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

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15 Oct 201012:08
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TLDRThe video script discusses using Hess's Law to calculate the enthalpy change for the formation of methane from solid carbon (graphite) and hydrogen gas. It explains how to reverse and combine combustion reactions of carbon, hydrogen, and methane to derive the desired reaction, ultimately determining the enthalpy change as -74.8 kJ/mol.

Takeaways
  • πŸ” The problem involves calculating the enthalpy change for the formation of methane (CH4) from solid carbon (graphite) and hydrogen gas.
  • 🚫 Direct measurement of the enthalpy change for this reaction in the lab is not feasible due to its slow nature.
  • πŸ”₯ Hess's Law is used to determine the enthalpy change for the formation of methane by combining the enthalpy changes of other known reactions.
  • 🌐 Hess's Law states that the enthalpy change of a reaction is the sum of the enthalpy changes of the reactions it is composed of.
  • πŸ”„ The script suggests reversing the combustion reaction of methane to start with the end product (methane) and then adding other reactions to construct the desired reaction.
  • βš–οΈ The combustion reactions for carbon and hydrogen are used, and their enthalpy changes are adjusted based on the stoichiometry of the reactions.
  • πŸ”„ The combustion of carbon (graphite) produces carbon dioxide, and the combustion of hydrogen gas produces water, which are necessary intermediates in the formation of methane.
  • πŸ”’ The script demonstrates the process of multiplying the hydrogen combustion reaction by 2 to obtain the required two molecules of water.
  • πŸ”„ The script shows how the reactants and products of the combined reactions cancel out, leaving only the desired reactants (graphite and hydrogen gas) and the product (methane).
  • πŸ“Š The final enthalpy change for the formation of methane is calculated by summing the adjusted enthalpy changes of the individual reactions, resulting in a negative value indicating an exothermic process.
Q & A

  • What is the main problem being addressed in the script?

    -The main problem is to calculate the enthalpy change for the formation of methane (CH4) from solid carbon (graphite) and hydrogen gas, using Hess's Law.

  • Why can't the enthalpy change for the formation of methane be measured directly in the laboratory?

    -The enthalpy change cannot be measured directly because the reaction is very slow, making it difficult to measure the temperature change or any meaningful data in the lab.

  • What method is suggested to find the enthalpy change for the formation of methane?

    -The suggested method is to use Hess's Law, which involves using the enthalpy changes of related combustion reactions to find the enthalpy change for the formation of methane.

  • What are the given reactions and their enthalpy changes used in the script?

    -The given reactions and their enthalpy changes are: 1. Combustion of carbon: C (graphite) + O2 -> CO2, Ξ”H = -393.5 kJ/mol 2. Combustion of hydrogen: H2 + 1/2 O2 -> H2O, Ξ”H = -285.8 kJ/mol 3. Combustion of methane: CH4 + 2 O2 -> CO2 + 2 H2O, Ξ”H = -890.3 kJ/mol

  • How is Hess's Law applied in this problem?

    -Hess's Law is applied by reversing the combustion reaction of methane and combining it with the combustion reactions of carbon and hydrogen in such a way that the intermediate products cancel out, leaving the formation reaction of methane.

  • Why do we reverse the combustion reaction of methane?

    -We reverse the combustion reaction of methane because in the original problem, methane is a product, not a reactant. Reversing the reaction gives us the formation of methane as a product.

  • How do we handle the stoichiometry in this Hess's Law problem?

    -We ensure that the stoichiometry of the intermediate products matches the target reaction. Specifically, we multiply the combustion reaction of hydrogen by 2 to get the correct amount of water (2 H2O) needed.

  • What is the total enthalpy change for the formation of methane, according to the script?

    -The total enthalpy change for the formation of methane is -74.8 kJ/mol.

  • What is the significance of a negative enthalpy change in this context?

    -A negative enthalpy change indicates that the reaction is exothermic, meaning it releases energy.

  • What is the final form of the reaction for the formation of methane after applying Hess's Law?

    -The final form of the reaction is: C (graphite) + 2 H2 (gas) -> CH4 (gas).

Outlines
00:00
πŸ”₯ Calculating Enthalpy Change for Methane Formation

The script discusses a problem from the Kotz, Treichel, Townsend Chemistry textbook, focusing on calculating the enthalpy change for the formation of methane from solid carbon (graphite) and hydrogen gas. The enthalpy change for this reaction is not directly measurable due to its slow nature. Instead, the problem suggests using Hess's Law, which states that the enthalpy change of a reaction is the sum of the enthalpy changes of the reactions it is composed of. The script proposes starting with the combustion of methane, reversing the reaction to represent methane as a product, and then using the enthalpy changes of the combustion of carbon and hydrogen to construct the desired reaction. The goal is to determine the enthalpy change for the formation of methane from its elements.

05:02
πŸ” Constructing the Reaction Using Combustion Data

This paragraph delves deeper into the process of using Hess's Law to calculate the enthalpy change for methane formation. It explains how to reverse the combustion reaction of methane and multiply the combustion reaction of hydrogen by two to match the required stoichiometry for water in the formation of methane. The script details how to combine these reactions, ensuring that reactants and products cancel out appropriately, leaving only the desired reactants (graphite and hydrogen gas) and the product (methane). The enthalpy changes for the individual reactions are then used to calculate the overall enthalpy change for the formation of methane.

10:06
πŸ“š Applying Hess's Law to Determine Enthalpy Change

The final paragraph concludes the process by applying Hess's Law to sum the enthalpy changes of the individual reactions calculated in the previous paragraphs. It emphasizes the importance of reversing the combustion reaction of methane and adjusting the reaction of hydrogen combustion to fit the stoichiometry needed for methane formation. The enthalpy changes for the combustion of carbon (-393.5 kJ/mol) and hydrogen (-571.6 kJ/mol) are combined with the reversed methane combustion reaction (+890.3 kJ/mol) to find the net enthalpy change for the formation of methane. The result is a negative enthalpy change of -74.8 kJ/mol, indicating that the formation of methane is exothermic, though not as significantly as the combustion reactions.

Mindmap
Keywords
πŸ’‘Enthalpy Change
Enthalpy change refers to the difference in the thermodynamic property enthalpy between the reactants and products in a chemical reaction. It is a key concept in the video, as it is the main focus of the problem being discussed. The script explains the difficulty in measuring the enthalpy change for the formation of methane from graphite and hydrogen gas due to the slow nature of the reaction, and instead, suggests using Hess's Law to calculate it.
πŸ’‘Methane (CH4)
Methane is a hydrocarbon with the chemical formula CH4, consisting of one carbon atom bonded to four hydrogen atoms. In the video, methane is the target product of the reaction under discussion. The script describes the process of calculating the enthalpy change for its formation from graphite and hydrogen gas, highlighting its importance in the context of the problem.
πŸ’‘Graphite
Graphite is a crystalline form of carbon used in the script as the starting material for the formation of methane. It is significant in the video's narrative because the enthalpy change for the reaction involving graphite is part of the calculation to determine the enthalpy change for methane formation.
πŸ’‘Hydrogen Gas
Hydrogen gas, with the chemical formula H2, is a diatomic gas and one of the reactants in the formation of methane. The script discusses how hydrogen gas combines with graphite to form methane, and its role in the combustion reactions used to calculate the desired enthalpy change.
πŸ’‘Combustion Reaction
A combustion reaction is a chemical reaction that occurs between a substance and an oxidant, typically producing heat and light. In the video, the combustion reactions of carbon, hydrogen, and methane are given, and their enthalpy changes are used to calculate the enthalpy change for methane formation through Hess's Law.
πŸ’‘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. In the video, Hess's Law is applied to calculate the enthalpy change for the formation of methane by summing the enthalpy changes of known combustion reactions.
πŸ’‘Endothermic Reaction
An endothermic reaction is a chemical reaction that absorbs energy from its surroundings. In the context of the video, the script mentions the endothermic nature of the reverse combustion of methane, which is used to calculate the enthalpy change for methane formation.
πŸ’‘Exothermic Reaction
An exothermic reaction is a chemical reaction that releases energy to its surroundings. The video discusses the exothermic nature of the combustion reactions of carbon and hydrogen, which release energy and are used in the calculation of the enthalpy change for methane formation.
πŸ’‘Carbon Dioxide (CO2)
Carbon dioxide is a chemical compound with the formula CO2. In the video, carbon dioxide is a product of the combustion of carbon and is used in the script to illustrate the steps of calculating the enthalpy change for methane formation using Hess's Law.
πŸ’‘Water (H2O)
Water is a chemical compound with the formula H2O. The script describes the formation of water in the combustion of hydrogen and its subsequent use in the reverse combustion reaction to calculate the enthalpy change for methane formation.
πŸ’‘Kilojoules
Kilojoules are a unit of energy, used in the script to express the magnitude of enthalpy changes in the reactions being discussed. The video uses kilojoules to quantify the enthalpy changes of the combustion reactions and the formation of methane.
Highlights

Introduction of the problem from Chapter 5 of the Kotz, Treichel, Townsend Chemistry and Chemical Reactivity textbook.

Explanation of the goal: to find the enthalpy change for the formation of methane (CH4) from solid carbon as graphite and hydrogen gas.

Clarification that the enthalpy change for this reaction cannot be measured in the laboratory due to the slow reaction rate.

Introduction of Hess's Law as a method to calculate the enthalpy change indirectly.

Step-by-step approach to constructing the desired reaction using given combustion reactions.

Reversal of the combustion of methane reaction to have methane as a product.

Explanation that reversing the reaction changes the sign of the enthalpy change to +890.3 kJ/mol.

Combustion of graphite to form carbon dioxide with an enthalpy change of -393.5 kJ/mol.

Combustion of hydrogen gas to form water with an enthalpy change of -285.8 kJ/mol.

Multiplication of the hydrogen combustion reaction by 2 to match the number of water molecules needed.

Calculation of the enthalpy change for the multiplied hydrogen combustion reaction: -571.6 kJ/mol.

Verification that the sum of the reactions cancels out all intermediate products, leaving the desired reactants and products.

Summation of the enthalpy changes from the individual reactions to find the total enthalpy change for the formation of methane.

Final calculation showing the enthalpy change for the formation of methane as -74.8 kJ/mol, indicating an exothermic reaction.

Conclusion that the enthalpy change calculation using Hess's Law confirms the exothermic nature of the methane formation reaction.

Transcripts

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Hess's LawChemical ReactionsEnthalpy ChangeMethane FormationCombustion ReactionsThermodynamicsChemistry EducationEnergy AbsorptionSolid CarbonHydrogen Gas