Kinetic Study of the Reaction of Atomic Hydrogen with Vinylidene Chloride
Abstract
The kinetics of the reaction H (1s1) + C2H2Cl2 have been measured from 1 torr to 10 torr by the fast
flow discharge system. Hydrogen atoms were generated by the discharge of hydrogen molecules.
Experiments at room temperature yielded k = 1.7x 10-12 cm3 molecule-1 sec-1 with an uncertainty of
0.7x 10-12 and kw = -2.4 sec-1.
Introduction
Vinylidene chloride (C2H2Cl2) is a halogenated volatile, clear, colorless liquid with a sweet odor. The
structure of vinylidene chloride is shown as following:
Cl
H
\
/
C = C
/
Cl
\
H
Vinylidene chloride is used for the formation of modacrylic fibres and copolymers. Up to
approximately 5% of manufactured vinylidene chloride is emitted into the atmosphere annually.1 It is
also used for the packing of foods, as metal coatings in storage tanks, building structures, and tapes,
and as molded filters, valves, and pipefitting. Vinylidene chloride is an important monomer in the
manufacture of methyl chloroform, saran, and other plastics. Emissions of vinylidene chloride
adversely affect humans. Exposure to vinylidene chloride causes central nervous system depression,
skin and eye irritation, and altered liver function. Vinylidene chloride is consumed in the production of
films, fibers, plastics, and paper coatings.2 Moreover, inhalation of vinylidene chloride at high
concentrations may result in neurological effects.3
Study of the reaction of vinylidene chloride with hydrogen in important because vinylidene chloride is
highly volatile and the atmospheric half-life of C2H2Cl2, following a reaction with hydroxyl radicals,
was found to be 2 days. 3
1
Reaction
The reaction H + C2H2Cl2 → Products has been studied using the fast flow discharge technique to find
out the rate constant. In order to make laboratory methods simplified, pseudo first order conditions are
employed to determine the rate constant, in which the concentration of C2H2Cl2 is made to be much
larger than that of the hydrogen. Kinetic information for C2H2Cl2 is obtained by measuring the relative
concentration of the hydrogen atoms in a flow tube.
Methods Used
Fast flow discharge technique
As the name indicates, the fast flow technique is operated at high flow speeds. A discharge is used to
generate hydrogen atoms from hydrogen molecules and hence the name fast flow discharge system.
Kaufman and co-workers developed this technique in determining the rate constant of many fast gas
phase reactions.4
Experimental approach to this system involves a flow tube in which one of the reactants enters at the
upstream end of the flow tube and the second reactant is added through a movable inlet. In this
approach the detector is fixed at the downstream end of the flow tube and moving the mixing point for
reactant A and reactant B relative to the fixed detector varies the reaction time.
k
H + C2H2Cl2 → Products ---------------------- (1)
On derivation, equation (1) becomes 
d [ A]
 k[ H ][C 2 H 2 Cl2 ]
dt
“If a small concentration of hydrogen atoms are generated in excess of C2H2Cl2, then even if reaction
(1) is allowed to go to completion, the concentration of C2H2Cl2 remains constant at its initial
concentration [C2H2Cl2]0.
Integrating reaction 1 and treating [C2H2Cl2]0 as constant results,
ln[ H ]
 (k[C 2 H 2 Cl 2 ]0 )t
[ H ]0
2
Hydrogen decays exponentially with time with a rate given by –k [C2H2Cl2]0 as in a case of first-order
reaction. Thus under pseudo-first-order conditions, a plot of ln [H] against time for a given value of
[C2H2Cl2]0 should be linear with a slope equal to (-k [C2H2Cl2]0). These plots are carried out for a
series of concentrations of [C2H2Cl2]0 and the values of the corresponding decays determined. The
absolute rate constant of interest, k, is the slope of a plot of these decay rates against the corresponding
values of [C2H2Cl2]0.”4
The kinetics of the wall loss, measure from the decay of the hydrogen atoms in the absence of added
reactant, is generally observed to be first order. When these wall losses are significant, the integrated
form of the rate expression becomes,
ln[ A]
 (k[ B]0  k w )t
[ A]0
Where kw is the loss of hydrogen atoms at the walls of the flow tube. The term, k[B]0+kw, is
collectively called as kps1.
Diffusion Correction 6
Diffusion is the movement of individual molecules from one region to another. Binary diffusion refers
to the movement of two individual molecules into each other. The binary diffusion coefficient [Dij] is
given as
3
(T
[ Dij ]  0.002628
Where  ij 
mi' m 'j
(m  m )
'
i
'
j
2 ij
)
P( si ) ij2  ij(1,1) (T * )
and T * 
*
k BT
(eps ) ij
Where diffusion Dij is expressed in terms of cm2/s-1, pressure p in terms of atm, and temperature T in
terms of K, m is the molar mass and kB is the Boltzmann constant.
The constant values of eps/kB, si, Ωij(T*) for H2 species are:
eps/kB = 59.7 K
si = 0.2827nm
3
Ωij(T*) is expressed as  ij(1,1) (T * )  a0  a1T *  a 2T *2  a3T *3  a 4T *4
*
T* = 295.15/59.7 = 4.94388 < 5.0
Coefficient values for a0, a1, a2, a3, and a4 are taken according to T* less than 5.0
Substituting the above values to reduce the diffusion correction equation results in,
[ Dij ] 
0.1663299(T 1.5 )
P(1.266038  3.19882  10 3 T *  8.6857  10 6 T *2  12.022  10 9 T *3  6.56727  10 12 T *4 )
Relation between D and k is given by the following equation:
k1  (k )[1  (
Dij
1
 2 )k ]
48  Dij V
Methods of Generation of Atoms and Free Radicals
The electrodeless discharges, such as microwave discharges, are used to generative highly reactive
hydrogen atoms. Discharging the stable hydrogen molecule produces a mixture of hydrogen atoms and
undissociated hydrogen molecule. A dilute mixture of the hydrogen molecule in an inert gas such as
argon is discharged to produce the small concentrations of excited hydrogen atoms. These
electronically excited hydrogen atoms are very short lived and rapidly emit fluorescence in all
direction as they return to ground state. The photomultiplier tube (PMT) is used to monitor the
emission in a direction perpendicular to the exciting radiation to see only the re-emitted light but not
the exciting radiation.
Experimental Technique
Fast flow system is used to find out the rate constant of the reaction. This system consists of a flow
tube in which reactant H and C2H2Cl2 are mixed in the presence of an excess bath gas, ultra high
purified argon (research grade). The system consists of a long Pyrex flow tube of length 75cm and
inner diameter 1.9cm. The side arm of the flow tube is of length 22.23cm long and inner diameter
1.27cm. Hydrogen atoms are produced in the side arm of the flow tube by microwave discharge
through a dilution of H2 in argon. The second reagent, C2H2Cl2, is introduced into the flow tube using
4
the movable inner injector, outer diameter is 1.5 cm. A mechanism was designed to ensure the smooth
movement of the injector through flow tube and to maintain low pressure in the flow tube during
kinetic runs. Measuring the concentration with the moveable inlet at several different positions while
the reactant flow rate is held constant typically carries out a rate constant measurement. If the radial
concentration gradients are assumed to be negligible in order to get the linear flow velocity of the
carrier gases same as the H atoms. Pressures are maintained at around 1 to 10 torr. Diffusion
coefficients must be considered due to the loss of hydrogen atoms at the walls of the flow tube. To
minimize the diffusion effects flow tube walls are coated with halogenated wax. This is to prevent the
loss of atoms on the surface of flow tube. Microwave discharge is used to generate highly reactive
hydrogen atoms. Hydrogen molecule is diluted with bath gas to produce very small concentrations
(about 3.5 x 1011 molecule/cm3) of hydrogen atoms. Fluorescence technique is used to detect the
hydrogen atoms, in which hydrogen atoms absorb light (at approximately 121.6 nm) in the visible and
ultraviolet region and emits fluorescence when excited.
A schematic diagram of the fast flow discharge system is shown below:
5
Laboratory Procedure
Vacuum all the lines before transferring gases or before starting experiment to ensure that there are no
impurities present in the lines. C2H2Cl2 is then purified with liquid nitrogen at a temperature of 77K
using several freeze, pump and thaw operations. Purified C2H2Cl2 is then mixed with bath gas (argon)
to make a 2% mixture. Similarly mixtures of hydrogen and argon are prepared at 0.1%. Vacuum all the
lines including the lines connected to microwave, after preparing gas mixtures for the experiment.
To turn on the microwave, open the valves connected to microwave discharger and let some amount of
gas flow through the line. Turn on the flow (3.75lt/min) of compressed air, to keep the system cool,
and turn on the high voltage on discharge to produce the energy required to produce hydrogen atoms
from hydrogen molecules. Fluorescence is generated using tesla coil. Reflected power is adjusted to
below one watt, to prevent the destruction of the microwave power supply unit. To turn on the other
microwave discharge, the bath gas (argon) is allowed to flow through the line while passing
compressed air around the system for cooling. The discharge is initiated with a tesla coil. Reflected
power is adjusted to below one watt and mixture of hydrogen and argon is allowed to flow through the
system. Reactant C2H2Cl2 is allowed to flow through the inner flow tube of the reactor. Signals are
observed using PMT (Photo Multiplier Tube) for several different positions of inlet flow.
Coating Procedure
As mentioned earlier, Dr. Abdell Goumri coated the inside of flow tube and outside of injector with
halocarbon wax to reduce loss of hydrogen atoms on the walls of flow tube and injector as described in
Dr. Ashutosh Misra’s dissertation5.
Gas Handling
Argon is used as the bath gas for all the experiments and is used as supplied as UHP (Argon Research
Grade, 99.9999% purity). Compressed air is used as supplied (Air compressed, Medical Grade).
C2H2Cl2 is purified by distillation from trap cooled liquid nitrogen at 77K and then subjected to several
freeze pump and thaw operations with methanol slush at a temperature of –300C.
6
Results
The decay of hydrogen atom concentration is given by
[ H ]  [ H ]0 exp( k ps1 )
l
v
Where l is the length and v is the velocity. In each experiment length was varied over a range of 5 cm
to 30 cm. The effective first order rate constant for loss of H atoms at the outer surface of the injector,
kw, is measured at the beginning of the each experiment. This is done by introducing H atoms in the
flow tube in the absence of vinylidene chloride and observed the change in [H] as the injector was
gradually withdrawn. As the length increases it is expected that the growth of atomic hydrogen
increase because of the decrease in surface area of the injector exposed to H atoms. Hence as the
residence time after injection increases, [H] increases, and kps1 in equation decreases. The
measurement of initial kps1 in each run was followed by stabilization of the flow tube using a large
flow, about 50 sccm, of the diluted (4%) C2H2Cl2 over a period of 45 – 60 min. This stabilization
procedure helps to reduce the scatter between kinetic runs.
For this reaction, the decay of hydrogen atoms as a function of reaction time in a fast flow discharge
system at a pressure of ~ 10 torr and the observed loss of the hydrogen atoms at the walls of flow tube
are shown below.
H + C2H2Cl2 => Products
y = -82.57x + 8.7761
R2 = 0.9939
8.0E+00
Ln(I-So), arbitrary units
7.5E+00
7.0E+00
6.5E+00
6.0E+00
5.5E+00
5.0E+00
4.5E+00
4.0E+00
0.002
0.007
0.012
0.017
0.022
0.027
Reaction time, s
0.032
0.037
0.042
0.047
7
y = 1.8257x + 8.0804
R2 = 0.9955
H + C2H2Cl2 => Products
Kw Analysis
8.4E+00
8.3E+00
Ln(I - S0), arbitrary units
8.2E+00
8.1E+00
8.0E+00
7.9E+00
7.8E+00
7.7E+00
7.6E+00
7.5E+00
0.0E+00
2.0E-02
4.0E-02
6.0E-02
8.0E-02
1.0E-01
1.2E-01
1.4E-01
Reaction time, s
Twelve determinations of k1 for the reaction H + C2H2Cl2 → products at the room temperature are
summarized in table 1.
Table 1
Date
P
T
Velocity [H2], 1011 [C2H2Cl2], 1013
k, 10-12 without
Torr
K
cm/sec
Diff. Correction
molecule molecule cm-3
cm-3
k, 10-12 with Diff. Uncertainty,
Correction
Reaction Time
kw
10-12
Sec
s-1
R2
cm3 molecule-1 s-1 cm3 molecule-1 s-1
120602
3.05
297
1095
1.5
1.7-14.5
1.486
1.676
0.07
0.0045-0.0237
-4.23
0.995
121602
4.07
295
1059
1.2
1.7-10.0
1.65
1.6095
0.16
0.0047-0.0236
-4.63
0.914
121802
5.05
296
1052
1.5
1.9-10.1
1.63
1.7111
0.15
0.00472-0.023 6
-3.19
0.972
122002
7.05
297
344
2.3
2.6-10.0
1.72
1.8670
0.09
0.0144-0.0289
-3.27
0.993
122702
1.41
298
940
6.4
1.9-10.9
1.41
1.5450
0.14
0.0051-0.0133
-1.39
0.972
010603
10.1
297
193
8.2
1.8-10.9
1.42
1.5250
0.03
0.0046-0.0256
-0.23
0.992
010703
6.06
296
322
4.5
1.4-8.2
1.76
1.8590
0.01
0.0155-0.0310
-0.55
0.989
011003
8.54
295
283
5.7
1.6-9.4
1.39
1.6090
0.04
0.0176-0.0317
-0.44
0.983
012802
5.02
295
640
2.5
1.4-10.9
1.66
1.8320
0.08
0.0077-0.0325
-1.59
0.989
013002
5.05
295
1262
1.2
1.4-11.1
1.34
1.4280
0.08
0.0039-0.0233
-3.83
0.992
020402
5.06
295
429
3.7
1.3-10.3
1.72
2.0130
0.05
0.0300-0.1155
-0.82
0.989
020402
5.06
295
1271
1.2
1.1-11.1
1.34
1.3810
0.06
0.0039-0.0234
-5.00
0.994
8
The graph between pressure and rate constant with uncertainty is shown as follows:
2.5
RESULTS OF VINYLIDENE CHLORIDE
2.0
-12
3
Rate Constant, 10 cm /molecule-sec
3.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
Pressure, Torr
The [C2H2Cl2] is varied between 1.1 x1013 molecule cm3 and 11.1 x1013 molecule cm3 and [H2] is
varied between 1.2 x1011 molecule cm3 and 8.3 x1011 molecule cm3 during the kinetic runs. After
correction for axial diffusion, the weighted mean k1 is 1.6713 x10-12 cm3 molecule-1 sec-1, with a
standard deviation of 0.08 x10-12 cm3 molecule-1 sec-1.
Standard deviation is a result of the uncertainty in each of the data points used in graph to determine
the final rate constant. The standard deviation (σ) is the root mean square of the deviations, where the
root mean square is the square root of the mean or average of the square of an argument. Deviation (di)
of any measurement xi from the mean μ of the distribution is the difference between xi and μ.
Deviation, d i  xi  
Lim
Variance,  2  N  (
1
N
x
2
i
)2
Standard Deviation,    2

If in a series of observations a statistical weight Wi is assigned to each value, a weighted mean Xw can be calculated from
the ratio of sum of Wi.Xi to the sum of Wi.
9
Discussions
From the table it can be seen that for the change in pressure from 1 torr to 10 torr, k is almost constant
with in a range of ± 0.341 x1013 molecule cm3. The rate constant is determined at the room temperature
using fast flow discharge system. The effect of change in velocity on the rate constant is observed from
last four readings of table 1. As the velocity increases, the rate constant decreases with in a range. As
the concentration of vinylidene chloride increases, the reaction time in seconds decreases making the
reaction to obtain steady state at faster rate. Loss of hydrogen atoms at the walls of the flow tube
decreases by coating the outer side of injector and inner side of flow tube.
References
1. Environmental Health Criteria, “Vinylidene Chloride”, International Program on Chemical Safety,
EHC 100, 1990. http://www.inchem.org/documents/ehc/ehc/ehc100.htm, Page no. 6 of 127.
2. Chemical Origins and Markets, fifth edition. G.M.Lawler, ed. Chemical Information Services, Menlo
Park, California, 1977. Page no. 118.
3. D. R. Tierney, T. R. Blackwood and M. R. Piana, “Status Assessment of Toxic Chemicals: Vinylidene
Chloride”, Industrial Environment Research Laboratory, Ohio, December 1979.
4. Barbara J. Finlayson- Pitts, James N.Pitts, Jr. “Atmospheric Chemistry – Fundamentals and
Experimental Techniques”, John Wiley & Sons, 1986. Page no. 238.
5. Ashutosh Misra, Ph.D. Dissertation, University of North Texas, Denton, Texas, May 1997.
6. William C. Gardiner, Jr. “Combustion Chemistry”, Springer-Verlag, 1984. Page no. 32-42.
10