1. Data Table Needed Standard Heats of Formation Problem Set Applications of Thermodynamics

2. Enthalpy Changes and Thermodynamic Applications Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

3. 6/22/2015 Thermodynamic Applications  Reading Assignment: Chang, Chapter 6.7, 9.10  Hess’ Law is used to calculate the heat released by a chemical reaction.  Applications of thermodynamic principles

4. Enthalpy H = E + PV Heat energy content of a molecule internal molecular motion electronic energy pressure-volume work required to maintain the volume of the molecule Only defined for a constant pressure process

5. Enthalpy Values Absolute enthalpy values cannot be measured, but changes in enthalpy can be measured relative to an arbitrary reference state

6. Enthalpy Heat of Formation Values H 2 (g) O 2 (g) C (gr) Reference State An element in its most stable phase at 25 ºC and 1 atm pressure  H f º = 0 H 2 O 2 (l) StandardState CH 3 OH (l) C 10 H 8 (s) StandardState An element, molecule or ion at 25 ºC and 1 atm pressure  H f º = -187.78 kJ·mol -1  H f º = +78.53 kJ·mol -1  H f º = -238.66 kJ·mol -1

7. HfºHfº change (final value - initial value) enthalpy standard state 25 ºC and 1 atm formation Is enthalpy a function of temperature? Enthalpy of Formation What do the initial and final values refer to?

8. Enthalpy of Formation Examples  H f º at 25 ºC and 1 atm for one mole of O 2 (g) oxygen:  H f º = 0 O 3 (g)3/2 O 2 (g) ozone:  H f º = +142.7 kJ·mol -1 CH 4 (g) C(gr) + 2 H 2 (g) methane:  H f º = -74.81 kJ·mol -1 Which of the reactions is exothermic?

9. Write the equation and determine the enthalpy of formation at 25 ºC and 1 atm for one mole of HCHO(g) formaldehyde: Hfº =Hfº = C(dia) diamond: Hfº =Hfº = KBr(s) potassium bromide: Hfº =Hfº = C(gr) + H 2 (g) + 1/2 O 2 (g) -108.57 kJ·mol -1 C(gr) +1.897 kJ·mol -1 K(s) + 1/2 Br 2 (l) -392.17 kJ·mol -1

10. 6/22/2015 Enthalpy of Reaction CO(g)C(gr) + 1/2 O 2 (g)  H f º = -110.52 kJ·mol -1 CO 2 (g)C(gr) + O 2 (g)  H f º = -393.51 kJ·mol -1 CO 2 (g)CO(g) + 1/2 O 2 (g)  H rx º = ? What does a negative enthalpy change indicate? Can carbon monoxide burn exothermically?

11. 6/22/2015 Hess' Law ReactantsProducts Elements  H rx +  H f,products  H f,reactants  H rx =  H f,products -   H f,reactants

12. 6/22/2015 Hess' Law CO(g) + 1/2 O 2 (g)CO 2 (g) C(gr) + O 2 (g)  H rx  H rx =  H f,products -   H f,reactants  H rx = [  H f,CO2 ] - [  H f,CO + 1/2  H f,O2 ] = [-393.51 ] - [-110.52 + 1/2 · 0 ] = -282.99 kJ · mol -1

13. Enthalpy of Reaction Hess’ Law During the winter, a typical house will require about a half million kilojoules per day for heating. How many moles of natural gas (methane) would be needed? Is there an advantage to produce liquid water or steam as a by-product?

14. Acetylene Determine the heat released when one pound (454 g) of acetylene is burned. To obtain a "hotter" flame, oxygen is often burned with acetylene. Why is the flame hotter?

15. Propane C 3 H 8  The standard heat of combustion of gaseous propane to make liquid water is -2220. kJ·mol -1 Determine the standard heat of formation of gaseous propane. How much pressure-volume work is involved when this reaction occurs at 25 °C? Determine the change in internal energy for this reaction. The heat flow in this process is equal to which thermodynamic term, q p or q v ?

16. Enthalpy of Neutralization (Literature value: -60.2 kJ·mol -1 ) Estimate the enthalpy of neutralization of aqueous sodium hydroxide and hydrochloric acid.

17. Thermite Reaction The Thermite Reaction has been used as a method to weld together railroad rails. It involves a single displacement reaction in which iron(III) oxide is burned with aluminum powder. How much energy is released by this process?

18. 6/22/2015 Thermodynamic Applications Comparison of Liquid Fuels  methanol  ethanol  isooctane

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20. Adiabatic Expansion  The same process occurs, except there is no heat flow allowed between the system and the surroundings.  On expansion, the gas will cool and follows a non-isothermal PV curve. PV  = constant for an ideal diatomic gas,  =1.67

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22. Adiabatic Expansion  In each of the examples, a different pressure change pathway is followed by the gas.  How much work will be done if the process is reversed to complete the cycle?

23. 6/22/2015 Carnot Cycle  Consists of two isothermal and two adiabatic steps, occurring alternatively.  One of each type of step is involved in compression and expansion.

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