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

04/18/23

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 energycontent ofa molecule

internal molecular motion

electronic energy

pressure-volume workrequired to maintain

the volume of the moleculeOnly definedfor a constantpressure process

Enthalpy Values

Absolute enthalpy values cannot be measured,but changes in enthalpy can be measured

relative to an arbitrary reference state

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

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?

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?

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

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?

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?

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?

Enthalpy of Neutralization

(Literature value: -60.2 kJ·mol-1)

Estimate the enthalpy of neutralization of aqueous sodium hydroxide and

hydrochloric acid.

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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