in a balanced chemical reaction. The script discusses adjusting coefficients to ensure that the manipulated reactions yield the correct stoichiometry for the desired net reaction.
💡Net Reaction
The net reaction is the overall chemical reaction that results from combining or adjusting other reactions. In the video, the net reaction is the final goal, and the script demonstrates how to use Hess's Law to calculate its enthalpy change by manipulating and combining other reactions to achieve this net reaction.
Highlights

Introduction to Hess's Law and its application in calculating enthalpy changes of reactions.

The enthalpy change for the reaction of nitrogen gas with hydrogen gas to produce ammonia is negative 92 kilojoules.

The decomposition of water into hydrogen and oxygen gases has an enthalpy change of positive 286 kilojoules.

Using two given reactions to calculate the enthalpy change of ammonia reacting with oxygen gas to produce nitrogen gas and water.

Hess's Law allows adjusting and combining reactions to find the enthalpy change of a target reaction.

Focusing on reactants and products that appear only once simplifies the application of Hess's Law.

Reversing a reaction and adjusting coefficients changes the sign and magnitude of the enthalpy change.

Example calculation: Reversing and doubling the first reaction to match the target reaction's nitrogen gas requirement.

Adjusting the second reaction by reversing and multiplying by six to align with the target reaction's water requirement.

Combining adjusted reactions to achieve the net reaction and calculate the enthalpy change using Hess's Law.

Final enthalpy change calculation for the reaction of ammonia with oxygen gas is negative 1532 kilojoules.

Additional example: Using Hess's Law to determine the enthalpy change for the reaction of hydrogen gas with atomic oxygen to form liquid water.

Adjusting the first reaction by reversing and halving to match the hydrogen requirement in the target reaction.

Revising the second reaction by reversing and halving to provide the necessary atomic oxygen.

Summing the adjusted reactions to find the enthalpy change of negative 533.5 kilojoules for the target reaction.

Further example involving ozone decomposition and reactions with oxygen and nitrogen monoxide to determine the enthalpy change of a different reaction.

Focusing on specific species in the reactions to simplify the application of Hess's Law for complex scenarios.

Adjusting reactions involving ozone and atomic oxygen to cancel out unwanted species and achieve the net reaction.

Final calculation of the enthalpy change for the reaction of nitrogen monoxide with atomic oxygen to form nitrogen dioxide is negative 233 kilojoules.

Transcripts

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