close

Вход

Забыли?

вход по аккаунту

?

THE MEASUREMENT OF THE HEAT CAPACITIES OF 1-NITROPROPANE AND NITROETHANE

код для вставкиСкачать
DOCTORAL DISSERTATION SERIES
T h e H m m m i Df I k M
C a jn rih a Of b M k o p fin e
TITLE.
h
AUTHOR. It.
i
Y
i
M
i
b
m
r
l
h
n
e
3 W
Jwi
UNIVERSITY U u e h i m s i l y d a t e i m
DEGREE A M aM - — PUBLICATION NO.
ip in
i r 1 r • '9
u UNIVERSITY MICROFILMS
ANN ARBOR
• MICHIGAN
THE MEASUREMENT OF THE HEAT CAPACITIES OF
1-KITHOPHOFANE AND N1THOETHANE
G. Victor Beard - Thesis (Ph.D.)
Purdue University, 1940
Heat capacity is defined as q/aT or C^ a
Its
measurement usually involves measuring q and-^T directly; however,
it can be measured by determiningthe variable in this equation
Cp
*
q
.
J T
a
V .
This method offers a
comparatively new approach
~2TT
to the problem (1) and is the method used here,
measured simultaneously in one operation,
.j V and
q and-^V are
a
T are measured
simultaneously in a second operation.
The apparatus consists of two parts:
one part measures direct­
ly the variables q and^aV and the second part measures directly** V
and A
T. The general plan of the
first apparatus is apparent from
Plate
1. q is measured by taking
volts times coulombs.
Volts are
measured by using a potentiometer and coulombs are measured by
using a gas coulometer containing nickel electrodes immersed in a
15 per cent NaOH solution.
The source of current is a 110 volt
D.C. generator with three six volt, lead storage batteries "floated"
across the circuit in the manner shown in Plate 1.
Plate 2 shows a 500 ml. round bottom flask through the bottom
i ■■
of which are sealed two tungsten electrodes.
These electrodes are
spot welded to two short pieces of nickel wire which in turn are
spot welded to a 0.3 ohm, No. 32, platinum resistance wire.
The
flesk has fused to it near the bottom a diagonal bored, capillary
stopcock.
flask.
It has the same type of stopcock near the top of the
On top of this stopcock is a small glass reservoir.
The
flask also has a section of carefully calibrated small diameter,
capillary tubing fused to it at the top of the flask.
This
capillary tubing has attached to it the measuring scale used to
calibrate the tubing.
2
-
This whole apparatus fits into a water-tight
glass jar which has had the top remodeled so that the capillary
tubing and measuring device emerge through the top.
The glass
jar and contents are kept in a water bath whose temperature is
controlled to 0.003°C.
A measured quantity of heat is generated in the platinum
heating coil and the meniscus rise in the capillary tubing is
carefully noted.
This gives us q and -4 V,
Certain precautions are necessary in this operation.
The
whole system is allowed to come to equilibrium with the top
stopcock open.
This is closed and if the meniscus in the capillary
tube is stationary the system is in equilibrium.
The meniscus
height is recorded, the current is turned on, the voltage is read,
the current is turned off, and the final meniscus height is recorded.
It is essential that the meniscus continues to rise, or at least
remain stationary for several seconds after the current is turned
off.
This insures that heat is not being lost by convection.
The current is usually run from 5 to 10 seconds.
The amount
of gas collected is from 5 to 10 ml, and the meniscus rise from
5 to 10 cm.
The second piece of apparatus measures-aV and -aT (Plate 4),
It is essentially a dilatometer and consists of a 500 ml, flask
on which is fused a stopcock for filling and a calibrated capillary
tube.
The apparatus is put in the glass jar and the whole put
in the water bath and allowed to come to equilibrium at a
temperature about 0.5#C, above the desired temperature.
The
meniscus height is read; it should be near the top of the capillary
tube.
The temperature of the bath is lowered to about 0.5*0.
below the desired temperature and the meniscus height again read.
-
3
-
After correcting for hydrostatic effects and expansion effects
we get^aV and A H ,
If the densities of the liquids being investigated are known
with sufficient accuracy this apparatus is not used.
dV/dT is then
determined as follows:
V *
M
dV
15
dT
■ M
V*
dD .
dft
The 1-nitropropane and nitroethane were obtained from Commer­
cial Solvents.
They were dried successively with CaCl 2 and Drierite.
The 1-nitropropane was fractionated in a 30 plate column using a
high reflux ratio and it was collected over a 0.2*0. boiling range.
The nitroethane was fractionated twice in this same column and the
0
second fraction was collected over a 0.3 C. boiling range.
1-nitropropane was probably very pure.
some 2-nitropropane remaining in it.
The
The nitroethane probably had
However, since the partial
molal heat capacities of these two substances are close, it is
believed the error produced is well under the precision of the
apparatus.
Temperature
Heat Capacities
Hitroethane
1-Hitropropane
25
0.441 t 1%
0.470 +• Vfo
35
0.437 + l/o
0.440 4 Vfo
45
0.437 + 1%
0.462 + V/o
50
0.438 + T/o
0.489 + Ifo
The minimum in the heat capacity curve for 1-nitropropane is
to be attributed to the tautomeric equilibrium
H
OH
CH3 CEaCHJS0j_"g CHjCH^C s N-0
(2)
No satisfactory explanation can be given at present for the
observed straight line representing the heat capacity of nitroethane.
1-lTCTROFiiCFAIIE A D
ITITROETH^S
Abstract
Experimental
Heat capacity is defined as ojfr T or Cp
=
Its
measurement usually involves measuring; q ana A T directly; however,
it can be measured by determininr; the variable in this equation
Op
=
q
& V
y*y *
•
This net nod offers a comparatively new approach
to the probler. (1) and is the method used here,
simultaneously in one operation*
c and A V are measured
a V and A T are measured simultaneous*
ly in a second operation*
Apparatus
The apparatus consists of two parts: one part measures dix'ectly the variables q ana
and A
V and the second part measures directly <=» V
T.
The general plan of the
first apparatus is apparent from
Plate 1.
q is measured by tailing
volts times coulombs.
Yolts are
measured by using a potentiometer ana coulombs are measured by using
a gas couloneter containing nickel electrodes i mersou in a 15 per cent
HaOH solution.
The source of current is a 110 volt D.G. generator
with three six volt, lead storage batteries "floated" across the cir­
cuit in the manner shown in I-late 1.
Plate 2 shows a 500 ;1. round bottom flash through the
bottom of which are sealed two tungsten electrodes.
These electrodes
2.
are spot welded to two short pieces of nickel wire which in turn are
spot welded to a 0.3 ohm, No. 32, platinum resistance vjire.
The flask
has fused to it near the bottom a diagonal bored, capillary stopcock.
It has the same type of stopcock near the top of the flask.
this stopcock is a small glass reservoir.
On top of
The flask also has a section
of carefully calibrated small diameter, capillary tubing fused to it
at the top of the flask.
This capillary tubing* has attached to it the
measuring scale used to calibrate the tubing.
This whole apparatus
fits into a water-tight glass jar which has had the top remodeled so
that the capillary tubing and measuring device emerge through the top.
The glass jar and contents are kept in a water bath whose temperature
is controlled to 0.G05°C.
A measured quantity of heat is generated in the platinum
heating coil and the meniscus rise in the capillary tubing is carefully
noted.
This gives us q and A V.
Certain precautions are necessary in this operation.
The
whole system is allowed to cone to equilibrium with the top stopcock
open.
This is closed and if the meniscus in the capillary tube is
stationary the system is in equilibrium.
The iUeniscus height is record­
ed, the current is turned on, the voltage is read, the current is turned
off, and the final meniscus height is recorded.
It is essential that
the i..eniscus continues to rise, or at least remain stationary for sever­
al seconds after the current is turned off.
not being; lost by convection.
seconds.
This insures that heat is
The current is usually run from 5 to 10
The amount of gas collected is from 5 to 1C ml. and the menis­
cus rise from 5 to 1C cm.
The secono. niece of apparatus measures ^ V and A T (Plate 4).
3.
It is essentially a dilatometer and consists of a 500 ml. flask on
which is fused a stopcock for filling and a calibrated capillary tube.
The apparatus is put in the glass jar and the whole put in the water
bath and allowed to come to equilibrium at a temperature about 0.5°C.
above the desired temperature.
The meniscus height is road; it should
be near the toj of the capillary tube.
The temperature of the bath is
lowered to about G.5°C. below the desired temperature and the meniscus
height avain read,
nfter correcting for hydrostatic effects and ex­
pansion effects we yet A V and o f .
If tie densities of the liquids beiny investigated are known
\ ith sufficient accuracy t. is apparatus is not used.
dV/dT is then de-
texTiined as fellows:
V
=
D
jdV _
dT “
lr_ dD
D s dT ’
Materials
The 1-nitropropane and nitroethane were obtained from Commer­
cial Solvents.
They were dried successively ’..ith CaCl2 and Drierite.
The 1-nitropropane was fractionated in a 30 plate column usiny a high
reflux ratio and it mas collected over a 0.2°C. boiling range.
The
nitroethane was fractionated twice in this same column and the second
fraction was collected over a 0.3°G. boiling range.
was probably very pure.
pane remaining in it.
The 1-nitropropane
The nitroethane probably had some T-nitroproiIov;ever, since the partial modal heat capacities
of these two substances are close, it is believed the error pi’educed
is well under the precision of the apparatus.
4
Results
Heat Capacities
Nitroethane
1-Hitropropane
Temperature
25
0.441 + Vfo
0.470
+ Vfo
35
0.437 + Vfo
0.440
+ Vfo
45
0.437 + Vfo
0.462
+ Vfo
50
0.438 + Vfo
0.489
+ Vfo
Discussion of Results
The minimum in the heat capacity curve for 1-nitropropane
is to be attributed to the tautomeric equilibrium
CH30H2CH2]iC£
£
xi
OH
CII3CH2C = Al-0
(2)
Ho satisfactory explanation can be fjiven at present for the observed
straight line representing the heat capacity of nitroethane.
:'iscellaneous
This apparatus has the following desirable features:
1.
It is easy and cheap
to build, as well as easyto
operate.
2.
lio corrections or allowances have to be made for the heatCapacity
of the calorimeter.
3. ho corrections have to be ..cide for temperature uniformity throughout
the liquid whose neat capacity is boiny measured.
4. Pure liouids, liquid mixtures, corrosive liquids ox* non-corrosive
licuids can all uc run ".'ithout nifficuxties.
This apparatus has the folio..inq undesirable features:
1. Fairly larqe quantities of liquids are necessary.
2.
Vex*y viscous liquids
3. Goniometers carryinj
cannot be used.
five. amperes of currentfor
ten secondstime
5.
are not too precise.
4. It is difficult to accurately calibrate tubin.; of C.02 cm. radius.
ouru-ary
The heat capacities of 1-nitropropane and nitroethane have
been measured.
An japparatus for :.easurinj the iieat capacities of li­
on id has been constructed.
This apparatus measures the variables in
i
the equation
!
|
1
rr _ _&i_
V
j*1L
a- x
!
Deferences
1. I. H. Dchlesinqer, Ihysik. h. 1C , 210-15 (1909).
2. hilliams and Daniels, J. n.u Jhe;... j u c . 46,
PURDUE UNIVERSITY
T H IS IS TO CEKTIFY THAT TH E TH ESIS PREPARED UNDER MY SUPERVISION
3. Victor beard
BY
ENTITLED
l-I'IITiOPIfiirV-.THj AI’D ril’dOV,iM/DL
COMPLIES WITH TH E UNIVERSITY REGULATIONS ON GRADUATION THESES AND IS APPROVED
BY ME AS FULFILLING TH IS PART OF THE REQUIREMENTS FOR THE DEGREE OF
Doctor of I-hilosoohy
Professor in Charge of Thesis
Head of School or Deportment
June
19
41
T O TH E LIBRARIAN
IS
T H IS TH ESIS I S W 8 F T O BE REGARDED AS CONFIDENTIAL.
Professor in Charge
Registrar Form 10—2-39—1M
THE LtSASUHEMENT OF THE HEAT CAPACITIES OF
1-NITHOPROPAKE AHD NITROETHANE
A Thesis
Submitted to the Faculty
of
Purdue University
by
G-. Victor Beard
in partial fulfillment of the
requirements for the Decree
of
Doctor of Fhilosoph}’’
August, 1940
Acknowledgment
The author wishes to express deep appreciation and gratitude
to Professor Roy F. Newton for the inspiration and practical help
given during the course of thesis investigation.
The author also
wishes to express sincere appreciation for the cooperation given by
Mr. Fish and Mr. Hession of the technical staff of the department of
Chemistry,
Table of Contents
Page
Introduction
.................................
Theoretical.......
Experimental
1
•••••.......
.......
1
2
Apparatus and Materials
......
4
Description of Apparatus ................
4
Theory and Use of Apparatus .................
6
Calibration of the Apparatus .............
8
Assumptions Concerning the Apparatus
Materials
......
Miscellaneous
Calculations
....
10
.......
1C
.... .••••«.......••••••••..........
Results .....
••••«......
Discussion of Results
9
15
15
..................
Features cf the Apparatus
IS
...................
lb
S u m m a r y ..............................
Ref e r e n c e s
......
20
Index of Figure and Plates
Page
Figure 1.
Figure showing specific heats of 1-nitropropane
and nitroethane.......
21
Plate 1. Electrical detail of the apparatus
22
Plate 2, 3, 4 and 5.
23
Schematic drawing of apparatus .......
THE ISASTJRHI.SliT OF THE HEAT CAPACITIES OF
1-NITROPROPANE AND NITROETHANE
Introduction
Theoretical
Classical thermodynamics defines the average heat capacity
as q/fr T, and the limit of this ratio, as T2 approaches infinitely
close to Tj_, is called the actual heat capacity at the temperature
T± (1).
q is defined as the heat absorbed by the system,
q can
further be defined as:
q
£E
=
A E + Vi
is defined as the increase in internal energy of the system.
is defined as the work done by the system on its surroundings.
V7
Heat
capacities are usually measured under either one- of two conditions,
the system being kept at constant volume, or the system being kept at
constant pressure.
Mathematically these are defined as follows (1):
Statistical thermodynamics accepts these definitions but goes
one step further than classical thermodynamics by trying to picture
the ways that molecules and atoms can increase their internal energy.
This has led to the development of a new concept in thermodynamics;
the partition function.
Statistical thermodynamics derives (2):
2
°v
op -
-
«fflT
- ®*
■
;w P - m *
a>
.* 2RTiiiiyj. ♦ p(^z)p
Q, is the partition function, and it is defined as:
n
Q, =
%•
%
is defined as:
i=S?
1=0
%
Pi 5. ” R?*
pi is the sta-fcistical weight factor.
is the difference in energy between the i—
state and the ground
state.
The energy absorbed by gaseous molecules and atoms in rota­
tion, vibration, and electronic transition can be obtained by optical
means.
The statistical weights to apply to these summation equations
can be calculated by quantum mechanical methods.
Methods are also
available for calculating heat capacities of molecules and atoms when
they exist as perfectly ordered solids.
Unfortunately, in applying
these methods to liquids the mathejaatical difficulties are so great,
that, except in the case of a few ideal liquids, the equations have
never been solved.
This leaves us without a sound theoretical basis
for calculating the heat capacities of liquids.
Experimental
Two general methods for measuring the heat capacities of
liquids are in use today.
The first method directly measures q and
.
3.
A T, tne variables given in the classical definition of heat capacity.
A carefully measured quantity of heat is put into a measured quantity
of liquid and the tempei'ature rise is accurately measured.
Various
ways of measuring or eliminating the heat capacity of the calorimeter
have been devised.
These calorimeters are very good and very good
results are obtained with them; however, they are expensive to build
and not too easily operated by students.
The second method was suggested by Lewis and Randall (1)
and has recently been used (5).
Cp
This method uses the equation:
=
•
(■=£!) s
The pressure is changed suddenly and the corresponding temperature
change measured.
This is done adiabaticaliy or at constant entropy.
The temperature and the coefficient of expansion are independently
measured, thus all the variables in the equation are measured.
This
method also gives satisfactory results but again the equipment is ex­
pensive and the process not too easy to operate.
This thesis deals with still a third possible way of measur­
ing heat capacities of liquids.
The method was suggested by Dr. R. j?.
Newton but a later survey of
the literature revealed that it had pre­
viously been tried (3).
equipment needed is inexpensive and the
The
process is easy to operate.
The method is apparent from the mathe­
matical equation:
1'
W ?
/
=
’
/^Vv
"dT ^
q and dV are measured in one process; 4rr
1
aT
cess.
measured in a second pro-
Apparatus and materials
Description of Apparatus
The apparatus consisted of two separate parts:
a. An apparatus for measuring q/^ Y.
b. nn apparatus for measuring dV/dT.
Plate I shows the general details of the apparatus for
measuring q/ A v .
Two, six volt lead storage batteries are connected
in series with each other.
They are connected through a 20 ohm re­
sistance to a 110 D.C. current supply.
They are, also, connected in
series, with a resistance and in parallel to six volt lead storage
battery.
This six volt battery is connected in series to a coulo-
meter, a heating element, a hey and a variable resistance.
ing element has leads going to a student potentioraeter.
The heat­
This arrange­
ment of storage batteries reduces a five per cent fluctuation in the
110 D.C. current to a fluctuation of less than 0.1 per cent in the
heating element.
The heating element is in an apparatus called "A1', shown
in Plate II.
Apparatus "A” consists of a 500 ml. round bottom flask
through the bottom of which two tungsten wires are fused.
These
wires are spot vrelded to two short nickel wires, which in turn are
spot welded to a 0.5 ohm, number 32 platinum coil resistance wire,
hear the bottom of the 500 ml. flask is a ground glass, diagonal,
capillary stopcock.
Hear the top of the 500 ml. flask is a ground
glass, diagonal, capillary'- stopcock.
servoir on top of it.
This stopcock has a snail re­
On top of the 500 ml. flask is fused a short
section of heavy 0.1 era. diameter capillary tubing.
To this is fused
a 70 cm. piece of carefully calibrated capillary tubing, whose diameter
is about 0.04 cm.
Around this calibrated capillary tubing is solidly
fastened a 50 nil. section of a 0.1 ml. calibrated burette.
The stop­
cock (see Plate III) on the top of the apparatus, has fastened to it
a small piece of copper sheet which has a small hole drilled in the
periphery of the piece.
The stopcock is also held tightly in place
by two heavy springs attached to collars
ends of the stopcock.
placed over
the butt and fore
The purpose of the copper attachment to the
stopcock is to aid in opening and closing the stopcock under conditions
to be described later.
Apparatus "A" fits directly into a glass jar of about three
liters capacity, and whose height is about ten inches (see Plate V).
The top to this glass jar can be fastened so that it is water tight.
The original top also has soldered to it a short conical section, on
which is soldered a three inch cylindrical section.
gives more effective height to the glass jar.
This simply
'The water tight gasket
was made by shellacking the metal, and covering liberally with cotton.
After this dried, the cotton was shellacked and a piece of circular
sponge rubber placed on the shellacked cotton.
After this dried, the
sponge rubber was coated liberally with paraffin wax.
This gasket,
after a year of fairly hard usage, was still in good condition.
The apparatus for measuring dV/dT is essentially a dilatometer (see Plate IV).
A 5C0 ml. flask is used for
this flask is fastened,
near the bottom, a diagonal,
capillary stopcock.
this purpose.To
ground glass,
On top of the flask is fastened a calibrated
100 cm. capillary tube.
This tube is about 0.1 cm. in diameter.
The coulometer is a simple gas couloneter with nickel elec­
trodes in a 15 per cent NaOH solution.
Tiro ml. of octyl alcohol are
used to break tiie frothing of the UaGH solution.
regulating device is used.
Tiie usual pressure
The collected gas can be read to C.l nil.
i?or convenience, the meniscus height is read with a cathetometcr, although satisfactory results can be obtained by using a
meter stick v.'hen working with the dilatometcr apparatus.
Theory and Use of Apparatus
The measurement of q/ a T '..-ill be described first.
Apparatus
''A” is filled with the liquid whoso heat capacity is being .measured,
and "A" is then placed in the glass jar, which is filled with some
suitable liquid.
Tiie top is firmly attached and the glass jar and
all it contains is placed in a water bath which is controlled to
G.003°C.
The leads coming from the platinum heater arc now connected
as shown in Plate I.
packed full of cotton.
reservoir as needed.
The cylindrical metal top of the glass jar is
Liquid is added to or subtracted fron the
During this whole process the stopcock connect­
ing the reservoir and the flask is open.
.fuile the system is coming
to equilibrium the circuit is frequently closed.
purposes.
This serves two
It thoroughly saturates the iJaOH in the coulometer with
gas, and it gives an idea as to the potential across the platinum
heating wire,
lifter equilibrium is attained throughout the system,
the stopcock is closed by inserting a piece of glass tubing down through
the opening of the glass top of the jar.
first.)
(The cotton is removed
The glass tubing is pressed against the top of the copper
niece ana pushed until tne stopcock closes.
placed in the cylindrical top.
The cotton is then re­
The meniscus in the calibrated
capillary tube is now closely watched; if it is stationary the system
is in heat equilibrium and its position is carefully recorded.
The
coulometer is adjusted ana the position of the top meniscus of octyl
alcohol carefully recorded.
The student potentiometer is set to
approximately the right voltage.
eight to ten seconds.
The circuit is closed for from
The potentiometer is read, and the position of
the meniscus in the capillary tube is carefully read.
The meniscus
in this tube should continue to rise after the current is shut off,
or it should remain stationary for several seconds.
it immediately fall back.
by convection currents.
In no case should
This means heat is being lost to the wall
(If benzene is run in the apparatus, these
convection currents can be studied in detail.
The index of refraction
of benzene changes markedly with the temperature.)
The coulometer
is allowed to stand for thirty minutes, the volume levels adjusted
and the position of the sane meniscus noted,
culated from this data.
at one temperature.
noted.
q and z\V can be cal­
It is desirable to have several determinations
The rate of fall of the capillary meniscus is
This is allowed for in the next determination.
Usually after
thirty minutes the rate of fall is very small and can be accurately
estimated for a timed run.
For runs at a different temperature,
the small copper wire attached to the drilled hole in the periphery of
the copper semicircle, is pulled.
This opens the stopcock and the
apparatus is allowed to come to equilibrium without removing from
the bath to fill with more liquid.
The measurement of dV/dT is carried out in an analogous
manner.
The dilatometer is filled with liquid and is placed in the
glass jar, which is placed in the constant temperature bath.
The
system is allowed to come to equilibrium at a temperature about 0.4
to C.5°G. above the desired temperature.
The meniscus should be al-
most to the top of tne capillary tubing.
brated.)
(This tubing is also cali­
After equilibrium is established the bath temperature is
lov;ered from 0.8 to 1.0°0.
Equilibrium is established and the new
position of tiie meniscus determined.
apparatus should be laiown.
‘
The weight of the dilatometer
The whole is weighed after each determina­
tion so the weight of the contents is known for each temperature de­
sired.
iiie gives us b V / A T .
If the densities of the liquid under investigation are known,
for the desired temperatures, the coefficient of expansion can be
determined, without using the dixatometer apparatus.
This is done as
follows:
V
=
dV
dT
=
I"
Volume equals mass/density
_ M
“
dD
dT
Usually this information is not available of the density is not known
with sufficient accuracy.
Calibration of the Apparatus
------— ■
■ T- T ■
■ ■ - - — I r " T
a.
------«**-*•- -
The smaller capillary was calibrated by pushing various
lengths of mercury back and forth (one cm. at a time) along the capil­
lary.
The lengths were recorded, and the mercury weighed.
b* A number of water threads fifty to seventy centimeters
in length v;ere measured and then accurately weighed.
c. The apparatus was filled with distilled water; a number
of runs were made and the volume of the capillary calculated from the
known heat capacity of water.
d. The heat capacities of benzene and butyl alcohol were
9,
then determined as a check on the calibration.
The gross calibrations found with 'water were taken, superim­
posed on these were the minor irregularities shown by the mercury
thread.
The calibrations obtained by the mercury and water threads
did not check.
It is believed (3) liquids, which wet glass, fill
small capillary tubes more than do liquids v/hich do not wet glass.
The larger capillary tube was not calibrated over small
lengths since only large lengths are used.
using both mercury and water threads.
This tube was calibrated
The two results were in good
agreement.
Assumptions Concerning the Apparatus
There are several assumptions inherent in the use of this
apparatus.
They are:
a. The heat capacity of the platinum heating wire is small.
b. The volume change on heating is the same if one ml. is
heated 1.0°C., or if 100 ml. are heated 0.01°C.
c. The change of heat capacity with small pressures is
negligible.
Assumption (a) seems reasonable when we consider the small
weight of the platinum wire and the exceedingly low heat capacity of
platinum.
Assumption (b) also seems reasonable because where experimen­
tal data are available we find dV/dT to be almost constant for small
temperature changes.
Assumption (c) seems reasonable Then we examine the equation
<£§>. - <H>p u>
9
then determined as a checi; on the calibration.
The gross calibrations found with water were talcen, superim­
posed on these were the minor irregularities shown by the mercury
thread.
The calibrations obtained by the mercury and water threads
did not check.
It is believed (3) liquids, which wet glass, fill
small capillary tubes more than do liquids which do not wet glass.
The larger capillary tube was not calibrated over small
lengths since only large lengths are used.
using both mercury and water threads.
This tube was calibrated
The two results were in good
agreement.
Assumptions Concerning the Apparatus
There are several assumptions inherent in the use of this
apparatus.
They are:
a. The heat capacity of the platinua heating wire is small.
b. The volume change on heating is the same if one ml. is
heated 1.0°C., or if 100 ml. are heated 0.01°G.
c. The change of heat capacity with small pressures is
negligible.
Assumption (a) seems reasonable when we consider the small
vreight of the platinum wire and the exceedingly low heat capacity of
platinum.
Assumption (b) also seems reasonable because where experimen­
tal data are available we find dV/dT to be almost constant for small
temperature changes.
Assumption (c) seems reasonable when we examine the equation
-
T(- ^ > p
(1>
v/e see that for very small pressure changes the change of
the heat capacity is almost zero because of the small change of V
with respect to T.
materials
All water used m s
doubly distilled.
The butyl alcohol was
a C.P. chemical and m s distilled over Drierite.
The benzene was
thiophene free and had been previously purified.
It was assumed pure
enough for check purposes.
The nitroethane and the 1-nitropropane were the commercial
quality from Commercial Solvents Corporation.
They were dried
successively with calcium chloride and with Drierite and were then
fractionated in a thirty plate column using a high reflux ratio.
The
1-nitropropane was collected over a G.2°C. variation in the boiling
range.
Tiie middle fraction of the nitroethane was collected.
This
was run through the column a second time and collected over a 0.3°C.
variation in the boiling range.
pure.
The 1-nitropropane was probably very
The nitroethane probably had a per cent or so of S-nitropropane
left in it; however, since the partial molal heat capacities of these
two compounds are probably fairly close to each other, it is believed
the error from this source is well under the precision of the appara­
tus.
Miscellaneous
The stopcocks were greased with the mannitol, dextrin gly­
cerol mixture, described in Lange’s handbook, when benzene, nitro­
ethane, and 1-nitropropane were used.
They were greased with a high
rubber content grease when butyl alcohol and water were used.
No
grease was perfectly satisfactory for butyl alcohol.
YJhen the glycerol grease was used, the glass apparatus was
filled with kerosene.
For other greases water was used in the glass
jar.
The large heat capacity of the liquids in the glass jar
served to smooth out the 0.003°G. change in the water bath.
Apparatus "A" must be free fron all air bubbles before any
runs are m d e .
Failure to observe this precaution, results in incon­
sistent readings.
The largest source cf error was getting accurate measure­
ments of the voltage.
was at a maximum.
Yflien the current was first turned on the voltage
This gradually fell to a steady state condition in
about thirty seconds.
A dummy heater was always drawing current from
the storage cells prior to a heat capacity determination so it was not
due to polarization of the battery electrode.
The fault probably was
in the couloneter and was due to the evolved gases effectively reducing
the area of the nickel electrodes.
{A coulometer tiiat was accurate
when carrying five amperes of current for ten seconds was hard to
find.)
The overall voltage change was usually between one and two
per cent.
The average heating time was about ten seconds so the
following procedure was adopted.
The potentiometer was set at a fixed
value arrived at in the gassing up period.
The time was observed,
the run then continued for an equal tine period beyond this value.
This amounts to linear interpolation and we believe jives the true
voltage to +0.5 per cent.
Corrections were made for the change of volume of the 500 ml.
12
flask with temperature and. also for compressional effects on the liquid
due to capillary rise.
This was for the dilatoiaeter.
13.
Calculations
Five to eight determinations were made at each temperature,
with apparatus "A".
The best values were averaged arithmetically and
this mean value used.
Three temperature points were made with the
dilatometer; this gave three values for the coefficient of expansion.
If these values checked to +0.5 per cent no other points were taken.
A typical calculation will be given.
Gas reading before heating
=
24.76
Gas reading after heating
=
20.68
Difference
=
Capillary reading before heating
=
42.70
Capillary reading after heating
=
53.50
Difference
=
Room temperature
=
29°C.
Bath temperature
=
50°C.
Barometer reading
=
753 mra. Hg.
E.L'.F.
=
4.08
9.20
0.800.0 volts
The coulometer equation is:
HgO
96,500 coulombs
ml. mixed gas
This gives
_
<^_
AY
3hV * /X T
F
q
-> H 2 + 1/2 0 2
=
4.08
x
728
75q
x
£73
502 x
96,500
15 ,000 x
0.800
0.418
calories
14.
ZLV
=
A 1
A T
Cp
=
9.20
x
1.05
1.12
x
io“3
AAA
9.20
x ZAL x
760
x
10"3
ml.
SA:____
gram 0.1°C.
273
302
9.6,500
16,800
0.800
0.418
1.12 x 10~3
cal.
1.05 x 10"3 gia. 1°C.
15.
Results
Temperature
Heat Capacities
Nitroethane
1-iTitropropa ne
25
0.441
£ 1$
0.470 £ 1$
35
0.437
£ lyj
0.440 ± 1/j
4o
0.437
£ Vp
0.462 £ T/o
50
0.438
£
0.489 £ 1%
Discussion of Results
The results for 1 -nitropropane are of the same general
nature as the results found by ;/illiar.is (4) for nitromethane when a
thoroughly dry sample vas used,
he shall attribute our minimum, as
he did, to the tautomeric equilibrium.
H
CH3 GH2 CII2IT02
it
CHgCHgO
0
= n -O H
The heat capacities of 1-nit ropro pane are larger than those
for nitronethane at the same tempenatures.
The 1-nitropropane turned a slight yellow color* when it -was
kept in apparatus "A” for six hours at 50°G.
It did. not change color
when kept at 50° for 15 hours in the dilatometer apparatus.
Presum­
ably anyone or all of the wires (Hi., Pt., and i’i) catalyzed the change.
The heat capacities of nitroethane are peculiar* and within
the precision of our apparatus they lie substantial!;/ on a horizontal
straight line.
This is about what .'.illiams found with nitromethane
which had not been thoroughly dried.
However, v/e believe our sample
v/as thoroughly dry.
It might be that the aci nitro equilibrium for nitroethane
occurs over a larger temperature interval than for the other two nitro
compounds and that we are working near the bottom of* a large minimus
over this temperature range.
ITitroethane was run with a very small amount of* boric acid
■present in ox’der to prevent the coloration which occurred in the 1-nitrorropane w h e n held at 5 0 ° G . for several hours.
I t is conceivable -that
this influenced the equilibrium between the two tau-boaeric forms.
It
does not, however, seem very probable that such snail amounts could
so seriously affect the results.
The boric acid was present to the ex-
tent of C.l gram per five hundred grams of nitro compounds.
Features of the Apparatus
It is easily built and can be constructed with.just ordinary
laboratory equipment.
It is easy to run and suitable for general laboratory work.
It can be used with pure liquids or with solutions either organic,
inorganic, or combinations of the two.
Disadvantages of the apparatus are:
It is time consuming.
It requires fairly large quantities of liquid.
Suitable greases for stopcocks are hard to obtain for some liquids.
It is not suitable in its present form for running liquids of
high viscosity.
Summary
An apparatus to measure the heat capacities of liquids has
been built.
It operates to give the variables in the equation:
G
°P
-
1
*V
^
AT
The heat capacities of 1-nitropropane and nitroethane have
been measured for the temperatures of 25°, 35°, 45° and 55°C.
20.
References
1. "Thermodynamics", Lewis and Randall, McGraw Hill Book Company,
(1923).
2. "Statistical Thermodynamics", Fowler and Guggenheim, (1939).
3. I. H. Schlesinger, Fhysik. Z. 10, 210-15 (1909).
4. Williams and Daniels, J. Am. Chem. Soc. 43, 2644 (1925).
5. J. S. Burlew, J. Am. Chem. Soc. 62, 681 (1940).
»
O.SOO
N ifroe thane
t-Nitropane
so
30
Ternper atture
Fig. I.
Pfate
1
o
7b pofaft f /o m o th
To b a t t e r y
Stopcock /
Defail -from Plate 2
Plate Z
P/ate
3
500 ml.
flask
P/ate 4
P/ate S
George Victor Beard was born in Coalville, Utah, on
August ±0, 1908.
His elementary nncl secondary education mere
I
obtained in the public schools of his birthplace.
In ±928 he obtained the A.B. degree from the "University
of Utah, where he majored in chemistry,
Iron 1928 to 1984 he
taught chemistry in various high schools in the states of Utah,
Idaho, and VJyoming.
However, during the school veer 1930-1931
be worked as a teaching assistant in the Department of Chemistry
at the University of Utah.
At this time he completed work for
the A.S. degree.
I'rom 1934-1936 he was an Instructor at the University
of Utah in the Depsi’tment of Elementary E due at ion.
In 1936-
1937 he was a teaching fellow at the California Institute of
Technology.
The following year he was Instructor of chemistry
at Snow College in Ephraim, Utah.
The next two years were spent
as a teaching assistant at lairdue University in the department
of Chemistry.
After completing, work for bis x'n.D. degree from iur.iue
University in 1940, he.entered the employ of tne University of
Utah as Instructor of physical chemistry.
Документ
Категория
Без категории
Просмотров
0
Размер файла
1 382 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа