Calculating Enthalpy Changes Using Heats of Formation Method

LearnChemE
28 Nov 201108:46
EducationalLearning
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TLDRThis screencast teaches how to perform an energy balance on a reaction using the heat of formation method. It involves material balances, creating an enthalpy table with elemental state references, and calculating enthalpy changes from 25°C to reaction temperatures. The example reaction is CO + H2O → CO2 + H2, with calculations showing how to determine the heat exchange.

Takeaways
  • 🔍 The screencast demonstrates the heat of formation method for performing an energy balance on a chemical reaction.
  • 📚 There are two methods for energy balance: the heat of reaction method and the heat of formation method, both yielding the same result.
  • 🌡 The example reaction involves CO and H2O gases converting to CO2 and H2, with specific temperature conditions for reactants and products.
  • 📉 The first step in the heat of formation method is to conduct material balances for the reactants and products.
  • 📝 An enthalpy table is constructed using heat of formation values and references to elemental states at 25 degrees Celsius and 1 atmosphere.
  • 🔥 The heat of formation method accounts for the energy change from elemental species to molecules and then to the final temperature.
  • ⚖️ The calculation of enthalpy includes the heat of formation plus the integral of the heat capacity over the temperature range.
  • 📊 For ideal gases, the heat capacity data can often be found in tables, simplifying the calculation process.
  • 🧪 The given reaction has 1 mole of CO and 1.5 moles of H2O with an 80% conversion rate, resulting in specific molar amounts of products.
  • 📈 The enthalpy table must include references to solid carbon, gaseous H2 and O2, and the molar flow rates in kJ/mol.
  • 🔢 The final calculation of Q (heat exchanged) is the sum of the products' enthalpies times their moles minus the sum of the reactants' enthalpies times their moles, resulting in -12.6 kJ for the example.
Q & A

  • What is the main topic of the screencast?

    -The screencast demonstrates how to perform an energy balance on a reaction using the heat of formation method.

  • What are the two methods mentioned for doing an energy balance on a reaction?

    -The two methods mentioned are the heat of reaction method and the heat of formation method.

  • What is the chemical reaction being discussed in the screencast?

    -The reaction discussed is CO plus H2O going to CO2 plus H2, all in gas form.

  • What is the first step in performing an energy balance according to the screencast?

    -The first step is to do the material balances.

  • What does the enthalpy table include?

    -The enthalpy table includes the references and the heat of formation values for the elemental states of the molecules involved in the reaction.

  • Why are elemental states used as references in the heat of formation method?

    -Elemental states are used because the method starts with the elemental species to form the molecules, where the heat of formation is applied.

  • What is the significance of the reference temperature and pressure in the calculation?

    -The reference temperature (25 degrees C) and pressure (1 atmosphere) are used as the starting point for calculating the sensible heat changes in the reaction.

  • How many moles of CO and H2O are involved in the reaction, and what is the conversion rate of CO?

    -There is 1 mole of CO and 1.5 moles of H2O involved. The conversion rate of CO is 80%.

  • What are the moles of CO2, H2, and H2O produced in the reaction?

    -The reaction produces 0.2 moles of CO2, 0.8 moles of H2, and 0.7 moles of H2O.

  • How is the final enthalpy value calculated for each substance in the reaction?

    -The final enthalpy value is calculated by adding the heat of formation of the molecule to the integral of Cp (heat capacity) from the reference temperature to the final temperature.

  • What is the final calculated value of Q for the reaction?

    -The final calculated value of Q for the reaction is -12.6 kJ.

Outlines
00:00
🔍 Energy Balance Using Heat of Formation Method

This paragraph introduces the heat of formation method for performing an energy balance on a chemical reaction, contrasting it with the heat of reaction method. The focus is on the reaction CO + H2O → CO2 + H2 in gaseous form. The process begins with material balances and involves creating an enthalpy table with elemental state references. The enthalpy calculation for each molecule includes the heat of formation and the temperature-dependent heat capacity integral from 25°C to the desired temperature. Ideal gas properties are used to simplify calculations. The example given involves 1 mole of CO with 50% excess H2O, resulting in 0.2 moles of unreacted CO, and 0.8 moles each of CO2 and H2 produced, with 0.7 moles of H2O remaining. The paragraph concludes with the setup for the enthalpy table, emphasizing the importance of including references in solid and gaseous elemental states at standard conditions.

05:01
📊 Calculating Enthalpy Changes for a Reaction

This paragraph delves into the specifics of calculating enthalpy changes for the given reaction at different temperatures. It explains how to determine the enthalpy for reactants entering at 300°C and products exiting at 500°C using the heat of formation and sensible heat from the reference temperature of 25°C. The process involves looking up the enthalpy values for ideal gases at the respective temperatures and adding them to the heat of formation values. The paragraph provides detailed calculations for CO, H2O, CO2, and H2, including their heat of formation and the sensible heat required to reach the process temperatures. The final step is to compile these values into an enthalpy table, which is then used to calculate the net heat exchange (Q) for the reaction by summing the products' enthalpies multiplied by their molar amounts and subtracting the sum of the reactants' enthalpies. The example concludes with a calculated Q value of -12.6 kJ, indicating an exothermic reaction.

Mindmap
Keywords
💡Energy Balance
Energy balance refers to the calculation of the total energy input and output in a chemical reaction or process. In the video, it is the central theme as it demonstrates how to calculate the energy balance of a reaction using the heat of formation method. The script mentions two methods for energy balance, with the heat of reaction method being discussed in a previous screencast.
💡Heat of Formation
Heat of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states. In the context of the video, the heat of formation method is used to calculate the energy balance of a reaction, starting with elemental species and accounting for the enthalpy changes as molecules are formed.
💡Material Balances
Material balances are calculations that ensure the conservation of mass in a chemical process, meaning the total amount of each component must be the same before and after the reaction. The script emphasizes that material balances should be the first step in performing an energy balance, as they establish the stoichiometry of the reaction.
💡Enthalpy Table
An enthalpy table is a reference that lists the standard enthalpies of formation for chemical compounds at a specified temperature, typically 25 degrees Celsius. In the video, the enthalpy table is constructed using the heat of formation method, including the elemental states of the compounds involved in the reaction.
💡Elemental States
Elemental states refer to the pure chemical elements in their standard physical forms. The script specifies that the reference states for the enthalpy calculations must be the elemental states, such as solid carbon, gaseous oxygen, and gaseous hydrogen, because the heat of formation method starts from these states.
💡Sensible Heat
Sensible heat is the heat that, when added to a substance, results in a temperature change without a phase change. In the script, sensible heat is used to calculate the enthalpy of the reactants and products at temperatures other than the reference temperature, adjusting for the heat required to raise or lower the temperature to the desired point.
💡Ideal Gases
Ideal gases are theoretical gases that obey the ideal gas law, PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature. The script mentions that for reactions involving ideal gases, the sensible heat can often be looked up in a table, simplifying the calculations.
💡Conversion
Conversion in a chemical reaction refers to the percentage of reactants that are converted into products. The script provides an example with 80% conversion of CO, meaning that out of the CO that enters the reaction, 80% is transformed into CO2 and H2.
💡Enthalpy Calculation
Enthalpy calculation involves determining the heat content or enthalpy of a system. The script describes how to calculate the enthalpy for each substance involved in the reaction by adding the heat of formation to the sensible heat required to reach the final temperature from the reference temperature.
💡Q (Heat Exchange)
Q represents the heat exchanged in a thermodynamic process. In the script, the final calculation of Q is determined by summing the products' enthalpies multiplied by their moles and subtracting the sum of the reactants' enthalpies multiplied by their moles, resulting in the net heat exchange for the reaction.
Highlights

Demonstration of energy balance on a reaction using the heat of formation method.

Comparison with the heat of reaction method shows identical results.

Reaction involves CO plus H2O producing CO2 and H2, all in gas form.

Material balances are the first step in performing an energy balance.

Enthalpy table includes references in elemental states, such as solid carbon, gaseous O2 and H2.

Enthalpy calculations start with elemental species and account for heat of formation.

Enthalpy calculations also include the integral from a reference temperature to the final temperature.

Use of ideal gas tables simplifies calculations for reactions involving ideal gases.

1 mole of CO and 1.5 moles of H2O are used in the reaction with 80% conversion of CO.

Enthalpy table is constructed with references at 25 degrees C and 1 atmosphere.

Enthalpy values are calculated by adding sensible heat to the heat of formation.

Heat of formation values for CO, H2O, CO2, and H2 are used in the enthalpy table.

Sensible heat values for CO and H2O are derived from an enthalpy table of ideal gases.

Final enthalpy values are adjusted for the reaction temperature of 300 degrees C for reactants and 500 degrees C for products.

Enthalpy table is populated with calculated values for each substance involved in the reaction.

Net heat Q is determined by summing the enthalpies times their moles for products and subtracting the sum for reactants.

Result of the energy balance calculation is -12.6 kJ.

Transcripts

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Energy BalanceHeat of FormationChemistry TutorialMaterial BalanceEnthalpy TableElemental StatesIdeal GasesReaction AnalysisThermochemistryEducational Content