lecture 2 - thermochemistry
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9.0 THERMOCHEMISTRY
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Concept of Enthalpy
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Important Terms
Heatis energy transferred between two bodies of
different temperatures
Systemis any specific part of the universe
Surroundingsis everything that lies outside the
system
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Open systemis a system that can exchange
mass and energy with its surroundings
Closed systemis a system that allows theexchange of energy with its surroundings
Isolated systemis a system that does not allowthe exchange of either mass or energy with itssurroundings
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Energyis the ability to do work
SI unit of energy is kg m2
s2
or !oule(J) Non SI unit of energy is "alorie(Cal)
Cal ! "#$" J
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Thermochemistry
A study of heat change in chemical reactions.
Two types of chemical reactions:
Exothermic Endothermic
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Exothermic reactions
Enthalpy of products
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Consider the following reaction:
A (g) ! (g) " C (g) H # ve
(reactants) (prod$ct)
reactantsenthalpy
reaction pathway
#-veH
products
Energy profile diagram for exothermic reaction
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%nthalpy of products #enthalpy of reactants& $His
positi%e
Energy is a&sor&edby the system from the surrounding
Endothermic %eactions
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Consider the following reaction
A (g) ! (g) " C(g) H # ve
(reactants) (prod$ct)
Energy profile of diagram endothermic reactions
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Enthalpy& H
The heat content of a system or total energy in thesystem
Enthalpy, H of a system cannot !e measured whenthere is a change in the system.
Example: system undergoes com!ustion or ionisation.
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Enthalpy of %eaction& 'H and
tandard Condition Ent'alpy o( rea"tion)
'he enthalpy change associated with a chemicalreaction#
Standard ent'alpy* +H, 'he enthalpy change for a particular reaction that
occurs at $*and atm(standard state)
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Thermochemical E$ation The thermochemical e"uation shows the enthalpy changes.
Example * H+,(s) " H+,(l) H # -./01
# mole of H+,(l)is formed from # mole of H+,(s)at $%C,
&H ' (.$# )*
However, when # mole of H+,(s)is formed from # mole ofH+,(l), the magnit$deof H remainsthe same with the
oppositesignof it.
H+,(l) " H+,(s) H # -./ 01
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Types of Enthalpies
There are many )ind of enthalpies such as:
Enthalpy of formation
Enthalpy of com!ustion
Enthalpy of atomisation
Enthalpy neutralisation
Enthalpy hydration
Enthalpy solution
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Enthalpy of 2ormation& 'Hf
The change of heat when / moleof a compound is
formed from its elements at their standard states.
H+(g) 3 ,+(g)" H+, (l) 'Hf # +4- 01 mol/
The standard enthalpy of formation of any element in
its most sta!le state form is 5E%,.
+H ./ ' $ +H Cl./ ' $
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Enthalpy of Com6$stion& 'Hc
The heat released when # mole of su!stance is
!urned completely in excess oxygen.
C(s) ,+(g) " C,+(g) +H"# 787 01 mol/
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Enthalpy of Atomisation& Ha The heat change when # mole of gaseo$s atomsis formed
from its element
Hais always positive !ecause it involves only !rea)ing of!onds
e.g:
0a,s/ 0a,g/ Ha' #$1 )* mol-#
2Cl.,g/ Cl,g/ Ha' #3 )* mol-#
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Enthalpy of 9e$trali:ation& 'Hn
The heat change when # mole of water, H. is formed from the
neutrali4ation of acid and !ase .
HCl,a"/ 0aH,a"/ 5 0aCl,a"/H.,a"/ &Hn' 67 )* mol#
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Enthalpy of Hydration& Hhyd
The heat change when # mole of gaseo$s ionsis
hydratedin ;ater.
e.g:0a,g/ 0a,a"/ Hhyd' 8$( )* mol-#
Cl-,g/ Cl-,a"/ Hhyd' 3(3 )* mol-#
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Enthalpy of ol$tion& Hsoln
The heat change when # mole of a s$6stanceis
dissolves in water.
e.g*9Cl,s/ 9,a"/ Cl,a"/ Hsoln' (1$ )* mol-#
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Enthalpy of $6limation& Hs$6l
'he heat change when one mole of a substance
sublimes (solid into gas)#
s/
5 g/
su!lH
Hsu&l
' #$( )* mol#
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Calorimetry
A method used in the la!oratory to measure the
heat change of a reaction.
Apparatus used is )nown as the calorimeter Examples of calorimeter
;imple calorimeter
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The outer ;tyrofoam cup
insulate the reaction
mixture from the
surroundings it is
assumed that no heat is
lost to the surroundings/ Heat release !y the
reaction is a!sor!ed !y
solution and the
calorimeter
imple calorimeter
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A 6om6 calorimeter
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Important Terms in Calorimeter
pecific heat capacity& c
pecific heat capacity& cof a su!stance is the amo$nt of
heatre"uired to raise the temperature of one gramof the
su!stance !y one degree Celsi$s(1g /C/). Heat capacity& C
Heat capacity&Cis the amo$nt of heatre"uired to raise
the temperature of agiven $antityof the su!stance !y
one degree Celsi$s(1
C/)
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+ ! mc,'
Heat released !y
su!stance'
Heat a!sor!ed
!y calorimeter
+ ! heat released by substance
m! mass of substance
C! specific heat capacity,' ! temperature change
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!asic rinciple in Calorimeter
Heat released
!y a reaction
'Heat a!sor!ed
!y surroundings
- Surroundings may re(er to
t'e)
i. Calorimeter itsel( or-
ii. T'e ater and "alorimeter- /rea"tion m"$T or C$T
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Example /
n an experiment, $.#$$ g of H.and excess of .were compressed
into a #.$$ = !om! and placed into a calorimeter with heat capacity
of 8.4 x /=1.C/. The initial temperature of the calorimeter was
6.$$$$C and finally it increased to 6.#66 $C. Calculate the
amount of heat released in the reaction to form H., expressed in )*
per mole.
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ol$tion
Heat released ' Heat a!sor!ed !y the
calorimeter
# C'T
' 1.$7 > #$8*$C-#/ > $.#66$C/
' #.8# > #$8*
' #8.# )*
H+(g) 3,+(g) " H+,(c)
mole of H. ' $.#$$ .$#(
' $.$81( mol
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moles of H+, # mole of H+
.=8- mol of H+, released /=./ 01 energy
/ mol H+, released # 01
# +4= 01
'Heat of reaction& 'H # > +4= 01 mol/
$81(.$
#.#8
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E1ample 2
# Calculate the amount of 'eat releasedin a reaction inan aluminum "alorimeter with a mass of 034.0gand
contains 5400.0 m6of water# 'he initial temperature of
the calorimeter is 27.08Cand it increased to 24.38C.
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?iven*
;pecific heat capacity of aluminum ' $.663*g-#%C-#
;pecific heat capacity of water ' 8.#7 *g-#%C-#
?ater density ' #.$ g m=-#
&T ' @.7 -6.$ /%C ' .7%C
ol$tion
# m;c;T mcccT
' #@$$.$ g/8.#7 *g-#%C-#/.7 %C/
3$7@.$ g/$.663 *g-# %C-#/.7%C/
' 8(@(.@# *
' 8.@ )*
.eat absorbed bywater
.eat absorbed byaluminium
calorimeter
.eat released ! /
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HE;;; =A?
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Hess =aw
Hesss =aw states that when reactants are converted to
products, the change in enthalpy is the same whether the
reaction takes place in one step or in the series of steps.
The enthalpy change depends only on the nature of thereactants and products and is independent of the ro$te
ta0en.
#HA B
C
3H
.H
+= 321 HHH
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i. @ist all the thermochemical e$ations involved
Alge!raic Bethod
#--#6($)*molH/
3/
/
@/(
.
#--7()*molH//
#/
.
#-313)*mol-H///
.
=++
=+
=+
gOH
gCO
gO
gHCiii
gOH
gO
gHii
gCO
gO
SCi
Step
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ii. rite the enthalpy of formation reaction for C+H-
/(
/3
/ gHC
fH
gH
sC
=
+
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iii. Add the given reactions so that the res$lt is the desiredreaction.
-78)*
3H
H
#H
fH
78)*-/(
/3
/
DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
#6($)*3H/@/(/3g/Ciii/
-7()*3
H/
3/
3/
33/
)*-313#
H/
/
/
/
=
++=
=
+
+=++
=+
=+
gHC
fH
gH
sC
gOgHCgOHreverse
gOH
gO
gHii
gCO
gO
SCi
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Energy Cycle Bethod
0raw the energy "y"leand apply Hesss 6a to calculate the
unknown value#
+
C
(
s
)
7
H+
(
g
)
C+
H-
(
g
)
+
C
,+(g) 7H+,(g)
+
,+(g) 7B+,+(g)H
H
H
H
B
+
,+(g)
,
,
,
,
f
/
+
7
#
+
(
>
7
8
7
)
#
7
(
>
+
4
-
)
#
>
(
>
/
D
-
.
)
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#-)*mol-78
#6($767--@7(
%3
&H/%
H3/%#
H'%f
&H
=
+=
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Example /
The thermochemical e"uation of com!ustion of car!onmonoxide is shown as !elow.
C,s/ 2 .,g/ C,g/ '
given :
C,s/ .,g/ C.,g/ +H' -318 )* mol-#
C,s/ 2 .,g/ C.,g/ +H' -73 )* mol-#Calculate the enthalpy change of the com!ustion of car!on tocar!on monoxide.
H
H
H
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E1ample 2
Calculate the standard enthalpy of formation of methane if the
enthalpy of combustion of carbon& hydrogen and methane are
as follows1+H 2C(s)3 ! 455 kJ mol4
+H 2.(s)3 ! 45 kJ mol4
+H 2C."(s)3 ! 4675 kJ mol4
cH
cH
cH
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Example 7
tandard enthalpy of formation of ammonia, hydrogenchloride and ammonium chloride is -8(.# )* mol-#, -1.3 )*mol-#, 3#8.8 )* mol-#respectively. ?ritethe thermochemicale$ation for the formation of each s$6stance and calc$latethe enthalpy changefor the following reaction.
0H3g/ HCl g/ 0H8Cls/
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Exercise
/.Calculate the enthalpy of formation of !en4ene
if :
+H C.,g// ' -313.3 )* mol-#
+H H.,l// ' -76.6 )* mol-#
+H C(
H(,l/
/ ' -3(6.3 )* mol-# fH
fH
fH
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Born-Haber
Cycle
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=attice Energy, Hlattice
is the energy re+uired to completely separate one mole of asolid :ioni" "ompound;into gaseous ions
e#g1
NaCl(s) Na/(g) / Cl4(g) .lattice! /66 kJ mol4
(lattice dissociation)
Na/(g) / Cl4(g) NaCl(s) .lattice! 466 kJ mol4
(lattice formation) 'he magnitude of lattice energy increases as
the ionic charges increase the ionic radii decrease
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There is a strong attraction !etween small ions and
highly charged ions so the
H is more negative.
H for Bg is more negative than H for 0a.
!ecause Bg.is smaller in si4e and has !igger charge
than 0a, therefore
HlatticeBg/ F Hlattice 0a./
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0aand Cl-ions in the solid crystal are separated fromeach other and converted to the gaseous state Hlattice/
The electrostatic forces !etween gaseous ions and polarwater molecules cause the ions to !e surrounded !y watermolecules Hhydr/
.soln! .lattice/ .hdyr
Hydration Grocess of onic ;olid
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8attic
e%ne
rgy.eatof.ydra
tion
.eat of Solution
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Example *
>i%en-
i. Ent'alpy o( (ormation
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Example: A Born-Haber cycle for NaCl
Na(s) + Cl2(g)
Na(g) + Cl2(g)
NaCl(s)
energy
E=0
Na(g) + Cl(g)
Na+(g) + e + Cl(g)
Na+(g) + Cl-(g)
HaNa
HaCl
IonisationEnergy ofNa
Electron !nity ofCl
"attice energy
Hf NaCl
#ro$ Hess%s "a&'
HfNaCl= HaNa + HaCl +IENa+
ECl + "attice Energy-ve
+ve
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Calculation:
]
k!!!H
k"#$k122k%&&k1&'k$11H
EAHEHHH
HEAHEHH
la*ce
la*ce
)Cl(a+
&
fla*ce
la*ce)Cl(a+
&
f
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Exercise*
Constru"t a DornHa&er "y"le to e1plain 'y ioni" "ompound
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