LUKYANOVA et al.
The energy equivalent W of the calorimeter was
The contents of CO2 in the combustion products
of the sample I proved to be close to 100.00%, (run 1)
in perfect accordance with the above mentioned result
of chemical analysis for C, so that mass m of the burnt
substance in runs 1, 2 is just the result of weighing the
tablet.
In runs with samples II and III deficit of CO2 was
observed. Assuming that traces of moisture (or other
incombustible admixture) were present in these
samples, the mass of the burnt substance was
calculated from the results of the CO2 analysis.
The influence of admixture of KPiv on the
energy of combustion the sample II was estimated on
the assumption that DfH0 (KPiv) is equal to DfH0
(AgPiv). Then the combustion energy of KPiv by
determined by combustion of the thermochemical
standard benzoic acid and was equal to
58254.9±10.6 J W–1 for the calorimeter with the
empty bomb. The massic combustion energy of benz-
oic acid was –26432.5±1.9 J g–1 under certificate
conditions.
For the combustion experiment a weighed amount
of AgPiv was pressed into pellet on hydraulic press,
sealed in a Terylene-film bag and placed into a quartz
crucible above the pellet of benzoic acid. Benzoic acid
served as an auxiliary material. Its standard massic
energy of combustion Dcu0= –26412.0±1.9 J g–1 was
derived from that at the certificate conditions. The
value of Dcu0 of Terylene film –22927.9±6.3 J g–1 and
the mass of CO2, formed from 1 g of the film
2.2897±0.0006 g were determined in [5]. The un-
certainty intervals of presented values as well as
everywhere in this paper are given as ±t×s, where s is
the standard deviation of the mean and t is the
Student’s coefficient for the 0.05 significance level.
1 cm3 of the water was introduced into the bomb.
The initial pressure of oxygen was 3.5 MPa.
The sample was ignited with a platinum wire heated by
the discharge of a capacitor. The energy of ignition
was constant in all experiments both with the inves-
tigated substance and in the calibration runs. Under
these conditions the substance was burnt down to CO2,
H2O and metallic silver. It was shown by laser mass-
spectrometry, that 99.99% of solid product of com-
bustion represented pure silver. Most part of silver was
in the quartz crucible as an ingot (about 98%), and
some part – as thin powder on the walls and bottom of
the bomb. In some experiments traces of soot were
observed in crucible, and the corresponding energy
correction (0.3 to 1.7 J) was introduced. The initial
temperature did not differ from 298.15 K by more than
0.02 K, the temperature rise was about 1.2 K. The
energy of combustion of auxiliary materials (Terylene
film and benzoic acid) was about 80% of the total
amount of the energy evolved.
equation:
K(C5H9O2)(cr)+6.5O2(g)+aq=
KHCO3(sol-n)+4CO2(g)+4H2O(l) is equal to
–22715 J g–1, and the correction amounts to 11 J g–1.
This correction is rather small even for the maximal
admixture, hence there is no need to take into account
all other minor admixtures
The q(HNO3) correction was calculated as the
energy of formation of nitric acid from N2(g), O2(g)
and H2O(l).
The q(st) correction was calculated according to
Hubbard et al. [7]. The enthalpies of dilution of
HNO3, vaporization of H2O and solution of O2 and
CO2 in water were taken from the handbook [8].
The massic energy of combustion was calculated
in each run by the following equation:
Dcuo = –(W'DR – q(b.a.) – q(f) – q(HNO3) +
+ q(s) – q(st))/m-corr.(KPiv)
The value Dcuo= –13320±27 J g–1 is obtained as
the mean of 7 runs with all three samples. The molar
energy of combustion according to the reaction:
Ag(C5H9O2)(cr)+6.25O2(g)=5CO2(g)+4.5H2O(l)+Ag
(cr) was found to be DcUo(AgPiv, cr)=
–2783.8±5.6 kJ mol–1. Based on this value, the
standard enthalpies of combustion and formation of
silver pivalate were derived: DcH0(AgPiv, cr)=
–2786.9±5.6 kJ mol–1 and DfH0(AgPiv, cr)=
–466.9±5.6 kJ mol–1; the standard enthalpies of
formation of CO2(g), H2O(l) were taken from [9].
The obtained value of DfH0(AgPiv, cr) was used
to calculate the enthalpy of breaking bond Ag-Piv by
After the calorimetric experiments the gaseous
products of combustion were analyzed for CO2 and
CO. The contents of CO2 were determined after
Rossini method [6] with the accuracy ±4×10–4 g.
Qualitative tests for CO(g) with indicator tubes were
negative within the limits of their sensitivity 6×10–6 g.
The amount of HNO3, formed from N2, O2 and H2O,
was determined by titrating the bomb solution with a
0.1 N solution of NaOH.
the
reaction:
Ag(CH3)3CC(O)O(cr)=Ag(cr)+
(CH3)3CC(O)O×(g), (DrH). The value DfH0 of pivalic
acid radical ((CH3)3CC(O)O×), necessary for this
calculation, is not known. It was estimated on the
assumption that the difference between the DfH0
values of pivalic acid and its radical is the same as for
n-butanoic acid. The enthalpies of formation of
n-butanoic and pivalic acids –475.8±4.1 and
–491.3±6.6 kJ mol–1 correspondingly were taken
Results and discussion
The results of seven combustion experiments are
reported in Table 1.
744
J. Therm. Anal. Cal., 92, 2008
Lukyanova
