1. © 2009, Prentice-Hall, 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.

2. © 2009, Prentice-Hall, Inc. Standard Enthalpies of Formation Standard enthalpies of formation,  H f °, are measured under standard conditions (25 °C and 1.00 atm pressure).

3. Sample Exercise 5.10 Identifying Equations Associates with Enthalpies of Formation For which of the following reactions at 25°C would the enthalpy change represent a standard enthalpy of formation? For each that does not, what changes are needed to make it an equation whose ΔH is an enthalpy of formation? Solution 6 C(graphite) + 6 H 2 (g) + 3 O 2 (g) → C 6 H 12 O 6 (s)

4. Sample Exercise 5.10 Identifying Equations Associates with Enthalpies of Formation Write the equation corresponding to the standard enthalpy of formation of liquid carbon tetrachloride (CCl 4 ). Answer: C(graphite) + 2 Cl 2 (g) → CCl 4 (l) Practice Exercise

5. © 2009, Prentice-Hall, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g)  3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g)  3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g)  4 H 2 O (l)

6. © 2009, Prentice-Hall, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g)  3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g)  3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g)  4 H 2 O (l)

7. © 2009, Prentice-Hall, Inc. Calculation of  H Imagine this as occurring in three steps: C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g)  3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g)  3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g)  4 H 2 O (l)

8. © 2009, Prentice-Hall, Inc. C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) C 3 H 8 (g)  3 C (graphite) + 4 H 2 (g) 3 C (graphite) + 3 O 2 (g)  3 CO 2 (g) 4 H 2 (g) + 2 O 2 (g)  4 H 2 O (l) C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) Calculation of  H The sum of these equations is:

9. © 2009, Prentice-Hall, 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.

10. © 2009, Prentice-Hall, 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 C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (l) Calculation of  H

11. Sample Exercise 5.11 Calculating an Enthalpy of Reaction from Enthalpies of Formation (a) Calculate the standard enthalpy change for the combustion of 1 mol of benzene, C 6 H 6 (l), to form CO 2 (g) and H 2 O(l). (b) Compare the quantity of heat produced by combustion of 1.00 g propane to that produced by 1.00 g benzene. Solution

12. Sample Exercise 5.11 Calculating an Enthalpy of Reaction from Enthalpies of Formation Solution Using the standard enthalpies of formation listed in Table 5.3, calculate the enthalpy change for the combustion of 1 mol of ethanol: Answer: –1367 kJ. Practice Exercise

13. Sample Exercise 5.12 Calculating an Enthalpy of Formation Using an Enthalpy of Reaction The standard enthalpy change for the reaction is 178.1 kJ. From the values for the standard enthalpies of formation of CaO(s) and CO 2 (g) given in Table 5.3, calculate the standard enthalpy of formation of CaCO 3 (s). Solution 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): Answer: –156.1 kJ/mol Practice Exercise