Download Standard enthalpies of of calcium, magnesium

J Jndmn Jnsr Scr., Sept.-Oct. 1990, 70, 409-417
olndlan Institute of Science.
Standard enthalpies of
of calcium, magnesium
K. V. RAJAGOPALAN,
R. KALYANARAMAN
AND M. SUNDARESAN
Analytical Chemistry Diwsion, Bhahha Atomic Research Centre, Trombay, Bombay 400085.
Recelved on October 3, 1986: Rensed on February 26, 1988, June 5, 1989, and January 29, 1990.
Abstract
Standard enthalpies of formation of sulfosal~cylatesof calc~um,magnesium, zinc and aluminium have been
determined by rotatlng bomb oxygen calorimetry and the mean d~ssociatianenthalpies of the iespect~veM-0
bands have been computed using smtable empirical thennochemical cycles. The homolytic and heterolytic
contnbutions to the mean dlsruptlon enthalpies have been calculated and the eifect of the introduction of the
sulphonyl group into salicyl~ca c ~ don the dissociation energies has been ~nvestigated.
Key words: Rotatmg bomb calorimetry, combustion, sulfosalicylates,Ca, Mg, Zn and Al, enthalples of formation,
enthalpies of chelatmn in xaseous and condensed phases, M - 0 bond, mean-bond dissociation enthalples
1. Introduction
The techniques of static oxygen bomb calorimetry and reaction calorimetry have been used
for the determination of enthalpies of formation and chelation of several inner complexes.
This work was initiated by Jones' in USA, followed by Pilcherz in UK and o u r l a b ~ r a t o r y ~ , ~ .
Studies in these fields have established that the mean M - 0 bond dissociation energies for
the same metal with different ligands are more or less constant within the associated
uncertainties. da Silva and Reiss reported the standard molar enthalpies of formation of
bis(benzoylacetonato)beryllium(II)and tris(benzoy1acetonato)-aluminium(II1)and the mean
molar M - 0 bond dissociation enthalpies, da Silva et a16 have also established that the
introduction of a methyl or a phenyl group into a P-diketonato complex does not influence
the M - 0 bond dissociation energies and this may be regarded as applicable to compounds
containing these groups. da Silva has reviewed excellently the thermochemistry of
8-diketones, substituted a-diketonates and metal a-diketonates in a recent publication7.
Static bomb calorimetry has been found to be unsuitable for the combustion of
compounds containinghalogens, sulphur, boron, etc., for want of thermodynamicequilibrium
inside the bomb after combustion. Hence, a rotating bomb calorimeter was fabricated in
this laboratory following the design of Good et a18.
Salicylic acid, its derivatives and complexes formed by them find many applications in
410
K. V. RAJAGOPALAN er ni
i n d u ~ t r y ~ .medicine"
'~,
and analytical chemistry12.Among its derivatives, sulphosalicylic
acid forms well-defined inner complexes most suitable for the thermochemical studies of
compounds containing sulphonic acid group, though not directly bonded to the metal
atom. In the present work, the combustion enthalpies of the sulphosalicylatesof Al, Ca, Mg
and Zn have been determined by rotating bomb oxygen calorimetry and the bond
dissociation enthalpies of the M - 0 bonds computed.
2. Experimental
All the reagents used in this work were of BDH Analar quality unless otherwise specified.
The methods of preparation and the analysis of the compounds under study have been
reported by Rajagopalan et all3.
Enthalpies of combustion of the compounds were determined in a specially fabricated
rotating type of oxygen bomb calorimeter. The temperature rise of the calorimeter was
measured with the help of Stantel F-15-type thermistor as one arm of a Wheatstone bridge
circuit, the imbalance potential being recorded by means of 0-lOmV self-balancing
potentiometric-type recorder. Thermal calibrations were made by the electrical method.
The apparatus was calibrated and tested by measuring the enthalpies of combustion of
BCS benzoic acid and sulphur. The enthalpy of combustion of pure rhombic sulphur using
this calorimeter with paraffin oil as the ignition aid was found to be - 887.0 f0.4kJmoleC1
for the reaction
as against the reportedI4 literature value of
- 887.8 _f 0.3kJmoleC1
The compounds were made into pellets of lOmm diameter and 5-8mm thickness using
a hand press and fired by passing a high current through a fuse wire. Ten ml of distilled
water was taken in the bomb during the experiment. The bomb was filled with oxygen to
a pressure of 30 atmospheres. The nitrogen originally present in the bomb was not removed
in order to facilitate the complete conversion of sulphur to sulphuic acid as the oxides of
nitrogen formed during the combustion will act as a catalyst. The final bomb solution was
analysed for sulphuric and nitric acids; atomic absorption spectroscopy showed that metal
ions were absent. Chemical and X-ray analyses were made of the solid oxides as these were
produced at high temperatures and hence were insoluble in the bomb solution.
3. Results and discussion
Corrections were applied in each case for the formation of sulphuric and nitric acids in
the bomb after combustion. The end products were found to be the respective oxides in
all the cases.
Table I lists the thermochemical properties of the compounds, and column 2 in it the
experimentallydetermined heats of combustion after proper cone~tions'~.
It was estimated
STANDARD ENTHALPIES O F FORMATION O F SULFOSALICILATES
41 1
Table I
Thermmhrmical ralues of compounds (kJ m o l e ' ar 29SdK)
AH,
,,,
Compound
Sulphosal~cyl~c
acrd dihqdiate
Alamino suiphosalic~llc
aad, tetrahydrate
Cslaum sulphosalicylate.
dihydratc
Magnesium sulphosalicylate
tetrahydrate
Zinc suiphosallcyiate
tetrahydrate
Standard
dewation
-AH 'f
Standard
deviation
AH
-
,.,,
AHche,,s,
-
2201
3
1982
3
2963
4
2487
4
2465
4
2068
4
- 81
- 262
2571
2
2500
2
54
- 181
2507
2
2310
2
244
353
- 98
26
that Washburn corrections were much smaller than experimental errors and hence their
appreciation was deemed unnecessary. A general illustration of the calculation of the
enthalpies of formation is given below:
Combining equations 1,2 and 3, one can arrive at
b
MC,HbS,Od(C)+ eO,(g)+MO,(C)+ aCO,(g)+ H,0(1)2(1) + cS(C)
where e stands for the summation of the appropriate a, b, c,d and x values.
From this, AH,,,,,
AH,,,,.
can be calculated.
= AH:MO,(c)
b
+ aAHyCO,(g) +ZAH;H,O(l)
- AHYMC,HbS,Od(c).
Therefore,
AH;MC,H,S,O,(S) = AHjMO,(c) + aAH3CO,(g)
+~AH;H,o(I) - AH^^.^.
Thc values so obtained are listed in column 3 of Table I. Standard deviations for the values
rcportcd in this work were computed from the stmdard deviations of the terms used in
thermochei;~ical calculations. using the f ~ r r n u l z ' ~
where F is a function of Lhe type nF = o . ~f b j + cz with x, y and z, independent of one
another and aF,ox, U Y and uZ, being the standard deviations in F, X, Y and Z. The
standard deviations of the known quantities were taken as zero as 1s the normal practice.
The conventional practice of accepting twice the standard deviation of the mean as the
experimental uncertainty has normally been followed. Rut when more than one quantity
is involved as in the present case, the standard deviation is calculated aS per the above-cited
equation.
The empirical thermochemical cycle, proposed by Jones' and later extended by Athavale
ei 1113. was appiied in this case wilh some modifications lo calculate the enthalpies of chelation
of the complexes in the gaseous phase. A typical cycle for magnesium is given in Fig. 1.
The enthalpies of chelation in the condensed phase could he calculated from the above
values, again using a modilied form of thermochemical cycle which is shown in Fig. 2. In
this case, it has been assumed that the enthalpy of sublimation of the complex equals that
of the ligand and the vaporisation enthalpy of the appropriate number of water molecules.
The hydration energies are small compared to the other enthalpy values encountered in
the present work. And hence, the difference between the binding energies of the dihydrates
and tetrahydrates has been taken as negligible (Table I). The thermochemical values, e.g.,
A H ~ H 2 0 ( 1 )etc.,
,
used for these calculations have been taken from the literature. The
<D)(O-Hj and <D)(H-H) values used were 460.0 and 431.0kJmoleC1, respectively16.
Even though these values may vary from compound to c o r n p o ~ n d ~ .l '9 ~the values used
by us are the most average value for many of the organic compounds.
FK.1. Empinwl themochem~calcycle for enthdpm ofchelatlon I" gaseous phase for maeeslum sulphosal~ylate,
tetrahydrate.
S IANDARD ENTHALPIES O F FORMATLON OF SULFOSALlClLATES
413
FIG.2. Empncal Born-Haber cyclefor enthalp~esofchelation ID condensed phase for rnagneslum sulphosalicylate,
tetrahydrate.
For the calculation of the bond dissociation enthalpies the structures of the sulphosalicylates
were taken as
where M = Mg, Ca or Zn.
In the case of the alummium complex the structure was found to be as follows.
II
0
This is in accordance with the structure of aluminosalicylic acid. Considcring the two
M - 0 bonds in each case as equivalent, the mean molar-bond dissociation enthalpy in the
compound can be defined as half the molar enthalpy of the disruption reaction. In other
words, the heats of chelation in the gaseous phase of a particular complex can be taken
as the summation of various bond dissociation and formation enthalpies mvolved in the
gaseous metal complex. The enthalpies of vaporisation of the ligand and also those of the
complexes were assumed safely as 100 f 15 kJmoleC1. This is purely an estimate of
reasonable nature. AHc,,,, of salicylic acid has been reported in l i t e r a t ~ r e ~as~ - ~ ~
95.7 f 0.8 kJ mole-' and 94.8 0.4 kJ mole- Based on this we have assumed the above
cited value for AH,,,, of sulphosalicylic acid. This conclusion is based on the study of various
similar ligands and complexes undertaken in our laboratory and the uncertainty is not
expected to bc abovc 2 l5kJmole-'. Thus the values of the mean-bond dissociation
+
'.
K. V. RAJAGOPALAN er a1
414
enthalpies of the bonds Al--0, Mg-0, C a - 0 and Z n - 0 in the respective complexes can
be calculsted. These values together with the respective values reported for compounds
not containing sulphonic acid group are given in Table 11.
Hon;olylic and heterolytic bonds are related to each other in the following way73.
where C:=, I, is the ionisation potential summation of the metal atom and E, is the electron
affinity of the ligand.
The ionisation potential summation values for zinc, calcium, magnesium and aluminium
are readily available in the literature. The electron affinity parameters, E,, of the ligand
sulphosalicylic acid could not be obtained. However, as the bonding takes place through
oxygen, this value could be taken as that of oxygen, viz., 142kJmole-'. Any variation on
this value would be a constant factor affecting equally all the calculations.
Table I11 summarises the homolytic and the heterolytic contribution to the bond
enthalpies between the respectwe metals and oxygen in the sulphosalicylates.
Table 11
Mean dkocintion enthrlpies (kJ m o l e ' at 298-K)
Compound
Bond
Value
Standard
obtained deviation
Value
reported
Alumino sulphosalicylic acid.
tetrahydrate
Calnurn sulphosal~cvlate.
. .
dihydrate
Magnesium sulphosaiicylate,
tetrahydrate
Zinc sulphosalicylate
tctrahydrate
AI-0
277
10
27911, 266'
Ca-0
376
14
4W6
Mg-0
335
I2
292'
Zn-0
232
8
?212'
Tabk I1
Hundytk and heterdytic values (in W mole-' at 298%)
Merai
Z;., I'
Aluminxm 5138
Calcium
1735
Magoaium 2188
Zinc
2639
nE
AH(M:Lh.,
AH(M:L)
427
285
285
285
277
376
335
214
5562
2214
2587
2831
STANDARD ENTHALPIES O F FORMATION O F SULFOSALICILATES
415
The results in Tabie I1 indicate that the introduction of a highly polar group, uiz.,
sclphonyi. does not make any novable impact on the mean-molar dissociation enthalpies
for the M-0 bond where M = Ca, Zn, Mg or Al. The constancy of the bond-dissociation
enthalpies indicates that for a certain metal, this value is not affected by the structure of
thc ligand.
It can be expected that the metal-oxygen bonds in the inner complexes are more or less
similar to the corresponding bonds of the crystalline metal oxides7 on considering the
process
60C
560
480
7- 400
E"
- 320
7
5
X
a
5
0
240
0
2
,^
I60
V
80
0
80
160
240
320
400
(D) (M. 0 . oxide, c)/kJ mol-'
FIG. 3. Variation of <D) (M 0. complex, c) with <D) (M.O. oxide, c).
480
560 600
K V RAJAGOPALAN et ni
416
and defining a parameter
(D>(M-0, oxide, C) =
A%,,%
__
coordination number of metal
A plot of ( D ) ( M -0, complex, g) uersus D (M-0, oxide, C) should be a straight line.
Figure 3 shows the correlation between these quantities. It could be assumed from the
linearity that the metal is surrounded by the oxygen atoms of two ligands and so, in other
words, gets totally oxidised. This is similar to the degree of oxidation obtained to the
corresponding metal oxide. Because of the valency difference between aluminium and the
other metal (aluminium being trivalent), the relation was not applied to aluminium.
In a very recent work2', Airoldi and Santos have determined the mean-bond dissociation
cnthalpy of the Ca-0 bond in hexa bis (urea) calcium bromide as 295 i9kJmole-I.
However, the calculation of this quantity for ionic complexes requires a knowledge of the
boud length which is not available at present. Consequently, thern~ochemicalquantities
involving this class of complexes are very limited. In view of the ionic nature of the above
compound, the differencc between their value and our value is as anticipated.
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