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Potassium Nitrate Decomposition Paper PURCHASED Fro Acs.org Michaelstarr1969

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  • 8/10/2019 Potassium Nitrate Decomposition Paper PURCHASED Fro Acs.org Michaelstarr1969

    1/4

    838

    ELIS. FREE~LIN

    Vol. 713

    and the temperature of

    150,

    at which the vapor

    pressure

    is

    78 mm., was chosen for the experiment.

    These were carried out a t a pressure of 5 3 mm. of

    disulfide and twice as much of thiol. Under these

    conditions the exchange is measurable though very

    slow,

    the half-time being abou t 40 hours.

    s

    a test

    of the homogeneity of t he reaction, the surface-

    volume ratio

    was

    varied by partially packing the

    vessel with Pyrex glass rings. Th e results are

    given in Fig. 4. Although the errors involved in

    these experiments are rather large, and therefore

    the extrapolation of the ra te to zero surface-volunie

    ratio rather arbitrary, the fundamentally hetero-

    geneous character of th e reaction is evident.

    Th e Non-base-catalyzed Exchange.-It is often

    reported in the literature that aromatic disulfides

    undergo homolytic dissociation at low tempera-

    tures (-looo), with th e formation of relatively

    stable sulfenyl radicals . This view is based on

    optical?' and magnetic?? evidence, m hich

    is

    not,

    however, entirely convincing,23

    o

    that the matter

    still seems controversial.

    It

    appeared interesting

    to determine t he ra te of exchange for diphenyl

    disulfide under those experimental conditions in

    which previous workers have proposed that sul-

    fenyl radicals exist in equilibrium with disulfide.

    In fact , if the disulfide dissociates in free radi-

    cals, an alternative path, (C), for the isotopic

    exchange is offered

    ( C ) C~,HSS-SC~H~r CsHjS. (13)

    CsHsS. + CsHs*SH

    CcHsSH

    + +CsHaS.*

    14)

    Since the hydrogen transfer

    (14)

    may be considered

    very fast, a very high rate of exchange may rea-

    sonably be predicted if t he concentration of

    C6H5S'radicals is so high

    as

    to cause t he observed

    optical anomalies.

    2 4

    I n

    the preceding sections the possibility of this

    alternative mechanism

    was

    not even mentioned,

    not because it

    was

    overlooked, but because it had

    ( 2 2 )

    A. Schonberg,

    E

    R upp a nd

    W.

    Gumllch,

    Ber.

    66B 1932

    (23) H G Cutforth and

    P.

    W Selwood,

    THIS

    OWRKAL T O 278

    2 4 ) 1 2 Lecher, Sczet tce, 1 2 0 , 220 (1954).

    (1933).

    (1948).

    been proved in some preliminary experiments tha t

    at low temperature the reaction

    is

    base catalyzed

    and occurs through the intervention

    of

    C~HSS-

    ions. Other experiments were therefore made in

    n hich the concentration of CeH5S-

    was

    kept as low

    as

    possible in order to make the exchange through

    mechanism

    B) a

    minimum. This was attained by

    performing the reaction in anhydrous xylene IT-here

    dissociation of C6H5SHmay be thought to occur at

    a

    very small extent. Experiments a t 100 showed

    a

    slow rate: for concentration

    of

    reagents

    7.3 X

    S

    the half-time

    was 10

    hours and

    R = 7 3

    X

    lo-

    mole 1.-1

    set.-'.

    This figure shows that if

    mechanism (C) be assumed, the stationary concen-

    trat ion of sulfenyl radicals must be extremely

    small and could not possibly be responsible for the

    observed optical anomalies. Rath er, the rate is

    so

    slow tha t i t can be perhaps accounted for by mech-

    ansim B) since the concentration of CsH5S-,

    though small, cannot be considered zero. Thi s

    view is supported by the value of the ra te of ex-

    change, in the same experimental conditions, be-

    tween 12-butyl disulfide and the corresponding

    thiol. Experiments gave =

    4.3

    x tha t is,

    less than

    1 / 2 0

    as fast. It is then probable that

    the mechanism is the same in both exchanges and,

    since there is no evidence whatsoever for thermal

    homolytic fission of aliphatic disulfides, th at mech-

    anism B) s likely to be operative also in these ex-

    perimental conditions. It is realized, however,

    that only

    a

    complete kinetic study can unequivo-

    cally assign the mechanism.

    Acknowledgment.-The author s are indebted to

    Prof.

    11.

    Calvin for having suggested the experi-

    ments with the cyclic disulfide and for the use of

    the recording spectrophotometer, to h Ir . P. l f .

    Hayes of the Radiation Labora tory of th e Uni-

    vers ity of California for a sample of trimethylene

    disulfide and to Professors

    11.

    Calvin and

    R. E.

    Powell of the Department of Chemistry, University

    of California a t Berkeley, for helpful discussions

    an d suggestions during the preparation of t he

    manuscript.

    PAD0VA4,

    IT.4LI'

    [CONTRlBUTION FROM

    TIIE

    PYROTECHTICS CHEMICAL RESEARCHABORATORY, PICATINSY .IRSENAL]

    The Kinetics

    of

    the Thermal Decomposition of Potassium Nitrate and

    of

    the Reaction

    between Potassium Nitrite and Oxygen1

    B Y

    ELI S. FREE MAN

    RECEIVEDCGUST 9, 1956

    The kinetics of the thermal decomposition of potassium nitrate were studied in oxygen, at a constant pressure

    of

    one

    atmos phere , over the temp era ture range of 650 to 800 . The rat e of reaction wa s follomwl

    by

    observing changes in volume

    as

    a

    function

    of

    time. From 650 to

    750

    the products

    of

    decomposition were found

    to be

    potassium nitrit e, oxygen and

    traces of nitrogen dioxide.

    Equilibrium was also attaine d between potassium nitrat e, potassium nitrite an d oxygen. At

    800 ,

    decomposition was more extensive, with potassium ni trite dec omposing to form nitrogen, oxygen and potassium oxide.

    The reaction between potassium n itrit e and oxygen was investigated fr om 550 to

    790,

    by nie asuring th e r at e of consumptior1

    o f

    oxygen to form potassitmi ni trate.

    From 650 to 750,

    equi-

    libriuin was attained between the reac tants an d potassium nitrat e

    . .4t

    790, decomposition

    of

    potassium nitrite was evident.

    'l%e equilibrium cons tants

    of

    the system were calculated from the

    data , and

    on the basis of their t emp era ture dependency,

    tlie heats of reaction for decomposition and oxidation were determined. I reac tion tnecllanism is proposed an d the kinetics

    of the reactions as well as the energies of activation were ev:iluated. In addition,

    sonic

    of th e results of thi s study ar e coni-

    pared with those obtained in a previous investigation of sodium nitra te a nd sodiurn nitrite.

    Fro m 550 t o 60O0, the reaction

    gocs

    slowly to completion.

    1)

    (a) This paper was presented, in part , before the Division of

    Physical and Inorganic Chemistry at th e North Jersey

    Meeting in

    hliniature

    of

    th e American Chemical Society in Xervark, N.

    J , ,

    anu-

    ary

    1956,

    and a t the Delaua re Valley Regional hieeting in Philadel-

    phia , Pa . , February 1956; (b) Th e Newark Colleges of Rutgers

    Uni-

    versity, Xervark 2 , h-. .

  • 8/10/2019 Potassium Nitrate Decomposition Paper PURCHASED Fro Acs.org Michaelstarr1969

    2/4

    Feb. 20, 1957 THERMALECOMPOSITION

    F

    P O T A S S I U M NITRATE KINETICS s39

    Introduction

    Due to th e extensive use of alkali nitrates in

    pyrotechnics, explosives, rocket ignitors as well as

    in metallurgical hea t t reating, there is considerable

    interest in the high tem peratu re behavior of these

    salts. Previous work dealt, primarily, with the

    identification of the reaction products and the de-

    termination of t he decomposition temperature^.^-^

    Much

    of

    the former work, however, was carried

    out in quartz which led to confusing results due

    to a reaction between silica and t he nitra tes.

    This investigation is concerned with the reac-

    tion kinetics of the the rmal decomposition of

    po-

    tassium nitrate. Th e reaction between potassium

    nitrate and oxygen was also studied to aid in the

    elucidation of the mechanism of decomposition.

    Some of t he result s

    of

    this investigation are com-

    pared with the results obtained from a similar

    study of sodium nitrat e and sodium nitrite.6

    Experimental

    The potassium nitrate and potassium nitrite were pur-

    chased from the Fisher Scientific Co. and were of

    C.P.

    Grade. The oxygen,

    99.8y0

    pure, was obtained from the

    Matheson

    Co.

    Of several types of reaction vessels tested,

    it was found that stainless steel 317 was suitable for this

    stu dy. Spectroscopic analyses of melts which were heated

    to the experimental temperatures showed that the extent

    of at ta ck on the stainless steel vessels was negligible over

    the experimental times. Furthermore, t he inner surface

    did not appea r to catalyze the reaction since varying the di-

    mensions of the vessel had no significant effect on the rate

    of decomposition.

    The experimental procedure and apparatus are identical

    to that used previously6; the gases, however, were analyzed

    b y t he Orsa t m e t h ~ d . ~ he reactions were carried out in

    oxygen, a t one atmos phere of pr essure, in Type 317 stain-

    less steel tubes. The dimensions of these vessels are 0.1

    cm. wall thickness, inside diameter, 1.6 cm. and 1.3 cm.,

    length, 13 cm. Unless otherwise mentioned, the reactio ns

    were conducted in t he tu bes having a n inside diame ter of

    1.6 cm. X-Ra y analysis was used t o identify the solid

    products.

    Results and Discussion

    Figure

    1

    shows a graph

    of

    the increase in volume

    v s . time for th e therm al decomposition of potas-

    sium ni tra te in oxygen. At temperatures of 650,

    700 and 750') the reaction products were found, by

    X-ray and gas absorption analyses, to

    be

    potas-

    sium nitrite, oxygen and traces of nitrogen dioxide

    (less than

    1 ).

    The increase in volume, there-

    fore, is almost entirely due to the evolution

    of

    oxygen. After a period of time no fur the r changes

    in volume were observed, indicating that the sys-

    tem had attained equilibrium. At

    800,

    the de-

    composition of potassium nit ri te becomes impor-

    ta nt as indicated by the formation of nitrogen.

    This was confirmed by analyses of the gases result-

    ing from the decomposition

    of

    potassium nitrite.

    Changing th e dimensions of t he reaction vessel did

    not alter t he reaction ra te significantly.

    The variation in weight

    of

    potassium nitrate

    and nitrite was autoniatically recorded as samples

    (2) Kurte Leschewsiki Bcv . ,

    72B

    763 (1939).

    (3) Kurte Leschewsiki

    B e v .

    Ges P r e w n d e n

    Te c h

    Ho c h Schule

    Berliii

    (4) K. Butkov ActaPhysiochem.

    U.R.S.S.,S,

    137 (1936 ) ;

    C.

    A . 8 1

    5 ) P. L. Robinson H .

    C .

    Smith and H. V. A . Brescos J Chem. SOL.,

    0 )

    E.

    S . Freeman J Phrs. Ch em . , 60

    1487

    (1950).

    (7)

    E.

    S. Freeman and

    S . Gordon, THISOURNAL 78 1813

    (1956).

    1, 168 (1942).

    5639 (1937).

    836 (1926 ) .

    J

    10 20

    30 40

    50 60

    70 80 90

    Time, min.

    Fig. 1.-The decomposition of potassium ni trate in oxygen

    (1

    atm.), temp.,

    OC.:

    0, 650;

    A,

    700; 0 750; 0 ,800

    were heated from room temperature to 1000, at a

    ra te of 15'/min.

    -4

    Chevenard thermobalance

    was employed for thi s purpose. When decomposi-

    tion was complete (970') the total weight losses

    corresponded, within

    3 )

    to the formation

    KzO.

    Potassium nitrite, heated in t he presence of

    oxygen from 550 to

    790 )

    forms potassium nitrate.

    Th e course of this reaction is shown in Fig. 2, a

    graph of the change in volume per g. of potassium

    nitrite

    V S .

    time. The rate

    of

    reaction increases

    with temperature, but the extent of reaction de-

    creases. At 550 and GOOo the reaction is con-

    tinuous and eventually goes to completion. From

    650 to 750, as in the case of decomposition of

    potassium nitrate, the system attains equilibrium.

    At 790, a rapid decrease in volume is first ob-

    served, followed by a period of

    15

    min. dur ing which

    no volume changes occur. This is then followed

    by an increase in volume due primarily to t he evolu-

    tion of nitrogen, which is att ributed to th e decom-

    position of potassium nitrite.

    At 750') the reaction was also carried out in

    a

    tube having an inside diameter of 1.30 cm. rather

    than t he usual 1.6 cm. Th e specific rat e was found

    to decrease from 0.190 min.-l to 0.125 min.-l.

    The ratio

    of

    specific rates for the reaction in b oth

    size vessels is 1.52 and is equal t o th e ratio of th e

    corresponding contact areas between the melt and

    gaseous atmosphere. This indicates that the

    oxidation process is heterogeneous, taking place,

    principally, at the liquid-gas interface. A similar

    surface dependency was observed for the case

    of

    the reaction between sodium nitrite and oxygen.6

    Th e equilibrium constants, defined in eq. 1 were

    determined from the d ata in Figs.

    1

    and

    2.

    K ,

    = equilibrium constant

    Nl and N2 = mole fraction of potassium nitrate or nitrite,

    depending on whether decomposition or

    oxidation is cotlsidered

    For this purpose the standard states of the salts

    were taken as pure molten potassium nitrate and

    potassium nitrite and for oxygen one atmosphere

    fugacity. Th e assumption of ideal behavior of

    the melt is made since the ni trate and n itrite ions

    are both univalent and their effective diameters

  • 8/10/2019 Potassium Nitrate Decomposition Paper PURCHASED Fro Acs.org Michaelstarr1969

    3/4

    s40 ELIs.

    FREERIAN

    Vol.

    79

    0

    10

    20

    :

    0

    40

    30

    100

    110

    120

    50

    100 150 200 250

    300

    Time, min.

    Fig. 2.-The reaction between potassium nitri te and

    oxygen (1 atm. ), temp., C. :

    (23,

    5 5 0 ;

    69

    00;

    0

    50;

    A ,

    700;

    0

    50;

    0

    90.

    are approximately th e same. Furthermore, sys-

    tems generally approach ideality at high tem-

    peratures. Th e act ivi ty coefficients were there-

    fore taken as unity . Th e respective mole fractions

    of potassium n itr ate and ni trite were determined

    from the amount of oxygen evolved or consumed

    during reaction up t o the time of equilibrium.

    Th e equilibrium constants, determined from the

    reaction between potassium nitrite and oxygen are

    reasonably close to the reciprocals of t he equilib-

    rium constants as obtained from the decomposi-

    tion studies. These values are

    14.0, 4.6, 1.9

    and

    14.1, 5 . 2

    and

    2.4

    a t

    750,

    700

    and

    650,

    respectively.

    It

    should

    be

    noted th at a small error in the meas-

    ured volume results in a relatively large error in

    the calculated equilibrium constants. For exam-

    ple, a t 650 a difference of

    5yo

    in the volume

    at

    equilibrium resulted in

    a

    l2Yb variation between

    the cqiiilibriuni constants.

    Figure 3 log Ke

    us.

    T- shows t he temperature

    dependency of the equilibrium constants. The

    hea ts of reactions evaluated from the slopes of the

    lines are 30.8 and

    -32.8 kcal. mole- for the de-

    composition of potassium nitrate and the reaction

    between potassium nitrite and oxygen, respectively.

    Correspondiiig temp., C .

    747 707 71 637

    .8

    F I y

    G

    0.9so

    1.020 1.060 1.100

    Fig.

    3.-Temperature dependency

    of

    equilibrium

    coi l -

    stants: A , right coordinate, KN03 = KXOz

    +

    / 2 0 2 ;

    0

    left coordinate, l / * 0 2 +

    KN02=

    KNOa.

    Th e corresponding values calculated from hea ts of

    formation datas are 31 kcal.- and -31 kcal.

    mole-l. For this calculation an approximate

    temperature correction has been applied where i t

    was assumed that the difference between the heat

    capacities of the sal ts is negligible, and a value of

    3.59

    cal./mole deg.

    was

    taken

    as

    the average heat

    capacity of oxygen over th e experimental tempera-

    ture range.

    Using the mean experimental value, 31.3 kcal.

    mole-, for the heat

    of

    reaction and

    118.2

    kcal.

    mole- for the dissociation energy of oxygenlo th e

    dissociation energy of the

    N-0

    bond in potassium

    nitrate was calculated to be

    90.0

    kcal. mole-.

    From similar experiments with sodium nitrate and

    sodium nitrite6 the

    IS-0

    bond energy was deter-

    mined to be 83.6 kcal. mole-. It appears then

    th at t he sodium ion weakens the

    N - 0

    linkage, prob-

    ably due to its greater polarizing effect on the ni-

    trate ion.

    If

    one assumes tha t the mechanism of the de-

    composition of potassium nitrate is similar to that

    of sodium n it ra tq6 where decomposition and oxida-

    tion involve a two-step chain reaction and eq.

    2

    and

    3 are ra te determining, the following sequence of re-

    actions is indicated.

    T - 1 x

    103,

    O K . ~ .

    K S O Y+ 0 2 kl_ KNO3 f 0

    KXOs

    +

    Ok r _KSO a

    ( 3 )

    KNOs

    kr_

    KXOa

    +

    0

    KNOa

    + 0 K,_ SO?+

    0:

    (2)

    4)

    ( 3 )

    ~

    (8) Kational Bu reau of Stan(l;rr(ls: Circular .;00, Selected

    Values or

    Chemical Therm odynamic Properties, b y F. D. Rossini,

    D .

    D. WW-

    ma n,

    W.

    H. Evans,

    L.

    Levine and I . Jaffe , February,

    1952.

    (9)

    Handbook

    of

    Chemistry and Physics, 37th

    Ed. ,

    Chemical

    Rubb er Poblishing Co., 1955-195t3, p . 2107.

    (10) I.. Pauling, Nature of the Chemical Eon d, Cornell Uni-

    versity Press, Ithaca. N.

    Y . , 19.18.

    1 ( i l .

  • 8/10/2019 Potassium Nitrate Decomposition Paper PURCHASED Fro Acs.org Michaelstarr1969

    4/4

    Feb.

    20, 1057 THERMAL

    ECOMPOSITION

    F POTASSIUM

    ITRATE KINETICS

    841

    kl, k2, ka and k4 are specific rates referring to the

    formation of potassium nitrate and nitrite. Th e

    reaction

    2 0

    +=

    O 2

    was not considered since the con-

    centration of atomic oxygen should be negligible

    compared to t he concentration of potassium n itrite

    and nitrate.

    The rat e equation for the formation of potas-

    sium nitrate, based on the above reactions is

    dNKNOs

    = k l N K N o , N o ,

    +

    k z N K N o 2 N o

    dt

    N =

    mole fraction

    kl , k2 , ka1k4 = rate constants

    k 3 N K N 0 3 k K N O sN O (6)

    20

    40

    60 80 100 120 140

    160

    180

    Time, min.

    Fig. 4.-Kinetics of reaction between potassium nitrite

    and oxygen (1 atm.). Temp., C.:

    3,

    550,

    e 00;

    Islo,

    650;

    A ,

    700; 63 50.

    12

    10

    2

    X

    - 8

    I

    j

    6 6

    5

    M

    0

    A

    5 4

    hl

    2

    5 10

    15 20

    Time, min.

    Fig. 5.-Kinetics of the decomposition of potassium ni trate

    in oxygen

    1

    atm.), temp.,

    O C : 0,

    600;

    A ,

    700;

    E , 750.

    1 1 ) The derivation

    of

    the rate expressions

    is

    given in re f .

    6

    Corresponding temp., C.

    727

    637 561

    0.900

    1.000 1.100

    1.200

    T-*

    x 103,

    O K . - ~ .

    Fig. 6.-Temperature dependency of the ra te const ants:

    A , reaction between potassium nitrite and oxygen; 0,

    decomposition of potassium nitrate.

    By

    applying the steady-state approximation for

    atomic oxygen at a constant oxygen pressure dur-

    ing the reaction, the terms N O , and No may be

    incorporated into the specific rates giving:

    dNKNOs k l ' N K N o 2

    +

    k Z 1 N K N o 2 3 N K N O s

    dt

    ~ ~ ' N K N O ~7)

    k i t , k z ' , k4' = new specific rates

    At equilibrium dNKN03/dt

    =

    0.

    equation is therefore obtained

    K I =

    2.3xe/at) log x,/ x, X )

    The following

    8 )

    K1

    =

    k'i

    + k z

    xe = no. of moles of potassium nit ra te a t equilib rium

    x =

    no.

    of moles of potassium nit rat e formed

    a

    =

    initial no.

    of

    moles of potassiun nitr ite

    t

    = time

    For the decomposition of

    KN03,

    one obtains

    KZ

    ( 2 .3

    Y e b t )

    log

    y e / ~ e

    Y)

    (9)

    Kz

    =

    k3

    +

    k ,

    b = initial no. of moles of potassium nitr ate

    y

    =

    no. of moles of po tassium ni trit e formed

    ye

    = no. of moles of potassum nitr ite a t equilibrium

    Figures

    4

    and 5 show the result of substi tut ing

    the da ta in eq.

    8

    and

    9

    and plotting as a function of

    time.

    It

    should be noted th at at 550 and 600,

    the reaction between potassium nitrite and oxygen

    goes to completion and consequently

    X e

    = a.

    This reduces eq.

    7

    to the usual form of a first-order

    non-reversible expression for these cases.


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