13th 14th thermodynamics
TRANSCRIPT
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hemical Thermodynamics
Part II: Fundamentals and
Spontaneity
Louie Paolo D. CarpioInstructor 3
Institute of Chemistry, UP Diliman
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OBJECTIVES for today
1. Discuss the different laws of
thermodynamics
2. Define entropy3. Define Gibbs free energy
. Define spontaneity in terms of
thermodynamic parameters
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SI!!S that you "ill need
1. !ol"ing for G# ! #$ and wor% of
processes.
2. Determine whether a chemicalreaction is spontaneous or not.
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&hermodynamics
&'$()*D+,-)C!/ the study of
the energy transfers accompanyingphysical and chemical changes
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Laws of &hermodynamics
1. 0eroth law / thermal euilibrium
2. 1stLaw energy
3. 2ndLaw / entropy
. 3rd
Law / perfect crystal
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1stLaw of &hermodynamics
Law of Conser"ation of $nergy/energy in the uni"erse is constant
$system 4 $surroundings
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1stLaw of &hermodynamics
n any process# the total change inthe energy of a system 4$5 is eualto the sum of the transferred heat
between the heat and thesurroundings
$system 6 w
w wor%
7wor% / all types of energy e8cept heat
heat
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9or% and heat at constant pressure
w Pe8ternal4:5
r8n nL(4';r8n5
#: < ;# wor% is done by the
system since w = ;#: = ;# wor% is done on thesystem since w < ;
# $nthalpy of reaction
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Con"entions
Energy of thesystem
decreased
Energy of thesystem
increased
Change inenergy of the
system (Esys)
Heat is
released
Heat is
absorbedHeat (q)
Work is done
by the system
Work is done
on the systemWork (w)
(-)(+)
Thermodynamic
variable
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Difference between
' and
$
BombCalorimeter
Coffee-cupcalorimeter
Instrument
used to
quantify
Heat at
constantvolume, w =
Heat at
constant
pressure
Heat
EHCriteria
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(elation between
' and
$
' $ 6 4P:5
#' and $ are e8pressed in >oules
# P is in Pascal 41;132? Pa 1 atm5
# : is in m34;.;;1 m3 1 liter5
#4P:5 45 wor%
@rom $ 6 wA
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Con"ersion of con"entional
chemistry units into >oules
7( B.31 mol1E1
4gas constant in terms of >oules5
f P is in atm and : is in liters#their product can be con"ertedinto >oules by multiplying the ratio
of the gas constant (.
( )( ) >oules
EmolatmL;.;B21
Emol
CB.31
:P litersatm =
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P: for reactions in"ol"ing solids
and liuids only
&a%e note that the product ofpressure and "olume for solidsand liuids can be ignored since
their molar "olume is typically ;.1F of that for a gas.
P: "ery small
4for solids and liuids5
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P: for reactions in"ol"ing gases
4P:5 4ngas5(&
@rom the ideal gas euation
ngas
4ngaseous products
5 / 4ngaseous reactants
5
where
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(elation between
' and
$ for
chemical reactions w gases
' $ 6 4P:5
' $ 6 4ngas5(&
since 4P:5 4ngas5(&
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!pontaneous Processes
definition / a spontaneous processis one that mo"es the reaction
system to euilibrium
# Hsually proceeds with a decreasein energy
# Proceed from more ordered to
more random states 4greater
entropy5
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$,&(*P+
denoted by the symbol !
# )easure of disorder
# Change in entropy is a state
function
! !final/ !initial
# HnitA E
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$,&(*P+
@or re"ersible processesA
&
1I!
processre"ersible=
7 re"ersible processes cause
infinitesimally small changes in
thermodynamic states P# :# & or n.$8ampleA adding 1;; of heat but the
temperature almost did not increase
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$,&(*P+
n general#
2. !gas< !liuid< !solids
3. ncreasing the temperature of a
substance increases its entropy
7 $ntropy is hard to uantify. 'ow do
we compute the entropies ofsubstancesJ
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3rdLaw of &hermodynamics
- completely ordered purecrystalline solid has an entropy of
Kero at ; E 4absolute Kero5.
# 3rd law of thermodynamics is the
basis of the numerical scale of
entropy
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3rdLaw of &hermodynamics
!ince ! is a state function and
assume that the initial state of a
substance is a perfect crystalline
solid at ; EA
! !final/ !perfect crystalline solid
! !final/ !initial
! !final/ ;
! !o standard molar entropy
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!tandard )olar $ntropies
# $ntropy of a substance at 1 atm
and a gi"en temperature 4usually
2?oC5
# Denoted by the !o
# Hnits of >oule per mole per Eel"in
4molE5
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!tandard )olar $ntropies
# n general# as molecules becomemore comple8 in structure# there
are more ways for atoms to mo"e
about with respect to one another#leading to a higher entropy.
$8. molecular o8ygen "s oKone
;
3*;
2* !!
>reactants>i
iproductsir8n !n!nI!
# $ntropy is also a state function.
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$8ample
!ol"e for the ! of the reactionbelowA
!o"aluesA
'2*24l5 1;.M Emol
'2*4l5 M.1 Emol
*24g5 2;?.; Emol
2 '2*24l5N 2 '2*4l56 *24g5
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$,&(*P+ and !pontaneity
# &he entropy change of the uni"erse4system and surroundings5
determines the spontaneity of a
chemical reaction.
# n spontaneous changes# the
uni"erse tends toward a state
of greater disorder.
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2ndLaw of &hermodynamics
# - net increase in the entropy of theuni"erse results to a spontaneous
reaction
!uni"erse !system6 !surroundings
!uni"erse< ; 4spontaneous5
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Change in entropy of a chemical r8n
# $ntropy of a chemical reaction
increases ifA
. the molar "olume of the products is
greater than that of the reactants
?. the temperature of a reaction system
increased
M. &he reaction produces gases or thesolid is diminished. @ormation of
gases are fa"ored since !gas< !liuids >reactants
o
f#>i iproducts
o
f#i
o
r8n
GnGnG
# !tandard free energy change at 1 atmand 2?oC.
# Gfo is the standard energy of
formation or free energy contentQ ofsubstances similar to 'f
o.
$ l
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$8ample
!ol"e for the Go
of the reactionbelowA
Gfo"aluesA
'2*24l5 12;. %mol
'2*4l5 22B.M %mol
*24g5 ; %mol
2 '2*24l5N 2 '2*4l56 *24g5
Gibb ' l h lt $ ti
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Gibbs'elmholtK $uation
# Pro"ides the relationship between G# ' and ! at a gi"en temperature
in EA
Gr8n 'r8n &!r8n
f the reaction is in standard
conditions
Go
'o/ &
!o
Gibb @ d t it
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Gibbs @ree energy and spontaneity
@or a spontaneous reaction# G = ;
G
' &
!
- chemical reaction would always bespontaneous ifA
2. 'reaction= ;
3. !reaction< ;
Gibb @ d t it
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Gibbs @ree energy and spontaneity
n general# spontaneous chemical
reactions in"ol"eA
Roth ' and ! must be considered to be
able to predict the spontaneity of a reaction.
!ome endothermic reactions arespontaneous. !ome reactions in"ol"ing a
decrease in entropy can be spontaneous. G
must be computed to accurately predict the
spontaneity of a chemical reaction.
1. (elease of heat or e8othermic 4' = ;5
2. ncrease in entropy 4! < ;5
$ l
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$8ample
!ol"e for the Goof the reaction belowA
2 CB'1B4l56 2? *24g5N 1B '2*4l56 1M C*24g5
!"#"$- %&'#&(H2O(l)
%$(#)*- ("(#'$O2(!)
%'#$*O2(!)
('&- %*"#""H#"(l)
$o(%&'mol)Hfo (%&mol)$ubstance
$ l
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$8ample
!ol"e for the
!
o
of the reaction belowA
*@24g56 '2*4g5N *24g56 2 '@4g5
'o 323.2 %mol
Go 3?B. %mol
$8press your answer in Emol.
Ch i Gibb @ $ G
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Change in Gibbs @ree $nergy#
G
(ecallA 4system is a chemical reaction5
( ) = > >reactants>i iproductsir8n GnGnG
G < ;# nonspontaneous
G = ;# spontaneous
G ;# euilibrium
Gibb @ d ilib i
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Gibbs @ree energy and euilibrium
@or a reaction in euilibrium# G ;
G
' / &
! ;
- chemical reaction is in euilibriumif the reaction is re"ersible and both
re"ersible reactions occur at the
same rate.
(eactants Products
$ ilib i ti
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$uilibrium reactions
@or a reaction in euilibrium# G ;
&he rate of formation of products iseual to the rate of degradation of
reactants at euilibrium.
(eactants Products
Phase changes at their phase changetemperatures are euilibrium
reactions.
Ph h d ilib i
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Phase changes and euilibrium
'2*4l5 '2*4s5at ;oC
Phase changes at their phase change
temperatures are euilibrium reactions.
GfreeKing Gmelting ; 4at ;oC5
'ence# at ;oC# melting and freeKing
occurs at the same rate. 4
G ;5
Gibbs 'elmholt S e ilibri m
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Gibbs'elmholtK S euilibrium
G ' / &! ;
' / &
! ;
' &!
!
'
&
=
Gibbs 'elmholtK S euilibrium
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Gibbs'elmholtK S euilibrium
!ignificance of euationA
2. &his is the minimum temperature at
which a reaction would be
spontaneous if !r8n< ;.
3. &his & is the estimated phase
change temperature in Eel"in for a
phase change reaction.
reaction
reaction
!
'& =
$8ample
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$8ample
$stimate the boiling point of C'3
*'
gi"en the following thermodynamic
data belowA
C'3*'4l5N C'3*'4g5
%("#&$ +.mol- %#!! k+molCH(/H0g1
$%!#& +.mol- %(&! k+molCH(/H0l1
2oHfosubstance
$8ample
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$8ample
$stimate at which temperature in Eel"in
will the reaction below will becomespontaneousA
CaCl24s5N Ca4s56 Cl24g5
$*#! +.mol- )"'#& k+molCaCl%0s1
%%(#) +.mol k+molCl%0g1
*$#*% +.mol k+molCa0s1
2oHfosubstance
$,D *@ CL-!!
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$,D *@ CL-!!
# Chapter co"eredA Chapter 1?
# ,e8t meetingA Problem !et dayO
# ,e8t topicA $lectronic !tructure of-toms
#TH$!&*,!JJJJ