Chapter 17 thermochemistry sections 17.3 & 17.4

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Chapter 17 thermochemistry sections 17.3 & 17.4

  1. 1. Chapter 17Honors ChemistryThermochemistry Copyright©2000 by Houghton 1 Mifflin Company. All rights reserved.
  2. 2. . Section 17.3Heat in Changes of State Copyright©2000 by Houghton 2 Mifflin Company. All rights reserved.
  3. 3. Heats of Fusion and Solidification How does the quantity of heat absorbed by a melting solid compare to the quantity of heatreleased when the liquid solidifies? Copyright©2000 by Houghton 3 Mifflin Company. All rights reserved.
  4. 4. The molar heat of fusion (∆Hfus) is the heat absorbed by one moleof a solid substance as it melts to a liquid at a constant temp. Copyright©2000 by Houghton 4 Mifflin Company. All rights reserved.
  5. 5. The molar heat of solidification(∆Hsolid) is the heat lost when one mole of a liquid solidifies at a constant temperature. Copyright©2000 by Houghton 5 Mifflin Company. All rights reserved.
  6. 6. That is,∆Hfus = –∆Hsolid. Copyright©2000 by Houghton 6 Mifflin Company. All rights reserved.
  7. 7. 17.4
  8. 8. 17.4
  9. 9. The amount of heat necessary tovaporize one mole of a given liquid is called its molar heat of vaporization (∆Hvap). Copyright©2000 by Houghton 10 Mifflin Company. All rights reserved.
  10. 10. The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its molar heat of condensation (∆Hcond). Copyright©2000 by Houghton 11 Mifflin Company. All rights reserved.
  11. 11. That is,∆Hvap = –∆Hcond. Copyright©2000 by Houghton 12 Mifflin Company. All rights reserved.
  12. 12. 17 Heats of Vaporization and Condensation.3
  13. 13. –Observe the phase changes as ice is converted to steam when heat is added.
  14. 14. 17.3
  15. 15. 17.5
  16. 16. 17.5
  17. 17. Section 17.4 Hess’s Law Reactants Products The change in enthalpy is the same whether the reaction takes place in one step or a series of steps.The change in enthalpy, Δ H, is independent of pathway. Copyright©2000 by Houghton 18 Mifflin Company. All rights reserved.
  18. 18. Figure 6.7 The Principle of Hess’s LawIt does not matter if H for a reactionis calculated in one step or a series ofsteps. Copyright©2000 by Houghton 19 Mifflin Company. All rights reserved.
  19. 19. The Principle of Hess’s Law Copyright©2000 by Houghton 20 Mifflin Company. All rights reserved.
  20. 20. Strategy for using Hess’s Law• Manipulate equations so that they add up to the desired equation. Copyright©2000 by Houghton 21 Mifflin Company. All rights reserved.
  21. 21. Calculations via Hess’s Law1. If a reaction is reversed, H is also reversed. N2(g) + O2(g) 2NO(g) H = 180 kJ 2NO(g) N2(g) + O2(g) H = 180 kJ2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. 6NO(g) 3N2(g) + 3O2(g) H = 540 kJ Copyright©2000 by Houghton 22 Mifflin Company. All rights reserved.
  22. 22. 3. Focus on the reactants and productsof the required reaction.Let’s do some Practice Problems Copyright©2000 by Houghton 23 Mifflin Company. All rights reserved.
  23. 23. Section 17.4 Standard Enthalpies of Formation• Hf • Change in enthalpy that accompanies the formation of one mole of compound from its elements with all substances in their standard states. Copyright©2000 by Houghton 24 Mifflin Company. All rights reserved.
  24. 24. A degree symbol on a thermodynamicfunction example Hsays that the process was carried outunder standard conditions. Copyright©2000 by Houghton 25 Mifflin Company. All rights reserved.
  25. 25. Standard StatesCompound For a gas, pressure is exactly 1 atmosphere. For a solution, concentration is exactly 1 molar. Pure substance (liquid or solid), it is the pure liquid or solid.Element The form [N2(g), K(s)] in which it exists at 1 atm and 25°C. Copyright©2000 by Houghton 26 Mifflin Company. All rights reserved.
  26. 26. Change in EnthalpyImportant!!!!!--------Enthalpies of many reactions can be calculated from enthalpies of formation of reactants and products. Hrxn° = np Hf (products) nr Hf (reactants) Copyright©2000 by Houghton 27 Mifflin Company. All rights reserved.
  27. 27. Hf for an element in its standard stateis Zero.Elements in their standard states are notincluded in the Hrxn calculations. Copyright©2000 by Houghton 28 Mifflin Company. All rights reserved.

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