Hess's law and reaction enthalpy change | Chemistry | Khan Academy
TLDRThis educational script discusses Hess's Law, explaining how the energy change in a chemical reaction is independent of the path taken. It demonstrates how to calculate the enthalpy change of reactions using standard heats of formation, applying the concept to examples like ammonia formation and propane combustion, highlighting the exothermic nature of these reactions.
Takeaways
- 🔍 Hess's Law states that the energy change of a process is independent of the path taken, reflecting the fact that energy is a state variable.
- 🌡 Enthalpy, including its change, is a state function and does not depend on the process or steps taken to reach a state.
- 🔄 Hess's Law can be applied to reactions where the overall enthalpy change can be calculated using the sum of the enthalpy changes of formation of the products and reactants.
- 📚 Heats of formation can be found in tables and are used to calculate the enthalpy change of a reaction using Hess's Law.
- ⚗️ The script provides a step-by-step example of calculating the enthalpy change for a hypothetical reaction involving A + B → C + D using known heats of formation.
- 📉 The heat of formation for elements in their standard state is zero, simplifying calculations for reactions involving elemental reactants or products.
- 📊 The calculation of enthalpy change for a reaction must account for the stoichiometry of the reaction, multiplying the heat of formation by the number of moles of each substance.
- 🔢 The script demonstrates the calculation of the enthalpy change for the reaction of ammonia with oxygen to form nitrogen monoxide and water, using actual values from Wikipedia.
- 🔥 An example of combustion of propane is used to illustrate the calculation of heat released during the reaction, emphasizing the exothermic nature of such reactions.
- 📉 The heat released or absorbed in a reaction can be scaled based on the amount of reactant used, as shown with the calculation for a given mass of propane.
- 🛠 Understanding Hess's Law and the concept of heat of formation is crucial for predicting and calculating the thermal effects of chemical reactions in various applications.
Q & A
What is Hess's Law and what does it imply about the energy change in a process?
-Hess's Law states that the energy change of a process is independent of the path taken from one state to another. It implies that whether a process occurs in one step or multiple steps, the total change in energy remains the same. This is because energy, including enthalpy and internal energy, is a state variable, not dependent on the path taken.
Why are enthalpy and internal energy considered state variables?
-Enthalpy and internal energy are considered state variables because their values depend only on the current state of the system and not on the path taken to reach that state. This means that the change in these variables is independent of the process history.
How can Hess's Law be used to calculate the enthalpy change of a reaction?
-Hess's Law can be used to calculate the enthalpy change of a reaction by breaking the reaction into known steps, such as the formation of reactants and products from their elemental forms. The total enthalpy change of the reaction is the sum of the enthalpy changes of these steps, which can be found from standard heats of formation.
What is the significance of the heat of formation in calculating enthalpy changes?
-The heat of formation is the change in enthalpy when one mole of a substance is formed from its constituent elements in their standard states. It is crucial in calculating enthalpy changes because it allows us to determine the energy required to form or decompose substances, which is then used in applying Hess's Law.
Why is the heat of formation of an element in its standard state zero?
-The heat of formation of an element in its standard state is zero because it is defined as the starting point from which all other heats of formation are measured. Since no energy is required to form an element from itself, this value serves as a reference point in thermodynamic calculations.
How does the number of moles affect the calculation of the enthalpy change for a reaction?
-The number of moles affects the calculation of the enthalpy change for a reaction because the heat of formation values must be multiplied by the number of moles of each substance involved in the reaction. This ensures that the total enthalpy change accounts for the correct amount of energy associated with the actual quantities of reactants and products.
What is the relationship between the heat of formation and the stability of a substance?
-A substance with a positive heat of formation requires energy to be formed from its elements, indicating that it is less stable than its constituent elements. Conversely, a substance with a negative heat of formation releases energy upon formation, suggesting that it is more stable than its elements in their standard states.
Can you provide an example of how to apply Hess's Law to calculate the enthalpy change of a reaction involving ammonia and oxygen?
-Yes, the script provides an example where the enthalpy change for the reaction of ammonia gas with molecular oxygen to yield nitrogen monoxide and water is calculated. The heat of formation values for ammonia, oxygen, nitrogen monoxide, and water are used, taking into account the stoichiometry of the reaction. The final enthalpy change is obtained by summing the products' heat of formation values and subtracting the reactants' heat of formation values.
What is the significance of the stoichiometry in a chemical reaction when calculating enthalpy changes?
-The stoichiometry of a chemical reaction is crucial when calculating enthalpy changes because it determines the number of moles of reactants and products involved. This affects the calculation by ensuring that the heat of formation values are correctly multiplied by the respective number of moles for each substance in the reaction.
How can you determine the amount of heat released or absorbed in a reaction given the mass of a reactant?
-To determine the amount of heat released or absorbed in a reaction given the mass of a reactant, first calculate the number of moles of the reactant using its molar mass. Then, multiply the moles by the enthalpy change per mole for the reaction. This will give the total heat involved in the reaction for the given mass of the reactant.
Outlines
🔍 Introduction to Hess's Law and Enthalpy Change
This paragraph introduces the concept of Hess's Law, which states that the energy change of a process is independent of the path taken to reach from one state to another. It emphasizes that enthalpy, like internal energy, is a state variable and explains how Hess's Law can be applied to calculate the enthalpy change of a reaction when only the heats of formation are known. The paragraph uses a hypothetical reaction A + B → C + D to illustrate the process of calculating the enthalpy change by breaking down the reactants into their elemental forms and then recombining them to form the products, utilizing the known heats of formation.
📚 Application of Hess's Law in Calculating Reaction Enthalpy
The second paragraph delves into the practical application of Hess's Law by providing a step-by-step example using the formation of ammonia and its subsequent reaction with oxygen to produce nitrogen monoxide and water. It explains how to look up the heats of formation for each compound involved in the reaction and how to calculate the overall enthalpy change by considering the moles of each product and reactant. The paragraph also clarifies the importance of accounting for the stoichiometry of the reaction when performing these calculations.
🔥 Combustion of Propane: A Practical Example of Hess's Law
This paragraph presents a real-world example of Hess's Law by calculating the heat released during the combustion of propane to form carbon dioxide and water. It details the process of finding the heats of formation for propane, oxygen, carbon dioxide, and water, and then uses these values to determine the enthalpy change of the combustion reaction. The explanation includes how to adjust the calculation based on the number of moles of products formed from one mole of propane and concludes with the total heat released from the reaction.
📘 Calculation of Heat Released for a Given Quantity of Propane
The final paragraph extends the previous example by addressing how to calculate the heat released from the combustion of a specific amount of propane, such as 33 grams. It explains the process of converting grams of propane to moles and then using the molar heat of combustion to find the total heat released for the given quantity. The paragraph provides a clear calculation that demonstrates the exothermic nature of the propane combustion reaction and the amount of energy released.
Mindmap
Keywords
💡Hess's Law
💡Enthalpy
💡Heat of Formation
💡State Variable
💡Ammonia
💡Nitrogen Monoxide
💡Water
💡Propane
💡Carbon Dioxide
💡Mole
Highlights
Hess's Law states that the energy change of a process is independent of the path taken from one state to another.
Energy, including enthalpy and internal energy, is a state variable, meaning it is independent of the process steps or path.
Hess's Law is useful for calculating the enthalpy change of reactions using known heats of formation.
The concept of heat of formation is explained as the change in enthalpy from elements to compounds.
Standard heat of formation values can be found in tables and used to calculate unknown enthalpy changes.
An example reaction of A + B yielding C + D is used to illustrate the application of Hess's Law.
The calculation involves summing the heats of formation of products and subtracting those of reactants.
The sign of the enthalpy change indicates whether a reaction is exothermic (releases energy) or endothermic (absorbs energy).
A practical application of Hess's Law is demonstrated with the reaction of ammonia with oxygen to form nitrogen monoxide and water.
Heat of formation values for ammonia, oxygen, nitrogen monoxide, and water are provided from a reference table.
The calculation of the reaction's enthalpy change involves multiplying molar quantities by their respective heats of formation.
The importance of considering the stoichiometry of the reaction when applying Hess's Law is emphasized.
A second example involves the combustion of propane to carbon dioxide and water, illustrating the exothermic nature of combustion.
The heat of formation for propane, oxygen, carbon dioxide, and water is used to calculate the enthalpy change of combustion.
The calculation shows that for every mole of propane combusted, a specific amount of energy is released.
An additional example calculates the energy released from the combustion of 33 grams of propane, demonstrating the application of molar mass.
The final example concludes with a rewritten reaction equation that includes the energy released, emphasizing the exothermic process.
Transcripts
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