[CONTRIBUTIONS F R O M THE HAVEMEYER LABORATORIES VERSITY, NO. 98.1
OF
COLUMBIA UNI-
THE DISSOCIATION O F LEAD N I T R A T E BY J. LIVINGSTON R. MORGAN.
In a recent paper with the above title,I Baekeland has given the vapor pressure of solid lead nitrate at different teniperatures ; but for some reason has made no application of the possible theoretical relations to his results. As the application of some of these shows much concerning the equilibrium, which is lost by his purely qualitative treatment, and as Mr. Baekeland has evidently no intention of considering the question further, the results of such application are presented in this paper. When heated, lead nitrate dissociates according to the following scheme :
+ + 2N0,.
PbO 0 Pb(NO,), Baekeland’s results are as follows :
TABLEI. Dissociation pressure of Pb( NO,),
___
_______
Temperatures. OC.
:
Pressures in millimeters
6.2 6-9 11.8
32.6 78.4 514.0
I
soo.02 I 180.0
In addition to these vapor pressures of the pure lead nitrate, he also gives the vapor pressure in the presence of an excess of oxygen and an excess of nitrogen peroxide. The Jour. Am. Chem. Soc. 26, 391-399 (1904). Interpolated from Mr. Baekelnnd’s curve, 1. c. 396.
i
Dissociation of Lead Nitrate
4'7
average values only of these are given in Tables I1 and I11 in order to reduce the experimental error to as low a value as possible. TABLE11. Vapor pressure of Pb(NO,), in presence of an excess of oxygen (inmm.) t= 357' C. ~~
Excess of free oxygen
Total pressure
Dissociation pressure of Pb(NO,),
Excess of free nitrogen peroxide
Total pressure of gases
Dissociation pressure of Pb(NO,),
it is apparent that r , K = 1/P,P,"r,
?
where K is the dissociation constant for the temperature in question, r rand r 2the active masses of the solids, Pb(N03)2and PbO, p , the partial pressure of the oxygen, and PZthat of the nitrogen peroxide. Omitting the active masses of the solids as constant, we should have then at any one temperature,
l/p7-,"= constant.
I.
At 357" C, from Table I, we find the total pressure of the gases from pure Pb(NO,), equal to 0.514 meter of mercury. Of this one-fifth must be oxygen and four-fifths nitrogen peroxide ; hencepI = 0.103, pz = 0.412, and
J Livingston R.Morgan
418
-
____
I/p,p," = I/o.103(0.412)~=constant
=0.062.
This constant value should also be given when one of the products of dissociation is previously present. From Table 111 we find that in presence of 0.635 (meter of mercury) of nitrogen peroxide we have 0.150 of gases from the Pb(N03)2. Of this the oxygen is one-fifth (0.150) = 0.030, and the nitrogen peroxide is four-fifths (0.150) = 0.120, which when applied in the equation gives, 1/0.03(0.120
+0.635)~=0.098.
That this high value is due to a slight experimental error is shown by the fact that a change of pressure of I O mm in the nitrogen peroxide does not change the pressure of the gases froin the Pb(N03),. We may assume then that an excess of the nitrogen peroxide keeps the reaction in its normal form. When an excess of oxygen is present, on the other hand, the reaction apparently becomes abnormal. From Table 11, by aid of the law of mass action, we have __ dO.032
and ~
~'0.024
+ = + 467(0.096)'= 0.0og1. 425(0.128)*
0.011
I t has already been observed that under certain conditions a basic nitrate of lead 3Pb0.2N205,or (Pb0)3Nz05,can be formed which has a very much smaller vapor pressure than the normal salt, From these results it would seem that an excess of oxygen favors the formation of this from the normal salt, and causes a different reaction to take place, and consequently gives a different constant. Application of van 't Hoff Equation T h e values in Table I can be used in van 't Hoff's equation to find the heat of dissociation of lead nitrate. We have
ix R X 2 . 3 0 2 X T,X T2( l o g 2P4=
T2
T 2 - TI
log
I1
where i is the number of gram-molecules of gas formed from one
419
Dissociation o f Lead Nitrate
of the original substance ( 2 . 5 in this case) ; R the molecular gas constant, i. e., Ostwald 0 . 0 2 cal ; 2.302, the reciprocal of the modulus of thesystem of logarithms; T, the lower absolute temperature, for which the dissociation pressure is PI; Tzthe higher, corresponding to PIand p the heat of dissociation for the mean tempera-
+. T Before using this equation, however, we ture, viz. T 1 ; 2
must consider just what value should be found. T h e heat of formation of Pb(N03)Zhas been determined by experiment only from the elements. T o get the value from PbO, 0 and 2N02, it is iiecessary to know the heat value of PbO and aNO,. We find Pb 2N + 60 = Pb(NO,), 1055’ Pb o = PbO 503’? 2N 40 = aNO, - 52. From these equations we get as the experimental value of the heat of formation of Pb(NO,), PbO 0 aNO, ==Pb(NO,), 640 (Ostwald calories = IOO cal.) This value, it is to be remembered, is under atmospheric pressure. T h e g, found bv aid of equation 11, however, is for the case that no external work is done by the gases as they form, or done on them when they disappear. To get the above result of 604 Ostwald calories in terms of no external work, it is necessary then to subtract from it the heat value of the work done by the atmosphere during the decrease in volume. If this is done, the negative value will correspond to the p in equation 11. For the loss of every gram-molecule of gas the work, in terms of heat, is R T Ostwald calories, where R = 0.02. I n the case of lead nitrate we have a loss in volume equal to that of 2.5 gram-molecules of gas ; i. e., $0, aNO,, so that the amount to be subtracted from the 604 in order to compare it with p (equation 11) is 2 . 5 X 0.02 T, where T is the average tem-
+
+ +
+
+ +
+
+
+
perature, the 1
T1 ~
+
2
T, of equation 11.
Ostwald’s Lehrbuch, 11, 341. Ostwald. 1. c. 348. Berthelot. Comptes rendus, go, 779 (1880).
TI _-___
I
(absolute) T2
calories 1
Average
Equation 11.
____
j
673
1
599 646
Observed
I
535 569 630
I
,
579 593 5231 '47
578 576 574 572
This value in Table I was interpolated from the curve given in the paper, so great stress may not be laid upon it.
