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

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

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

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

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

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 = kJ·mol -1  H f º = kJ·mol -1  H f º = kJ·mol -1

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?

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 = kJ·mol -1 CH 4 (g) C(gr) + 2 H 2 (g) methane:  H f º = kJ·mol -1 Which of the reactions is exothermic?

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) kJ·mol -1 C(gr) kJ·mol -1 K(s) + 1/2 Br 2 (l) kJ·mol -1

6/22/2015 Enthalpy of Reaction CO(g)C(gr) + 1/2 O 2 (g)  H f º = kJ·mol -1 CO 2 (g)C(gr) + O 2 (g)  H f º = 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?

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

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 ] = [ ] - [ /2 · 0 ] = kJ · mol -1

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?

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?

Propane C 3 H 8  The standard heat of combustion of gaseous propane to make liquid water is 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 ?

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

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?

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

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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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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?

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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