Chapter 6: Thermochemistry

1. Chapter 6: Thermochemistry • Internal Energy, Heat, and Work • Heat Capacity • Enthalpy • Measuring Enthalpy Changes (Calorimetry) • Calculating Enthalpy Changes • Hess’ Law • Standard Heats of Formation • Fuels

2. Example 1 A system releases 255 calories of heat and does 428 calories of work. What is the change in internal energy of this system? Answer: -683 calories

3. Example 2 A gas at 2.00 atm pressure expands from a volume of 5.00 L to a volume of 15.00 L as it is heated.  Calculate the work done during this expansion assuming that the pressure remains constant. Answer: -2.03 x 103 J

4. Example 3 One mole of H2O(g) at 1.00 atm and 100.°C occupies a volume of 30.6 L. When one mole of H2O(g) is condensed to one mole of H2O(l) at 1.00 atm and 100.°C, 40.66 kJ of heat is released. If the density of H2O(l) at this temperature and pressure is 0.996 g/cm3, calculate ΔE for the condensation of one mole of water at 1.00 atm and 100.°C.

5. Example 4 How much energy (in joules) is needed to heat a cup of water for coffee?  Assume the following: the cup holds 250 mL of water the density of water is 1.00 g/mL the water is initially at 19°C the ideal temperature for brewing coffee is 97°C Answer: 81.6 x 103 J

6. Example 5 A 19.6 g sample of an unknown metal was heated to 61.67°C.  When the hot metal was placed in calorimeter containing 26.7 g of water, the temperature rose from 25.00°C to 30.00°C.  Find the specific heat of the metal. Answer: 0.900 J/g°C

7. How a refrigerator works http://homepage.smc.edu/merlic_jennifer/Chem11/lectures/refrigerator.htm

8. Example 6 Consider a 64.0 g sample of H2O(g) at 128°C. What phase or phases are present when 171 kJ of energy is removed from this sample? Specific heat capacities: ice, s=2.1 J/g°C liquid, s=4.2 J/g°C steam, s=2.0 J/g°C DHvap = 40.7 kJ/mol DHfus = 6.02 kJ/mol

9. Example 7 How much heat is released when 2.50 g of N2 reacts with excess H2 to form NH3 (g)? N2 (g)  +  3 H2 (g)  --->  2 NH3 (g)  ;  DH° = -91.8 kJ Answer: 8.20 kJ are released

10. Example 8 If 25.0 mL of 2.50 M HCl reacts with 35.0 mL of 1.25 M NaOH, how much heat is released? HCl (aq) +  NaOH (aq)    H2O (l) + NaCl (aq) ; DH° = -53.8 kJ Answer: 2.35 kJ are released

11. Example 9 When 5.00 g of NaNO3 was dissolved in 100.0 g of water at 22.0°C, the temperature dropped to 19.1°C.  Find the enthalpy change for the dissolving of exactly 1 mole of NaNO3.  Answer: +22 kJ

12. Example 10 When 33.0 mL of 1.20 M HCl was added to 42.0 mL of a solution containing excess NaOH in a coffee cup calorimeter, the temperature of the solutions rose from 25.0°C to 31.8°C.  Find DH for the reaction as written below. HCl (aq)  +  NaOH (aq)  --->  H2O (l)  +  NaCl (aq)  ;  DH = ? Assume the density and specific heat of the solutions are the same as those of water. Answer: -54 kJ

13. Example 11 Given: Fe2O3(s) + 3 CO(g)  2 Fe(s) + 3 CO2(g) ; DH° = -23 kJ 3 Fe2O3(s) + CO(g)  2 Fe3O4(s) + CO2(g) ; DH° = -39 kJ Fe3O4(s) + CO(g)  3 FeO(s) + CO2(g) ; DH° = +18 kJ Find DH° for: FeO(s) + CO(g)  Fe(s) + CO2(g) Answer: -11 kJ

14. Example 12 Use Hess' Law and standard enthalpies of formation to determine DH° for the following reaction. 4 NH3 (g)  +  5 O2 (g)  --->  4 NO (g)  +  6 H2O (g) Answer: -906.0 kJ