Sulphur Hexafluoride (SF6)
M.A. Taddei
Department of Energy Engineering – Florence University, Via S. Marta 3 – 50139 Florence (Italy)
Abstract
It is shown the outcome of a research concerning SF6 destruction of ozone according the following original
manuscripts previously registered in unpublished form:
1)
“SF6: an ozone killer? With the certificate of registration from the US copyright office (Iuly, 20 1995).
2
“SF6: a gaseous dielectric suspected enemy of atmospheric ozone”: certificate of registration (may 19, 1997),
wrote with the goal of giving further scientific response to the interrogative of the first manuscript.
3)
“SF6: entitled as above-continuation: “certificate of registration (April 19-06).
The Kyoto protocol (December 11, 1997) included the SF6 in the list of the six greenhouse gases, whose total
production-consumption will be subjected to reduction.
Nevertheless, as regards SF6 destruction of ozone, the updated literature emphasizes that, owing to the fast
deactivation of atomic fluorine by the attack of CH4, the fluorine itself will not be able to destroy any significant
amount of ozone.
Against such thesis, the manuscript 3) confirms the contents of two original articles 1) and 2).
1. Sulphur Hexafluoride (SF6): an ozone
killer?
1.1. Introduction
Following an already existing body of law in
the European Community field, Italian law
number 549 (December 28, 1993) foresees that
before the end of 1994 the production of
chlorofluoro carbons (CFCs) will be prohibited,
and that before the end of 1999 the production of
those substances which should have been
substituted for them, the hydrochlorofluorocarbons
HCFCs,
(less
damaging
than
chlorofluorocarbons, yet still dangerous to the
ozone) will be prohibited as well.
This body of law aims to protect us from the
environmental hazards which threaten the
integrity of atmospheric ozone. These hazards
derive in particular from chlorofluorocarbons and
also nitrogen-oxides.
1.2. The chemistry of the ozone
The ozone (O3) is an allotropic form of oxygen:
a molecule constituted by three oxygen atoms (O),
different from molecular oxygen (O2), one of the
principal components of air, constituted by only
two oxygen atoms (O).
The presence of ozone grows notably in the
stratosphere and more precisely in the area
approximately 15 to 30 Km from the Earth’s
surface, and the greatest concentrations have been
verified between 20 and 25 Km. Below these
heights the concentrations rapidly diminish.
In the absence of winds, the ozone
concentration is determined by the balance
between the chemical processes which form it and
those which destroy it.
The balance is substantially expressed through
the following chemical reactions:
O 2 + Hν = O + O
O + O 2 = O3
O 3 + Hν = O + O 2
O + O3 = O 2 + O 2
(1)  → Process of O formation
3
(2)
(3) 
→ Process of O3 destruction
(4)
Reactions (1) and (3) express the capacity of
solar ultraviolet radiation of an appropriated wave
length to photodissociate either the molecular
oxygen O2 or the ozone.
Reaction (2) expresses the recombining of
atomic oxygen with molecular oxygen in order to
form ozone.
Reaction (4) serves to reduce the ozone
concentration by converting atomic oxygen and
ozone into molecular oxygen. This last one is a
very slow reaction.
The balance between ozone which forms and
that which decomposes results in the
2
establishment of a more or less constant
concentration in the interval of atmospheric height
between 15 and 30 Km. This stratum of the ozone
represents a protective shield for man and for the
environment, strongly reducing, with its
absorption power, the solar ultraviolet radiations
which would otherwise arrive on Earth. A
reduction of this protective screen could cause
damages to man (skin cancers, etc.) but it could
also provoke serious and irreversible alterations in
the development of the fauna and’ of vegetation
and, according to some, could result in climatic
alterations as well.
1.3. The “hole” in the atmospheric ozone
During the past several years, available data
have revealed a significant decrease of the
stratospheric ozone in the Antarctic and, more
recently, in the Arctic. The geographic extension
and progressive character of this phenomenon,
constitute an unexpected and alarming signal.
Initial studies of the atmosphere indicated that
the responsibility lay with pollutants capable of
moving from the atmosphere to the stratosphere
and which have the capacity to react with ozone
and destroy it.
These studies led to an investigation of the role
of free radicals which are produced in the
stratosphere, above the ozone stratum. These free
radicals are produced by photodissociation, on the
part of present strong UV radiation, of the
molecules of pollutants resulting from increased
industrial and agricultural activity. In particular, it
was found that ozone concentration can be
strongly reduced by some catalytic cycles
generically expressed in the following form:
X + O3
→ XO + O 2
XO + O
→ X + O2
(5)
This cata1yic cycle is equivalent to reaction (4)
O + O3
→ O2 + O2
which, as we have already seen, reduces the
concentration of ozone. The characteristic of
catalyst X is that it greatly accelerates reaction
(4), in small quantities and low concentrations,
even if, in the end, it remains unaltered.
1.4. Chlorofluorocarbons and Halons
The existence of the catalytic cycle of chlorine,
capable of shifting the chemical balance towards
minor ozone concentrations, was suggested by
F.S. Rowland and M.J. Molina in 1974 along with
the hypothesis that the major source of CL was
the photodissociation of the chlorofluorocarbons
on the part of the strong UV radiations in the
stratosphere and above the ozone stratum:
Along the lines of cycle (5) we will have:
CL + O 3
→ CLO + O 2
CLO + O
→ CL + O 2
(6)
The chlorine freed by the photodissociation
consumes ozone in order to form chlorine
monoxide which combines with the atomic
oxygen released in reaction (1) in order to form
again chlorine atoms which restart the cycle of
ozone destruction; in this way, each chlorine atom
can destroy many ozone molecules.
The analogous BR catalytic cycle has its
origins in the photodissociation of halons
(fluorocarbons that contain bromine) which are
also produced industrially.
The chlorofluorocarbons and the halons are
organic halogenated fluorine compounds,
characterized by an elevated chemical and thermic
stability which increases with the fluorine content,
and are therefore, for the most part, insusceptible
to decomposition, before arriving in the
stratosphere, where they remain for long periods,
constituting a serious threat to the ozone stratum.
They are non-flammable.
These CFCs and halons are largely used in
industrial processes, creating refrigeration in
refrigerators and air conditioners, in fire fighting
devices, thermal isolating plastics, and nonessential uses (i.e. spray confections, etc.).
The concentration of these components,
measured in different localities, shows consistent
increases over the last decade. Even if the input
rate were to reach a constant level, because of
long periods of permanence in the atmosphere, the
concentrations would continue to increase.
Despite the minimal risk of dispersion from air
conditioning and refrigeration devices, the risk of
gas dispersion with regard to accidents or the
demolition of machinery is nevertheless so great
that even the possibility of recuperating devices
by recycling gas was excluded. And international
legislation preferred to release laws aiming to first
limit and then prohibit the use of CFCs and halons
for the protection of the stratospheric ozone and
the environment.
It has already been announced in the press that
new “ozone-friendly” refrigerators, constructed
3
without CFCs and HCFCs, are already being
produced in Germany.
S + O3
→ SO + O 2
1.5. Sulphur hexafluoride
SO + O
→ S + O2
It is surprising at this point to find that similar
precautions have not been taken with regard to the
ever-increasing use of sulfur hexafluoride (SF6),
on both a national and international level, as a
gaseous dielectric in high pressure operations to
produce medium and high voltage circuitbreakers, high voltage metalclad switchgears, high
voltage current transformers: all of which are
installed in power plants and distributing
substations of electrical energy.
Notwithstanding the expensive precautions
taken by manufacturers to ensure the tightness of
the containers in order to minimize leaks, the risk
of accidents still remains in the event of lightning,
earthquakes, war, or destruction of the machinery.
Two hypotheses can be taken into account:
A) The dispersion of sulfur hexafluoride in the
atmosphere creates toxic products like sulfur
dioxide. It is worth noting that in this hypothesis
its use shouldn’t be allowed, due to its toxicity.
B) The sulfur hexafluoride is a stable compound
of fluorine, non-flammable and therefore largely
insusceptible to decomposition, before arriving in
the higher zones of the stratosphere above the
ozone stratum where it would be decomposed by
the UV radiation present, releasing sulfur atoms,
similar to that which happens with the CFCs.
As a consequence of these two hypotheses
these points are to be considered and are worthy
of an answer:
1) If, in the case of hypothesis A, a catalytic cycle
of this sort
SO 2 + O
→ SO + O 2
SO + O 3
→ SO 2 + O 2
(7)
will occur. The sulfur dioxide would combine
with the atomic oxygen released in reaction (1) to
form sulfur monoxide, which would consume the
ozone to form again sulfur dioxide which begins
again the cycle of ozone destruction.
This catalytic cyc1e is equivalent to reaction
(4)
O + O3
→ O2 + O2
which diminishes the ozone concentration, but is
made much more rapid, whereas SO and SO2
remain unconsumed.
2) If in the case of hypothesis B, a catalytic cycle
of reaction analogous to cycle (6) of CFCs:
(8)
will occur. It also leads to ozone destruction.
One can observe that catalytic cycle (8), when
inverted, can be triggered by the photodissociation
of SO2 by means of UV radiation:
SO 2 + Hν = SO + O
3) If, in the event both hypotheses A) and B) are
verified, both cataclytic cycles (7) and (8) will
occur.
4) If, bearing in mind current EC and national
legislation regarding CFCs and HCFCs, sulfur
hexafluoride, considering the hypotheses stated
herein and the high pressures involved, should
also be subject to the same legislation.
1.6. Conclusions
In order to find scientific evidence to support
these hypotheses it is necessary to perform
experimental laboratory trials which simulate
actual conditions. However, it is worth noting that
the exceptional reduction of vast zones of ozone
concentration found in 1992 and at the beginning
of ‘93 was attributed by researchers to the
eruption in June 1991 of Mount Pinatubo
(Phillipines) which spread notable quantities of
sulfur dioxide into the atmosphere and the
stratosphere for months, confirming the suspicion
that this and the products of its photolitic
dissociation can be considered catalysts in the
process of ozone destruction.
Having stated the above, it is realistic to ask if,
after the prohibition of the production/use of only
one class of chemical compounds, the problem of
effective protection of the environment is
resolved; or, if more new problems will emerge,
since
the
dynamic
balance
between
oxygen/ozone/radiation could alter with emissions
and progressive accumulation of other organic
compounds, with the prevision that, because of
their particular conditions of use, there will be
long delay between their market release and
environmental impact.
In other words, it is realistic to ask if it is worth
increasing the use of a technique like that of sulfur
hexafluoride, taking into account that this
insulation technique can in some cases
satisfactorily resolve today’s industrial problems
at minor economic cost but which, on the other
hand, can lead to future environmental damages.
4
As a matter of fact, from the present increasing
use of this technique will derive at a future time
the subsequent increase of element components of
sulfur hexafluoride which could catalytically
destroy the ozone.
2. Sulphur Hexafluoride (SF6): a gaseous
dielectric suspected enemy of atmospheric
ozone
2.1. Introduction
It is easy to evaluate the course and
characteristics of ventilation and air leaks in both
a small area and a large building at night and
during work hours, using methods which employ
gases opportunely selected as tracers(16).
Since the inhabitants of a city spend most of
their time indoors, such data are important in
determining exposure of the population to the
totality of air pollutants and verifying qualitatively
and quantitatively the need to adjust or modify
existing ventilation systems.
Omitting an illustration of the different
techniques which are currently in use, these
techniques in general involve mixing the tracer
gas with the air of the environment, which is then
monitored to determine its variation concentration
throughout the area.
An ideal tracer should have the following
principal characteristics (10):
1) be easily measurable, even at low
concentrations
2) be inert and not be a normal constituent of air
3) be non-toxic and non-flammable
4) have approximately the same molecular weight
(Mr) as air.
No tracer gas meets all these requirements.
Until a few years ago certain CFCS such as
CFC 12 (freon CF2C12, Mr 120.92), CFC 1 15
(C2F5C1, Mr 154.46) and Halon 1301 (CF3Br , Mr
148.92) were widely used in the above mentioned
tests. These gases are heavier than air (Mr
28.964), but are stable, inert, non-flammable and
dissociate only at high temperatures, rendering
them non-toxic: their threshold limit value is
equal to 1.000 ppm (ref. 12 section 16-17).
Because of these qualities CFCS were for a long
time considered ideal for the above-mentioned as
well as other industrial uses, thus, paradoxically,
damaging the stratospheric ozone.
Inert gases do not break down easily in the
troposphere. Rather, they move upwards into the
stratosphere, surpassing approximately 25 km, the
height of greatest ozone concentration. At this
point they encounter ultraviolet radiation which,
no longer absorbed by ozone, breaks down the
stable molecules of CFCS, releasing chlorine
atoms which catalytically destroy ozone. The
process of ozone destruction is well-known, and
described in a preceding article by this author.
Following prohibition of the use of CFCS and
halons, sulfur hexafluoride (SF6) is currently the
most commonly used tracer gas. Like CFCS, it has
the principal characteristics required of an ideal
tracer gas. It is heavier than air (Mr 146.05), but
in the same molecular weight range as CFCS. It is
very stable at room temperature, chemically inert
under normal conditions, non-flammable and nontoxic: its threshold limit value is equal to 1.000
ppm, the same as that of the above-mentioned
CFCS (ref. 12 section 16- 20).
Furthermore, it is not easily water-soluble
[Note: the gaseous phase reaction with H2O which
produces SO3 and HF, while favoured by a
negative AG, is an exceptionally slow reaction
(ref. 9)].
For its extraordinary stability and excellent
insulating properties, SF6 is used increasingly
nationally and internationally as a gaseous
dielectric in high pressure operations to produce
medium and high voltage circuit breakers, high
voltage metalclad switchgears, and high voltage
current transformers, all of which are installed in
power plants and electric energy distributing
substations. The experience acquired in the field
of metalclad switchgears gave rise to the
development of gaseous insulated cables where
sulfur hexafluoride at high pressures serves as the
insulating medium.
The increasing employment of SF6 as a tracer
and insulating gas, and the possibility of
subsequent future increase in the concentration of
its constituent elements in the atmosphere, obliges
us to ask ourselves some questions regarding the
possible danger of SF6 as an agent of ozone
destruction. This article attempts to address this
issue.
2.2. The stability of SF6
Thermodynamics provides the elements
necessary for the study of the chemical
equilibrium of possible SF6 dissociation reactions
at both room and high temperatures, enabling us
to verify its stability and subsequent threat to the
integrity of stratospheric ozone.
Research phases:
1) The starting point is represented by the
thermodynamic data relative to the enthalpies of
formation under standard conditions (1 bar, 298.
5
15 K) of F, S, SF, SF2, SF3, SF4, SF5, SF6 and
their values provided by the most recent
recommendations in the technical literature,
These recommendations supply new important
data after re-examination of preceding data and
therefore should, at the present time, receive
common consent, but future changes to the
recommendations could well arise.
This data is reported in Table I along with sources
of information.
2) On the basis of this data, the reaction enthalpies
relative to the atomization of SF6 by consecutive
steps, i e, dissociating one fluorine atom at a time,
are reported in Table II. It is necessary to
remember that the bond dissociation energy or the
bond strength for the reaction AB → A+B, where
A and B may be atoms or free radicals, is defined
as:
0
0
0
0
0
D298
( A − B ) = ∆ r H 298
= ∆ f H 298
( A) + ∆ f H 298
(B ) − ∆ f H 298
( AB )
0
is the enthalpy of the reaction in
where ∆ r H 298
which the bond is broken.
From the examination of Table II one can deduce
the following:
a) The successive bond dissociation energies
0
(SFn −1 − F ) of SF6 (where n is the number
D298
of fluorine atoms) alternate in magnitude, being
greater when n is even and lesser when n is odd,
in agreement with published technical literature
(14) (13).
0
(SF5 − F ) appears to
b) The bond strength D298
be stronger than previously indicated:
420 ± 10
kJ
on the basis of the most recent
mol
recommendations (17).
c) The enthalpy of atomization (∆ a H ) of SF6, or
the change of enthalpy that accompanies
total demolition of the SF6 molecule into its
gaseous monatomic components S and F is, by
Hess’s law, equal to the sum of the bond strengths
shown in Table II.
Its value of 1974.100
kJ
mol
is more rapidly
obtainable
by
adding
the
enthalpies
corresponding to the following phases:
0  kJ 
∆ r H 298


 mol 
SF6 (g) → S(rhomb) + 3F2 (g) =
3F2 (g) → 6F (g) 79.4 x 6 =
S (g)
=
S(rhomb) →
1220.5
476.4
277.2
Net
SF6 (g) = S (g) + 6F (g) =
1974.100
where one bears in mind that the standard
enthalpy of formation of the elements S(rhomb) and
F2(g) is nothing by definition regardless of the
temperature.
d) Reported in Table II are the standards enthalpy
changes for all possible SF6 dissociation reactions
which are identical to the net result of the
summation of all the enthalpies of reaction that
precede them, and therefore identical to the sum
of strengths of the bonds which are broken.
3) In order to calculate the feasibility and degree
of progress of the chemical processes, it is
necessary to know the entropy S(T) and enthalpy
H(T) of each substance taking part in these
processes, to determine the value of the Gibbs
energy G(T), related by definition to the aforesaid
values by the well-known relation
G (T ) = H (T ) − TS (T ) , which can also be shown
as:
−
G (T )
H (T )
= S (T ) −
T
T
(3.1)
In order to calculate processes in which chemical
transformations occur, it is also necessary to know
the equilibrium constant K(T) which, in a generic
chemical reaction between gases of the type
aA + bB ⇔ cC + dD
(3.2)
is related to the value of the Gibbs energy of the
substances taking part in the reaction by the
simple relation:
RT ln K (T ) = − ∆ r G (T ) = −
[∑ G (T )products − ∑ G (T )reactants] =
= −[cG (C , T ) + dG (D, T ) − aG ( A, T ) − bG (B, T )]
where a, b, c, d are the stoichiometric coefficients,
∆ r G (T ) is the change of Gibbs energy in
reaction 3.2, and R is the gas constant
= 8,3143
J
Kmol
For (3.1) the above relation is shown as:
∆ G (T )
∆ H (T )
(3.3)
R ln K (T ) = − r
= ∆ r S (T ) − r
T
T
where ∆ r S (T ) and ∆ r H (T ) are the changes of
entropy and enthalpy of the system at temperature
T as a result of reaction 3.2.
One must, however, keep in mind that the
dependence of Gibbs energy on the temperature is
6
generally tabulated in a different form, based on
the following considerations:
0
If one subtracs from (3.1) the value H (0) or
T
0
H (298.15K )
from both members of the
T
equation, the equality does not change and (3.1)
can be written in the form of the following two
functions:
G (T ) − H (0 )
H (T ) − H (0 )
Φ 0 (T ) = −
= S (T ) −
T
T
or
Φ (T ) = −
G (T ) − H (298.15 K )
H (T ) − H (298.15 K )
= S (T ) −
T
T
(3.4)
These functions, called Gibbs energy functions,
provide the standard Gibbs energy at temperature
T with respect to standard enthalpy at 0 K or to
standard enthalpy at 298.15 K. The relation that
links the two expressions is the following:
Φ (T ) = Φ 0 (T ) +
H (298.15 K ) − H (0 )
T
(3.5)
The function Φ 0 for each substance is
tabulated for different values of T (by steps of 100
K up to 6000 K) (11). Even the difference of the
enthalpic values [H(298,15 K)-H(0)] is tabulated
(11) and thus it is not difficult to pass from the
value of Φ0 to the value of Φ on the basis of (3.5).
The values of Φ calculated in this way were
reported in Table III both for the standard
temperature of 298.15 K and for a range of
temperatures warying from 1900 to 2400 K, with
the goal of verifying the temperature at which the
dissociation products become predominant. By
examining Table III we can observe that at the
standard temperature of 298.15 K, in (3.4) being
[H (T ) − H (298.15 K )] = 0' , the value of
function Φ is identical to the value of entropy
S(298.15 K).
In the case of a reaction, (3.4.) transforms into
the following:
 G (T ) − H (298.15 K )
∆ r Φ (T ) = ∆ r −
 =
T

∆ [H (T ) − H (298.15 K )]
= ∆ r S (T ) − r
T
Keeping in mind that
∆ r H (T ) = ∆ r H (298.15 K ) + ∆ r [H (T ) − H (298.15 K )]
and furthermore that
∆ G (T )
 G (T ) − H (298.15 K ) ∆ r H (298.15 K )
− r
= ∆ r −
=
 −
T
T
T

∆ H (298.15 K )
= ∆ r Φ(T ) − r
T
we can show 3.3 as
∆ G (T )
R ln K (T ) = 2.303 × R × log10 K (T ) = − r
=
T
∆ H (298.15 K )
∆ H
= ∆ r Φ (T ) − r
= ∆ r S (T ) − r
T
T
(3.6)
being known as, ln K (T ) = 2.303 log10 K (T )
and 2.303 × R = 19.1478
J
.
K mol
The value of ∆ r H (298.15 K ) is a constant
already reported in Table II for all possible
dissociation reactions of SF6.
4) The application of formula (3.6) to the possible
dissociation reactions of SF6 based on the values
of Φ(T ) and ∆ r H (298.15 K ) cited above is
highlighted in Table IV, where upon examination
one can deduce the following:
a) At the standard temperature of 298.15 K, the
∆ r G (T ) of all the dissociation reactions is positive
T
(non spontaneous reactions), whereas, since the
sign of the exponent in K(T) is negative [opposite
∆ r G (T )
to the sign of
], K (T ) is <1 and is as
T
small as the value of the exponent is large. Since
the partial pressures (the concentrations) of the
dissociation products appear in the numerator of
the equilibrium constant, these partial pressures
are extraordinarily small in the equilibrium with
respect to the partial pressure of SF6.
Having separately evaluated the enthalpic and
entropic contribution to the equilibrium and
seeing that the change of entropy is very small
with respect to the change of enthalpy, we can
conclude that SF6 is very stable at room
temperature.
b) Since dissociation reactions are endothermic, in
accordance with Chatelier’s principle, an increase
in temperature favours the endothermic formation
process of the dissociation products, so that more
heat is absorbed by the system during reaction,
tending to counterbalance the original temperature
7
increase. Therefore, at some elevated temperature,
the dissociation products become more abudant
than SF6.
According to thermodynamics, upon raising the
temperature, the ∆ r G (T ) of the dissociation
T
reaction passes from positive values, ∆ r G (T ) > 0
T
(non spontaneous reactions) to negative values
∆ r G (T )
< 0 (spontaneous reactions) and the
T
numerical value of the equilibrium constant K(T)
increases, passing from values of K (T ) < 1 to
values of K (T ) > 1 .
One can calculate the discriminant temperature
∆ G(T )
is greater
between the interval in which r
T
than 0[K (T ) < 1] and that in which is less than
0[K (T ) > 1] keeping in mind that at said
temperature the following condition is realized in
(3.6):
∆ r G (T )
∆ H (T )
= ∆ r S (T ) − r
=
T
T
∆ H (298.15 K )
= ∆ r Φ (T ) − r
=0
T
−
therefore:
∆ r H (T )
T
∆ H (298.15 K )
or ∆ r Φ(T ) = r
T
log10 K (T ) = 0 K (T ) = 1
∆ r S (T ) =
In this condition the partial pressure of the
dissociation products is equal to the partial
pressure of SF6.
In Table IV the two temperatures, within which
the interval of the above condition occurs, are
shown for each possible dissociation reaction.
When the temperature increases with respect to
said values, the dissociation products production
increases with respect to SF6.
2.3. The photolysis of Sulphur Hexafluoride
In Table II, as mentioned in point 2.d), for
every possible dissociation reaction, the enthalpies
of the reaction expressed in
kJ
show us the
mol
energies necessary to break the bonds involved in
the relative processes.
Let us remember now that, by Plank’s relation,
the energy per mole of eletromagnetic radiation
(photon) with wavelength λ or wavenumber ν~ is
expressed by the relation:
E = hνNa = h
c
Na = hcNa ν~
λ
where
h
= Plank’s constant
c
= speed of light
hc
= 1,98648x10-23Jcm
Na
= Avogadro’s number = 6.022x1023mol-1
ν
= frequency in Hz
λ
= wavelength in m
= number of waves in cm-1
ν~
The frequency v, the wavelength λ and the
speed are connected by the relation:
ν =c
The quantity
1
λ
1
= cν~
λ
= ν~ is known as wavenumber
and is commonly expressed in cm-1.
The
constant
hcNa =11.96266 is the
conversion coefficient ν~ = 1cm−1 = 11.96266 J :
mol

ref .12 sec tion 1 − 34) used to convert the molar
energies expressed in
J
mol
into energies per
molecule expressed in wave numbers (cm-1).
In Table II, the division of the bond strenghts
by the said constant supplies the wavenumber
corresponding to electromagnetic radiation
(photon) and its inverse, that is, the wavelength.
The photodissociation of the bonds is possible
by means of that radiation which has en energy
greater than or equal to the indicated wave
numbers and therefore a wavelength less than or
equal to the values shown in the Table.
From further examination of Table II, one can
deduce that SF6 is not able to dissociate under the
action of solar light due to the following
considerations:
1) Upon examining the inferior value of SF5-F
kJ
, we can show
bond energy of
410
mol
410.000
1
ν~ ≥
= 34273 cm−1 and λ ≤
= 291 nm
11.96266
34273× 102
as
a
wavelength
which
is
shorter than those present in solar light as a
8
consequence of the fact that ozone strongly
absorbs solar ultraviolet radiation below 300 nm.
Wavelengths of the value cited above can be
found only above the ozone layer.
2) Under solar light the dissociation of SF6 with
production of SFn radicals with n<5 cannot occur
because the higher energies necessary for the
breaking of the relative bonds require
progressively more energetic radiation with a
wavelength progressively shorter than 29 1 nm:
radiation which is found above the ozone layer at
progressively higher altitudes.
The fact that SF6 cannot dissociate under solar
light can be considered a positive quality
considering that SF5 and SF4 radicals are highly
toxic, their threshold limit values having a ceiling
value equal to 0.01 ppm and 0,1 ppm respectively
(ref. 12 section 16-20). In particular, SF4 is
extremely reactive: in water it instantly
hydrolyzes, forming SO2 and HF: (SF4 + 2H2O →
SO2 + 4HF).
In short, under normal conditions, use of SF6
causes the same problems as use of inert gases
such as CFCS and halons. The recovery and
subsequent reemployment of the gas cannot be
foreseen indefinitely and does not resolve the
problem. Since it does not decompose, SF6 arrives
and accumulates in the high zones of the
stratosphere above the ozone layer, where it is
decomposed by the UV radiation present here.
Two possible mechanisms of ozone destruction
may occur:
A) The release of fluorine atoms following the
photolysis of SF6 and its SFn radicals (with n
varying from 1 to 5) and
B) The release of sulfur atoms following the
atomization of SF6.
A) Liberation of fluorine atoms could cause a
catalytic cycle of ozone destruction of the
following type:
F+O3 → FO+O2 (1)
FO+O → F+O2 (2)
in which O could remain an oxygen atom in its
ground state (O3P).
From a kinetic point of view, it is necessary to
remember that the rate of a chemical reaction, e. g
A+B→ Products, is given by
Rate of products production = d [ Products ] = k [ A][ B ]
dt
where the rate constant k depends on the
temperature. Therefore the rate of a reaction
varies in proportion to the concentration of the
reactants and the rate of reaction (1) can be
compared, at a concentration equal of the
reactants, with the rate of analogous Cl and Br
reactions, by examining the rate coefficients data
k taken from the technical literature (1) and
displayed in the following Table (1 cm3 molecule1 -1
s = 6.023 x 1020 dm3 mo1-1 s-1)
Reaction
k/298
cm3 molecule-1
s-1
Br+O3=BrO+O2
1.2 x 10-12
Cl+O3=Cl=+O2
1.2 x 10-11
F+O3=FO+O2
1.3 x 10-11
Temp.
dependence of
k cm3
molecule-1 s-1
1.7 x 10-11
 800 
−

T 
exp 
Temperature
range
K
195-392
2.9 x 10-11
 260 
−

T 
exp 
205-298
2.8 x 10-11
 230 
−

T 
exp 
250-365
Furthermore it is necessary to bear in mind that,
in agreement with the technical literature (15), in
the ozone layer there is much lower concentration
of O than O3, compensating for the greater values
of the rate coefficients of reaction (2), which are
compared in the following table:
Reaction
k/298
cm3 molecule-1
s-1
BrO+O→Br+O2
ClO+O→Cl+O2
FO+O→F+O2
3 x 10-11
3.8 x 10-11
5 x 10-11
Temp.
dependence of
k cm3
molecule-1 s-1
Temperature
range
K
3.8 x 10-11
200-300
Seeing that, in a chain reaction, the slower
reaction is kinetically more important, inasmuch
as it determines the rate of the global reaction, it is
therefore reaction (2) that must be taken into
consideration in establishing the above-mentioned
comparison. The technical literature in reference
(1) shows that the reactivity of FO with the
oxygen atom is similar to that of the analogous
ClO and BrO reactions and it can be inferred that
the temperature dependence of k, for the BrO and
FO reactions, will be small in the temperature
range of 200-300 K, which is also the
stratospheric temperature range.
B) Releasing of sulfur atoms can occur following
atomization of the hexafluoride molecule.
In order to understand how sulfur can
catalitically destroy ozone, it is necessary specify
that in the high layers of the stratosphere there are
oxygen atoms in an excited state O(1D) with an
elevated degree of reactivity because they have
kJ more energy with respect to their ground
190
mol
state.
9
The electronic spectroscopy of the 02 molecule
in defining the potential energy difference
between the lowest vibrational state and the
beginning of an absorption continuum
(corresponding to its dissociation energy) in fact
demonstrated that, when the wavelength of the
absorbed radiation is lower than 175 nm
~

−1 kJ
= 683, the O2 molecule in an
ν > 57127 cm ;
mol


excited state dissociates itself into an O atom in its
ground state 3P and into an oxygen atom in an
excited state [O(1D)] with the reactivity
characteristics cited above (6).
The ozone too can be photolyzed by UV
radiation in O2 and into an O atom in excited state
(1D).
The following Table, taken from the technical
literature (1), summarizes the possibility of an
O(1D) formation following the photolysis of O2
and O3.
∆ r H °(0 K ) *
Reactions
O2+hν = O(3P)+O(1D)
O2+hν = O(1D)+O(1D)
O3+hν = O(1D)+O2(5.1)
threshold
λ (nm)
kJ
mol
683
873
291
175
137
411
It is this O(1D) atom which makes possible a
catalytic cycle of ozone destruction of the form:
S+O3→SO+O2
SO+O(1D)→S+O2
(5.2)
2.4. The possibility of S02 formation
To elaborate on the possibility of SO2
formation
following
sulfur
hexafluoride
dispersion in the environment, as discussed in the
preceding article, it is necessary to specify that the
only possibility this gas has to escape from
hermetically sealed receptacles, where it is
*
The obtainable values of the bond energies of diatomic
molecules by means of a spectroscope are relative to the
temperature of 0K and are identical to the dissociation energy
of molecules found in the lowest energetic level.
Therefore, the values of ∆ r H 0 (0 K ) shown above are
identical to the enthalpies of the relative reactions at 0K, on
the basis of the following values of the enthalpies of
formation at 0K (7):
( )
kJ
O 3 P : ∆ f H 0 (0 K ) = 246.79
mol
( )
O 1 D : ∆ f H 0 (0 K ) = 246.79 + 190 = 436.79
O2 :
kJ
∆ f H (0 K ) = 0
mol
O3 :
∆ f H 0 (0 K ) = 145 .348
kJ
mol
contained at high pressure, is by activation of the
pre-established breakage security valves which
allow the free release of gases and eliminate the
danger of explosions in the event of abnormal
internal pressures. If, to cite an example, due to a
defective coordination of the insulation, lightning
were to provoke an internal discharge, the arc, if
not instantly interrupted, could raise the sulfur
hexafluoride temperatures far higher than those
previously examined, decomposing it into its
atomic components (sulfur and fluorine).
If the dielectric regeneration does not reset
itself, returning the sulfur and the fluorine to the
SF6 state, hexafluoride decomposition products
will be released from the security valves. In the
case of weather disturbances, water and humidity
are always present and when sulfur bums in air, it
forms sulfur dioxide based on the following
reaction:
S + 6F + 3H2O = SO2 + 6HF + O
The formation of SO2+O is favoured by the
high temperature with respect to SO3 formation.
Both the SO2 and HF gases are toxic.
The successive reaction of SO2 with the
atmospheric oxygen to form sulfur trioxide is
currently considered too slow to be important in
atmospheric chemistry.
It was instead recently discovered that the only
important oxidant of SO2 is the OH radical formed
as a result of the reaction of oxygen atoms in
excited state O(1D) formed by the photolysis of
ozone (reaction 5.1) and water vapor:
O(1D) + H2O = 2OH.
The OH product, known as hydroxyl, is then
available, causing the transformation of SO2 into
H2SO4 as follows:
SO2 + OH + M → HSO3 + M
HSO3 + O2 → SO3 + HO2
SO3 + H2O → H2SO4
which contributes to the well-known formation of
acid rain.
The part of SO2 which escapes from this
reaction and moves up into the high layers of the
stratosphere, can be photolyzed by UV radiation
as follows:
SO2+hν = SO+O and trigger the inverted catalytic
cycle (5.2).
Being that the bond strength D298 (O—SO) is
equal to 552 ± 8 kJ (ref. 12 sect. 9-67), the
mol
0
kJ
mol
photolysis of SO2 can occur by means of absorbed
radiation
having
a
wavenumber
10
 552000  and therefore a wave
≥ 46143cm −1 

 11.96266 
length ≤ 216 nm.
But SO2 can also, in the high layers of the
stratosphere, combine with atomic oxygen in an
excited state O(1D) freed by the photolysis of
molecular oxygen or ozone and cause the
following catalytic cycle
SO2 + O(1D) = SO + O2
SO + O3 = SO2 + O2
which can also lead to destruction of ozone.
2.5. Conclusions
SF6 is currently used ever increasingly as an
inert tracer and a dielectric, but a quantitative
evaluation of future uses of this gas is not
foreseeable.
We can, however, note the following:
1) the number of SF6 molecules contained in 1 m3
at 5 bar at 298 K is equal to:
N=
(
)( )
PV
5 × 105 Pa 1 m3
× Na =
× 6.022 × 1023 mol −1 =
J 
RT

 8.3143
 × 298 K
Kmol 

= 1,215 × 1026
where:
R = gas constant = 8.3143
J
m 3 Pa
= 8.3143
Kmol
Kmol
Na = Avogadro’s number = 6.022 × 1023 mol-1
Furthermore, each SF6 molecule contains 6
fluorine atoms and one sulfur atom, each of which
could cause a catalytic cycle of ozone destruction.
2) There are no differences between
characteristics of SF6 and those of CFCS and
halons, although only use of the latter are
prohibited, due to resultant ozone destruction.
Therefore, when one reads in specialized texts that
the insulation technique of the future is SF6, it is
realistic to ask if the danger of SF6 with regard to
consequent environmental damage such as ozone
destruction was merely underestimated, or not
taken into consideration.
3. Sulphur hexafluoride: a gaseous dielectric
suspected enemy of atmospheric ozonecontinuation
3.1. Introduction
The technical literature in references (4),(15)
recognizes that in principle the fluorine atoms
liberated in the stratosphere as a result of the
decomposition of fluorine containing molecules,
could catalytically destroy ozone by mechanisms
based on the reaction O3+O→2O2 or 2O3→3O2.
However, it is put into evidence that F-atom
chains will be much shorter that Cl-atom chains,
because the reaction of hydrogen atom abstraction
from CH4 or other hydrogen containing molecules
in the stratosphere, is much faster for F-atoms and
produces HF, which is a very stable molecule.
The reactivation of fluorine by the attack of OH
on HF molecules is strongly endothermic and
therefore the reaction OH+HF=F+H2O is
extremely slow at atmospheric temperatures and
negligible in the stratosphere.
From the data taken from the technical
literature and displayed in the following table –
A) F+CH4→HF+CH3 (2)
∆H°= -131.5 kJ · mol-1
K1= 6,7 x 10-11cm3 molecule –1 s-1 at 298 K
K1= 1.6 x 10-10 exp(-260/T) cm3 molecule–1 s-1
over the temperature range 180-410 K
B) F+O3→FO+O2 (3)
∆H°= -113 kJ · mol-1
K2= 1 x 10-11cm3 molecule-1s-1 at 298 K
K2= 2.2 x 10-11exp(-230/T) cm3 molecule-1s-1
over the temperature range 250-370 K,
- we can note that, at an equal concentration of Fatoms, reaction A will be faster that reaction B
(usually the starting reaction) if, at a given
altitude, K2 [O3] < K1 [CH4], occurs.
Thus the catalytic cycle terminates and fluorine
is permanently deactivated before it can destroy
any significant amount of ozone.
3.2. Remarks
It is well known that in the middle and upper
stratosphere (25 km until 35 km altitude), there
are oxygen atoms in an excited state O(1D) with
an elevated degree of reactivity, because they
have 190 kJ · mol-1 more energy with respect to
their ground state. They are the result of the
decomposition of ozone in the reaction of
absorption of a UV-C or UV-B photon:
O3+UV photon (λ < 320 nm) →O*2+O(1D),
wherein the oxygen molecules also are produced
in an excited state.
The collision of fluorine atoms (or Cl–Br) with
the atoms O(1D) causes the reaction F + O(1D) =
FO, with the simple photonic energy transfer of
the energy in excess above the ground state.
11
It is a very exothermic reaction (∆H°= - 409.28 kJ
· mol-1, with a presumably high rate constant.
Besides, taking into consideration that in the
middle and upper stratosphere presumably
[O(1D)] > [CH4] occurs, we can conclude that
reaction F+O(1D) = FO is faster than above
mentioned reaction A (F+CH4→HF+CH3).
Therefore, it is realistic to take into
consideration the following mechanism I, wherein
the first of its three steps starts with the photo
dissociation of two different ozone molecules:
1)
O3+hv = O(1D)+O*2
O13+hv = O(1D2) + O*2
2)
F+O(1D1) = FO
F1+O(1D2) = FO
3)
FO+FO→ [FOOF] →2F+O2
∆H°= -59 kJ · mol-1
___________________________________________
Net: 2O3→3O2.
The slow step in the mechanism is step 3,
which is the combination of two FO molecules.
As known, the [FOOF] molecules have little
thermal stability; therefore, even at moderate
temperatures they may dissociate back to their
2F+O2 components, before light absorption and
photolysis has time to occur.
Mechanism II:
If the rate K2[O3] < K1[CH4] does not occur, we
can also take into consideration the following
mechanism:
F+O3→FO+O2
O3+hv→O(1D)+O*2
FO+O(1D) →F+O2→ (∆H°=-468.5 kJ · mol-1)
___________________________________
Net: 2O3→3O2
3.3 Note
Eventually, the F-atom chain can be renewed by
the reaction of O(1D) with HF:
O(1D)+HF→OH+F.
This exothermic reaction (with ∆H°=-46.9 kJ ·
mol-1), is very similar to the reaction
OH+HCl→H2O+Cl, with ∆H°= -67,2 kJ · mol-1,
by which the Cl-atom is reactivated.
3.4 Conclusions
1) The oxygen atoms O(1D) in an electronically
excited state can be considered a valid antagonist
of the methane and his combining process with
the Cl, Br, F radicals in order to form catalytically
inactive molecules HCl, HBr, HF.
2) The deactivation of the fluorine is not always
so fast as to allow the conclusion that fluorine
itself will not be able to destroy any significant
amount of ozone.
3) Taking into consideration the long life of SF6
(4)
and of generally fully fluorinated compounds,
these gases, besides being significant greenhouse
gases clearly indicated in the Kyoto Accord,
should be subject to the legislation of Montréal
Protocol.
4) As further support of such thesis according to
the original article concerning SF6 destruction of
ozone previously registered, the annexed table V
emphasizes that the four gases CCL 4 – CFC 114
– CFC 115 – Halon 1301 (see list annexed to
Montreal protocol concerning the gases destroyers
of ozone) are heavier and have smaller bond
energy (therefore smaller stability) that the fully
fluorinated compounds (SF6, C2 F6).
It follows that the above mentioned gases
prohibited from Montreal protocol, despite their
bigger molecular weight and smaller stability, in
order to be destroyers of ozone, must at least rise
to mid – stratosphere before decomposing, since
UVC does not penetrate to lower altitudes.
The smaller molecular weight and bigger
stability of the fully fluorinated compounds (SF6,
C2 F6) therefore causes that the photolysis of these
gases from the UVC and the consequent
liberation of fluorine atoms occurs at higher
altitudes that one of the above mentioned gases
prohibited from Montreal protocol; where more
energetic radiations with shorter wavelength can
break the bond and liberate the fluorine atoms.
They are therefore more able to destroy ozone
at high stratospheric altitudes where a few percent
of CH4 emissions eventually rise.
12
TABLE I – The enthalpies of formation under standard conditions (298, 15K and 1 bar)
SPECIES
∆0f (298.15K )
kJ
mol
(1) Handbook of Chemistry and Physics, ed. 1995-1996(12), Section 5-14 with reference to Chase M.W. et al., J. Phys. Chem. Ref. Data
(1)
SF6
-1220.5
SF5
-879.9(2)
SF4
-746.0(3)
SF3
-490.2(4)
SF2
INFORMATION
-296.6(5)
14(1985), Suppl. N° 1(7): -1220.473±0.8
(2) Handbook of Chemistry and Physics, ed. 1995-1996, Section 9-71(12) with reference to Tsang W. and Herron J.T., J. Chem. Phys. 96,
4272, 1992(17): -879.9±20 kJ
mol
(3) Handbook of Chemistry and Physics, ed. 1995-1996, Section 9-71(12) with reference to Tsang W. and Herron J.T., J. Chem. Phys. 96,
4272, 1992(17): -746±12 kJ
mol
(4) Recalculated on the basis of ∆0 H (298.15 K ) of SF4=746 above mentioned and of J.B. Bott experiments on the dissociation of SF4 in J.
f
Chem. Phys. 54, 181, 1971(5). For SF3-F bond strength at oH this study suggests: D00 (SF3 − F ) = 79 ± 2 kcal = 330.536 ± 8 kJ . The
mol
mol
0
kJ
value D298 (SF3 − F ) is on the basis of Table III equal to: 335.219
(330.536 + ∆ r [H (298.15K ) − H (0)] = . 330.536 + 4.683)
mol
This last value is equal to a fourth of the enthalpy of SF4 atomization at 298.15K in agreement with the Ianaf assumption(7).
In fact:
SF4
→ S rhomb + 2 F2
S rhomb → S gas
2F2
→ 4 F 79.4 × 4 =
13.0(6)
S
277.2(7)
F
79.4(8)
∆H 0298
746.0
277.2
317.6
kJ
mol
(5) Chase et al., J. Phys. Chem. Ref. Data, 14(1985), Suppl. N° 1(7): -296.646±16.7 kJ
mol
(7)
kJ
(6) Chase et al., J. Phys. Chem. Ref. Data, 14(1985), Suppl. N° 1 : 12.970±6.3
mol
(7) Handbook of Chemistry and Physics, ed. 1995-1996(12), Section 5-22 with reference to Codata Key Values for Thermodynamics 1989(8):
277.17±0.15 kJ
mol
(8) Handbook of Chemistry and Physics, ed. 1995-1996(12), Section 5-13 with reference to Codata Key Values for Thermodynamics 1989(8):
(79.38±0.3 kJ ) and Chase et al., J. Phys. Chem. Ref. Data, 14(1985), Suppl. N° 1(7): (79.39±0.3 kJ )
mol
mol
NetSF 4 → S gas + 4 F = 1340.8
SF
kJ
mol
1340.8 : 4 = 335.2
13
TABLE II – Bond strengts
DISSOCIATION REACTION
0  kJ 
∆ R H 298

=
mol
0
D298
kJ
mol
1) SF6→SF5+F
-879.9+79.4+1220.5=
420


2) SF5→SF4+F
-746+79.4+879.9=
Net Σ 1)+2)
SF6→SF4+2F
633.3
3) SF4→SF3+F
-490,2+79.4+746=
Net Σ 1)+2)+3)
SF6→SF3+3F
968.5
4) SF3→SF2+F
-296.6+79.4+490,2=
Net Σ 1)+2)+3)+4)
SF6→SF2+4F
1241.5
5) SF2→SF+F
13+79.4+296.6=
Net Σ 1)+2)+3)+4)+5)
SF6→SF+5F
1630.5
6) SF→S+F
277.2+79.4-13=
Net Σ 1)+2)+3)+4)+5)+6)
SF6→S(g)+6F(g)
1974.100
213.3
335.2
273
389
343.6
Bond
Photolysis
ν~
cm −1
SF5-F
420000
= 35109
11.96266
SF4-F
213300
= 17830
11.96266
SF3-F
335200
= 28020
11.96266
SF2-F
273000
= 22821
11.96266
SF-F
389000
= 32517
11.96266
S-F
343600
= 28722
11.96266
THRESHOLD
1
2
10 × 35109
1
10 2 × 17830
1
2
10 × 28020
1
2
10 × 22821
1
10 2 × 32517
1
2
10 × 28722
1
λ=~
ν
nm
= 284
nm
= 560
nm
= 356
nm
= 438
nm
= 307
nm
= 348
nm
14
TABLE III – Gibbs energy functions based on
SPECIES
SF6
Mr=146.050
S°(298.15 K) =
291.671
SF5
Mr=127.052
S°(298.15 K) =
322.268
SF4
Mr=108.054
S°(298.15 K) =
296.707
SF3
Mr=89.055
S°(298.15 K) =
285.610
SF2
Mr=70.057
S°(298.15 K) =
256.576
SF
Mr=51.058
S°(298.15 K) =
225.279
S
Mr=32.06
S°(298.15 K) =
167.828
F
Mr=18.99840
S°(298.15 K) =
158.750
Φ (T ) = −
0
H 298
.15 K
0
P = 1 bar
G (T ) − H
G (T ) − H (298.15K )
; Φ 0 (T ) = −
T
T
0
0
0
0
(0) ;
(from IVANTEHRMO ref. 11)
Φ(T ) = Φ 0 (T ) +
H
0
(298.15K ) − H (0) 
0
T
J 


 Kmol 
298.15 K
1900 K
2000 K
2100 K
2200 K
2300 K
2400 K
Φ0(T)
234.855
415.094
421.849
428.332
434.562
440.560
446.341
Φ(T)
291.671
424.009
430.319
436.398
442.262
447.925
453.399
Φ0(T)
259.175
433.042
439.049
444.798
450.310
455.604
460.697
Φ(T)
322.268
442.942
448.454
453.755
458.86
463.782
468.534
Φ0(T)
245.114
383.913
388.734
393.350
397.779
402.034
406.129
Φ(T)
296.707
392.009
396.425
400.675
404.771
408.722
412.538
Φ0(T)
240.170
353.874
357.673
361.306
364.787
368.129
371.343
Φ(T)
285.610
361.004
364.447
367.757
370.945
374.019
376.988
Φ0(T)
219.521
303.245
305.936
308.508
310.971
313.334
315.605
Φ(T)
256.576
309.059
311.46
313.768
315.992
318.137
320.208
Φ0(T)
193.516
257.376
259.254
261.045
262.756
264.394
265.966
Φ(T)
225.279
262.360
263.989
265.554
267.060
268.511
269.911
Φ0(T)
145.499
186.969
188.088
189.151
190.163
191.129
192.054
Φ(T)
167.828
190.473
191.416
192.321
193.188
194.023
194.828
Φ0(T)
136.887
177.418
178.519
179.564
180.560
181.511
182.420
Φ(T)
158.750
180.848
181.778
182.668
183.523
184.345
185.136
0
0
H 298
.15 K − H 0
 kJ 


 mol 
16.940
18.811
15.383
13.548
11.048
9.470
6.657
6.518
15
TABLE IV – The equilibrium constants
DISSOCIATION
REACTION
SF6 = SF5 + F
T
K
298.15
K=
2000
PSF5 × PF
PSF6
SF6 = SF4 + 2F
2100
298.15
K=
1900
PSF4 × P 2
F
PSF6
2000
298.15
F
PSF6
2100
298.15
2000
kJ
= 1241.5
mol
F
PSF6
2100
298.15
2100
kJ
= 1630.5
mol
K = 10 +0.0013
298.15
392.009+2x180.848-424.009 − 633300 =
F
PSF6
SF6 = S + 6F
2200
-94.087
2000
285.610+3x158.750-291.671 − 968500 =
298.15
364.447+3x181.778-430.319 − 968500 =
2000
367.757+3x182.668-436.398 − 968500 =
2100
256.576+4x158.750-291.671 − 1241500 =
298 .15
311.460+4x181.778-430.319 − 1241500 =
2000
313.768+4x182.668-436.398 − 1241500 =
2100
225.279+5x158.750-291.671 − 1630500 =
298.15
265.554+5x182.668-436.398 − 1630500 =
2100
267.060+5x183.523=-442.262 − 1630500 =
2200
298.15
167.828+6x158.750-291.671 − 1974100 =
=828.657-6621.163=-5792.506
2300
298.15
194.023+6x184.345-447.925 − 1974100 =
2300
194.828+6x185.136-453.399 − 1974100 =
2400
=852.245-822.541=+29.704
K = 1016.844 × 10 −110.931
K = 10 −0.189
+0.6795
K = 10 +0.6795
-145.091
K = 10 −145.091
K = 10 24.555 × 10 −169.646
-0.25
K = 10 −0.25
+0.949
K = 10 +0.949
-186.13
K = 10 −186.13
K = 10 31.33 × 10 −217.46
-0.6526
K = 10 −0,6526
+0.880
K = 10 +0.880
-247.6193
K = 10 −247.619
K = 10 37.986 × 10 −285.605
-1.7721
K = 10 −1.7721
+0.0667
K = 10 +0.0667
-302.515
K = 10 −302.515
K = 10 43.277 × 10 −345.792
-0.3204
K = 10 −0.3204
+1.5513
K = 10 +1.5513
=825.168-858.304=-6.136
2400
K = 10 −94.087
-0.189
=742.413-741.136=+1.277
∆ r H (298.15 K ) =
kJ
= 1974.100
mol
PS × P 6
F
1900
396.425+2x181.778-430.319 − 633300 =
=742.496-776.428=-33.932
PSF × P5
PSF6
+0.00130
=727.358-5468.723=-4741.365
∆ r H (298.15 K ) =
K = 10 9.888 × 10 −73.568
2100
=200.025-200=+0.025
296.707+2x158.750-291.671 − 633300 =
=608.042-591.190=+16.852
SF6 = SF + 5F
K = 10
K = 10 −0.526
=608.253-620.75=-12.497
PSF2 × P 4
∆ r S (T )
∆ H (T )
− r
19
.
1478
19
K = 10
× 10 .1478 T
−63.68
-0.526
=599.905-4164.011=-3564.106
∆ r H (298.15 K ) =
∆ r G (T )
19
K = 10 .1478 T
−
2000
=199.913-210=-10.087
453.755+182.668-436.398 − 420000 =
=479.363-461.19=+18.173
SF6 = SF2 + 4F
K=
448.454+181.778-430.319 − 420000 =
=479.462-484.25=-4.788
PSF3 × P3
-63.68
=470.189-3248.364=-2778.175
2000
kJ
= 968.5
mol
K=
298.15
=329.662-316.65=+13.012
∆ r H (298.15 K ) =
K=
322.268+158.750-291.671 − 420000 =
=329.696-333.315=-3.619
SF6 = SF3 + 3F
K=
J
Kmol
∆ G (T )
log10 K (T ) = − r
19.1478 T
=322.536-2124.098=-1801.56
∆ r H (298.15 K ) =
kJ
= 633.3
mol
∆ (298.15 K )
G (T )
= ∆ r Φ (T ) − r
T
T
=189.347-1408.686=-1219.333
∆ r H (298.15 K ) =
kJ
= 420
mol
− ∆r
16
TABLE V
BOND ENERGY
SPECIES
CHLORO
FLUORO
CARBONS
BOND
°
D298
kj
mol
*
threshold
DENSITY COMPARED
TO AIR
Cl-CF2Cl
346 ±13.4
119627
= 345
346
4.18
CFCl3 (CFC 11)
Cl-CFCl2
305±8
119627
= 392
305
4.74
CCl4
Cl-CCl3
305,9±7.5
119627
= 391
305.9
5.31
C2F4Cl2 (CFC 114)
Cl-C2F4Cl
326±8
119627
= 367
326
5.9
C2F5Cl (CFC 115)
Cl-C2F5
346±7.1
119627
= 345
346
5.31
CF3Br (halon 1301)
Br-CF3
295.4±13
119627
= 405
295.4
5.13
F-SF5
420±10
119627
= 285
420
5.11
F-C2F5
530,5±7.5
119627
= 225
530.5
4.76
FULLY
SF6 (sulfur
Fluorinated
hexafluoride)
Compounds
119627
nm
°
D298
CF2Cl2(CFC 12)
(CFCs)
Halons
λ=
C2F6
hexafluoroethane
* Handbook of Chemistry and Phisics 76th Edition (1995-1996) Section 9 Pages 66-67
17
Note : The present work has been developed with
the collaboration of Prof. Roberto Spinicci of the
Department of Energy Engineering.
(9)
References
(10)
(1) Atkinson R. et al., 1992. J. Phys. Chem. Ref.
Data 21, 1125-1568.
(2) Atkinson R. et al., 1999. J. Phys. Chem. Ref.
data 28, 340.
(3) Atkinson R. et al, 2000. J. Phys. Chem. Ref.
data 29, 183.
(4) Baird C., Cann M., 2005. Environmental
chemistry WH Freeman and Company New
York.
(5) Bott J.F., 1971. J. Chem. Phys. 54, 181.
(6) Brix P. and Herzberg G., 1954. J. Chem.
Phys. 21, 2240 (1953), Canad. J. Phys. 32,
110.
(7) Chase M.W., et al., 1985. J. Phys. Chem.
Ref. Data 14, Suppl. N. 1.
(8) Cox J.D., Wagman D.D., Medvedev V.A.,
1989.
Codata
key
values
for
(11)
(12)
(13)
(14)
Thermodynamics, Hemisphere Publishing
Corp., New York.
Greenwood N.N., Earnshaw A., 1984.
Chemistry of the elements, Pergamon Press
Ltd.
Grimsrud D.T. et al., 1980, ASHRAE Trans,
86 :1, 258-267, Atlanta.
Gurvich L.V., Veyts I.V., Alcock C.B.,
1989. Thermodynamics Properties of
individual substances – 4a Edition, vol. I,
Hemisphere Publishing Corp., New York.
Handbook of Chemistry and Physics, 19951996. 76th Edition, CRC, Press. Inc..
Herron J.T., 1987. J. Phys. Chem. Ref. Data,
16:1.
Kiang T. and Zare R.N., 1980. J. Am. Chem.
Soc. 102, 4024.
(15) Molina M.J., Rowland F.S., 1974.
Nature, 249, 810.
(16) Nagda N., Rector H, Koontz M., 1987.
Guidelines for monitoring Indoor Air
quality, Hemisphere Publishing Corporation,
New York.
(17) Tsang W. and Herron J.T., 1992. J. Chem.
Phys. 96, 4272.