1. Chapter 5 Thermochemistry John D. Bookstaver St. Charles Community College Cottleville, MO Lecture Presentation © 2012 Pearson Education, Inc.

2. Thermochemistry © 2012 Pearson Education, Inc. Energy Energy is the ability to do work or transfer heat. –Energy used to cause an object that has mass to move is called work. –Energy used to cause the temperature of an object to rise is called heat.

3. Thermochemistry © 2012 Pearson Education, Inc. Kinetic Energy Kinetic energy is energy an object possesses by virtue of its motion: 1 2 E k =  mv 2

4. Thermochemistry © 2012 Pearson Education, Inc. Potential Energy Potential energy is energy an object possesses by virtue of its position or chemical composition. The most important form of potential energy in molecules is electrostatic potential energy, E el : KQ1Q2KQ1Q2 d E el =

5. Thermochemistry © 2012 Pearson Education, Inc. Conversion of Energy Energy can be converted from one type to another. For example, the cyclist in Figure 5.2 has potential energy as she sits on top of the hill.

6. Thermochemistry © 2012 Pearson Education, Inc. Conversion of Energy As she coasts down the hill, her potential energy is converted to kinetic energy. At the bottom, all the potential energy she had at the top of the hill is now kinetic energy.

7. Thermochemistry © 2012 Pearson Education, Inc. Units of Energy The SI unit of energy is the joule (J): An older, non-SI unit is still in widespread use: the calorie (cal): 1 cal = 4.184 J 1 J = 1  kg m 2 s2s2

8. Thermochemistry © 2012 Pearson Education, Inc. Definitions: System and Surroundings The system includes the molecules we want to study (here, the hydrogen and oxygen molecules). The surroundings are everything else (here, the cylinder and piston).

9. Types of Systems Open – matter and energy are exchanged with surroundings Closed – only energy is exchanged Isolated – neither matter nor energy are exchanged What type of system is the human body? © 2012 Pearson Education, Inc.

10. Thermochemistry © 2012 Pearson Education, Inc. Definitions: Work Energy used to move an object over some distance is work: w = F  d where w is work, F is the force, and d is the distance over which the force is exerted.

11. Thermochemistry © 2012 Pearson Education, Inc. Heat Energy can also be transferred as heat. Heat flows from warmer objects to cooler objects.

12. Thermochemistry © 2012 Pearson Education, Inc. First Law of Thermodynamics Energy is neither created nor destroyed. In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa.

13. Thermochemistry © 2012 Pearson Education, Inc. Internal Energy The internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E.

14. Thermochemistry © 2012 Pearson Education, Inc. Internal Energy By definition, the change in internal energy,  E, is the final energy of the system minus the initial energy of the system:  E = E final − E initial E final = products E initial = reactants

15. Thermochemistry © 2012 Pearson Education, Inc. Changes in Internal Energy If  E > 0, E final > E initial –Therefore, the system absorbed energy from the surroundings. –This energy change is called endergonic.

16. Thermochemistry © 2012 Pearson Education, Inc. Changes in Internal Energy If  E < 0, E final < E initial –Therefore, the system released energy to the surroundings. –This energy change is called exergonic.

17. Thermochemistry © 2012 Pearson Education, Inc. Changes in Internal Energy When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is,  E = q + w.

18. Thermochemistry © 2012 Pearson Education, Inc.  E, q, w, and Their Signs

19. Thermochemistry Sample Exercise 5.2 Relating Heat and Work to Changes of Internal Energy Calculate the change in the internal energy for a process in which a system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings.

20. Thermochemistry © 2012 Pearson Education, Inc. Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings, the process is endothermic.

21. Thermochemistry © 2012 Pearson Education, Inc. Exchange of Heat between System and Surroundings When heat is absorbed by the system from the surroundings, the process is endothermic. When heat is released by the system into the surroundings, the process is exothermic.

22. Thermochemistry © 2012 Pearson Education, Inc. State Functions Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem.

23. Thermochemistry © 2012 Pearson Education, Inc. State Functions However, we do know that the internal energy of a system is independent of the path by which the system achieved that state. –In the system depicted in Figure 5.9, the water could have reached room temperature from either direction.

24. Thermochemistry © 2012 Pearson Education, Inc. State Functions Therefore, internal energy is a state function. It depends only on the present state of the system, not on the path by which the system arrived at that state. And so,  E depends only on E initial and E final.

25. Thermochemistry © 2012 Pearson Education, Inc. State Functions However, q and w are not state functions. Whether the battery is shorted out or is discharged by running the fan, its  E is the same. –But q and w are different in the two cases.

26. Thermochemistry © 2012 Pearson Education, Inc. Work Usually in an open container the only work done is by a gas pushing on the surroundings (or by the surroundings pushing on the gas).

27. Thermochemistry © 2012 Pearson Education, Inc. Work We can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston: w = −P  V

28. Thermochemistry © 2012 Pearson Education, Inc. Enthalpy If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure–volume work, we can account for heat flow during the process by measuring the enthalpy of the system. Enthalpy is the internal energy plus the product of pressure and volume: H = E + PV

29. Thermochemistry © 2012 Pearson Education, Inc. Enthalpy When the system changes at constant pressure, the change in enthalpy,  H, is  H =  (E + PV) This can be written  H =  E + P  V

30. Thermochemistry © 2012 Pearson Education, Inc. Enthalpy Since  E = q + w and w = −P  V, we can substitute these into the enthalpy expression:  H =  E + P  V  H = (q + w) − w  H = q So, at constant pressure, the change in enthalpy is the heat gained or lost.

31. Thermochemistry © 2012 Pearson Education, Inc. Endothermicity and Exothermicity A process is endothermic when  H is positive.

32. Thermochemistry © 2012 Pearson Education, Inc. Endothermicity and Exothermicity A process is endothermic when  H is positive. A process is exothermic when  H is negative.

33. Thermochemistry Sample Exercise 5.3 Determining the Sign of  H Molten gold poured into a mold solidifies at atmospheric pressure. With the gold defined as the system, is the solidification an exothermic or endothermic process?

34. Thermochemistry © 2012 Pearson Education, Inc. Enthalpy of Reaction The change in enthalpy,  H, is the enthalpy of the products minus the enthalpy of the reactants:  H = H products − H reactants

35. Thermochemistry © 2012 Pearson Education, Inc. Enthalpy of Reaction This quantity,  H, is called the enthalpy of reaction, or the heat of reaction.

36. Thermochemistry © 2012 Pearson Education, Inc. The Truth about Enthalpy 1.Enthalpy is an extensive property. 2.  H for a reaction in the forward direction is equal in size, but opposite in sign, to  H for the reverse reaction.

37. Thermochemistry © 2012 Pearson Education, Inc. The Truth about Enthalpy 3.  H for a reaction depends on the state of the products and the state of the reactants. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H= -890 kJ CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  H= -802 kJ 2H 2 O(l)  2H 2 O(g)  H= +88 kJ

38. Thermochemistry Hydrogen peroxide can decompose to water and oxygen by the reaction 2 H 2 O 2 (l) 2 H 2 O(l) + O 2 (g)  H = –196 kJ Calculate the quantity of heat released when 5.00 g of H 2 O 2 (l) decomposes at constant pressure. Sample Exercise 5.4 Relating  H to Quantities of Reactants and Products

39. Thermochemistry © 2012 Pearson Education, Inc. Calorimetry Since we cannot know the exact enthalpy of the reactants and products, we measure  H through calorimetry, the measurement of heat flow.

40. Thermochemistry © 2012 Pearson Education, Inc. Heat Capacity and Specific Heat The amount of energy required to raise the temperature of a substance by 1 K (1  C) is its heat capacity.

41. Thermochemistry © 2012 Pearson Education, Inc. Heat Capacity and Specific Heat We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K (or 1  C).

42. Thermochemistry © 2012 Pearson Education, Inc. Heat Capacity and Specific Heat Specific heat, then, is Specific heat = heat transferred mass  temperature change Cs = q m  Tm  T q = C s x m x  T

43. Thermochemistry A.Hg(l) B.Fe(s) C.Al(s) D.H 2 O(l)

44. Thermochemistry Sample Exercise 5.5 Relating Heat, Temperature Change, and Heat Capacity (a) Large beds of rocks are used in some solar-heated homes to store heat. Assume that the specific heat of the rocks is 0.82 J/g–K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0  C. (b) What temperature change would these rocks undergo if they emitted 450 kJ of heat?

45. Thermochemistry © 2012 Pearson Education, Inc. Constant Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.

46. Thermochemistry © 2012 Pearson Education, Inc. Constant Pressure Calorimetry Because the specific heat for water is well known (4.184 J/g-K), we can measure  H for the reaction with this equation: q = m  s   T =  H q soln = -q rxn

47. Thermochemistry Sample Exercise 5.6 Measuring  H Using a Coffee-Cup Calorimeter When 50.0 mL of 0.100 MAgNO3 and 50.0 mL of 0.100 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 22.30  C to 23.11  C. The temperature increase is caused by the following reaction: AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) Calculate  H for this reaction in AgNO3, assuming that the combined solution has a mass of 100.0 g and a specific heat of 4.18 J/g  C.

48. Thermochemistry © 2012 Pearson Education, Inc. Bomb Calorimetry Reactions can be carried out in a sealed “bomb” such as this one. The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction.

49. Thermochemistry © 2012 Pearson Education, Inc. Bomb Calorimetry Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy,  E, not  H. For most reactions, the difference is very small.

50. Thermochemistry Sample Exercise 5.7 Measuring q rxn Using a Bomb Calorimeter A 0.5865-g sample of lactic acid (HC 3 H 5 O 3 ) is burned in a calorimeter whose heat capacity is 4.812 kJ/  C. The temperature increases from 23.10  C to 24.95  C. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole.

51. Thermochemistry © 2012 Pearson Education, Inc. Hess’s Law  H is well known for many reactions, and it is inconvenient to measure  H for every reaction in which we are interested. However, we can estimate  H using published  H values and the properties of enthalpy.

52. Thermochemistry © 2012 Pearson Education, Inc. Hess’s Law Hess’s law states that “if a reaction is carried out in a series of steps,  H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”

53. Thermochemistry © 2012 Pearson Education, Inc. Hess’s Law Because  H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.

54. Thermochemistry a. A.No change. B.Sign of changes. C.Value of increases. D.Value of decreases.

55. Thermochemistry b. A.No change. B.Sign of changes. C.Value of doubles. D.Value of decreases by half.

56. Thermochemistry Sample Exercise 5.8 Using Hess’s Law to Calculate  H Carbon occurs in two forms, graphite and diamond. The enthalpy of the combustion of graphite is –393.5 kJ/mol, and that of diamond is –395.4 kJ/mol: C(graphite) + O 2 (g) CO 2 (g)  H = –393.5 kJ C(diamond) + O 2 (g) CO 2 (g)  H = –395.4 kJ Calculate  H for the conversion of graphite to diamond: C(graphite) C(diamond)  H = ?

57. Thermochemistry Sample Exercise 5.8 Using Hess’s Law to Calculate  H Carbon occurs in two forms, graphite and diamond. The enthalpy of the combustion of graphite is –393.5 kJ/mol, and that of diamond is –395.4 kJ/mol: C(graphite) + O 2 (g) CO 2 (g)  H = –393.5 kJ C(diamond) + O 2 (g) CO 2 (g)  H = –395.4 kJ Calculate  H for the conversion of graphite to diamond: C(graphite) C(diamond)  H = ?

58. Thermochemistry © 2012 Pearson Education, Inc. Enthalpies of Formation An enthalpy of formation,  H f, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.

59. Thermochemistry © 2012 Pearson Education, Inc. Standard Enthalpies of Formation Standard enthalpies of formation,  H f °, are measured under standard conditions (25 °C and 1.00 atm pressure).

60. Thermochemistry Sample Exercise 5.10 Equations Associated with Enthalpies of Formation Write the equation corresponding to the standard enthalpy of formation of liquid carbon tetrachloride (CCl 4 ).

61. Thermochemistry © 2012 Pearson Education, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l) C 3 H 8 (g)  3C (graphite) + 4H 2 (g)

62. Thermochemistry © 2012 Pearson Education, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g)  3C (graphite) + 4H 2 (g) 3C (graphite) + 3O 2 (g)  3CO 2 (g) C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)

63. Thermochemistry © 2012 Pearson Education, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g)  3C (graphite) + 4H 2 (g) 3C (graphite) + 3O 2 (g)  3CO 2 (g) 4H 2 (g) + 2O 2 (g)  4H 2 O(l) C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)

64. Thermochemistry © 2012 Pearson Education, Inc. C 3 H 8 (g)  3C (graphite) + 4H 2 (g) 3C (graphite) + 3O 2 (g)  3CO 2 (g) 4H 2 (g) + 2O 2 (g)  4H 2 O(l) C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l) Calculation of  H The sum of these equations is C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)

65. Thermochemistry © 2012 Pearson Education, Inc. Calculation of  H We can use Hess’s law in this way:  H =  n  H f,products –  m  H f °,reactants where n and m are the stoichiometric coefficients.

66. Thermochemistry © 2012 Pearson Education, Inc.  H= [3(−393.5 kJ) + 4(−285.8 kJ)] – [1(−103.85 kJ) + 5(0 kJ)] = [(−1180.5 kJ) + (−1143.2 kJ)] – [(−103.85 kJ) + (0 kJ)] = (−2323.7 kJ) – (−103.85 kJ) = −2219.9 kJ Calculation of  H C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)

67. Thermochemistry Use Table 5.3 to calculate the enthalpy change for the combustion of 1 mol of ethanol: C 2 H 5 OH(l) + 3 O 2 (g) 2 CO 2 (g) + 3 H 2 O(l) Sample Exercise 5.11 Calculating an Enthalpy of Reaction from Enthalpies of Formation

68. Thermochemistry Sample Exercise 5.12 Calculating an Enthalpy of Formation from Enthalpies of Reaction Given the following standard enthalpy change, use the standard enthalpies of formation in Table 5.3 to calculate the standard enthalpy of formation of CuO(s): CuO(s) + H 2 (g) Cu(s) + H 2 O(l)  H  = –129.7 kJ