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Understanding Enthalpy and Standard Enthalpies of Formation in Chemical Reactions

This text explores the concept of enthalpy (∆H) and its significance in chemical reactions. It outlines how the enthalpy change for a reaction can be calculated using standard enthalpies of formation for reactants and products. It details a specific example involving methane combustion: CH4 + 2O2 → CO2 + 2H2O, and shows how to calculate the overall enthalpy change using the formation enthalpies of the involved substances. The importance of measuring enthalpy changes and drawing conclusions from them in thermodynamics is emphasized.

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Understanding Enthalpy and Standard Enthalpies of Formation in Chemical Reactions

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  1. Enthalpy

  2. Enthalpy Enthalpy

  3. Enthalpy reactants Enthalpy

  4. Enthalpy reactants Enthalpy products

  5. Enthalpy reactants ∆H Enthalpy products

  6. Enthalpy For some reactions, ∆H cannot be measured.

  7. Enthalpy If the reaction happens in steps, the total ∆H is equal to the sum of ∆H for each step. reactant Enthalpy intermediate product final product

  8. Enthalpy Standard enthalpies of formation have been measured for many different compounds.

  9. Enthalpy Standard enthalpies of formation have been measured for many different compounds. For any element in its standard state, ∆Hf = 0.

  10. Enthalpy CH4 + 2O2 → CO2 + 2H2O

  11. Enthalpy CH4 + 2O2 → CO2 + 2H2O ∆Hf(products) - ∆Hf(reactants) ∆Hf(reaction)

  12. Enthalpy CH4 + 2O2 → CO2 + 2H2O Standard enthalpies of formation:

  13. Enthalpy CH4 + 2O2 → CO2 + 2H2O Standard enthalpies of formation: -75 + 0 → -394 + -286

  14. Enthalpy CH4 + 2O2 → CO2 + 2H2O Standard enthalpies of formation: -75 + 2(0) → -394 + 2(-286)

  15. Enthalpy CH4 + 2O2 → CO2 + 2H2O Standard enthalpies of formation: -75 + 2(0) → -394 + 2(-286) -75 → -680

  16. Enthalpy CH4 + 2O2 → CO2 + 2H2O Standard enthalpies of formation: -75 + 2(0) → -394 + 2(-286) -75 → -680 ∆Hf(reaction) = -605

  17. Homework: Page 512 problems 31, 34, 35

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