Download KV 1.17 Elevation of the boiling point

KV
1.17
Elevation of the boiling point
Materials
Frame for complete experiments
Rear cover for complete experiment panel
Panel for complete experimental set-ups
Shelf with hanging device
Clamping holder, d = 18...25 mm
Apparatus carrier with fixing magnets
Apparatus holder, variable
Spring plugs, 50 pieces
G-clamp
Round bottom flask, short-necked,
DURAN, 250 ml, GL 25/12
Gaskets for GL 18,
8 mm hole, 10 pcs, 1 only
Apparatus for boiling point elevation
Silicone tubing, d = 7 mm
Pinchcock, width 15 mm
Temperature probe, immersion type,
Teflon Pt100
Digital thermometer 4-2
45500.00
45501.00
45510.00
45505.00
45520.00
45525.00
45526.00
45530.00
02014.00
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1
1
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1
4
35812.15
1
41242.03
36820.00
39296.00
43631.15
1
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11759.04
13617.93
1
1
Heating mantle in metal housing, 250 ml
Support clamp
for the housing of heating mantle
Power regulator
Beaker, DURAN, tall form, 250 ml
Pellet press
Spoon with spatula end,
l = 150 mm, steel, micro
Funnel, glass, d = 80 mm
Laboratory balance with RS 232,
80/160/320 g
Weighing dish, 80 x 50 x 14 mm
Tweezers, l = 200 mm,
straight, blunt, st. steel
Wash bottle, 500 ml
Boiling stones, 200 g
Urea, 250 g
Water, distilled, 5 l
Vice
47550.93
1
47557.01
32247.93
36004.00
04403.04
1
1
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1
33393.00
34459.00
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1
48800.93
45019.05
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40955.00
33931.00
36937.20
30086.25
31246.81
1
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Fig. 1
Phywe series of publications • Complete Experiments Chemistry/Biotechnology • © PHYWE SYSTEME GmbH • D-37070 Goettingen
13102
73
KV
1.17
Elevation of the boiling point
Safety measures
Wear protective goggles.
Observe the detailed information on safety measures in
the appendix.
(Should the experiment be carried out with other substances, ensure that the appropriate safety instructions are
followed)
Preparation
Several separate determinations must be made to investigate the dependence of the elevation of the boiling point
on the concentration. It is best to increase the concentration of the test solution by adding small portions of the substance to be dissolved (in this case, urea). These portions
can be prepared prior to the experiment using a pellet
press.
The pellet press consists of a cylinder and a long and a
short plunger. To prepare a pellet, fit the short plunger into
the cylinder through the end without side slits. Fill a level
spatula of substance into the press, then close it with the
long plunger. Clamp the press in a vice to press the
plungers against each other. The resulting pellet has a
mass of between 0.200 g and 0.450 g. Weigh it to an accuracy of 1 mg!
At least 4 pellets should be prepared prior to the experiment.
Set-up
Position the clamping holders on the panel for complete
experiments as shown in Fig. 2. Subsequently assemble
and fix the apparatus to the panel as shown in Fig. 1. When
assembling the apparatus for boiling point elevation,
ensure that the inlet opening in the wall of the vessel is
below the sealing ring. In addition, ensure that the gasket
of the 250 ml glass flask is replaced by a gasket with an
8 mm hole (see the list of materials).
Fig. 2
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13102
Phywe series of publications • Complete Experiments Chemistry/Biotechnology • © PHYWE SYSTEME GmbH • D-37070 Goettingen
KV
1.17
Elevation of the boiling point
Procedure
First weigh the inner vessel without content, then pour
about 40 ml of water into it and assemble the apparatus for
the elevation of the boiling point (refer here to Fig. 3 and to
the operating instructions supplied with this apparatus).
Pour 150 to 200 ml of water into the 250 ml round bottom
flask and add a few boiling stones. The hose clip on the
lower silicone tube must be open. Neither of the tubings
should reach to the bottom of the beaker!
Heat the water in the round bottom flask with maximum
power, at step 10 on the power regulator. As soon as the
water boils, the power regulator setting can be reduced to
step 8. After boiling for some minutes, close the lower silicone tube with the hose clip. Hot steam flows into the inner
vessel and heats up the water there. The actual experiment begins when the water in the inner vessel has
reached a constant temperature of around 100°C. This
takes about half an hour! It is therefore recommended that
the water be heated up prior to the start of the lesson, or
that at least hot water be prepared. This can then be filled
into the apparatus instead of cold water – important is the
determination of the mass (not the volume) of the water
filled into the inner vessel. The time to start of measurement is greatly shortened.
As soon as a constant temperature has been attained in
the inner vessel, switch the temperature measuring instrument to the operating mode for change in temperature
(see the operating instructions for the measuring instru-
ment). This has a resolution of one hundredth of a degree.
Open the upper closure of the inner vessel, which holds
the temperature probe and, as quickly as possible, drop in
one of the urea pellets and close the vessel. After a short
time, read off the difference in temperature between the
now elevated boiling point and the boiling point of the plain
water. Repeat this procedure with as many urea pellets as
intended. At least four are recommended for a graphical
plot.
On completion of the measurements, first open the hose
clip on the lower silicone tube. This avoids a sucking over
of liquid from the inner vessel into the round bottom flask
that would otherwise result from the volume contraction
when the apparatus cools. Should this happen, it would
prevent an accurate determination of the contents of the
inner vessel. Only after having opened the hose clip, lower
the heating mantle and switch it off! Now disassemble the
boiling point apparatus(Caution! Still hot!) and re-weigh the
inner vessel. The difference in mass of the full and the
empty vessel gives the mass of solution. The mass of
water is given from the mass of the solution less the mass
of the ureas pellets used.
Observations
After each addition of a compressed portion of urea, the
boiling temperature drops briefly and then increases by a
few hundredths of a degree. The new boiling point remains
constant.
Fig. 3
Phywe series of publications • Complete Experiments Chemistry/Biotechnology • © PHYWE SYSTEME GmbH • D-37070 Goettingen
13102
75
KV
1.17
Elevation of the boiling point
Results
The following results were obtained from an actual experiment:
There was 50.94 g of water in the inner vessel and during
the experiment seven portions of urea of between 0.237 g
and 0.442 g were added (see Table 1).
A straight line is obtained on plotting the values of the elevations of the boiling points above that of water against the
total concentration of urea (see Fig. 4).
Explanation
On adding urea to water, this first dissolves. Subsequent to
this, a new boiling point equilibrium, at a temperature that
is changed by some hundredths of a degree, is attained. It
is clear from the graph that this elevation of the boiling
point is proportional to the concentration of the solution.
Solutions of solid substances in water have a higher boiling point than pure water. The boiling point is higher, the
higher the concentration of the solution. The elevation of
the boiling point is only dependent on the concentration of
the solution. One mole of dissolved particles increases the
boiling point temperature by 0.515 K (the ebullioscopic
constant for water = 0.515 g·K/mole). This enables the
molar mass of the added substance to be calculated from
the elevation of the boiling point and the mass of added
substance (ebullioscopy).
For more on the theory of boiling point elevation and freezing point depression, or on ebullioscopy, please refer to the
technical literature.
set-up or procedure. For details on this, refer to the technical literature and the operating instructions for the
“Apparatus for boiling point elevation”.
From the results of the measurement example, seven values are given, between 55.32 g/mole and 64.70 g/mole.
The average of these gives a molar mass of 60,66 g/mole
for the substance used (literature value: 60.05 g/mole). A
Beckmann thermometer can be used for temperature
measurement instead of the digital measuring instrument.
In this case, refer to the operating instructions for the
“Apparatus for boiling point elevation” for details on the procedure.
The graphical evaluation of the measured values can be
simplified by use of “measure” software. This software is
license-free for the purpose of evaluating and graphically
representing measured values (freeware). It is available as
download-file under URL “www.phywe.de”, or can be
installed from the demo-CD supplied with the digital thermometer 4-2. Figure 4 was created with this software.
Fig. 4
Notes
Hydroquinone or common salt can be used as alternatives. When using the latter, take the correction factor for
the ionic activity into consideration!
The same apparatus can also be used to determine molar
masses from the elevation of the boiling point. As long as
no other chemicals are used, no change is required in the
Table 1
76
Mass of urea
in g
Conzentration of urea
in mole/1000 g water
Boiling point elevation
in K
Total boiling point elevation
in K
0.442
0.144
0.07
0.07
0.383
0.270
0.07
0.14
0.300
0.368
0.05
0.19
0.418
0.504
0.07
0.26
0.237
0.582
0.04
0.30
0.256
0.666
0.04
0.34
0.237
0.743
0.04
0.38
13102
Phywe series of publications • Complete Experiments Chemistry/Biotechnology • © PHYWE SYSTEME GmbH • D-37070 Goettingen