37
EXPERIMENT 5: ENTHALPY OF NEUTRAIISATION
POTENTIAL HAZARDS
Safetyglassesmustbewornotalltimesinthelaborotory.T.0MsotutionsofNooH,HCl,and
CH3CooHareusedinthtisexperiment.Avoidcontoctwithyourskin,Washtheaffectedoreo
thoroughly with cold water if spilloge occurs'
INTRODUCTION
The enthalpy change accompanying a chemical.reaction
in solution can be determined by ca
ying
out the particular reaction in a caloiimeter.
ln this process there is.a transfer of energy between
the
eacting substances and a definite mass of liquid
- usually water' The enthalpy can then be found
y
measuring the change iniemperature of the tiquia.
we also need to determine the heat capacity of
the calorimeter System, since it also undergoes
the temperature change during the reaction.
ln this experiment, the heat capacity of a calorimeter
is determined using the known heat of solution
ofpotassiumnitrate.Theheatofneutralisationofsodiumhydroxidewithbothhydrochloricand
acetic acids is then determined using the same
calorimeter system'
The reactions in this experiment are ca
ied out
at a constant (atmospheric) pressure in a simple
Vacuumflaskwhichisanexcellentthermalinsulator.Hencetheprocessisca
iedoutunde
Reactants (Tr)
Reactants (Tr)
lsothermal Process , Products (Tr)
Adiabatic Process
(Calorimeter)
adiabatic(qo=AHo=0)ratherthanisothermal(Tconstant)conditions.Enthalpiesofreactionare
usuallytabulatedforisotl.rermalprocesses(e'g.at298K)butinthiscase,thereactantsandproducts
are at different temperatures. The following
diagiam shows the relationship between the
enthalpy
change accompanying the process in the aJabati.
.utotir"t"t (AH.), and the one accompanying the
isoth-ermal process (AHr), which we wish to determine'
38
We can see that the adiabatic process is thermodynamically equivalent
to an isothermaI process plus
a term in which the reactants have their temperature changed from
Tr to Tz
I2
aHc = AHrz + JrPoo, (1)
But, for our adiabatic process 9p = 0, i'e' AH. =
g' Hence'
.T"
6gr, = -J_'CrdT (2)
lfthetemperaturechangeissmall,Cowillbeindependentoftemperaturesothat
AH1 = -cp(Tz - T1) (3)
Let's see how we can use equation (3) in this experiment'
Heat CapacitY of Calorimete
Here we are using a chemical reaction with known AHr'
KNos(s) + 250Hzo(l) ) ruo, in 250Hzo; AH1= +34'6 kJ mol-1 (4)
ir'
Frpm (3), kfrowlng AHr, Cp can be found. The heat capacity iri
thls case ls thts sum of the hpat caPacitY
of the calorimeter:vesselt ,na of tnu solutldn in the calorlmeter' lf weassume thet
solutlon has the
;;;;;"r; .;p..'rv f if ;, *ili;.ilt*ri g'1, t'u'-'' the heat capacltv af the solutioh ls simplv equal
to the mars 1my oiwater plus KNOr (ln g)times the heat capaclty s (ln J K'*
g'')'
ltii .. - '":' -,':1t' ' r"'j
Hbnce, equation (3) becomes
-'
i .. ,
AH1 = -(m.s + CXTz - l) (s)
The heat capacity of the calorimeter, C, can therefore be found'
Enthalpy of Neutralisation
ln this experiment a solution of acid of mass ma, at an initial temperature T"
is mixed with a solution
of base of mass mb at temperature T6, the base having been equili
ated
in the calorimeter before
mixing. Therefore, for the purposes of calculation, it is necessary to treat the
system as if it consisted
of two separate quantities of solution each having undergone a different
temperature change' say
AT"=(Tt-T.)fortheacidand4T6=11.Tu)forthebase.Sincethecalorimeterisatthesame
temperatureasthebase,itundergoesatemperaturechangeequaltoATu'
From equation (3), the heat of neutralisation is then given by
s + c)aro]lH= -[(m, s)Ar" + (m5 (6)
Temperature
Time
Distilled water (800 ml)
KNO3 (18 g)
30 s
21.5
19.90
60 s
21.5
19.90
90 s
21.5
19.90
120 s
21.5
19.90
150 s
21.5
19.90
180 s
21.5
19.90
210 s
21.5
19.90
240 s
21.5
19.90
270 s
21.5
19.90
300 s
21.5
19.90
Temperature
Time
(500 ml distilled water +150 ml NaOH)
(160 ml) HCl
NaOH +HCl
30 s
21.5
22.6
24.5
60 s
21.6
22.8
24.3
90 s
21.6
22.8
24.3
120 s
21.6
22.8
24.3
150 s
21.6
22.8
24.3
180 s
21.6
22.8
24.3
210 s
21.6
22.8
24.3
240 s
21.6
22.8
24.3
270 s
21.6
22.8
24.3
300 s
21.6
22.8
24.3
Temperature
150 NaOH+ 500 ml wate
In a beaker 160 ml Acetic acid
NaOH+HCl
30 s
21.6
22.9
24.1
60 s
21.5
22.9
23.9
90 s
21.5
22.9
23.9
120 s
21.5
22.9
23.9
150 s
21.5
22.9
23.9
180 s
21.5
22.9
23.9
210 s
21.5
22.9
23.9
240 s
21.5
22.9
23.9
270 s
21.5
22.9
23.9
300 s
21.5
22.9
23.9