Synthesis, characterization, and thermodynamic study of ammonium benzoate C7H5O2NH4(s)

Article abstract of DOI:10.1016/j.tca.2010.01.021

Benzoic acid and concentrated ammonia were chosen as reactants and a novel compound (ammonium benzoate) was synthesized using the method of liquid phase reaction. FTIR, elemental analysis and X-ray powder diffraction technique were applied to characterize the structure and composition of the compound. Low-temperature heat capacities of the solid compound were measured by a precision automated adiabatic calorimeter over the temperature range from 80 to 399 K. A polynomial equation of the heat capacities was fitted by the least-squares method. In accordance with Hess' law, a thermochemical cycle was designed, the enthalpy change of the reaction of benzoic acid with ammonium acetate was determined as Δ_r H_m ° = (15.59 ± 0.50) kJ mo l^{- 1}, and the standard molar enthalpy of formation of the compound was calculated as Δ_f H_m ° [ C₇ H₅ O₂ N H₄,s ] = - (472.65 ± 0.62) kJ mo l^{- 1} by an isoperibol solution-reaction calorimeter.

Full text of DOI:10.1016/j.tca.2010.01.021

Thermochimica Acta 502 (2010) 14–19
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Synthesis, characterization, and thermodynamic study of ammonium
benzoate C7H5O2NH4(s)
Wei-Wei Yanga, You-Ying Dia,∗, Yu-Xia Konga, Xiao-Yang Guoa, Zhi-Cheng Tanb
a College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, Shandong Province, PR China
b Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2009
Received in revised form 14 January 2010
Accepted 15 January 2010
Available online 25 January 2010
Benzoic acid and concentrated ammonia were chosen as reactants and a novel compound (ammonium
benzoate) was synthesized using the method of liquid phase reaction. FTIR, elemental analysis and X-ray
powder diffraction technique were applied to characterize the structure and composition of the com-
pound. Low-temperature heat capacities of the solid compound were measured by a precision automated
adiabatic calorimeter over the temperature range from 80 to 399 K. A polynomial equation of the heat
capacities was fitted by the least-squares method. In accordance with Hess’ law, a thermochemical cycle
was designed, the enthalpy change of the reaction of benzoic acid with ammonium acetate was deter-
Keywords:
C7H5O2NH4(s)
Adiabatic calorimeter
Low-temperature heat capacity
Isoperibol solution–reaction calorimeter
Standard molar enthalpy of formation
mined as ꢀrH◦ = (15.59 0.50) kJ mol−1, and the standard molar enthalpy of formation of the compound
m
was calculated as ꢀfH◦ [C7H5O2NH4,s] = −(472.65 0.62) kJ mol−1 by an isoperibol solution–reaction
m
calorimeter.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
relevant theoretical studies and application development of the
title compound. In order to calculate enthalpy changes, equilib-
Benzoic acid has a prominently biological effect in the growth
and development of humans and animals. It is an important pre-
cursor for the synthesis of many other organic substances [1]. It
has an antiseptic action, inhibits the growth of mold, yeast and
some bacteria [2], and is often applied for ulcers, wounds, blis-
ters, and chapped hands in the form of the tincture. In addition,
it can be used for the treatment of chronic bronchitis and phthisis.
Ammonium benzoate has a similar action to benzoic acid as shown
above. It can be used as a significant raw material to produce the
electrolytic capacitors in electronic industry. It is an excellent anti-
corrosion treatment agent for metal and rubber. In medicine, it can
be served as stimulants, diuretics, antirheumatic drugs, etc.
Miller et al. [3] have synthesized ammonium benzoate by the
reaction of solid benzoic acid, either as powders or as single crystals,
with ammonia gas at constant pressure to give 1:1 microcrystalline
ammonium salt, and characterized the composition and structure
of the title compound by microanalysis, infrared spectra, nuclear
magnetic resonance spectra, X-ray powder camera, and elemental
analysis. They investigated the reaction rate and possible reaction
mechanism of solid benzoic acid with ammonia gas. However, up
to now, thermodynamic properties of ammonium benzoate have
not been found in the literature, which restricted the progress of
rium constants, and theoretical yields of reactions in which the
substance is involved, the thermodynamic properties for the sub-
stance are urgently needed, and closely related to other physical,
biological, physiological and chemical properties. The aim of the
present work is to synthesize the compound C7H5O2NH4(s) by the
method of liquid phase reaction, to measure low-temperature heat
capacities of the compound by adiabatic calorimetry, and to deter-
mine the dissolution enthalpies of the reactants and products of
the reaction of ammonium acetate with benzoic acid by isoperi-
bol solution–reaction calorimetry. Finally, some thermodynamic
parameters such as the enthalpy change of the reaction and the
standard molar enthalpy of formation of the product C7H5O2NH4(s)
were derived from these experimental results.
2. Experimental
2.1. Synthesis and characterization of the compound
C7H5O2NH4(s)
Benzoic acid reacts with concentrated ammonia at a molar ratio
of n (benzoic acid):n (concentrated ammonia) = 1:1. Concentrated
ammonia is slightly excessive for the purpose of complete reaction
of benzoic acid. A certain amount of benzoic acid was firstly put
into a 250 mL beaker. Then excess concentrated ammonia and
appropriate amount of distilled water were successively added
into the beaker with stirring to obtain a clear solution. The above
solution was heated and condensed on the electric furnace until
∗
Corresponding author. Fax: +86 635 8239121.
E-mail addresses: diyouying@126.com, yydi@lcu.edu.cn (Y.-Y. Di).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.01.021

W.-W. Yang et al. / Thermochimica Acta 502 (2010) 14–19
15
Table 1
Characteristic vibration absorptions of main groups obtained from FTIR spectra of
benzoic acid and ammonium benzoate (cm−1).a.
Compound
C6H5COOH
ꢁC
ꢁ−OH ꢁC
ıC–H
ꢁN–H
O
C
1689
3070 1603, 1585, 1464 812, 805, 708, 685
–
C6H5COONH4 1595
–
1554, 1449, 1398 717, 707, 692
3128
a
ꢁ, stretching vibration; ı, out-of-plane bending vibration.
a crystal membrane appeared. The final solution was cooled
naturally to room temperature and filtered; the crude product
was washed with anhydrous ethanol (A.R.) three times. The white
solid product was recrystallized using anhydrous ethanol, and
white crystals were obtained. Finally, the sample was placed in
a vacuum desiccator at 60 ◦C to vacuum dry for 6 h. Theoretical
contents of C, H, and N in the compound were calculated to be
60.42%, 6.52%, and 10.07%, respectively. Element analysis (Model:
PE-2400, PerkinElmer, USA) has shown that the practical contents
of C, H, and N in the compound were measured to be 60.35%,
6.50%, and 10.03%, respectively. This showed that the purity of the
sample prepared was higher than 99.5%.
Fig. 1. XRD spectra of benzoic acid and ammonium benzoate.
FTIR (Nicolet 5700 FT-IR, USA, KBr) was used to determine the
bond mode of ammonium cation with benzoic acid, the range
of the wavelength was 400–4000 cm−1. Vibration characteristic
absorptions of main groups obtained from the FTIR spectra of the
compound and benzoic acid are listed in Table 1.
2.2. Adiabatic calorimetry
A precision automatic adiabatic calorimeter was used to mea-
sure heat capacities of the compound over the temperature range
78 ≤ (T/K) ≤ 400. The calorimeter was established in the Thermo-
chemistry Laboratory of the College of Chemistry and Chemical
Engineering, Liaocheng University, China. The principle and struc-
ture of the adiabatic calorimeter have been described in detail
elsewhere [4,5]. Briefly, the calorimeter comprised mainly a sam-
ple cell; a platinum resistance thermometer; an electric heater;
inner, middle and outer adiabatic shields; three sets of six-
junction chromel-constantan thermopiles were installed between
the calorimetric cell and the inner shield, between the inner and
middle shields, and between the middle and outer shields; and a
high vacuum can. The miniature platinum resistance thermome-
ter (IPRT No. 2, produced by the Shanghai Institute of Industrial
Automatic Meters, 16 mm in length, 1.6 mm in diameter and a
nominal resistance of 100 ꢄ) was used to measure the tempera-
ture of the sample. The thermometer was calibrated on the basis of
ITS-90 by the Station of Low-Temperature Metrology and Measure-
ments, Academia Sinica. The electrical energy introduced into the
sample cell and the equilibrium temperature of the cell after the
energy input were automatically recorded by use of a Data Acqui-
sition/Switch Unit (Model 34970A, Agilent, USA), and processed on
line by a computer.
To verify the accuracy of the calorimeter, the heat capacities
of a reference standard material (␣-Al2O3) were measured over
the temperature range 78 ≤ (T, K) ≤ 400. The sample mass was
1.71431 g, which was equivalent to 0.0168 mol based on its molar
mass, M(Al2O3) = 101.9613 g mol−1. Deviations of the experimental
results from those of the smoothed curve lie within 0.2%, while the
uncertainty is 0.3%, as compared with the values given by the for-
mer National Bureau of Standards [6] over the whole temperature
range.
Heat-capacity measurements were continuously and automati-
cally carried out by means of the standard method of intermittently
heating the sample and alternately measuring the temperature. The
heating rate and temperature increments were generally controlled
at 0.1–0.4 K min−1 and 1–3 K. The heating duration was 10 min,
and the temperature drift rates of the sample cell measured in an
equilibrium period were always kept within 10−3 to 10−4 K min−1
during the acquisition of all heat-capacity data. The data for heat
capacities and corresponding equilibrium temperatures have been
corrected for heat exchange of the sample cell with its surround-
It can be seen from Table 1 that the organic component of
the compound possesses distinctly different characteristic absorp-
tion peaks relative to that of benzoic acid. The strong absorption
peak of the O–H stretching vibration (ꢂO–H) which appeared at
3070 cm−1 in benzoic acid has disappeared in ammonium ben-
zoate. It showed that the oxygen atom of the organic component
in the title compound was directly linked with nitrogen atom of
NH4+. The absorption peak of the C O stretching vibration, ꢂC
,
,
shifted to the low wave number in the title compound, 1595 cm−O1
as a result of the formation of the delocalization ␲ bond includ-
ing the benzene ring and carboxylate (–COO−), which lowered
the density of the electron cloud around –COO−. The absorption
peak of the N–H stretching vibration in the compound appeared at
ꢂN–H = 3128 cm−1. In addition, different shifts of other character-
istic absorption peaks occurred, which were also ascribed to the
change of the surrounding near the C–O bond after the formation
of the compound.
The X-ray powder diffraction (XRD) technique was used to
determine whether the new synthesized compound is novel. XRD
spectra of benzoic acid and ammonium benzoate have been plot-
ted in Fig. 1. Step length of powder diffraction angle was 0.01 rad,
wavelength was 0.154056 nm (Cu K␣ 1 radiation), electric voltage
was 36 kV, and electric current was 20 mA. The scanning rate was
4 rad min−1 and a graphite monochromator was used for the filter-
ing. It was found from Fig. 1 by comparison of the two charts that
two obvious diffraction peaks in the angle range of 2ꢃ = 16–18 rad
and seven other weak characteristic diffraction peaks in the angle
range of 2ꢃ = 24–35 rad were seen in the diffractogram of benzoic
acid. One distinct new diffraction peak appeared near 2ꢃ = 12 rad,
three other weak characteristic diffraction peaks in the angle range
of 2ꢃ = 18, 29, and 36 rad were seen in the diffraction pattern of
ammonium benzoate. Because any kind of substance has its own
unique X-ray diffraction pattern, the X-ray diffraction pattern of the
mixture is just a simple superposition of characteristic diffraction
peaks of its components. In other words, identification of different
materials can be made from X-ray diffraction patterns. Therefore,
novel characteristic diffraction peaks completely different from
those of benzoic acid appeared in the diffraction pattern of ammo-
nium benzoate, which showed that a new substance was produced
by the liquid phase reaction of benzoic acid with concentrated
ammonia.

16
W.-W. Yang et al. / Thermochimica Acta 502 (2010) 14–19
Table 2
Experimental heat capacities of ammonium benzoate.
T (K)
80.24
Cp,m (J mol−1 K−1
)
T (K) Cp,m (J mol−1 K−1
)
T (K) Cp,m (J mol−1 K−1
)
65.17
66.20
67.46
68.89
69.94
71.51
72.72
74.19
75.80
77.01
78.04
79.33
80.86
81.80
83.16
84.27
85.30
86.37
87.62
88.74
90.01
91.47
92.90
94.89
96.99
98.85
101.0
102.8
104.6
106.2
108.2
110.5
112.3
113.9
116.1
118.2
120.0
122.0
170.7 124.1
173.7 125.5
176.7 127.1
179.7 129.4
183.8 131.9
187.8 134.2
190.8 136.4
193.8 138.6
196.7 140.4
199.7 142.2
202.7 144.2
205.7 146.1
208.7 148.1
211.6 149.9
214.6 152.0
217.6 154.0
220.6 155.6
223.6 157.5
226.6 159.3
229.6 161.3
232.5 163.1
235.5 164.9
238.7 166.7
242.5 169.3
246.2 170.9
249.2 172.6
252.1 174.3
255.0 176.3
258.0 177.8
261.1 179.4
264.1 180.9
267.1 182.9
270.1 184.6
273.1 186.5
276.1 188.1
279.0 189.4
282.0 191.0
285.0 192.6
287.9 194.1
291.0 195.7
294.0 197.3
297.0 199.6
300.0 200.9
302.9 203.1
305.8 204.6
308.8 205.9
311.7 207.7
314.7 209.4
317.7 210.9
320.8 212.6
323.7 214.0
326.7 215.5
329.8 217.0
332.8 218.8
335.8 220.3
338.7 222.1
341.7 223.7
344.6 225.4
347.6 226.8
350.5 228.4
353.5 229.7
356.6 231.2
359.5 232.7
362.6 234.1
365.6 235.6
368.6 237.6
371.5 239.1
374.5 240.9
377.5 242.4
380.5 244.0
383.4 245.7
386.3 247.6
389.3 249.4
392.4 251.2
395.3 252.7
398.3 254.1
81.77
84.03
86.20
88.38
90.56
92.57
94.64
96.64
98.62
100.6
102.4
104.3
106.2
108.0
109.8
111.6
113.4
115.2
116.8
118.6
120.3
123.0
126.3
129.2
132.2
135.1
138.0
141.0
144.0
147.0
149.9
152.9
155.9
158.9
161.8
164.8
167.7
Fig. 2. The curve of experimental molar heat capacities of ammonium benzoate
C7H5O2NH4(s). “o” represents the values of the literature and “ꢀ” represents the
values of our experiment.
experimental molar heat capacities (Cp,m) vs. reduced temper-
ature (X), X = f(T) = [T − (T1 − T2)/2]/[(T1 − T2)/2] (where T1 = 399 K
and T2 = 80 K), has been obtained:
Cp,m(J mol−1 K−1) = 166.852 + 94.760X − 11.979X2 − 0.067X3
+ 4.942X4
(1)
in which X = (T − 239.5)/159.5. This equation is valid between
T = 80 K and T = 399 K.
The standard deviations of experimental molar heat capacities
from the smoothed heat capacities calculated by the polynomial
equation were within 0.3% except for several points around the
lower temperature limit. The coefficient of determination for the
fitting, R2, was equal to 0.99998.
In addition, the heat capacity of ammonium benzoate was mea-
sured between 93 K and 293 K in 1932 by Crenshaw and Ritter
[9]. The comparison of the two results has been given in Fig. 2,
it can be seen from Fig. 2 that the heat-capacity values of the lit-
erature is slightly higher than ours. This is caused probably due to
the different structure and adiabatic performance of our adiabatic
calorimeter from theirs. In the reference, the calorimetric cell is sur-
rounded with only one adiabatic shield. However, in our device, the
calorimetric cell is surrounded by inner, middle and outer adiabatic
shields and the electric heating wires were coiled on the outer wall
of cell. Three sets of six-junction chromel–copel thermocouple piles
were installed between the calorimetric cell and the inner shield,
between the inner and middle shields, and between the middle and
outer shields to detect the temperature differences between them.
The same electrical energy is introduced into the sample cells of
the two devices, one which has poor adiabatic effect will lose more
heat to its surroundings by means of the heat radiation, the cor-
rected temperature difference will be smaller, and will lead to the
larger heat-capacity value. By comparison with the results in the
reference, our adiabatic calorimeter is more stable than theirs in
the measurements of the heat capacities, as shown from two curves
of Fig. 2.
ings [4]. The sample mass used for calorimetric measurements was
1.79816 g, which was equivalent to 0.01292 mol in terms of its
molar mass, M(C7H5O2NH4) = 139.1519 g mol−1
.
2.3. Isoperibol solution–reaction calorimetry
The isoperibol solution–reaction calorimeter consisted primar-
ily of a precision temperature controlling system, an electric energy
calibration system, a calorimetric body, an electric stirring system,
a thermostatic bath made from transparent silicate glass, a preci-
sion temperature measuring system and a data acquisition system.
The principle and structure of the calorimeter have been described
in detail elsewhere [7].
The reliability of the calorimeter was verified previously [7]
by measuring dissolution enthalpy of KCl (calorimetrical primary
standard) in double distilled water at T = 298.15 K. The mean dis-
solution enthalpy was 17,547 13 J mol−1 for KCl, which compares
with corresponding published data, 17,536 3.4 J mol−1 [8].
3. Results and discussion
3.1. Low-temperature heat capacities
3.2. The determination of enthalpy change for the solid-state
reaction of ammonium acetate with benzoic acid
All experimental results, listed in Table 2 and plotted in Fig. 2,
showed that the structure of the compound was stable over
the temperature range between T = 80 K and 399 K, namely, no
phase change, association nor thermal decomposition occurred.
The 114 experimental points in the temperature region were
fitted by means of least-squares, and a polynomial equation of
The reaction of ammonium acetate with benzoic acid is shown
as follows:
C7H6O2(s) + CH3COONH4(s) = C7H5O2NH4(s) + CH3COOH(l)
(2)

W.-W. Yang et al. / Thermochimica Acta 502 (2010) 14–19
17
Scheme 1.
The enthalpy change of reaction (2) can be determined by
respective measuring enthalpies of dissolution of benzoic acid and
acetic acid in 1 mol dm−3 NaOH, ammonium acetate in 1 mol dm−3
NaOH solution containing certain amounts of benzoic acid, and
C7H5O2NH4(s) in 1 mol dm−3 NaOH solution containing certain
amounts of acetic acid at 298.15 K.
Fig. 3. UV/vis spectra of solutions A and B obtained from the dissolution of the
[C7H6O2(s) and CH3COONH4(s)] mixture and the [C7H5O2NH4(s) and CH3COOH(l)]
mixture in the supposed reaction (2) in 100 mL of 1 mol dm−3 NaOH (diluted to 1:20).
The solid ammonium acetate and benzoic acid were respectively
ground within an agate mortar into a fine powder.
About 1 × 10−3 mol of benzoic acid was dissolved in 100 mL
of 1 mol dm−3 NaOH at 298.15 K. If “s” = calorimetric solvent,
1 mol dm−3 NaOH, then,
{C7H6O2(s)} + “sꢀꢀ = solution Aꢀ
Scheme 2.
The results obtained from five dissolution experiments are listed
in Table 3.
The stoichiometric number of CH3COONH4(s) in reaction (2)
or [n(CH3COONH4)/n(benzoic acid)] = 1:1] was regarded as a norm
for sample weighing, about 1 × 10−3 mol of {CH3COONH4(s)} was
dissolved in solution Aꢀ, i.e.,
agree with those of solution B, no difference in the structure and
chemical composition existed between the two solutions. These
results have demonstrated that the solutions A and B were same,
namely, ꢀH = 0, the designed Hess thermochemical cycle was rea-
sonable and reliable, and can be used to derive the standard molar
enthalpy of formation of the compound C7H5O2NH4(s). Then, solu-
tion A or solution B was defined as final solution. Therefore, the
above thermochemical cycle also can be expressed as (Scheme 2):
According to Hess’s law, the enthalpy change of the reaction can
be calculated as,
{CH3COONH4(s)} + solution Aꢀ = solution A
The results obtained from five dissolution experiments are listed
in Table 3.
About 1 × 10−3 mol of CH3COOH(l) was dissolved in 1 mol dm−3
NaOH at 298.15 K. Because of the volatilization of acetic acid, when
the liquid was weighed, the temperature in room must be con-
trolled at 17–18 ◦C to reduce the rate of volatilization, and the
sample cell in which acetic acid was put should be covered with
plug made of polytetrafluoroethylene, then,
ꢀH1 + ꢀH2 + ꢀH = ꢀrHm◦ + (ꢀH3 + ꢀH4)
ꢀrHm◦ = (ꢀH1 + ꢀH2 + ꢀH) − (ꢀH3 + ꢀH4)
Then,
{CH3COOH(l)} + “sꢀꢀ = solution Bꢀ
ꢀ
ꢀ
ꢀrHm◦
=
ꢀsolH◦
−
ꢀsolH◦
m(Products)
m(Reactants)
The results obtained from five dissolution experiments are listed
in Table 3.
Therefore, the enthalpy change of the reaction (2) can be derived
The dissolution enthalpy of C7H5O2NH4(s) in solution Bꢀ was
measured under the same condition as the above,
as,
ꢀ
ꢀ
ꢀrHm◦
=
ꢀsolH◦
−
ꢀsolH◦
= ꢀH1 + ꢀ
m(Reactants)
m(Products)
{C7H5O2NH4(s)} + solution Bꢀ = solution B
H2 − ꢀH3 − ꢀH4 = [−26.496 + (−7.362)] − 4.536
The results obtained from five dissolution experiments are listed
in Table 3.
− (−53.979)] = (15.59 0.50) kJ mol−1
.
The enthalpy change of reaction (2) can be calculated in accor-
dance with a thermochemical cycle and the experimental results.
The designed thermochemical cycle was shown in Scheme 1.
The results of UV/vis spectroscopy and refrangibility (refractive
index) are both of important information used to detect whether
the differences of the structure and composition between two kinds
of solutions existed. In this paper, all of the reactants and products
of reaction (2) can be easily dissolved in the selected solvent. The
measured values of the refractive indices of solution A and solu-
tion B were (1.3441 0.0004) and (1.3439 0.0003), respectively.
The results of UV/vis spectroscopy are shown in Fig. 3. UV/vis spec-
tra and the data of the refractive indices of solution A obtained
3.3. The standard molar enthalpy of formation of the compound
C7H5O2NH4(s)
A reaction scheme used to derive the standard molar enthalpy
of formation of C7H5O2NH4(s) is given in Table 4. The experimental
values of the dissolution enthalpies of the reactants and products
in reactions (2) were combined with some auxiliary thermody-
namic data of, ꢀfH◦ [C6H5COOH,s] = −(384.8 0.50) kJ mol−1
m
[10],
ꢀfH◦ [CH3COONH4,s] = −586.95 kJ mol−1
[11],
and
m
ꢀfH◦ [CH3COOH,l] = −(483.52 0.36) kJ mol−1 [12], to derive
m

18
W.-W. Yang et al. / Thermochimica Acta 502 (2010) 14–19
Table 3
Dissolution enthalpies of reactants and products of the reaction (2) in the selected solvents at 298.15 K.a.
System
Solvent
No.
m (g)
ꢀEs/ꢀEe
te (s)
37.781
55.373
55.313
45.281
30.360
Qs (J)
ꢀsolHm◦ (kJ mol−1
)
1
2
3
4
5
0.12233
0.12200
0.12243
0.12213
0.12270
1.4429
0.9774
0.9977
1.1905
1.8250
−26.485
−26.293
−26.811
−26.190
−26.918
−26.439
−26.319
−26.743
−26.188
−26.791
1 mol dm−3 NaOH
Benzoic acid
Avg. ꢀsolHm◦ ,1 = −(26.50 0.12) kJ mol−1
1
2
3
4
5
0.07151
0.07446
0.07788
0.07589
0.07326
0.5707
0.7040
0.8853
0.8955
0.9745
25.999
19.969
17.015
15.999
15.375
−7.208
−6.823
−7.318
−6.961
−7.279
−7.769
−7.071
−7.243
−7.070
−7.659
Ammonium acetate
Solution A
Avg. ꢀsolHm◦ ,2 = −(7.36 0.15) kJ mol−1
1
2
3
4
5
0.13936
0.13974
0.13956
0.13977
0.13974
−1.2424
−1.2375
−1.2820
−1.4402
−1.4300
7.421
7.109
7.206
6.653
6.997
4.479
4.274
4.488
4.655
4.861
4.473
4.256
4.475
4.634
4.841
Ammonium benzoate
Solution B
Avg. ꢀsolH◦ = (4.54 0.10) kJ mol−1
m,3
1
2
3
4
5
0.06066
0.06045
0.06073
0.06088
0.06058
0.8939
0.9320
0.9069
0.9109
0.8902
123.985
123.672
123.718
123.749
123.578
−53.895
−56.013
−54.525
−54.779
−53.460
−53.317
−55.642
−53.914
−54.032
−52.992
1 mol dm−3 NaOH
Acetic acid
Avg. ꢀsolH◦ = −(53.98 0.46) kJ mol−1
m,4
a
m: mass of sample; te: heating period of electrical calibration; Qs = −(ꢀEs/ꢀEe) × I2Rt: heat effect during the sample dissolution; ꢀEs: the voltage change during the
sample dissolution; ꢀEe: the voltage change during the electrical calibration; ꢀsHm◦ = Qs/n = −(ꢀEs/ꢀEe) × I2Rt(M/m), where R is the electro-resistance (R = 1213.09 ꢄ at
T = 298.15 K), I is the current (I = 20.015 mA), and M is the molar mass of the sample.
Table 4
Reaction scheme used to determine the standard molar formation enthalpy of ammonium benzoate.
No.
Reactions
ꢀfHm◦ or (ꢀsolHm◦ ꢅa)a (kJ mol−1
)
1
2
3
4
5
6
7
8
{C7H6O2(s)} + “s” = solution Aꢀ
−(26.50 0.12), ꢀH1
−(7.36 0.15), ꢀH2
(4.54 0.10), ꢀH3
{CH3COONH4(s)} + solution Aꢀ = solution A
{C7H5O2NH4(s)} + solution Bꢀ = solution B
{CH3COOH(l)} + “s” = solution Bꢀ
−(53.98 0.46), ꢀH4
−(384.8 0.50), ꢀH5
−586.95, ꢀH6
7C(s) + 3H2(g) + O2(g) = C7H6O2(s)
2C(s) + (7/2)H2(g) + O2(g) + (1/2)N2(g) = CH3COONH4(s)
2C(s) + 2H2(g) + O2(g) = CH3COOH(l)
−(483.52 0.36), ꢀH7
−(472.65 0.62), ꢀH8
7C(s) + (9/2)H2(g) + O2(g) + (1/2)N2(g) = C7H5O2NH4(s)
ꢁ
ꢂ
5
a
ꢅa
=
(xi − x¯)2/n(n − 1), in which n is the experimental number; xi, a single value in a set of dissolution measurements; x¯, the
i=1
mean value of a set of measurement results.
the standard molar enthalpy of formation of C7H5O2NH4(s),
ꢀfHm◦ (C7H5O2NH4, s) = ꢀH8 = ꢀrHm◦ + ꢀfHm◦ (C7H6O2, s)
+ꢀfHm◦ (CH3COONH4, s)
and the dissolution enthalpies of the reactants and products of
the designed solid-state reaction of ammonium acetate with ben-
zoic acid by isoperibol solution–reaction calorimetry. Additionally,
the thermodynamic functions and standard molar enthalpy of for-
mation of the product C7H5O2NH4(s) were derived from these
experimental results.
−ꢀfHm◦ (CH3COOH, l) = ꢀH1 + ꢀH2
−ꢀH3 − ꢀH4 + ꢀH5 + ꢀH6 − ꢀH7
Acknowledgments
= [−26.496 + (−4.362)] − 4.536
This work was financially supported by the National Natural Sci-
ence Foundations of China under the contract NSFC No. 20673050
and 20973089.
− (−53.979) + (−384.8) + (−586.95)
− (−483.52) = −(472.65 0.62)kJ mol−1
in which ꢀH1–ꢀH8 are the enthalpy changes of the reactions cor-
responding to number of the reactions in Table 4.
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