1. No category

LEC
02.08
Determination of the enthalpy of neutralisation
Related concepts
Enthalpy of neutralisation, calorimetry, heat capacity.
Principle
When a strong acid is neutralised with a strong base in dilute
solution, the same amount of heat is always released. If the reaction takes place under isobaric conditions, this heat is known as
the enthalpy of neutralisation. The chemical reaction which generates this heat is the reaction of protons and hydroxyl ions to
form undissociated water. It therefore correlates to the enthalpy
of formation of water from these ions.
Tasks
1. Measure the temperature change during the neutralisation of
a dilute potassium hydroxide solution with dilute hydrochloric acid.
2. Calculate the enthalpy of neutralisation.
Equipment
Cobra3 Basic-Unit
Power supply 12 V/2 A
Data cable, RS232
Temperature measuring module Pt 100
Software Cobra 3 Temperature
Temperature probe Pt 100
Calorimeter, transparent
Delivery pipette, 50 ml
12150.00
12151.99
14602.00
12102.00
14503.61
11759.01
04402.00
04402.10
1
1
1
1
1
1
1
1
Pipettor
Rubber bulb, double
Pinchcock, w = 15 mm
Heating coil with sockets
Work and power meter
Universal power supply
Connection cable, l = 500 mm, black
Magnetic heating stirrer
Magnetic stirrer bar, l = 30 mm, oval
Separator for magnetic bars
Support rod, l = 500 mm, M10 thread
Right angle clamp
Universal clamp
Laboratory balance
with data output, 800/1600/3200 g
Volumetric flask, 500 ml
Glass beaker, 100 ml, tall
Glass beaker, 600 ml, tall
Pasteur pipettes
Rubber bulbs
Stop watch, digital, 1/100 s
Wash bottle, 500 ml
Potassium hydroxide for 1 l
of 1 M solution, ampoule
Hydrochloric acid for 1 l
of 1 M solution, ampoule
Water, distilled, 5 l
PC, Windows® 95 or higher
36592.00
39287.00
43631.15
04450.00
13715.93
13500.93
07361.05
35720.93
35680.04
35680.03
02022.20
37697.00
37715.00
1
1
1
1
1
1
4
1
1
1
1
2
2
48803.93
36551.00
36002.00
36006.00
36590.00
39275.03
03071.01
33931.00
1
2
1
1
1
1
1
1
31425.00 1
30271.00 1
31246.81 1
Fig. 1. Experimental set-up.
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3020811
1
LEC
02.08
Determination of the enthalpy of neutralisation
Set-up and procedure
Set up the experiment as shown in Fig. 1 but for the time being
do not connect the heating coil to the work and power meter.
Prepare the potassium hydroxide solution required (cKOH =
2 mol/l) by dissolving one ampoule of potassium hydroxide for
1 l of 1 M solution in a 500 ml volumetric flask and topping off
with water to the calibration mark. Proceed in a similar fashion
with a second 500 ml volumetric flask using 1 ampoule of
hydrochloric acid for 1 l of 1 M solution to produce hydrochloric
acid of the same concentration (cHCl = 2 mol/l).
Connect the temperature probe to T1 of the measuring module.
Call up the Measure programme in Windows and enter
<Temperature> as measuring instrument. Set the measuring
parameters as shown in Fig. 2. Under <Diagram 1> select
Temperature T0a, the appropriate range for the temperature and
the X bounds and ‘auto range‘. Now calibrate your sensor under
<Calibrate> by entering a temperature value measured with a
thermometer and pressing <Calibrate>. After having made these
settings, press <Continue> to reach the field for the recording of
measured values. Arrange the displays as you want them.
Pour approximately 750 g water and 60 g of the 2 M potassium
hydroxide solution (both weighed to an accuracy of 0.1 g) into
the calorimeter. Using a delivery pipette and a pipettor, draw
around 50 ml of the 2 M hydrochloric acid from a small glass
beaker. The exact mass of the hydrochloric acid contained in the
delivery pipette is calculated from the difference between the
masses of the filled and the empty delivery pipette (accuracy
0.1 g). The 600 ml beaker is used as a pipette stand.
Place the filled calorimeter on the magnetic stirrer, put in the oval
magnetic stirrer bar and switch on the stirrer (Caution: Do not
switch on the heating unit by mistake!). Push the delivery pipette
through the cap of the calorimeter from below and mount the lid
on the calorimeter vessel. Now attach the pipette to the support
rod using a clamp in such a manner that the opening is above
the level of the liquid and that the stirrer bar can rotate unhindered. Insert the heating coil and the temperature probe into the
lid of the calorimeter and fix them in position.
When the temperature equilibrium has been reached (after
approximately 10 min) start the measurement by pushing <Start
measurement>. Wait 3 to 4 minutes, then blow the hydrochloric
Fig. 2:
acid out of the delivery pipette into the potassium hydroxide
solution in the calorimeter. To do this, first clamp a pinchcock
onto the tube of the rubber bulb, blow up the air reservoir of the
rubber bulb and quickly release the pinchcock. Continue to
measure until a new equilibrium has been reached. Subsequently perform electrical calibration to determine
the total heat capacity of the calorimeter. Supply 10 V AC to the
work and power meter for the electric heating. Push the <Reset>
button and then put the free ends of the heating coil
connection cables into the output jacks. The system is now continuously heated and the supplied quantity of energy is measured. The temperature increase in the system should be
approximately 2 K. When this value has been reached, switch off
the heating and read the exact quantity of electrical energy supplied. After a further three minutes, stop the recording of temperature.
Fig. 3 shows the graph as it is presented by the programme
when the measurement is stopped. If you use <survey> from the
toolbar you can read the temperature difference data.
Theory and evaluation
The value of the enthalpy of neutralisation ∆RH for the reaction
of strong acids with strong bases is independent of which strong
acid or base is used, because the heat of reaction is generated
by the reaction of hydrogen and hydroxyl ions to form water.
H+ + OH- S H2O
In the case of the neutralisation of weak acids and bases, additional heat effects arise from dissociation, hydration and association of molecules, so that the value of the enthalpy of neutralisation will differ to that given above.
The heat capacity of the system must be determined in order to
be able to determine the system change in enthalpy ∆h. This is
undertaken, after completion of the neutralisation reaction, by
introducing a specific amount of heat into the filled calorimeter
using electrical heating. The electrical energy Wel = I · U · t
which is converted into heat Q causes an increase in temperature ∆Tcal. From this the heat capacity of the system Csys can be
calculated using equation (1).
Q = I · U · t = Csys . ∆Tcal = Wel
(1)
Measurement parameters
Fig. 3:
2
∆RH = -57.3 kJ · mol-1
P3020811
Temperature-time curve of neutralisation and determining the heat capacity of the system
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
LEC
02.08
Determination of the enthalpy of neutralisation
Using the heat capacity of the system, the enthalpy of neutralisation ∆RH can be calculated from the temperature increase ∆T
of the neutralisation reaction for a known amount n of converted
hydrochloric acid.
∆RH n
cHCl
mHCl
rHCl
∆RH
Csys
Csys · ∆T
n
rHCl
Csys · ∆T
cHCl · mHCl
(2)
Amount of hydrochloric acid introduced
Concentration of hydrochloric acid (= 2 mol/l)
Mass of hydrochloric acid introduced
Density of hydrochloric acid (= 1.0344 g/ml for 2 M HCl
at 20°C)
Enthalpy of neutralisation
Heat capacity of system
For reasons of simplification it is assumed that the heat capacity of the dilute salt solution differs only negligibly from that of
water.
Data and results
Enthalpy of neutralisation:
∆RH = -57.3 kJ · mol-1
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen
P3020811
3
LEC
02.08
4
P3020811
Determination of the enthalpy of neutralisation
PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen