Data Table Needed Standard Heats of Formation Problem Set Applications of Thermodynamics.
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Transcript of Data Table Needed Standard Heats of Formation Problem Set Applications of Thermodynamics.
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Data Table Needed
Standard Heats of Formation
Problem Set
Applications of Thermodynamics
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Enthalpy Changes and Thermodynamic Applications
Edward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
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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
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Enthalpy
H = E + PV
Heat energycontent ofa molecule
internal molecular motion
electronic energy
pressure-volume workrequired to maintain
the volume of the moleculeOnly definedfor a constantpressure process
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Enthalpy Values
Absolute enthalpy values cannot be measured,but changes in enthalpy can be measured
relative to an arbitrary reference state
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EnthalpyHeat of Formation Values
H2 (g)O2 (g)C(gr)
ReferenceReferenceStateStateAn element in its
most stable phaseat 25 ºC and1 atm pressure
Hfº = 0
H2O2(l)
StandardStandardStateState
CH3OH(l)
C10H8(s)
StandardStandardStateState
An element, molecule or ion at
25 ºC and 1 atm pressure
Hfº = -187.78 kJ·mol-1
Hfº = +78.53 kJ·mol-1
Hfº = -238.66 kJ·mol-1
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Hfºchange
(final value - initial value)
enthalpy standard state25 ºC and 1 atm
formation
Is enthalpy a function of temperature?
Enthalpy of Formation
What do the initial and final values refer to?
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Enthalpy of FormationExamples
Hfº at 25 ºC and 1 atm for one mole of
O2(g) O2(g)oxygen:
Hfº = 0
O3(g) 3/2 O2(g)ozone:
Hfº = +142.7 kJ·mol-1
CH4(g) C(gr) + 2 H2(g)methane:
Hfº = -74.81 kJ·mol-1
Which of the reactions is exothermic?
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Write the equation and determine theenthalpy of formation at 25 ºC and 1 atm for
one mole of
HCHO(g)
formaldehyde:
Hfº =
C(dia)
diamond:
Hfº =
KBr(s)
potassium bromide:
Hfº =
C(gr) + H2(g) + 1/2 O2(g)
-108.57 kJ·mol-1
C(gr) +1.897 kJ·mol-1
K(s) + 1/2 Br2(l) -392.17 kJ·mol-1
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Enthalpy of Reaction
CO(g) C(gr) + 1/2 O2(g) Hfº = -110.52 kJ·mol-1
CO2(g)C(gr) + O2(g) Hfº = -393.51 kJ·mol-1
CO2(g)CO(g) + 1/2 O2(g) Hrxº = ?
What does a negative enthalpy change indicate?
Can carbon monoxide burn exothermically?
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Hess' Law
Reactants Products
Elements
Hrx
+Hf,productsHf,reactants
Hrx = Hf,products - Hf,reactants
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Hess' Law
CO(g) + 1/2 O2(g) CO2(g)
C(gr) + O2(g)
Hrx
Hrx = Hf,products - Hf,reactants
Hrx = [Hf,CO2 ] - [Hf,CO + 1/2 Hf,O2 ]
= [-393.51 ] - [-110.52 + 1/2 ·
0 ] = -282.99 kJ · mol-1
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Enthalpy of ReactionHess’ Law
During the winter, a typical house will requireabout 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?
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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?
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PropaneC3H8
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, qp or qv?
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Enthalpy of Neutralization
(Literature value: -60.2 kJ·mol-1)
Estimate the enthalpy of neutralization of aqueous sodium hydroxide and
hydrochloric acid.
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Thermite Reaction
The Thermite Reaction has been used as amethod to weld together railroad rails.
It involves a single displacement reactionin which iron(III) oxide is burned with
aluminum powder.
How much energy is released by this process?
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Thermodynamic ApplicationsComparison 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?
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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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