ANDERSONJUNIOR COLLEGE
JCl CHEMISTRY2OO4
CHEMICALENERGETICS
References
l. Ciemistry,I G R Briggs.
2. A-Le\€lchemistry,E.N.Ramsden
Ann& PatrickFullick
3. Chemistry,
4. A-Level
Chemistry,
HillandHolman
Bmwn,Lernay,Bursten
5. chemistry: TheCentralScience,
6. Chemistry,
Olrnsted
& williams
Content
I
Enthalpy
changes:
AH,of formation,
combustion,
hydration,
solution,
neutralisation,
atomisation,
bondenergy,latticeenergy,electronamnity.
U
Hess'Law,
indudingBom-Haber
cydes.
AssessmentObjectives
At the endof the lectures,shjdentsstlouldbe ableto
(a)
principally
explainthat somedlemicalreactions
are accompanied
by energychanges,
in the
(AH,negative)
formof heatenergy;theenergychanges
canbeexothermic
or endothermic.
(b)
explain
andusetheterms:
(i)
enthalpychangeof reactionand standardconditions,with particularreferenceto :
formation;
hydration;
neutralisation;
aombustion;
solution;
atomisation;
(ii)
bondenergy(AHpositive,
i.e.bondbreaking);
(iii)
latticeenergy(AHrEgative.,
i.e.gaseous
ionsto solidlaftice).
(c)
calculate
enthalpydlangesfrom appropriate
experimental
results,indudingthe use of the
relationshiD
Enthalpy
change= mc.AT
(d)
explain,in qualitative
terms,the effectof ionicdrargeand of ionicradiuson the numerical
magnitude
of a latticeenergy.
(e)
gmde energycydesandcarryoutcalculations
UseHess'Law
to conslruct
involving
suchcydes
and rele\€ntenergyterm (irdudingionisation
energyand electronaffinity),with partiojlar
referenceto
(i)
determining
enthalpydanges that cannotbe foundby directexperiment,
e.9.. an
enthalpydange of formationfromenthalpyciangesof combustion.
(ia)
theforrnation
of a simpleionicsolidandof itsaqueous
solution;
(iii)
average
bondenergies.
(0
constructand interpreta reactionpathwaydiagram,in termsof the enthalpydlange of the
reactionandof the activationenerov.
ENTHALPYCHANGES
Intrcductaon
chemicalreactions
are usuallyaccompanied
by changes
in energy,usuallyin the form
of heatenergy.Heatis a formof kineticenergywhereas
theenergyof chemical
bonds
is a form of potentialenergy.The study of the energychangesthat accompany
chemicalreactionsis calledchemicalenergeticsor thermochemistry
or chemicil
thermodynamics.
(a)
Enthalpy
: Heatcontentof a substance,
represented
bythe symbolH.
StandardEnthalpy: Heatcontentof a substance
understandardconditions
(see(d)),represented
by Ho.
Note:Standard
enthalpyof elements= O
(b)
AH = En1631Ot
changewhenreactants
reactto formDroducb.
= A H proaucs- A H reactants
= heat of reaction.
Unit of AH : kJ mol-ror (sometimes)kl
kJ : for the reactionwith the corresponding
numberof molesas depictedin the
equation.
kl mol-r:not necessarily
for fer moleof products!
dependson the definitionof the enthalpychange.
(c)
A H >0
(Endothermicreaction)
Heatabsorbedfrom the
surroundinos
AH<0
(Exothermicreaction)
Heatreleasedto the
surroundings
Enthalpy
Enthalpy
y'l.tt
Reactants : lo.r.varro$
rWL
(d)
Standardconditions
(i) a pressureof
I a+!.
tS"c'
(ii) a temperature
of
If solutions
areused,theconcentrations
mustbe
| ,'l d"-3
condibons.
aHorefersto enthalpy
changeunderstandard
In this case,when
Note: Reactions
neednot takeplaceunderstandardconditions.
pressure
quotingtheenthalpychanges,
willbe stated.
temperature
and
(e)
is the puresubstance
in a
Standard
states- Thestandardstateof a substance
specified
state(solid,liquidor gas)understandard
conditions.
Examples: chrailtttl , r.ta-solfl
It0 , cJzoA
dr, a1 ,cD7
- lr4rrA
-
0'r.
Elements in their standard states under standard conditions are
assagnedzero enthalpy,
(D
A thermochemical
equationis a balancedchemicalequauonthat includes
(i) the aH value
(ii) the physicalstateof the reactantsand products
(iii) temperature,if necessary.
Example:
1260kl of heat is evolvedwhen 4 molesof NH3reactwith 3 molesof 02 to form 2
molesof N2and 5 molesof HzOunder standardconditions.Hence,the H2Oformedis
in liquidstate. 4^)rlrl?)+ 30. I g) r
r Nr l4) + Ol'o t t)
Aq
E
-
.2b1
t! nol -r
Example:Enthalpychangeof combustionof ethene
1411 kJ of heat is evolvedfrom the completecombustionof 1 mole of CzHr,all
reactantsand productsare understandardconditions.
C.>l+(g) t 202 t!)
--+ 2 6,srt!\ r 2sro ll)
KT"roJ-l
AH = ,t$tt
Definition of Standard Enthalpv Chanqes
(a)
StandardEnthalpyChangeof Reaction
The amountof heat absorbedor evolvedwhen molarquantitiesof reactantsas
shownin the chemicalequationreacttogetherunderstandardconditons.
Examole:
.t+B lc) * 5. 1r)
_->
) 0203(t
= - tjo I
-r\ '; n
^..-+i/"
-1"
\l
K J- -hl . r ,
J,
g, L .l- t"
1r
,"da, pt'ls
(b)
-gnlr'd
cr'6{orJ
Standard
Enthalov
Change
(AHf)
of Formation
The amountof heatabsorbedor evolvedwhenone moleof a substance
is
formedliom its oonstituent elements, all in their standad states.
Examples:
Ag (s) + trl1', (g\ a
ACct( sl
AAt' -- - t>t tJ lr,ol
C(r) + 0a lg)
A t t!)
-t
o =
a|t1
- 7a+ k}hot
Enthalpy changes of formation are often theoretical.Such reactions may not take place
.c ls)+3 H" fg) + { ,.q) + crrlroH(t) fzrrla(c,x3ox)]
in practice.Forexample:
CzHsOH
cannotbe formedby just mixingC, H2and 02.
such
entharpy
chanses
orrormation
#'#lj,r;::ll#i;lll"i
r#'j*"'
enthalpychangesof reaction.
zHf are usually negative. Examplesof exceptionare the formation of the oxides of
nitrogen.(Endothermiccompounds)
(c)
StandardEnthalovChangeof Combustion
(aH.o)
The heat change when one mole of a substancein its standard state is
completelybumed in oxygen.The measurementsof heat being adjustedto the
standardconditionsof 29BKand 1 atmospheric
pressure.
Note : Combustionsare usuallyexothermic,i.e., AHc< O
Exampfes: }|" G)li g , t1\ ---t y,6 (1)
f Alu a aH .t rba.atb, ,l
aq"o -- - at6 tg fa1
"ri'l
4 H 4 (6 ) r > a L h t
frlr.lar r
(d)
.arl r.
--t co.(,1) + tl 10 fl )
rrr+talrfo ,hrr.$ pl dr'\ ur+bn 4 (i"]
StandardEnthalpvChangeof Hydration
Ls,,norr&I \
-{ 1s Lrl " ol
.
ab,.p e,.r4.r,r;c
,or.lf
The standardenthalpychangeof hydration
of an ion is the heatchangewhen
onemoleof freegaseousions is dissolved
in a largeamountof waterforming
a solutionat infinitediluuonat 298Kand1 atmospheric
pressure.
t h.'d
9r&o.J rn!, J.rro,,ndrd 14 .an+tr.
llr*vni.\
Examples
:
No,' (g) + oq -+
No+lall
cL' q, -+ c", +
tl- 0,f)
- *6gu1 l-ol^a -
,rrr = - 39+t J /,e/
--
tu^t. rctttd.{.ar I L.d jv*rf
irtalti* )
;oa- $*
9
r xo.k',r ic
Enthalpychangesof hydntion arealwaysnegative.
Theheatprofucedin the hydrationof ionscomesfrom the bondsformedbetweenthe
ionsandtte dipde on tlte polarwatermolectrles.
(e)
Standard
Enthalov
Change
of Solution
The enthalpychangeof solutionof a substance
is the heatchangewhenone
mofeof the substanceis dissolvedin a largeamountof solvent(usuallywater),
so that no fuftherheatchangeoccurson furtherdilution.Allmeasurements
are
pressure.
madeunderthe standard
conditions
of 29BKand1 atrnospheric
Exampfesi
Nact (s) * o+
uo,r tar)
, t f @I \
xa+1aq1
u
a H = + f . q rllt . t
Enthalpychangeof solutioncanbe positiveor negative.
C ac r-r.{ s )+
(f)
^,
-,
or/
C 4C trl ri )
C r' + aot) t {t-
tai \
r
8 t.1
lJ
/,-!.
4rr
StandardEnthalpvChangeof Neutralisation+'r'b\-rch!- rl rrtl r ,, ".i {,c' c'r' ' .'o++v$
"
The heat changewhen an acidreactswith a baseto form a salt and one mole
of water, understandardconditionsof 298Kand 1 atmosphericpressure.
*
E x a m p fe s:N .0 H 'r I.r
,o
{ ver r +
+
srvrotl
rtrgv -.; Narjov + t{.}O
fhe neutralisationprocess
- St,tyyfuo t
loV,
,J
1
.trrt : o.J d,$.5d?itr
Ht1"q1+ OH 1a; ----> HzOCI
- s1^^n ulst rre.ig.
is afwaysexothetmic.
Thetff valueinvolving the neutralisationof a strong acid with a strong atkali is almost
the same for all strong acidsand alkalis.
This is because
(a) the strongacidsand alkalisare virhrallycompletelyionisedand
(b) the reactionbetweenthe strongacidand strongalkaliis thereforethe same :
+ OH-13q1
H+16q1
----> HzOtrt
AHo has the value of - 57.1 kJ mol-r
For a weak acid or weak hse, ttf
4ax,
will be stighdy tessTnuneriJlyl.
ctt rJ
lt.
ewg<A
tttt"rd'r1t''.' a t+^ ltal*4
+ NaOHlay
CH3COOHlaqy
--> CH:COONalaqy
+ H2O1ry
AH = -55.2kJ mol'r
Thisis because
the ionisation
of a weakacidor baseis incomplete
andhence,some
energyis requiredto complete
process.
the ionisation
Theenergyis takenfromthe process
+ OH-1"qy
Hr1"q1
----> Hzeo
Thenet heatchange,or enthalpychangeof neubalisaUon,
is hencesmaller.
(S)
Standard Enthalpy Change of Atomisation 4 n^'+ u
at&l\q-ia (l,.f,{ +.rcr
L.nt
"'' ' td)
The heat ctrangewnen one mole of free gaseous atoms of an element is
formedfrom the elementunderstandardconditions.
Exampfes:k fr) --t R (d
*cr'ro '-+ cr h)
aH"= + t1gy7f n t
.aH'. + tzrNr1,,\,l.
Note : Enthalpychange of atomisationis not the same as the enthalpychangeof
vaDorisataon.
Forexample,consideriodine
Vaporisation
(moleculesformed)
Iz(r)----> Iz(s)
nHo= +62 kl mol-r
Atomisation(atomsformed)
thl261----> 161
lHo = +107 kJ mol-r
Ail enthalpy change of atomisation are positive, becauseenergy must be
absorbedto pull the atomsfar apartand breakall the bondsbetweenthem.
Enthalpy
changesof atomisation
of the noblegasesarezero.(why?)
(h)
StandardEnthalpyChangeof Hydrogenation
The heat change when one mole of an unsaturatedgaseouscompoundis
completelyconvefted into a saturatedcompoundby reactingwith gaseous
hydrogenundera pressureof one atmosphere.
Example:
c Ar-7 6 n
1)
'i
tl r (1 ) --e
c)tcs13 (4\
AH '
- rzb Vr /not
Measurcmentof EnthalpyChanoeof Reaction
The enthalpychangeof a reactioninvolvingsolutionscan be measuredusing
calorimetry
(a)
Calorimeter
vessel(to reducethe lossor gainof heatto/fromthe
- an insulated
for
determining
the quantityof energyreleased
or absorbed
surroundings)
used
or physical
duringa chemical
change.
(b)
A simple,inexpensive
calorimeter
canbe madefromtwo nestedstyrofoam
cups
cum stirrer(coffee-cup
calorimeter).
The inner
with a coverandthermometer
cup holds the aqueousreactants,the outer cup providesextra thermal
insulation.
(c)
(i) A knownquantity(volumeandnumberof moles)of reactantX is placedin
the vessel.
(ii) Initialtemperatureof X is determined.
(iii)A knownquantityof Y is added.
(iv) The mixtureis stirredand the final temperatureis then measured.
(iv)
Since the polystyrenecontaineris a poor conductorof heat and has
negligible
heatcapacity,
Q = mclT
Q = Heatchangeof the reaction
m = massof contents(usuallydiluteaqueoussolutions)of vessel
c = specificheat capacityof contentsof vessel
(assumeto be same as for water (4.18 JK-l g-t;since reactantsare dilute
aqueoussolutions).
aT = changein temperatureof contents
(e)
Calculatethe enthalpy change of reaction by dividing the heat change
determinedin (d) by the limiting number of moles of reactant or product
(depending
on the definitionof the enthalpychange).
Example : calculatethe enthalpychangeof the reactionAg* + cl- -+ Agcl from the
followingexperimental
data:
100,cm3of 0.5 mol dm-3AgNO3q"oy
was placedin a plasticbotue,l00 cm3of 0.5 mol
dm-' KC(*vwasthen addedand the mixturestined. Bothsolutionswere initiallyat the
same temperafure,After mixing,the temperafureof the mixturewas found to have
risenby 7.5 K. (specificheatcapacityof solution= 4.20 J Klg-r)
Solution:
Assumingdensityof solution= densityof water at 4oC= 1 g cm-3
massof solution -
zo0 5
( to5 l.- R, Ncr D ,ro! l'.h rr?)
Sincea plasticboftleis used,its heat capacitycan be assumedto be negligible.
Heatchangein the reactionQ = mc^t = roov{.rr
=
6rooI
t.s
6,r tJ
Agn
= o r r r 5.g = e .j5
NumberofmolesofAgNO:used
( = no. of molesof KCIused)
r7 -
Hence,heat producedper mol of A9NO:(or KCI)used=
i.e. Enthalpy
changepermoleof AgClformed, AHo=
=
+*-
-t t6 tt /w I
l.'
"'"rt r*
r
l :6 kJ
$Yt Nna,
E tlSn
Example : Calculatethe EnthalpyChangeof Neukalisationbetweenaqueous
NaOH
and aqueousHCIfrom the followingexperimentalresults.
250 cm3of 0.400mol dm-3sodiumhydroxidewas addedto 250 cm3of 0.400mol dm-3
hydrochloricacidin a polystyrenecup. The temperatureof the two solutionswas the
sameinitiallyandroseby 2.7K.
The reactionis HCl1a1
+ NaOHlaqy
---> NaCftay+ Hzeo
Numberof molesof waterproduced-
o .25 r o,r : o. I rol
since the cup is made of polystyrene,it can be assumedto have negligibleheat
capacity.
Assumethat the specificheat capacitiesand densitiesof the solutionsare the sameas
that of water,i.e.,4.18J g-1K-1and 1.0g cm-3respectively.
= Heatreceivedby solutions,e
Heatfrom neutrdlisation
= 5.ox.r.t t x 2 -1
= 6 6 foif
: s.t_1v!
lt
= mcaT
Heatevolvedpermoleofwater=J;+
= 56.+ kT
- -50lkJ mol-r
(Molar)Enthalpy
changeof neutralisation
I
\ e*o,lh.-i,
,,
"
usinga BombCalorimeter
Change
of Combustion
Determination
of Enthalpv
of a compoundcan be found usinga bomb
The enthalpychangeof combustion
(referChemistry
by Ann& PatrickFullickp113)
calorimeter
Hess'Law of Heat Summation
reactionis constant,
and
Hess'Lawstatesthatthe overallenergychangein a chemical
not dependenton the routetakenprovidedthe initialandfinalconditionsare the same
for each route/pathway.
For example,when reactantsA and B reactto form productsC and D by route 1
(throughX),or by route2 (throughY andZ),thenaccording
to Hess'Law
the enthalpychangeof thereactionin routeI
= the enthalpychangeof reactionin route2
X
**gt ,r 1a.16-;r 7s.rtr I
*n*
I
' auro*
AH'!
]
c+ D )
A +B
or* \
6ltrr
/
/
/"H',
|
I
Y -------------+
Z
e,+vleg
@t
J
chuq
'
alra
,L ..a1{rn
+ {H tr
:!.
+ AH ..
r
route2
Apolication of Hess' Law
Hess'Law can be usedto calculatethe enthalpychangeof a reactionwhichcannotbe
determinedexperimentally.
Example ,l : Given that the standard enthalpy change of combustionof carbon
(graphite) and carbon monoxideare -394 kl mol-r and -283 kl mol-r respectively.
Calculatethe standardenthalpychangeof formation(aH ro ) of carbonmonoxide.
Qs1+ VzO21n1
co tnl
[srlnr]
q,y*rz,d1nfl-\/
Yrozrn\
.' \ at"
O *{b
, ) ao",
afo tr'n
@ so aoto . Ft4#4
/ v,qot
lrf
>
|
ir..t
SnE
r r^]arrt
aH ro
Coztsl
Esl
= -293kJmol-l; AHro- ?U mol-r
AHlo= -394kJmol-r; AH2o
By Hess'law,
the enthalpychangein route1 = enthalpychangein route2
AHlo=6gro4AHro
-34{
:
-21 I
AH 1o =
r aHSo
- tll
kJ mol-l
Method2 : EnergyLevelDiagram ( & r,'t
Enthalpy
urr g,
'r.+l
2Vmrl
6
SLr
lic r'
r
rrlt|s)
M?"t
-C( gr aphits) ,0r
i
AH ro
I
;
i
COrsli
AHlo=-394
i
AH2o= -293
I
COz(s)
r' Wri.
Note: If the anowis goingdown,AHvalueis negative
andviceversa.
Fromthe energyleveldiagram,
10
t.d-l
AHlo = 61'1,o+ AHrg
t atlrc
- 3q1 z -z.rb
AHro ' -P rlwcl't /
Method3 : EquationMethod(LeastPrefened)
Q"l + Oasl -->
aHJ= -39akJmorr....(1)
CQts)
COley+ %O4e1->
AH6o= -fg3 kl mofr.... (2)
CQtgt
AHf = z kJ mol-r. ......(3)
----> COtgl
Qs1+ 7zO21n1
Byobservation,
Eqn(3) + Eqn(2) = Eqn(1)
or,
Eqn(3) = Eqn(1) - Eqn(2)
-->
Qsl + 7zO21n1
AHf
@ts)
= alt,t -d{;
= -3A +
-(-rfr)
I
II
= - | l l | J .r1 J
dto'5HJ-'6t'a
o rr" + (_>ttl __-9,
A,{lo.
!J.r,fi
- 1rr
fuample2 : Calculate
AH for the reaction
2F2(9) + 2H2O(l)--+
4HF(g) + O, (g)
Giventhat
Hz(9) + F2(S)-----'
zHF(q)
H = -537Umot-r
^ H = -572Umol-r
2H2(g) + Oz(g) +
2H2O0)
a
Solution
4l
--t
ztl,o (l)
t?, t \ .
\4
.Hr r.irr\
\/
4lrF lt
/,*t,
-+
o> {!)
_ ?x (_ 6]?)tsJ
2H.i€f'
rH:( a) a rFr'll)
3ll,
-
jfl..
r{ f_S!?) :
.H3
{
f':rrv+ )
or'1
-g11 + r,$
- So, rlralr
/t
t-1
o!{t)
7
)
'ttzt
:ane
0z ')1-A'o
Recallthat H = AHproaucr
- AHreactans
Sincethe standardenthalpycontentof a subtanceis its standard
enthalpychanges
of
Formation,
AHo = AHor (proau"t")- AHor(reacants)
Hence,the enthalpychanges
of reactioncanalsobecalculated
fromenthalpycnanges
of formation.
Example,l: Giventhat
&"'r'ip"'l
ts t
'
c<w\x
rtv * L
..rrl"o, Q') + Oz(g)----> COr(g)
.1.,*..o^
au'L"tdr
4 "r'
9 Hz{s)+ 1/zo21s'1-->
Hzoo
AH.o= -394 k) molr = aHr (COz)
AH.o= -296 kl mol'r = aHr (Hzo)
CzH4s;+ 21/zO4s1---->
2CO21s)
+ H2O111
AHqo= -1300kJ mol-1
Calculatethe enthalpyof formationfor CzHz.
Required: 2C19+ Hu(s)--> C2H2(e)
AHf = z
l}l.o
C2H21ey
+ 21/zO4s1----> 2CO21q)+ H2O61 AH6o= -1300 kJ mol-1
Enthalpy change of rreaction AHo = AHor(p.oaucts;
- AHo;1,"".1"no,
Enthalpychangeof reactionAHo = A H.o(C2H2).
8oHt, (prr.a,) - Zaq;@n
AHor(ploa,.s)= [24 Hf(COr) + AHf (H2O)] = a(-yt+).1-'16) , -roat
= [AHf (C2H2)+
AHor(,ea.cns)
= auto (r,rt,).
,ZVz AHI(O)]
o*,1
rJ
:i/o)
N.B.Theenthalpychanges
or rormaii6nof co2andH2oarethe sameasthe entharpy
changes
of combustion
of C andH2respectively.
AHf(C2H2)=
-rjoo
=
6artto t?.tu,t,)
- gf
-b7+
Atrr " ( <.*,)
a",+".*)
- .^t,!o (t,$t)
12
>26 EJ
^ol'r//
- +
Altemativemethodof Calculation
Usingenergycyclemethod:
AHf
(s*o/+)
2Qs1+ H4ey+ 21/2O2451J+
+ 2lA0.y,sl
C2H4s1
.':'-\\
gouLgr /- ,t-
r',.
LHf
2CQsy+ H201;6nl
tc"t)
AHlo= 2169.o
o;C) + (AHco
ofH2)= 2(-3q4., + (->16) :
-roe4 rI
AHao= tr;6o o; arHz = -1300kl mol'r
AH ao =6gtoaOtto
'J
Therefore,AHf = 69oo - AHso=
- tol { - (-reoo)
= + zr-6Ft
I
Ethyne(CzHz)
is an endothermic
compound
sinceaHrois positive.
Example2 : Calciumcarbide, CaCz, reactswifl waterto form ethyne,CzHz, and
ca(OH)2:
(s) + 2HzO(l)
(s) + CzHz(g)
CaCz
Ca(OH)2
LH =-177.28
-
(s) usingthegiven
Calculate
thestandardenthalpychangeof Bglglig! AHPof CaCz
aHroin kJ mol-r :
AHrofor HzO(l)
= -286
ca(oH),(s) = -9es
(g)
CzHz
AH;t .
= +226.7
Atlt" toAtdr -
4Hto neder& '
CaC, li) + )|lr0 (4
d/.,rn
+ a( ,
$\,
\
Ca (s) + CC (s) +
A HE o'
-tlgL
t r:6,T
l{lo
4A q o .
:
{,
+
C' /
'
(71
t ^'o
{1) + o . t 3 )
4v!1o
A Hlo lo ' UL L
lo- caCz
+
: - 60.l kq
( - r f6 ) r z + ( - tf ,2)
nL''
/
BOND ENERGYAND CALCUI.ATIONOF ENTHALPYCHANGEOF REACTION
.
.
Energymust be suppliedto break a chemicalbond becausea chemicalbond
representsa stablearrangementof electronssharedbetweenthe bondedatoms.
Energyis releasedin bondformation.
r
Bond energy is deftned as the amount of energy requiredto break ! mol of
chemicalbonds in gasqous molecules to form gaseousfragments.
o Molarbondenergies(BE)thereforepositive values.
r Polyatomicmoleculesoften have severaldifferenttypes of bonds,each with its
own energy.
Examoles:
(a)
H2(g)+
BE= + 435kllmol
2H (S)
H(g)+ H(S)._--.------>
=-435kJ/mol
Hz(g)
^H
(b)The bondenergyfor the O-H bondin water is one-halfthe enthalpychangefor the
reaction:
H 2 O(g ) +
2H( s) + o( q)
nH =+9243$hd
Hence
BE(O-H)=1/zLH =1/z x924.4 = +462.2k)lmol
(c) The bond energyfor the C-H bond in methaneis one-quarterthe enthalpychange
for the reaction:
CHa(g) -=+
C (g) + aH(g)
,lH = +1665kllmol
Hence
BE(C-H) = 1/qLH = l/q x 1665 = + 416 kJ
r Bondenergydependsnot onlyon the typeof atomsin the bond,but alsoon the
restof the molecular
strucfure.
e.g. in CH3CH2OH
BE (Ca-H) = + 410k1/mol
BE (Cb-H) = + 393 kllmol
HH
|
'
|
H- F- p-o-H
db
HH
T4
D5'F
i)
fl74t tt
rtlo4
't"+'o
. Althoughbond energiesdependson the enure molecularenvironment,
many
bondsbetweenspecific
atomshaveenergies
thatclusteraroundan average
value.
Henceaverage
bondenergies
overmanydifferentcompounds
areused.
. Averagebondenergiesare lessreliablewhenthey are appliedto substances
in
condensedphasesas moleculesin liquids and solids experienceattractive
intermolecular
forcesthatmodifotheenergies
of individual
bonds.
Tableof Average
Bond Eneroies
BOND
H.H
c-H
N-H
o-H
ct-H
Br-H
I-H
c-c
N-N
o-o
O=O
ENERGY(k/mol)
431
410
389
460
427
364
297
343
159
142
489
BOND
ENERGY(U/mol)
c-N
c-o
c-cl
ct-cl
Br-Br
l-l
C=O in COr
C=C
N=N
301
351
33s
238
188
146
607
732
799
828
941
. Energyis absorbed
in breakingthe bondsin the reactantmorecures,
whileenergy
is evolvedin formingthe bondsin productmorecules.
Hence,the entharpy
changefor a chemical
reactioncanbe estimated
as the sumof all bondenergies
of the reactants
minusthe sumof all bondenergiesof the products
AH .eaction= I BE (bondsbroken) -
I BE (bondsformed)
.{
AH 1-ea.tion= L BE reactants-
I BE pl66rrcts
Examples
1.
Determine
the entharpychangeof combustion
of propaneusingaveragebond
energies.
CrHe(9) + 5O2(g)---------+3CO2(g)+ 4HzO(q)
Bondsbrokenin reactantmolecules:
5oz
LlnS
) c- c
: 2x t + 3t t
g.-t{
. itrro
(atc+a'rrt = tl!.lt 1 5.
Z 8e
5o=o
i 5x4 81 lJ
t'J
Bondsformedin productmolecules:
tco:.
b c= o
-- 6 xa.l4 t!
4 I't 10
I c-H
= EuGo 17
2lVt
15
6-eurt 'l = + t +71 l7
3COz
4Hro
AH re"aion = X BE reaaans __a6i[
2.
I BE oroogcs
_ r. t+19
_ !063 1.T,.-f
I
Estimate
the enthalpychangefor the reactionusingaveEgebondenergies:
CHa(g) + Cl2(g) -------' CH:CI(g) + HCt(q)
c+
c't:
C { 3r?
)
i
3c-H
:3({ ,o)l .J
I c-c r
lacr.[s) = +r8?f lJ \
! ]'5 tt
+ . - t-t
I c t-c r
= q({ro) rJ
, lr3r5
I
-'. Z*
gc !
5
!. 4A.'6. -- fx
-
= 4)xl 1
,'. Zot e**+)
t*,+,,+)- /,te 6'*,nr\
+t0?8-
: _ lr4
| ]l -al
tqlz
!J..ol-l
,--rwc' i'
I
^I"t
L6
g
= -f E1z tJ.
I-ATTICEENERGIES
ANDBORN-HABER
CYCLES
(a)
(LatticeEnergy)
LatticeEnthalpv
Standard
The heat changewhen one mole of an ionic ompound is formedfrom its
freegaseousions understandard
constifuent
conditions.
+ Cllsy-->
Examples:
K+1ny
AHo= -701kl mol-r
KCk.l
By this definition,all latticeenergiesare ngqg9\grbecauseheat energyis evolved
whenthe ionscometogetherto formelectrovalent
bonds. Alr (rF) < o
(b)
FirstlonisationEnerqyof an Element
Theamountof energyrequired
to removeelectrons
fromonemoleof gaseous
atoms producing
onemoleof gaseousions with unitpositivecharge.
----> Na+1qy
+ e AHo= +496kl mol-r
Examples:
Na(s)
SecondlonisationEnerqvof an Element
The amountof energyrequiredto removeelectronsfrom one moleof singly
charged Fosativegaseous ions producingone moleof doubly charged
positivegaseousions
Exampfe:,o* (5\ -,
br1' '' $b lt '',l-l
cet h) + e-
2t$ ('!..e.'', > O
The ionisationDrocesses
are alwavsendothermic.
(c)
FirstElectronAff nitv of an Element
The amountof energyevolvedwhen one moleof gaseousatomsgain electrons
to form one moleof gaseousionswith unit negauvecharge.
Examples
: Ilny+ e ----> I'(s)
AHo= -314kl mol-r
Electronaffinity is not alwaysnegative(Can you think of an elementwhich has an
endothermicfirst electronaffinity?)
Born-HaberCycle
The latticeenergiesof somecompounds
cannotbe founddirectlyfiom experiment.
Hence,suchvaluesmustbe calculated
indirectly
fromotherknownenthalpychanges
of reactionby meansof an energycycle(gom-Haber
Cycle).
Eorn-Haber
cyclefor sodiumchloride
Na(s)
+
Or","J
------------|
Na*Cl-(s)
I lHu*n
Y
Na(s)
cl(s)
AHr"J
II aH"
Na-(s)
AHr
thcl2(9)
t-
+
Standardenthalpychange
of atomisationof Na
Standard
enthalpychangeof
atomisationof Cl.,
cl(s)
Na(s)--> Na(g)
= +109kJ mol-l
AHo
"o,''
vzct2(g)
---> Cl(S)
AHou6'. = +121 kl mol-l
Firstionisationenerg,iof lodium
Na(S)--> Na*(g)+ e
AHo1on= +494 kJ mol-t
Firstelectronaffinityof chlorine
Cl( q ) + e - - > c l- ( S )
AHoE = -364 kJ mol-t
Standardenthalpychangeof
formationof NaCl
Na(s)+ yrclr(g)--> NaCt(s)
AHo6= -411 ;t; to'-1
Lafticeenergyof sodiumchloride
Na*(g)+ Cl'(g)--> Nacl(s)
AHo 6111."= ?
1B
. Standardenthalpy changeof atomisaUonof Clzalso = = 7z x AHoa
AH06= 56n6.'6 enthalpy drange of bond dissociationof Clz
Cl21q)
--> 2cks)
LHo6= a242 kJ mol'r
Bytl€d tau
attf = allea.(Na)
+ AFlorm(Na)+AHoab,n(O) + aHoe (o) + AHotruol
=
(+ro4) + (+re{
:
( -t||r)
AHuni."o=
+ (+t2r) + (-36..) 4 .a rE(^wl)
- 1]l tJ,/',"rr,
Example:
Constructa Bom-Habercycle for the formation of calciumchloride(s).Usingvalues
from the data booklet,calculatethe sbndard enthalpychangeof formationof calcium
chloride(s)giventhat its latdceenergyis -2218 k/mol
l9
Interpretabon
of LatticeEnergv
sincelafticeenergyis the energyevolvedon formingan ioniclatticefromconstifuent
'
gaseous
ions,it is a measure
of the strengthof the ionicbondwithinthe ionicsolid.
Hence,
(a) Ioniccompounds
withhighlatticeenergies
will havehighmehingpoints.
(b)
nl.g 11 9
t rro{orc
+ lod r',rl h *l *,t
a atd.tle(,.tocr * &;b. 4pd;((,,a)
Thesolubility
of anioniccompound
depends
ontheretative
values
of ther"tti'lif*ifr'",e
and hydrationenergies,which togetherconstitutethe enthalpychangeof
soluuon.
, +,'rp.
Factors
affectingthe Magnitude
of LatticeEnergv
r The latticeenergydependson the attractiveforcesbetweenthe ions in the
crystal.Thestrongerthe aftractions,
the biggerthe latticeenergy.
o The attractiveforcesdependon the chargesof the ionq theirsizes,andthe way
theyarearranged
in a solid.
r Themagnihrde
of the lafticeenergyof an ioniclatticeis proportional
to
(chargeon cation)x (chargeon anion
' 1"*'u*
sum of ionicradii
Compound
Radiusof
cation(nm)
Radiusof
anion(nm)
t-:fu--L<\-r
s charg to silc
ratb
distance
between
LatticeEnergy
centresof ions (kJ mol-')
(nm)
NaF
0.095
0.136
0.231
-902
NaCl
0.095
0.181
0.281
-77L
NaBr
0.095
0.196
0.298
-7i3
NaI
0.095
o.216
0.323
-684
Mso
0.056
0.140
0.206
-3791
20
Solubilitv
of ioniccomoounds
Enthalpy changeof solution of an ionicsolid(X u+ Yb*) is represented
by
X a+ Yb+(s) + aq --)
X "* (aq) + Yb*(aq)
Processes
involvedwhenan ionicsoliddissolves
in water:
(Usingsodiumchloride
asanexample)
(1)
Separation
of ionsin the crystallatticeto form isolatedgaseousions.
+ Cllsl
NaCllsl-------+Na+1e,
Enthalpychange= -Latticeenergy.
This processis alwaysendothermicsinceheat energyis absorbedin breaking
the electrovalent
bondsand pullingthe ionsapart.
(2)
Solvation(hydration)ofthe gaseousions.
The gaseousions formed in stage (1) form electrostaticattractions(ion-water
dipoles)with the polarwater moleculesto evolveenergy.
Na+1ey
* aQ -------+Na* (aq)
Clisl +aq
> Cl-(aq)
Enthalpychange= Sumof enthalpychangeof hydrationof cationand anion.
Hydrataon is always exothermic
Small and highly chargedions undergo more intensive hydration, evolving
more heat,that is, their enthalpychangesof hydrationare more negative.
\--
The enthalpychangewhen an ionic solid dissolvesin water is the sum of the
enthalpychangein stages(1) and (2)
Enthalpy of Solution
= -Lattice Energy + Enthalpy Hydration of cation + Enthalpy Hydration of
anion
aH" sotn
NaC(,1
Na+1sq1
+ Cl-1aq1
\r
\,*rr"
-.*'\
\
/
mf
\
/
Na*1ny+ Clini
2l
.."t"t a{
ql,or-
ByHess'Law,rHlo= aHzo
+ aH:o
wnere
aHro= 5n11',u1pt
changeof solution
-_ ?
aHzo= -1u61."tn..nt
= _(_77g)
kJlmol
= (Totat)Enthatpy
aH3o
change
of hydration
iof ruj'inJ CfI
= (_3e0)+ (-384)
or
Enthalpy
of Solution
= -LatticeEnergy+ Entharpy
Hydration
of cation+ Entharpy
Hydration
of anion
= -( -rqa) + [(-ao)rr-l oJ
:
+ 5 t Jr^ol,
chanoeof solutionAHo
Solubility
(mol dm-3)
Licl
Nacl
KCI
AgCl
-862
-7 7 I
-778
-883
-770
-2t
+1
+23
+75
-69s
-830
-90s
19
o
5
1.3x10-5
(insoluble)
(a)
If the latticeenergyis numericallylargerthan the enthalpy
changeof
hydration,the enthalpychangeoi solutionis positive(endothermic).
-t;;':,{'nt
Eaoq5 ,u,/e. {
(b)
(c)
Lo.. n,
t6au
lr arr., !rr) 4r"o{ ,,.n.ot
(d)
} €-ta7 qi,,rt
o. t.'t,t+i
ff the latticeenergyis numerically
smallerthanthe enthalpychangeof
hydration,
the enthalpych.ange
of solutionis negativelexonermic"y.
FvA nallS +" V',y ,,a lo+Rr1 r*65
aVon oy, ,arr6i
;;J;
i;;;;
Generally,
ionicsoridswith highpositiveaHosoh
areinsorubre
in waterbecause
the energyevorved
duringthe hydrationof the ionscannotcompensate
for
the energyabsorbed
in separating
the ionsin the lattice.
(;91^lE (ro
9 7 *.
,
Generafiy,ionicsoridswith negativeor smafl-"rrf
positiveAHo.oln
are sorubrein
water (shortfallof energyto overcomeattraciionsbetween
ions in lafticecan
be absorbedfrom surroundings)
22