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Chapter 17

Chapter 17. Thermochemistry. Energy Transformations. When fuel is burned in a car engine, chemical energy is released and is used to do work. Measuring and Expressing Enthalpy Changes. A burning match releases heat to its surroundings in all directions. Chemical energy → ….

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Chapter 17

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  1. Chapter 17 Thermochemistry

  2. Energy Transformations When fuel is burned in a car engine, chemical energy is released and is used to do work.

  3. Measuring and Expressing Enthalpy Changes • A burning match releases heat to its surroundings in all directions. • Chemical energy → … Quick-write : Describe the energy conversions for this scenario. How much heat does this exothermic rxn release???

  4. Energy Transformations Thermochemistry: study of heat changes (ΔH; change in heat content) that occur during • chemrxns and • changes in physical states. • chemical potential energy (chemical energy) • energy stored in the chembonds of a sub

  5. Calorimetry Calorimetry • the precise measurement of the heat flow into or out of a system for chemical (e.g. burning) and physical processes (e.g. boiling, melting—phase changes).

  6. Calorimetry In calorimetry, • heat released by the system = heat absorbed by surroundings • Conversely, heat absorbed by a system = heat released by surroundings

  7. Calorimetry Calorimeter • insulated device used to measure the absorption or release of heat in chem or physical processes.

  8. Calorimetry Enthalpy (H) of the system. • The heat content of a system • cannot be measured directly with instruments. • can measured enthalpy change (ΔH);

  9. Calorimetry Calorimetry expts can be performed at a constant volume using a bomb calorimeter.

  10. Thermochemical Equations A thermochemicaleqn • chemeqn that includes the enthalpy change (ΔH). exothermic In a chemeqn, the ΔH for the rxn can be written as either a reactant or a product.

  11. Thermochemical Equations • The heat of rxnis the ∆H. Exothermic Enthalpy change

  12. Thermochemical Equations Exothermic Rxn

  13. Thermochemical Equations Endothermic Rxn

  14. Thermochemical Equations

  15. Thermochemical Equations • The (molar) heat of combustion is the heat of rxn for the complete burning of 1 mole of a sub.

  16. Energy Transformations Energy Transformations • In what direction does heat flow?

  17. Exothermic and Endothermic Process In an endothermic process, • system gains heat as the surroundings cool down.(+ve) • In an exothermic process, • system loses heat as the surroundings heat up. (-ve)

  18. Exothermic and Endothermic Process A system • part of the universe on which you focus your attention. • The surroundings • include everything else in the universe. • Law of conservation of energy • states that in any chem or physical process, energy is neither created nor destroyed.

  19. Exothermic and Endothermic Processes An endothermic process is one that absorbs heat from the surroundings.

  20. Exothermic and Endothermic Processes An exothermic process is one that releases heat to its surroundings.

  21. Units for Measuring Heat Flow 2 common units for heat flow • calorie ; Calorie (food) • joule.

  22. Specific Heat Capacity The specific heat of a sub (pure) • is the amt of heat it takes to raise the temp of 1 g of the sub 1°C. J/(g∙°C) q: qty of heat (J) m: mass of sub (g) ∆T: change in temperature (°C); ∆T = Tfinal – Tinitial

  23. Heat Capacity and Specific Heat • Water releases a lot of heat as it cools. • During freezing weather, farmers protect citrus crops by spraying them with water.

  24. Specific Heat

  25. Heat Capacity and Specific Heat Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.

  26. Calculate specific heat of a metal Problem 1: The temp of a 95.4-g piece of Cu increases from 25°C to 48.0°C when the Cu absorbs 849 J of heat. What is the specific heat of Cu ?

  27. Calculate Amt of heat to … Problem 2: How much heat is required to raise the temp of 250.0g of Hg 52°C? (CHg = 0.14J/(g·°C).

  28. Latent Heat Latent heat for phase change • melting, freezing, condensation, vaporization, involved heat changes. • the temperature of the sub remain constant; • only phase change. • Heat of fusion (ΔHfus), heat of solidification (ΔHsolid), heat of vaporization (ΔHvap), heat of condensation (Δhcond)

  29. Latent Heats

  30. Latent Heat Heat of fusion (ΔHfus) • ΔH for melting (+ve; gain heat); • e.g. ΔHfus of ice = 6.01 kJ/mol • temp of sub remains constant • e.g. ice at 0°C remains 0°C while melting though it keeps on absorbing heat • refer to graph in the next slide.

  31. Molar Heat of Fusion ΔHfus of ice = 6.01 kJ/mol Heat change (to melt 1 mole of ice) qty of Heat (kJ) = # moles of sub Heat absorbed (+ve)

  32. Heats of Fusion and Solidification Molar heat of fusion (∆Hfus) • amt of heat absorbed by 1 mole of a solid sub as it melts to a (l) at a constant temp. • e.g. ΔHfus of ice = 6.01 kJ/mol Molar heat of solidification (∆Hsolid) • amt heat lost when 1 mole of a (l) solidifies at a constant temp. • e.g. ΔHsolid of water (l) = - 6.01 kJ/mol

  33. Heats of Vaporization and Condensation

  34. vaporizing (e) liquid water temp increasing (d) (100°C) Temp increasing All ice 0°C Boiling water 100°C melting (c) (Liquid H2O + steam) All liquid water 0°C (b) (0°C) (Ice+water) 0°C (a) (Ice) temp increasing

  35. Quick-write Why f.p. of water = 0°C and m.p. of ice is also 0°C?

  36. Latent Heat Heat of vaporization (ΔHvap) • ΔH is +ve Heat of condensation (ΔHcond) • ΔH is -ve Heat of fusion (ΔHfus) • melting • ΔH is +ve Heat of solidification (ΔHsolid) • freezing • ΔH is -ve

  37. Heats of Fusion and Solidification The qty of heat absorbed by a melting (s) is exactly the same as the qty of heat released when the (l) solidifies ∆Hfus = –∆Hsolid

  38. Heats of Vaporization and Condensation How does the qty of heat absorbed by a vaporizing (l) compare to the qty of heat released when the vapor condenses?

  39. Heats of Vaporization and Condensation Molar heat of vaporization (∆Hvap). • amt of heat necessary to vaporize 1 mole of a given (l) at constant temp. Molar heat of condensation (∆Hcond). • amt of heat released when 1 mol of vapor condenses at the normal b.p.

  40. Heats of Vaporization and Condensation The qty of heat absorbed by a vaporizing (l) is exactly the same as the qty of heat released when the vapor condenses; ∆Hvap = –∆Hcond

  41. Heats of Vaporization and Condensation

  42. Heats of Vaporization and Condensation ΔH accompany changes in state.

  43. Heat of Soln Heat of Solution • During the formation of a soln, heat is either released or absorbed. • Molar heat of soln (∆Hsoln) • The enthalpy change caused by dissolution of 1 mole of sub.

  44. Heat of Solution When NH4NO3 crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve.

  45. Problem 3: The graph below shows a pure sub which is heated by a constant source of heat supplying 2000.0 j/min. Identify the area described in the questions below and complete the necessary calculations. UV = 0.36 min, VW = 3.6 min, WX = 3.6 min, XY = 19.4 min, YZ = 0.6 min

  46. a. being warmed as a solid ___________b. being warmed as a liquid __________c. being warmed as a gas ____________d. changing from a solid to a liquid _____e. changing from a liquid to a gas ______f. What is its boiling temp? _________________g. What is its melting temp? _________________h. How many joules were needed to change the liquid to a gas? ____________i. Where on the curve do the molecules have the highest KE? ______j. If the sample weighs 10.0 g, what is its heat of vaporization in J/g? ______

  47. Example 4 How many g of ice at 0°C could be melted by the addition of 5.75 kJ of heat? (ΔHfus of ice = 6.01 kJ/mol)

  48. CST problem 1 The specific heat of Cu is about 0.4 J/(g·°C). How much heat is needed to change the temperature of a 30-g sample of Cu from 20.0°C to 60.0°C? A 1000 J B 720 J C 480 J D 240 J

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