22 September, 1997 Chem 1A03E/1E03E THERMOCHEMISTRY (Ch. 6) 1 Thermochemistry Thermochemistry - Energy of Chemical Reactions ents nts: t, work, forms of energy cific heat and energies of phase changes halpy changes in chemical reactions ndard enthalpies of formation s’s law imating enthalpies of reaction from Bond En
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22 September, 1997Chem 1A03E/1E03E THERMOCHEMISTRY (Ch. 6)1 Thermochemistry - Energy of Chemical Reactions Contents Contents: heat, work, forms of energy.
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22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)1
ThermochemistryThermochemistry- Energy of Chemical Reactions
ContentsContents:• heat, work, forms of energy• specific heat and energies of phase changes • enthalpy changes in chemical reactions• standard enthalpies of formation • Hess’s law • estimating enthalpies of reaction from Bond Energies
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)2
CHEMICAL ENERGYCHEMICAL ENERGY
Chemical bonds are a source of energy• BOND BREAKING - requires energy• BOND MAKING - releases energy
In a chemical reaction:• if more energy is released in forming bonds than is used in breaking bonds then . . . reaction is EXOTHERMICEXOTHERMIC
• if more energy is used in breaking bonds than is released in
forming bonds then . . . reaction is ENDOTHERMIC
Energy is released as HEAT, LIGHT, SOUND, WORK
Energy can be provided by - LIGHT - photochemistry- WORK - electrochemistry- COOLING of surroundings
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)3
Energy and ChemistryEnergy and ChemistryENERGYENERGY is the capacity to do is the capacity to do workwork or transfer or transfer heatheat..
HEATHEAT is the form of energy that flows between 2 is the form of energy that flows between 2 samples because of a difference in temperature.samples because of a difference in temperature.
WORKWORK is the form of energy that results in a is the form of energy that results in a macroscopic displacement of matter such as gas macroscopic displacement of matter such as gas expansion or motion of an object expansion or motion of an object (force x distance)(force x distance)
Other forms of energy —Other forms of energy —• light light • electricalelectrical• kinetickinetic
Heat flows into the Heat flows into the SYSTEMSYSTEM (solid CO (solid CO22) from the ) from the
SURROUNDINGSSURROUNDINGS in an E in an ENDOTHERMICNDOTHERMIC process.process.
heatheat
SurroundingsSurroundings
SystemSystem
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)12
• ENERGY is the capacity to do work or transfer heat.• HEAT is the form of energy that flows between 2 samples because of a difference in temperature.• WORK is the form of energy that results in a macroscopic displacement of matter such as gas expansion or motion of an object (force x distance)
In CO2 sublimation & expansion, the same amount the same amount
of of ENERGY flows from flows from surroundingssurroundings to systemsystem
If expanding gas is enclosed, part of the energy transfer appears in the form of WORK OF EXPANSION
wexp = - PV (for an ideal gas)
If expanding gas is not enclosed, the energy transfer appearsonly as HEATHEAT (CO2 gas gets warm).
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)13
FIRST LAW OF THERMODYNAMICSFIRST LAW OF THERMODYNAMICS
q = q = E - wE - w
heat energy transferred
Energy change
work doneby the system
Energy is conserved!Energy is conserved!
OR E = q + wE = q + w
NB - q and w positive when they are
transferred FROM surroundings
TO system
SurroundingsSurroundings
Heat
qsys > 0
System
Workwsys > 0
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)14
ENTHALPYENTHALPYENTHALPYENTHALPYMost chemical reactions occur at constant P, so
Heat transferred at constant P is called qp with
qp = H = E - w = E + P V = E+PV)
where H = enthalpy H is defined as E + PV)
H = heat transferred at constant PH = change in heat content of the system
H = HH = Hfinalfinal - H - Hinitialinitial
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)15
H = HH = Hfinalfinal - H - HinitialinitialH = HH = Hfinalfinal - H - Hinitialinitial
If Hfinal > Hinitial then H is positive
Process is ENDOTHERMIC
If Hfinal > Hinitial then H is positive
Process is ENDOTHERMIC
If Hfinal < Hinitial then H is negative
Process is EXOTHERMIC
If Hfinal < Hinitial then H is negative
Process is EXOTHERMIC
ENTHALPYENTHALPYENTHALPYENTHALPY
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)16
Endo- and ExothermicEndo- and ExothermicEndo- and ExothermicEndo- and Exothermic
SurroundingsSurroundings
Heatqsys > 0
System
ENDOTHERMICENDOTHERMICENDOTHERMICENDOTHERMIC
HeatHeat qqsyssys < 0 < 0
SurroundingsSurroundings
System
EXOTHERMICEXOTHERMIC
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)17
But the reverse reaction, the decomposition of water :
H2O(g) + 242 kJ ---> H2(g) + 1/2 O2(g)Endothermic reaction — heat is a “reactant”, H = +242 kJ. This does not occur spontaneously.
Consider the combustion of H2 to form water . .
H2(g) + 1/2 O2(g) ---> H2O(g) 242 kJExothermic reaction — heat is a “product”. H = -242 kJ. This is spontaneous and proceeds readily once initiated.
USING ENTHALPYUSING ENTHALPYUSING ENTHALPYUSING ENTHALPY
BUT . . . Decomposition of water can be made to occur by coupling to another, spontaneous process . . .
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)18
How can we How can we make Hmake H22 gas ? gas ?
N. Lewis, N. Lewis,
American Scientist,American Scientist,
Nov. 1995, page 534.Nov. 1995, page 534.
H2O
O2H2
LIGHT
S e m ic o nduc t o r Me t a l
w i r e
22 September, 1997 Chem 1A03E/1E03E
THERMOCHEMISTRY (Ch. 6)19
Making H2 from liquidliquid H2O involves two steps.