.J
CHEMICALEQUILIBIURM
CHEMICALEQUILIBRIUM
PART1:
EQUILIBRIUM
LAWANDEQUILIBRIUM
CONSTANT
Obiectives
.
.
.
.
Explain, in tems of rates of fonyad and reversereactions,what is meant by a reversiblereaciion and
dynamicequilibrium:
Deduceexpressionsfor equilibdumconstanbin termsof concentration,
lG,and partialpressures,lg;
Calculatethe valuesof equilibriumconstanbin termsof concentrations
or padialpressuresfrom appmpriate
data;
Calculatethe quantitiespresentat equilibrium,
givenappropriatedata:
Textsand References
\*
.
.
.
A-LevelGuides,Chemistry,
J G R Bdggs,Longman,
3d ed,ChapterI
A-LevelChemistry,E N Ramsden,StanteyThomes(Publishers)Ltd,2med, pg 19t206
Chemistryin Aciion,MichaelFreemantle,MacmillanEducationalLtd,pg 250- 2*,25&261
IRREVERSIBLE
REACTION
.
.
.
Manychemicalreactionsgo to comoletion.
This meansthat the reactioncontinuesuntil one of the reactantsis completely
used up in the reaction.Whenthis happensthe reactionstops-- lt has gone to
completion.
Suchreactionsare irreverciblereactions.
A +B )C +D
This can be represented
graphically:
c
.9
E
{)
o
o
Examples:
0) F'!
t",r!
(t\ cacDg.
AJC/JC,1
.,"*;I +w lvrl
z{tl
a,
taClz
t.ns
r.rt
+ (Az +
q
>o
CHEMICALEQUILIBIUR'N
REVERSIBLE
REACTION
.
ManJ othe-r,reactions. lo ,ort go t
a.,.plctn,r
productsof the reactionthemserves
reaa@ctanis.
'
such chemicalreactionsthat take placein both directionsare calledreversible
reactions.Thisis indicatedby the reversiblesign,' !; ".
A tB
&'*'d "r'
<br.lultl
, because
- the
C+ D
,r |i
Thiscan be represented
graphically:
c
o
o
Aftertimet, forwardand backward(reverse)reactionsoccurat the san€ra,L
Boththe concentrations
ofthe reactants
(A andB) andproducts
(c andD) remain
arttdnt
.
. Thereare still certainquantitiesof reactantsremaining
(i.e.concentrations
of A andB * 0).
The system is said to reach
Examplesof
reversiblereactions:
OH 2 ra r
@ rro3
<
F-
'
:ut
,Jo:
r
oz
Otherexamples:
o
o
E
E
o
(1,
o
AJC/JC1
CHEMICAL EQWUAIURM
DYNAMIC
EQUILIBRIUM
.
.
.
In a reversiblereactionbothforwardand backwardreactionscontinueindefinitely
andthe systemis saidto reachdynamicequilibium.
Dynamicequilibrium
In an equilibriummixture,both the fonrard and backward
a
o
o
place.
o
reactionsare stilltaking
o
Systemdoesnotundergoanyobservablechange.
i.e.Concentration
of eachspecies(equilibrium
mixture)
t
remainsconstant.
I
.
t-
(*la o( &"oa
rih i Rrtr e{ gr.g
"o4rl
oo
o
o
,.rrr
Equilibrium
canonlybe achievedin a closedsvstemin whichthereis no exchange
of matteror energybetweenthe systemand its surroundings.
EQUILIBRIUM
LAWANDEQUILIBRIUM
CONSTANT,
K
,
o
The behaviorof a reversiblereactionat equilibriumcan be describedin termsof
equilibrium law. This enablesthe calculationof a quantityknown as the
equilibriumconstant,K.
.
K givestheequilibrium
concentration
of reactants
andproducts.
.
K can be expressedin termsof:
(1)Concentrations;
(2) Partialpressuresof gases;and
(3) Rateconstants.
(1)
K in terms of Concentrations
Consider
an equilibrium
system:
a A +b B s cC+dD
A, B, C and D represent
chemicalsubstances,
a, b, c and d representstiochiometric
coefiicients.
ay,'Lt"rlvt
La,r
fc l' I s J '
'
AJC/JC1
fr ] ' [ Bl'
3
I I = eqm concin mol dm
CHEMICAL EQUILIBIURM
The aboveis a mathematicalexpressionof the
lad
which relates the concentrationsof reactantsand procluctsIn an equilibrium
mixture.
The expressionshowsthat no matterwhatstartingconcentrations
are for A, B, C
and D, whenequilibrium
is established,
the aboveratioof concentrations
will be
aalarl
It is possible to determine the concentrationsof the substances
through
experiments.
K" is called the
a
(2)
.uiriLr:,r!,
oe.r+.*
K" is a constantat
( ,*t t-Unitof l(" is
for the reaction.
K in terms of Paftial pressures
(Applicableto reactionsinvolvinggases.)
aA (S)
t.e.
K r,
+ bB (S) s
?'
c.
oe
'A
9b
' ,E
cc (s) +
dD (s)
WhereP^is the partialpressureof
A, etc.
(Unitof gas: atmorpa)
Unitof Ko
.
l"n + o,\'-^'o-t
Partialpressures
of gasescanbe calculated
usino
Pr= Xr.x P1
Pr = Totalpressure
of gases
p = Molefractionof X = nx/nr
(x
(X
"P) ' Dp)
Hence,Kocan also be
expressedin terms of 1 and p:
(3)
( x , p) ^( X"t ) t
K in Termsof RateConsfants
K may be derivedfromkineticsif we knowthe rateequationsfor bothforward
and
reversereactions.
tl
e .g . A
+
B5C
tb
AJC/JC1
CHEMICAL EQUILIBIURM
. rf.orr
ol 'S,parl .cac{ioy , 1.
k1[tl[o)
' fub + asvs< nocl.t:* , r" = kb fc]
' At
4clrb,t,a
of 6,we'A
th7- .t*
Yt
=
alt
Grqx
'l.acloat
qQ .+al',
f ,
uaItft s] , rr[.]
t c 1 / 1 g 1 1 6 e 1=) F r /Ku
t(
= kl
/Kb
' Y.,;r',;
:;
)'4 z o'w
t,
Note: This expressionis only true if the chemicalreactionproceedin a single step. Many '1 "n
reactionsdo notproceedin onestepbut proceedthrougha complicated
mechanism.
O
Why doesthe expression
: K = k/k applicable
to onlysinglestepreactions?
POSITION
OF EQUILIBRIUM
.
Definesas the proportionof productsto reactantsin theequilibriummixture.
p+n
| ,\,,r
.
.,
tP)fs'l
't= 6Li
lf conversionof P andQ intoR andS is sma//,the p.o.m.liesto the /eft
r +A
o
, +S
,1
\--
K +3
(r
at
h
Jh4.rt )
lf conversionof P andQ intoR andS is large,the p.o.m.liesto the nght:
f+ A
P +S
l*
iI
b
l^ ' . )
Changein conditionse.g.pressure,temperature
etcwill affectthe p.o.m.
NOTES ON EQUILIBRIUM CONSTANTS KCAND KP
.
Equilibrium
lawonlyappliesto systemat equilibrium.
.
( and [$ are only affectedby_t"rr+".e_J_Theeffectof temperature
dependson theenthalpy
changeofthe reaction,
AH.
.
Magnitudeof G and Ko providesa usefulindicationof the e)dentof a chemical
reaction.
AJC/JC,1
CHEMICALEQUILTBruRM
Reactionhas a skong@
positionof equilibrium
liesto theright.
composition
willconsistof largely
reactants;
positionof equilibrium
liesto the left.
10-3<K <103
Significantamou@
P.o.m. can be affectedby
ca,rtt.7-t.'o,,cr.
. However,K is only affectedby temperatura
'
K for a reactionindicatesthe extentof a reaction,butgivesno
information
:-'
the rateof reaction.rt teilsus howfar but not howfast-thereactiong;;;. about
Examples:
(1)
Givethe( andunitsforthefollowing
reactions:
pcts
(b) PCl3+ Cl, [r.r:] [c1,]
tpttcf
[-r r."'r1[,,] r"-f
. r.t
Lu-rJ
.
l;r,il
f^r t*"t]
=
' L^au'zJ
(2)
f,,".,rr"-{l
--
;=
lillca*
L.c/i'''l
t
J
l * tt-,).' )
A sealedflaskcontained
2 moresof iodine,19mores
of hydrogen
iodideand4 moresof
hydrogen_at
equiribrium.
carcurate
theequiiibriu;
tt
for
thereaction.
Givethe
unitsforK.
""".tr.i,
Solution:
b+ tt"r va.,,.c ol rt
nr .
^,,r.
(*t7
ol ,i,l
o*
ot
.ri.
€4,".
2HI
fl61g 51 1.
rt
-'s
-
t7
t4Z
I
'n
t (c
,H I,
.4L
l,tr 7,
K.'F;m
AJC/JC1
H2+
[,r ]'
>t.
tiilil
: 45, rs
lo
m q,*tr
J
CHEUTCALEQUIUBIURIt'
3(a)
Whena mixtureof 50 g ethanoicacid and 20 g ethanolis allorcd to readt equilihium at
constanttemp,it ls that 0.469mol of ethancicacida.s foundto be gesent. Calorlatethe
equilibriumconstantfor the Baction at this temp€ratw€.
Solution
CH3COTH(4
t;l
CTH5OH(r)
to
tr$',
0o
ff'o.r13
ft. o,ers
ar ,\0r.,al .
0.033- r
o..fJt - I
.0, rfrl
O,8t3-,
(A'r"\,t-'e
qPrtq
o..+tl t
4 .tc.t&.,
,-
oo
r E o,'l+
V f.t)
0',.*h
o.rb+,/v
o' + r\lv
3
a.a-thh
=
cH3corqHs(4 + HzO(4
5
,o
Id tl f ur.J ,
.4h
U
+
o,tt.
/v
fttr"o,"x3l[u'01
[crrr a, n][6Hsc*r]
Io 3.+II o.rr+l
I J I LT J :
+.02 I
l+lt+]
(b)
3.fl) molesof ethanoland 1.00 mde of ethanoicacidweremixedtogetherat the
producedat equilibrium.
Calculatethe amountof ethylethanoate
sametemperaturc.
Solution
: i.
-tn
-*
(0 +CrHsOH(/)
CH3CO2H
!
'.ol|r.
'
-r,Q
lc t
l ,o o - >
t. o0 - r
t!
tr@ ' a )
v
t1.c-r)
*
lU. .l
fel, to, u] fa,$ ruJ
t+l[+]
:
l0.ortc+ ra.of - o
+.42 0<l\ v s.1o!
fL
lclr^,..
AJC/JC1
o0
tr
[cx1o1c,ns] frl no1
Fc ;.{.or :
,
3, oo
r. oo
' * ) t< ;
cH3corcrHs(4 + HrO(4
5
\,,.- *. 4 ..r *
o..y ,..o g
c{, (ad
b
',Et .l
o. {ot
ctll &o. tr
,.,rllr
. fd ur.l
CHEMICAL EQUILIBIURM
(4)
Dissociationof NaOr
0.05 mol N2o1is introducedinto a vesselof 1 dm3 at 25"c. The totar pressure
at
equilibrium
is 142.5kpa. Calculatel$ for the dissociation
of N2Oa.
NzOr(9)
s
2NOdg)
Solution:
To caVdh r,r0at
gcst'.r ,
Pi o+
^LO4
i
lrv ? o.l h
1 V' n r1
It,r.
_ o .o s x c ..)l r:4 1
r
(o.rf ,r.,
1 r tg r
= t..t rr x |0s Pa
1z
Ne 01 l9)
triid
li nel I
| ' t .4 r
(P r' - P)
I
p
P{,,
'
tq t, S - v t ,i
ta
?|t . . a +.
Kr'
t? ' b
t2 r,1 -
.:P
l,a 3 q tlo l- ?
t\r.3.i -9) + e?
-_
)
o
o.og
:
PLaoq s
:N 02 ft)
.
tf ,(
.
!4).!
rP.
_ tt .g l < ?a
3 1 .2 l Po
'
,
r! j , , 876
tD,
tr7 2t
P.r,
"{
1 0 3 ,3
/,
= t3't+
lt.tkla
-
I
Self-DirectedLeaming:
(5)
The equilibrium
mixturein a flaskwasfoundto consistof o.s0moleof so2, 0.12mole
Ozand 5.0.
weregaseo;s.The pressuiein the
Toles of SO3.All threesubstances
was8.0x 105
Pa.
Calculatethe partialpressuresof SO2,02 andSO3in the flask.
(il
(b)
Writean expression
for Kofor the equilib,rium:
'2SO,(g) + o, (g) S
2SOs(g)
(c)
calculate the varue of G for the mixtuie in the flask. state the units.
Ans: 5.85 x 1O'3Pa'1
AJC/JC1
CHEMICAL EQUILIBIURM
Self-Directed Learning:
(6)
The equilibriumconstantK for the esterificationreaction= 4.00. Calculatethe mass
ethanolthat mustbe mixedwith 2.0 mol of ethanoicacidto produce1.5 mol of ethyl
ethanoateat equilibrium.
cH3corH(4+c2H5oH(4
5
HzO(4
CH3COTCTH5(/)+
Ans:121.0 o
PRINCIPLE
LE CHATELIER'S
PART2:
Obiectives:
State Le Chatelieis Principleand applyit to deducequalitatively(fromappropriateinformation)the effectsof
pre€sure,on a systemat equilibrium:
changesin temperature,concentration,
Deducewtrethercfrangesin temperature,concenfationor pressureor the presenceof a catalystaffect the
valueof the eouilibrium
consiantfora readion:
.
.
LE CHATELIER'S
PRINCIPLE
Summarizesthe effect on the compositionof an equilibriummixture when
conditions
arechanged.
Staftr
[i,e.
-lrat
a'c|t
clor{rr b
a
g.rrtn
so'ic
rrft
rv' p?ri hL'i/ m
{o
rr rubj.c+ad
vnl Sdo yg r.rth 4t .t"d\'rt
t
a
chet'0(
f.t.tJqr
, atc.)
ih
,.4}i'{<r"'r
- +ra
Note: The systemcannotcompletely
cancelthe changein the erternalfactor,but it movesin
the direclionthatwill minimizethe change.
FACTORSAFFECTING
POSITION
OF EQUILIBRIUM
1.
lnfluencein Concentration
.
When the concentrationof the reactant is increasedor decreasedin an
equilibrium
occursso as to &rrrcr d 'tnd.a'€d$au.lg
system,reaction
lL.
.
co^ar.tro,{,oon ut
To ' wt.rr
AJC/JCI
*| t
1i"c+ r^folqnC
changz ^n
coq.'*V'5t"vti
e
CHEMICALEQUTUBruRM
Example:
Hr+lz
-+lr. irtru*
,to
'rAue
j!b!_
-r=
grs Ar,a
fur&.I,
H-
+
i^
no4
.r oeht6
fA']
e+l6\^
,
sL,p6
pctu,.
1
a5.
HI
{arourrl
t4 a !_ zn-----------.-:
L
New eqm composition : t H,1 D FArf
rnr*.,r i
ft)
Lt cx,,r<s
c
.9
E
o
o
By LCP,to reducetfreAecr
to producemoreH2.
ga$-a/d,
=)
Art
rct'eAx^ la,or,'.t .
!:
r-=+
lHr
Neweqm composition: fx,-l * furl 4.,.ol. ., L!r1;nr*,
K" : ,a.star+
2.
lnfluence in Pressure
a
when totalpressure
is changed,concentration
of ail speciesin thecontainer
wirl
changeto reducethechangein pressure.
lncreasein pressure(by decreasingthe vorume)brings morecures
croser
together.To rerievestress of additionarpressure,ieactionoccurs produce
to
er.rrr
*'olqrl.r
Of gasgs.
Hence,increasein pressurefavorsthe reactionthat producesfewermorecures
of
gasesandviceversa.
Example:
AJC/JC1
Nz+3Hz
5
2NHJ
10
CHEMICAL EQUILIBIURM
Totalmoles:
4
plt
9n<. *\t
llt
(
*r
rye {on
Pressure on {rrc dgh,|
n& I lg r-cP , ep'l{{.u
t
lrtc{
nvrrlcr cf trolc..to
;",15
, ' lr , il',,.,
'. i .,: " .:.,,...,,,,
Etfect
i:::;1,i,._'.t:
so ar to
...
l^as + gr.,v*lrlrr cq^.fq^f +. a gartr h r
,,lr 3h$t .t +"
J5!L
, ,.,hrch nA*ce 'l*
lhr- atat
&.C!4,.
p^..${n
-
Jt E',.raaf €n.b&.or,tl'
.ila + 3Hz r.rlle
:5
( P*,)
( pu) ( p,J
New eqm compositioni 4r, I ltx" I o'.^r. \ 4,,p,i" 14at'
K": ,axa*.
Sincethe leftsideof the equationhas4 particlescomparedto 2 particleson
the rightside,by LCP,eqmwillshiftsto the lult
to increasethe total
Pressure no. of molecules
so as to increasethe totaloressure.
J
ba<Y,,t l ea,&r
lo,",t d ,
d:. r 3$ r- '-r,!--- L\tH A
)
New eqm crmposition i (*, a ltr, iv,crc.s ,j n\1r )c.re'e
G: r.-rtor't
NOTE: Changein pressurehasno effecton the p.o.mfor gaseousreadionsif the
no.of moleculesin bothsidesof the equationis the same.
Example:
3.
Hu+ lz
5
2Hl
Influence in Temperature
When an equilibriums)Nstem
is subjectedto an increasetn temperature,the
brn
AnA
rcacuo.a
Example:
ot;v
t*
v{.. Vtrt(.
Nzor
2NO2 AH>O(Foruardreactionis endothermic)
(Backwardreactionis exothermic)
Change
Effect
Temperature 4
t
aht.
u",
ar.tro'f
$,b*n
,rsqtk
(h*fr
th
+- @,t4 noa g-dr4c
b $, .@
s*q&oo
&
0+ rr4(ra$ ad t rno t
a la
tvt
alr $,,y1)
-[le
tf*tn
A Yt.
o l!{-
6o(l,o)r" rcX
AJCiJCI
11
11
cd71' -
+
lA, o
CHEMICALEQUILIBIURM
:+
hrr.rad
Nl.ll-
,tn 1hsF rqtor.rA .
,^101
:=r
Neweqmcomposition
: [^r,o.]
Aac,eot,
r^.,r r *
iv' ( z .34.
@ ill
Temperature l,y
Byr-vr,
LCP,equlllDnum
equitibriumGhifts
snlTISIO
toGe
Ine
J
E\t)]
l?+l
tO prOdUCemOre reaCtantSGin;
thebackward
readionresultsin r;te;Gd;;;"rgi
temperature.
"nJi";"
to increase
the
ra.nb,^ lceer
r.lr 9" ..:1p,
$(r.I(n$
New eqm composition :
fu o,l'
f^lno.]
i,.61ta,.t, fi.ro:f d"rar:..
r,ilr dec+r
lC = Ifr;lNote:
a
fncreasein temperaturefavors the
ca[4ra1,q rector
a
Decreasein temperature
favorsthe ..orr,r,.,r.F,
Positionof equilibrium
andK
.rn"^".,
.
o
Reason for changes in K:
RecallK canbe expressed
as rateconstants,
K = k/k.
Increasein temperatureincreasesthe rate constantsand rates
reactions
rhroush
qi[fpq!€nqgl.
of both forwardand backward
[kr = Ae-Emr,k. = Ae€'m where Er + EJ
4.
Influenceof Catalyst
Lt4t
rateof reaction.
po ridt
a
o
t)
affect the positionof equilibriumand K.
Reason:
catalystlowersthe activationenergyof bothforwardandbackward
reactions:
:)
iv\ c'\1 ..r
r4+(
= ftrJ,L\ar- \
,Pt4'\'r
AJC/JC1
{^a
4
lr,{"
.h,oO
^41?rrL 0cr,{r, \rt
/ uoff,,I,.'a
.r"a^c,.r
.,r<r,r}.dirrt
4
cir.ctlg
.pa'c o a+ o,*r.flt r,rr
CHEMICALEQUIUBIURIT
5.
lnfluenceof lnerl Gas (atconstanttemperature)
1.
At constantvolume
r
Total pressureof the systemincreasesdue to increasein numberof gaseous
particles.
After additlonof lnert oasea(a I
OmlnalSvstem
@ o@
o@
@
@
00f-->
@
oa
o
@O
o@
@
@
a
\9
Equilibrium
System: @
.
S
(^o
,,60
O
Concentrations
or partialpressuresof reactantsandproductsremain ur'rlonttA
+ No alft,t en +lu prrtrora ee qrr'!b;ut' , aa| K.
2.
.
At constant pressure
To maintaina constantpressure,reactionvolume must increase.
After additionof inert qasesfll I
OrqinalSvstom
o
(^lrl
:@
o
o
mr--->
I
a
@:
o
o
o
io
a
oi"
o
a
@
o
@
.
Partialpressureofeach gasesdecreases.
.
Thiseffectis the sameas if the volumeincreasesis broughtby
,. l.r^'.4
+'r. taaprl
.
(effectdue to pressure).
lftsaac
System reacts to increase partial pressures of reactants/productsby favoring
oroductionof more molectrles
o
Positionof equilibriumwill shift in the directionproducing
AJC/JC1
'rc ,'lr.rc,rr..
CHEMICALEQUIUBTURM
SUMMARYOF THE EFFECTSOF CHANGINGCONDITIONS
ON AN
EQUILIBRIUM
SYSTEM
Considerthe reaction:
aA +
Changesimposeclon the
systemat Equilibium
bB
s
cc
+
Equilibium positionshifts
Concentrations
of
A and/orB increase
Concentrations
of
C and/orD increase
Pressureincreases
lf (c+d) < (a+b): To right
Verylittleeffect,if any,on
reactionsin liquidsolution.
lf (a+b) < (c+d): To tefl
Temperature
increases
lf fonrvard
reactionis
exothermic: To left
K decreased o Rate of both forwardand
If forwardreactionis
endothermic:
To right
K increased
.
.
Additionof catalyst
backwardreactions
increases.
Hence,equilibrium
is
achievedfaster.
K is also affectedby
temperature.
Equilibrium
is achievedfaster
dueto loweringof E".
Presenceof inertgas:
1. Constant
volume
No change
2. Constanttotal
pressure
lf (ctd) < (a+b): To Ieft
No change
lf (a+b)< (crd): To right
AJC/JC1
14
2. Volumemustincrease:
=) partialpressures
of
reactantVproducts
decrease)
CHEUICALECNI'UB'Uffi
(7)
The equlllbriummixturein a 2.oodmsflask at soo oc urasfoundto consistof 0.6q) mot
of SO2,0.200mol of Ozand 4.80 mol of SOa.
Calculatethe equilibriumconstantlG at S00.C for the Eaction.
2 S Oz(g )+Od s)s2 SOg( g)
(i)
Solutlon:
(i)
2sodg)
qf
etlt
Or(S)
5
2sO.(S)
a t00
0. lar
:
$+g
+
.+,80
PYr plT,
r ,3 t+Jl rrr.l -l
F'
ro.ro. tltar.+) talA + 90)
9!'rrt s|rt 4 4.
Iq ,rl
Pdttal R'Dr1r +
rrot P A
(o . a o ) ( & ai )( + 1t)
z , oc (o.t)t
0. ,
g.+)b x rrt ta
.qhr f'lt tllna?)
I,oo l,, rlt
.
( t s+ .:* r rot)t
p',t
,x?
(ii)
tgl.>* sttl 0a
ET_
=
6rr.et rro,)9(o.rrtrrot)
'
4'1b x,o-s qo-t
,
Howmarrymolesof 02 mustbe addedinto the flask at 50O"C in orderto
increasethe amountof SO3to 5.0Omoles?
lo\r.r {
llrrr . el'
rl 0 r,l t
O f r 1 .:
. r t - . + . *, .
o ,L c
g .4 ,
(c.
A"JClJC1
+
l tl d
=
:50s f5)
o r.o
4.tto
Irc fi
* .Fl,
(ry)(ryI'(+)
= 6+o
-l
15
l-'3
CHEI(ICALEQUI'-'BIUR',
e|lr} 5
L**ro h rt,L
a r01q) r
ot
llFGro
= o'{t
dr. xfo rrl r
T
. otor tr rt,r
:
. ?2
9,
? :Jor r { l
*.n +o'Lg
Oo
-9,
,.tc. g-{9rr)
{c 'l
-5
go
1
{.,O. I
-!]!,
z
(€)'
(ry)"(+)-- { ro
Kct
134,15
u t9 .l
3 '-6
I '
(iiD
lf 2.00 mol of SO2ard 1.00mol of 02 are mixedand allou/edto read at S(D"C.
wttat mustthe total pressurebe for 80 % conversionto SO3?
zl0r tl) +
. 7,*n
, fr
o,t&t/
rf
q
rl
I
*
,
( aa n l \
r,or
,.(. -t.Lc
. ,,.to
.
i*
Qt3)
10" Q)
l,oo
o
(t+)
"- =o-2o
{d.
a(o&)
:1.61
o ,tto + o ,to .
t.60
? ,.2 0
^
ft=T
tlr lT
(!,t
)(l ,$ r)(tb + o ?3)
" +.ol ( ,.t ?. I
AJC;/JC1
16
CHEMICAL EQUILIBIURM
HOMOGENOUS
EQUILIBRIA
substancespresentin onephaseonly.
Equilibrium
reactionsin whichparticipating
Examples: Reactions
in liquidor gasphase.
(/) +
CHgCOzH
NzOa(g)
(/) s
CTHsOH
s
+ HzO(/)
CHsCOzCzHs(D
2NO2(s)
HETEROGENOUS
EQUILIBRIA
.
phase.
present
participating
indifferent
substances
Equilibrium
reactions
inwtrich
Ca4J: ts't
Example:
ao o (s) r
co:. (j)
-""'>
fcaol p...
h-
5
(n31
lf a
?,- . lgli..^
I
Note: Whatis meantby the concentrations
of pure solid/liquid?
.
.
.
.
.
=
Concentration
of a puresolid/liguid
*Cgry (a constant)
Amount of solid/liquidmay change,but concentrations
of pure solids/liquids,
and of
solventsof dilutesolutionsare constant.
lt is obviouslysensibleto combineanyconstantswithK itself.
Therefore,crncentrations
of pure solid/liquiddo not appearin the equilibriumconstant
exoression.
The equilibriumconstantexpressionfor a heterogeneous
reactionis written in
such a way that the concentrations
of solids/pure liquidsare excludedin the
exDression.
Examples:
AJC/JC1
K . = [c o,]
or
l(t -- ?ro,
CHEMICALEQUTUBURU
(8) A mixtureof ironand steam
at 600 .c. The equilibrium
_wasallowedlo reachequilibrium
pressuresof hydrogenandsteam
were3.2kpa and2:4 kpa respectivery.
c"i*i"-r.e (,.
3Fe (s) +
4HrO(g)
Solutlon:
1o \.
(p=
FqOa(s)
+
4Hz(9)
( r .r )
:_
(P *" Jt
(9)
S
= ttb
I
'l
Thesolubilityof the silverharidesin waterdecreases
in theorderAgcr > AgBr> Agr.
Becauseof thisdifi.*19 in solubility,bromideionswill displacecilorideiins frori'sotio
silverchloride.Theequilibrium
constantlC for thisreaciion:
AgCt(s)
+
Br (aq) g
AgBr(s)
+
Ct(aq)
is 360 at 2s8K. rf 0-100mordmi Br (aq) is addedto soridAgcr, what wi[
be the
equilibrium
@ncentrations
of Br(aq)and'Ci(aq).
Solutlon:
A6cl ls) .
Bv- lq)
Itriltrt corc I
At A
c q-
Ag Br b)
-
a, log
"
{
o'l- i
IAr'
.l6b
16 -rlor
,[
fu..
.+
r-^r^l 0I
.
1
-- 36t,
t z
o,oe{\)
.l
*
.F
g/ -
d
orr" t
=
o, oqq?a
,,olJ\-l
v.t _ 5 sqq*L
2.?? r 1e.e ,.;g-3
AJC/JC1
cr- tq,,,
o
,' l !o -i
'
+
1B
CHEMICAL EQUIUAIURM
PART3: HABERPROCESS
Obiectives
.
Describeand explainlhe conditionsusedin lhe Hab€.process.as an exampleof the importance
of an
understanding
of chemicalequilibrium
in thechemical
industry.
THEHABERPROCESS
T F r it z Habers tu d i e d th e N 2 (g } + 3 H ,(g ))2 N H :(g)cqui l i bri umi nt}reearl y
19O0s and concluded that direct ammonia sjmthesis should be possible.
E
Howevcr, it was not until l9l4 ttrat the cngineering problems and catal56t
question had been solved by Carl Bosch, and arnmonia production began just
E
in
time for ttle atart of World War I. Ammonia is the starting matcrial to makc
-E
nitric
acid, a vital material in t}re manufacture of the e.folosives TNl and
f
E nit r ogly c c r i n .T h e re fo rc ,th i s i s tl ro u g h tt obethefi rstl .argc.scal euseofa
synthetic chemical for military purposes.
m
Haber's contract with t}re manufacturer of ammonia called for him to receive 1
A Fritz Haber (1868-1934) pfennig per kilograrn of ammonia, and he aoon
-untom:natcly,
became not only famous but rictrhc joined the Gcrman
E
Chemical
Warfare
Servicc
at
tlrc
start
of
World
War
1
and became its director
-L
E
in 1916. The primary mission of tfie service was to develop gas warfare, and in
-r-1915 he supervised the frrst use of Clz at tie battle of Ypres. Not only was this a tragedy of modern
warfare but also to Haber personally. His wife pteaded with him to stop his work in this area, and when
he refused, she committed suicidc.
IE
In 1918 he was awarded the Nobel Prize for the ammonia qmthesis, but ttte choice was criticized because
of his role in chemica-l warfare.
After world war 1, Haber did some of his best work, continuing on in thermodynarnics (Bom-Haber
99!el Jvas a product of tllis period). However, because he had a Jewish background, Haber left Gerrnany
in 1933, worked for a time in Englald, and died in Swi rland in 1934.
THE MODERNMANUFACTURING
PROCESS
Manyimportantindustrialreactionsare reversiblereactions.
Two importantprinciplesin choosinga processfor industrialreactionsare:
(1) Efficientand(2) Economicallyviable
Theconditions
in an industrial
reversible
reaction
aimto:
o Produe maximumamountof product(highyield);
o Usingminimumamountof time;
o At minimumcost.
Thecostcanbe minimized
by:
o Usingcheapestreagenfs(air, water);
o Makingreadionas rapidas possible(catalyst);
o Avoidingveryhigh temperature/pressure,
if possible.
AJC/JC1
19
CHEMICAL EQUILIBIURM
THEHABERPROCESS
Nz(g) +
3Hz(g) I
aH = 46 kJmol-l
2NH3(g)
Nitrogen from
fractional
distillation of liquid
alr
trr#--lqffidm
&d
tI from
"r*"*" 1I
I
gas
synthesls
I
OF AMMONIA
OPTIMUMCONDITIONS
FORSYNTHESIS
Conditionsfor the synthesisof ammoniahaveto be carefullychosenif the processis to
be efficientandthuseconomically
viable.
3 mostimportant
are:(1)yield,(2)rateof reactionand(3)energy.
considerations
(1)
Effect of Pressure:High Pressure
Desired
Out
Reason
) lncreaserateof
reaction
F An increasein pressurebringsgas
molecules
closertogether.Theycollide
and hencereac{ionis
morefrequently
faster.
D Pos'rtion
of equilibrium > Accordingto LCP,highpressureincreases
shiftedto the right
the relativeamountsof NH3at equilibrium,
(higheryield)
sincethefoMardreac{ionresultsin a
decreasein the no.of molesof gas (4 mol
) 2 mol).
Undesired ) Noteconomically
viable
AJC/JC1
plantandmaintenance
) Costof industrial
becomemoreexpensive,
e.g.pipeshaveto
be tougherto withstandhighpressure.
20
CHEMICALEQUILIBIURM
betweenhighyieldandlowcost.
Compromise
.
(21
EffectofTemperature: LowTemperafrtre
Desired
Outeome.'.',,...:,
virtuallyzero
(3)
t.,. :.,:.rrijliii:.r..:
tbvors
to LCP,lowertemperature
F Positionof equilibrium > According
production
heat
to
counteract
of
more
the
right
to
the
shifted
Hence,the exothermic
the lowtemperature.
(higheryield)
reaction(fonnardreaction)is favored.
Undesired > Rateof reactionis
o
ReaSOn
no.of energetic
> At lowtemperature,
whichleadto chemicalreaction
collisions
reduced.
aredrasticallv
betweenhighyieldandlowrate.
Compromise
Catalyst
Reason
Desired
. :..
:,,.,
a:,ti-.ltr.
F Increaserate of
reaction
> Catalystlowersactivationenergyof reaction
andhenceincreasethe no.of effective
collisions.
) Synthesiscanbe
caniedoutat a lower
temperature.
> Highrateableto achieveat low
temperature.
Conclusion
.
o
Accordingto the principlesof kineticsand chemicalequilibrium,the following
of NH3:
conditionswouldfavorthe manufacture
(1) Highpressure
(2) Moderotetemperoture
(3) Useof catolyst
Hence, optimum conditionschosen that compromisesbetween kinetics and
chemicalequilibrium
for HaberProcessare:
(1) Pressure :200 - 300otm
rote of reocfion.
(2) Temperature
: 450 - 500'Cis usedto obtqinreasonable
by cddifion
(3) Catalyst
: FinelydividedFecotolyst.Efficiencyis enhonced
promoters:
AlaOrondKzO.
of
of smollomount
YIELDOF AMMONIA
.
yieldof NHris only10 - 15%.
Despitetheseoptimumconditions,
AJC/JC1
CHEMICAL EQUILIBIURM
.
Thisyieldcan be increasedby allowingliquidNH3formedto be drawnoff the
systemso thatpositionof equilibrium
is shiftedto the rightin favorof production
of moreammonia.
unreactednitrogenandhydrogencan be recycled.Ail reactantgaseseventua[y
convertedto NH3.
'
USESOF AMMONIA
.. - .
.
Make feftilizefsi
o.ro,i r $+rcr.r , aurrritrr,l iaa+c
Makg nitfic acid: r, ,qr\! r{$! t a. y sxt c,, ryo1p (ilrl
,
.
l.[rt.
,r"o. ca^ U
.r,tl
c
ieF
,^'..'iqr
rr,,6
Makesyntheticfibres: ngn^, .400^_
Self-Directed
Learning
Exercise:
The HaberProcess.
for the production
of ammoniafromits elementscanbe represented
by the
equation:
Nz(g) +
3Hdg) s
2NH3(g)
ln the process,a mixtureof nitrogenandhydrogenis passedovera catalyst.TheNH3is
remov_ed
ftom.theproductsandunreacted
gasespassedthroughthe rea&ionveir"r
tiT:-j:
gasis removedfrJmthereactionvesJetlwhereit accumuiate"s;,
"g"in.
1,",T Statethe
ll": catalyst
?19o1
(axt)
normallyusedin the Haberprocess
(ii) Whatare the sourcesof the nitrogenandhydrogen?
(iiD Suggestwherethe argoncomesftom.
(iv) \a/hyis the NH3removedfromthe productsbeforepassingthe unreacted
nitrogen
andhydrogenoverthe catalystagain?
(b)
Figurebelowshowsthepercentage
of NH.in the equiribrium
mixtureunderdifferent
conditions.
z
Prgssuray'alm
(i) Vvhichtemperature,
200oCor S00oC,producesa largerpercentage
of NH3in the
equilibrium
mixture?stateonedisadvantage
of using-atemperature
wefiberowsoo oc.
(ii) \A/hichpressure,s0 or 1SOatm,produces largerpjrcent"ie
of NH3in fhe
equilibrium
mixture?
Stateone disadvantage
of usinga pressure
- greaterthan200atm.
AJC/JC1
22