cUM^ • ?/3 1°?^-

NATIONAL BUREAU OF STANDARDS REPORT

10 860

Afb

THE ENTHALPIES OF HYDROLYSIS OF ARSENIOUS OXIDE AND

ARSENIC TRICHLORIDE IN IN-SODIUM HYDROXIDE:

THE ENTHALPY OF FORMATION OF ARSENIC TRICHLORIDE

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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2

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Maryland, unless otherwise noted; mailing address Washing-
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3160111 September 1971

NBS REPORT

10 860

THE ENTHALPIES OF HYDROLYSIS OF ARSENIOUS OXIDE AND

ARSENIC TRICHLORIDE IN IN-SODIUM HYDROXIDE:

THE ENTHALPY OF FORMATION OF ARSENIC TRICHLORIDE

By

J. I. Minor, Jr., A. A. Gilliland and D. D. Wagman

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government,

and review. For this reason, th< whole or in part, is not author Bureau of Standards, Washingto

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ss accounting documents intended

subjected to additional evaluation

listing of this Report, either in

» Office of the Director, National

ty the Government agency for which

opies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

ABSTRACT

The heats of hydrolysis in IN NaOH were measured for three

forms of As^O^Cc) and for AsCl^(^). These were found to be:

As 20s

(e^ oetahedral) AH = -43.985±0.157 kJ*mol"^ (-10.513±0. 038 kcal*mol“^)

A^Os^^monoelinie) Ad = -45.990±0.073 kJ'mol ^ (-10. 992±0. 017 kcal*mol

As

20, (vitreous) £& = -63.957+0.222 kJ-mol

-1

(-15. 286+0. 053 kcal-mol"

1

) AsC1

3 (^) Z^H = -297.533+0.274 kJ*mol

_1

(-71. 112±0.065 kcal-mol"

1

)

The heat of formation of AsCl^ was determined from direct chlorination of metallic arsenic to be; AHf OQO 1c = -305 . 222±1.874 kJ.mol

(-72 . 950±0.448 kcal.mol).

From these data a value for the heat of formation of As^O^

(c, octahedral) is derived, AHf

2 ^g

^

= -658.416±3.790 kJ*mol ^ (-157.365±

0.906 kcal«mol and a value for the heat of transition As^O^ (c, monoclinic)

-* ^s^O^c, octahedral), Z^H = +2 .005±0. 173 kJ'mol ^(+0.479±0.041 kcal*mol ^).

1

INTRODUCTION

The thermochemistry of every element revolves around certain

so-called "key" compounds. The reliability of the total structure of

thermodynamic values assigned to the family of compounds of an element

depends greatly upon the accuracy of the values assigned to the key

compounds. A key compound of the element arsenic is As^O^ (c_, octahedra 1) .

The cell measurements of R. Schumann [1], and of Kirschning and

Plieth [2], the specific heat measurements of C. T. Anderson [3],

and the solubility measurements of J. Thomsen [4], of Stranski. Plieth and

Zoll [5], and of Schulman and Schumb [6] were considered by NBS in

selecting "best values" [7] for As^O^ and AsCl^ heats of formation.

These investigations were all performed prior to 1958.

In more recent years a series of investigations by Mortimer and

Beezer [8,9] have questioned the NBS value for As

? 0

2

. Their work was

done with 2N NaOH and followed the scheme:

As(c) + 3/2Br

2 (g) -*AsBrg(£)

AsBr^(^) + 4NaOH(aq) -» NaAs0

2

(aq) + 3NaBr(aq) + 2R

? 0(&)

As^^ (c,octa) + 2NaOH(aq) -* 2NaAsC>

2

(aq) + H

2 0(£)

As will be seen later, this is the route used in this present investigation;

substituting Cl

2

and AsCl^ for Br

2

and AsBr^ used above.

Four reactions were used in the course of these investigations:

l.a. As

2 0^ (c, octa . ) + 2NaOH(aq) 2NaAs0

2 (aq) + H

2 0 (j&)

b. As

2 0^ (c,mono.) + 2NaOH(aq) -* 2NaAs00 (aq) + H

2 0 (j6)

c. As

2

0

2

(vitreous) + 2NaOH(aq) -» 2NaAs0

2 (aq) 4- H2

0(j£)

2

2. AsC1

3 (4) + 4NaOH(aq) ->NaAs0

2

(aq) + 3NaCl(aq) + 2H

2 0 (j£)

3. 3NaCl(c) + NaAs0

2

(aq) NaAs0

2

(aq) + 3NaCl(aq)

4. As (c,Q') + 3/2Cl

2 (g) AsCl^CX)

Equations lb and lc, when used in conjunction with la, allow an

evaluation of the heats of transition between the various solid forms

of As

2

0

3

» However, equation la is the one used for all other calculations .

3

MATERIALS

Arsenic

:

metallic arsenic, identified by Fisher Scientific Co. as "purified" was used. Analysis in this laboratory indicated a

purity > 99.8$.

Arsenious oxide: samples of arsenolite, AS

2

O

3

(c, octahedral) , of

claudetite, As^O^ (c,monocl inic) , and of the vitreous

oxide, AS

2

O

2

(amorp . ) were supplied by A. Bestul and

S. Chang of NBS [10]

Chlorine: chlorine gas with an assay value of 99.85$ was obtained from

The Matheson Co., Inc.

Sodium chloride

S odium hydroxide* from Baker Chemical Co., Reagent Grade. The sodium

hydroxide solutions were prepared carbonate free and

stored protectively from CC^*

EXPERIMENTAL PROCEDURES

These investigations required two separate calorimetric assemblies,

one for the solution reactions (Eq. 1,2,3) and one for the formation

reaction (Eq. 4)

In each instance the controlled temperature water bath was the same.

The temperature was controlled to ±0.002°C by a system consisting of a

resistance bridge with a nickel sensing element as one arm, a D.C.

null detector and a current-adjusting type (C.A.T.) control unit. The

power was supplied by an audio-amplifier to a 126 Q heat coil.

4
I
The calorimeter vessel used in the solution experiments was described

in an earlier work by Johnson, Gilliland and Prosen [11]. The solution

calorimetric vessel was placed directly into the controlled temperature

water bath. Temperature measurements were made on the solution within

the vessel.

For the heat of formation experiments the reaction vessel consisted of

a closed-end glass tube (approx. 17mm dia.) with a 6mm side-arm approximately

one inch from the bottom. The side-arm leads directly to the outside of

the system and serves as a means of ingress for the gaseous reactant. The

open end of the vessel is a 19/22 standard taper (inner part). The cap

for the vessel is a 19/22 standard taper (outer part) leading through a

succession of coils -.and terminating outside of the system. A drainage

bulb is sealed onto the lowest of the coils to facilitate the collection

of the liquid products. A small glass wool sample support plug sits in

the vessel just above the side-arm. The exit end of the coils is fastened

to a series of gas scrubbers containing alkali.

The heat-of-forma tion reaction vessel was placed in a copper

3 calorimeter can (approx. 2000 cm ) which was suspended within a closed

submersible container. In usage the entire system is submerged in the

water bath. The calorimeter can is filled with distilled water and temperature measurements are made on the water in the calorimeter can. A 170 Q

immersion heater was used for the electrical calibration for these reactions.

All samples, with the exception of the metallic arsenc, were sealed

off in pre-weighed soft glass bulbs. The sealed bulb and sample were

then weighed along with the excess glass tip. The difference in weights was

5
§-

'
taken as the sample weight which was corrected to "in vacuo" weight.

Weighing was done on an analytical balance using NBS calibrated brass

weights. The As^ and NaCl were poured directly into the bulbs.

The AsCl^ was vacuum distilled into the bulbs from a break-off tip

ampoule. The metallic arsenic was weighed directly into the reaction

vessel using a Mettler single-pan balance. All weighings were to ±0.01 mg.

Each experiment consisted of an electrical calibration followed immediately by the chemical reaction. The sequence of events was: an

initial rating period, an electrical heating period, a middle rating

period (which served both as the final rating period for the calibration

and the initial rating period for the reaction), a chemical reaction period,

and a final rating period.

The rating periods were of twelve minute duration, the electrical

heating periods thirty minutes (during ten of which power was being

introduced), and the chemical heating periods from thirty to sixty minutes.

At the appropriate time after the middle rating period, the hydrolysis

reactions were initiated by depressing the bulb crusher. In order to

insure complete breakage the rod was passed entirely through the sample

holder two or three times. It was then withdrawn to the upper boundary

of the holder to prevent interference with the mixing of sample and solvent.

Prior to the first rating period in the AsCl^ formation experiments,

the vessel was flushed for approximately 5 minutes with He. This was then

diverted to a by-pass so that the rating was done with an internally

static atmosphere. The was introduced into the line near the end of

the middle rating period. The rate of flow of the gas mixture was adjusted

while flowing through the by-pass.

6
.
The reaction is quite spontaneous. At the desired time the flow of

the gas mixture is turned from the by-pass to the vessel. After approximately 2-4 minutes the Cl^ was shut off and the vessel was flushed with

He for 10 minutes. This was then shut off also, so that the final rating

period could be taken with a static atmosphere as were the previous two.

Temperature measurements were made using a 25 0 platinum resistance

thermometer in conjunction with an L & N G-2 Mueller Resistance Bridge and

an L & N Microvolt Indicating Amplifier which served as a galvanometer. In

the heat of formation experiments the galvanometer output was fed into an

M-H Brown Recorder.

These measurements were made at two-minute intervals during the rating

periods and at one-minute intervals during the heating periods. For the

rating periods, a galvanometer reading is taken 5 seconds before the minute.

The thermometer current is then reversed and a second reading is taken

5 seconds after the minute. In the experiments using the recorder, these

intervals were increased to 10 seconds before and after. The readings during

the heating period were taken with no current reversal.

Power for the electrical heating came from a Lambda Regulated Power

Supply which was fed by a Sola Harmonic Neutralized Constant Voltage

Transformer

.

Power measurements for the experiments were made using a White

4 1

Double Potentiometer (10 -10 p,v) in conjunction with the L & N Microvolt

Indicating Amplifier as galvanometer. In all other experiments the White

4 0

Potentiometer was replaced with a Wenner Potentiometer (10 -10 jjlV) . This

afforded an increase in resolution by a factor of ten, but necessitated a manual switching for readings of e^, the voltage readings for calculation

of the voltage drop across the heater, and e^, the voltage readings for

7
..

'
calculation of the current through the heater. The e^ readings were made

across a 0.1 G standard resistor, the e^ readings were made across a 4

10 Q standard resistor which was in series with a 10 Q standard resistor.

This 10010 Q system was in parallel with the heater. (See sketch).

The readings were made alternately, beginning with e^, at 30-second

intervals, for a period of ten minutes. This gave eleven e^ readings

and ten e^ readings. Their respective averages were used in the

computations. The observed values were corrected not only for potentiometer

zero and calibration corrections, but also for the variation of the

indicated emf from the true emf.

The products of the AsCl^ reactions (Eq. 2,4) were titrated with

+3 standard iodine solution to determined the As involved. Products from

Eq. 4 experiments were first reduced with hydrazine sulphate as described by

Morries [12].

Analysis of the products of the early AsCl^ (Eq. 2) hydrolysis experiments was not made immediately following the experiment. These gave

results that were appreciably low in comparison with the results obtained

when analysis immediately followed the experiment. (See Table 6).

Because of this, the original mass of sample was used for all calculations

in the AsCl^ hydrolysis series.

Analyses of the products of Eq. 4 experiments were used to determine

the amount of As used in the reaction.

8
'

r
Sketch 1

In Tables l y 2 } 3, and 4 the column headings have the following

meanings

:

Ms - the mass of sample as weighed

Es - the energy equivalent of the system as determined by electrical

ca librat ion

Rm - the mean temperature

E S corr

" t *ie energy equivalent of the system at the mean temperature

of the reaction; Es + Temp. Coeff. (Rm - Rm ,

)

react calib

ARc - the corrected temperature rise of the reaction

Q - the heat evolved by the reaction; Es x ARc

corr

M.W. - the molecular weight

Rf - the final temperature of the reaction

AT - the variance of the final temperature from 25 °C; ^

25 ”^

Corr^ - the correction for temperature of the heat evolution; AT x Acp

' o' x M W

AHoc. - the observed heat of reaction at 25 °C; (~——

!—'-) - corr.

Id Mis AH

25 - the heat of reaction at 25 °C adjusted for dilution effects.

Table 3 has the following additional designations:

m^ - the arsenic from analysis of the liquid product

m^ - the arsenic from analysis of the gaseous product

m - the total arsenic from analysis

a

y

r - the ratio M /m

m

g

Corr

2

s a the adjusted mass of arsenic appearing as AsCl^(g); (r x m^) /M.W. (arsenic)

- the correction to the observed heat for the vaporization of

part of the product; (m0 /M.W. (arsenic)) x AH

z. va p

10

Q

- the observed heat evolved plus the vaporization correction

(E scx ZR ) + [(m0 /M.W.) x Zm ] 2 vap

The following constants were used in calculating:

Es temperature coefficient

-2

Table 1 364.705 J . Q

Table 2 364.705

Table 3 630.

Table 4 100. for H

2

0

250. for As 0 (aq)

2 ^

Z^cp Reaction

-1

Table 1 0.757 KJ • molQ

Table 2 -2.341

Table 3 0.374

Table 4 negligible

R

25 = 0.503780

11

In Table 1, the A; C, and V following experiment designations

indicate, respectively, the octahedral, monoclinic, and vitreous

forms of As^O^.

The change of energy equivalent with temperature was calculated from data obtained in Expts. 2 and 3 of the AsCl^ hydrolysis.

This value was also used for the As^O^ experiments.

The value for Acp was calculated from data obtained in

Expt. 4A of the AS

2

O

3

series and Expts. 9 and 10 of the AsCl^

series. The equation used was:

(-Q

f

) - (-Q

I

) R

i

Acp

where Q is calculated using the calibration preceding the reaction

F

and Q is calculated using the calibration following the reaction.

Similar calculations were made for the solution of NaCl in

(ks^O^ + NaOH) . See Table 4.

12
'
Table 1

Heat of Solution of as

2 0^ in IN NaOH

M

/ E R E 4R

s s Calib. React. s corr

c g J-Q"

1 Q Q J-Q" 1 Q

1A 4.34296 20808.390 0.420383 0.466808 20825.321 0.046233

2A 3.53898 20817.776 0.454094 0.565559 20858.428 0.037787

3A 3.50111 20813.823 0.444080 0.548726 20851.988 0.037124

4A 3.84889 20795.591 0.355218 0.472916 20838.516 0.041042

1C 2 . 93445 20806.795 0.428834 0.546354 20849.655 0.032592

2C 2.83046 20807.966 0.419690 0.536971 20850.739 0.031378

IV 3.67722 20816.929 0.417244 0.539732 20861.601 0.056964

2V 3.71024 20804.717 0.407038 0.533878 20850.976 0.057541

3V 3.98752 20812.230 0.401812 0.530130 20859.028 0.061505

Q x M.

M

AT Corr^ ^25 m

25

KJ.mol -1 Q q KJ-mol " 1 KJ.mol- 1 KJ- mol

-

1A 43.861 0.496771 0.007009 0.005 43.855 43.855

2A 44.062 0.590056 -0.086276 -0.065 44.127 44.204

3A 43.744 0.573244 -0.069464 -0.053 43.797 43.879

4A 43.962 0.500160 0.003620 0.003 43.959

ave

44 . 003

= 43.985

1C 45.814 0.568474 -0.064694 -0.049 45.863 46.027

2C 45.731 0. 558582 -0. 054802 -0.041 45.772

ave

45.952

= 45.990

IV 63.936 0.574038 -0.070258 -0.053 63.989 64.050

2V 63.976 0.571842 -0.068062 -0.052 64.028 64.086

3V 63. 653 0.569800 -0.066020 -0.050 63.703

ave

63.735

= 63.957

Q

J

962.817

788.177

774.109

855.254

679.532

654.254

1188.360

1199.786

1282.935

± 0.157

± 0.073

± 0.222

13
m
Table 2

Heat of Solution of AsC1^(j2») in IN NaOH

M E' R E Q s s Calib. m

React. s corr

c g J-Q" 1 n Q J'Q” 1 Q J

1 2.56649 20799.786 0.342808 0.505472 20859.110 0.187816 3917.675

2 1.72883 20834.899 0.410100 0.522742 20875.980 0.132146 2758.677

3 3.26580 20814.934 0.398788 0.568840 20876.953 0.249239 5203.351

4 2.64335 20756.285 0.306010 0.457174 20811.415 0.202093 4205.841

5 1.34298 20798.222 0.418136 0.533256 20840.207 0.102468 2135.454

6 2.98697 20738.442 0.387590 0.555626 20799.726 0.228066 4743.710

7 1.21587 20816.435 0.422838 0.534436 20857.135 0.093496 1950.059

8 0.67854 20723.558 0.387260 0.479845 20857.234 0.052613 1092.100

9 2.59940 20804.594 0.334406 0.541312 20880.054 0.199422 4163.942

10 2.43583 20847.861 0.343832 0.543699 20920.753 0.186238 3896.239

Q' x M.W.

M

“p z5T Corr^ M

25 ^25

KJ- mol -1 Q Q KJ* mol -1 KJ •mol” 1 KJ* mol

1 276.720 0.600056 -0.096276 +0.224 276.496

2 289.268 0.590361 -0.086581 +0.203 289.065 290.017

3 288.832 0.693362 -0.189582 +0 . 444 288.388 288.971

4 288.436 0.560072 -0.056292 +0.132 288.304 288.985

5 288.252 0.587321 -0.083541 +0.196 288.056 288.266

6 287.898 0.671592 -0.167812 +0.393 287.505 287.689

7 290.745 0.585350 -0.081570 +0.191 290.554 290.766

8 291.768 0.510421 -0.006641 +0.016 291.753 291.998

9 290.391 0.645868 -0.142088 +0.333 290.058 290.560

10 289.968 0.641914 -0.138134 +0.323 289.645 290.171

ave = 289.714

14
cT

'

.
Table 3

As (c) + 3/2 Cl

2 (g) -AsC1

3

(£)

Exp . M

s m

l

z

m

2

m

a r m

g

g g g g moles

2 1.31200 0.43040 0.77472 1. 20512 1. 088688 0 . 0112575

4 1.87233 0.94738 0.91924 1. 86662 1 . 003059 0 . 0123069

5 1.92053 1.65088 0.33768 1.98856 0. 965789 0.0043530

8 2.09335 1.89653 0.65373 2. 55026 0. 820838 0 . 0071623

9 2.28166 1.60196 0.96670 2. 56866 0. 888269 0 . 0114612

10 1.95554 1.34002 0.52719 1. 86721 1. 047306 0 . 0073694

11 1.84983 1.73716 0.34206 2. 07922 0. 889675 0 . 0040618

13 2.03404 1.73007 0.27121 2. 00128 1 . 016370 0 . 0036792

14 2.36237 2.01723 0.34389 2. 36112 1. 000529 0 . 0045924

R

/

E E Q

/ Exp . Calib. m

React

.

s s corr

c Q Q j-cr 1 j-rr 1 Q J

2 0.404986 0.547674 89412.052 89501.945 0.050202 4493.177

4 0.485807 0.636473 89567.836 89662.756 0.079896 7163.696

5 0.492601 0.641226 89655.280 89748.914 0.088137 7910.200

8 0.553944 0.699592 89665.249 89757.007 0.092123 8268.685

9 0.415034 0.581845 89493.691 89598.782 0.097763 8759.446

10 0.494150 0.629142 88963.406 89048.451 0.097394 8672.785

11 0.583686 0.698118 89468.333 89540.425 0.081974 7339.987

13 0.508752 0.638965 89391.408 89473.442 0.089790 8033.820

14 0.437188 0.582690 89428.736 89520.402 0.105433 9438.405

Exp. 4T Corr^ Corr

2 Q Q x M.W.

M

^

25 =^;

Q Q Kj # Cr 1 J J KJ •mol

” 1 KJ •mol"

2 0.590805 -0.087025 -0.033 494.564 4987.741 284.824 284.857*

4 0.694686 -0.190906 -0.071 540.667 7704.363 308.291 308.362

5 0.699630 -0.195850 -0.073 191.236 8101.436 316.044 316.117*

8 0.758092 -0.254312 -0.095 314.654 8583.339 307.200 307.295

9 0.649733 -0.145953 -0.055 503.513 9262.959 304.163 304.218

10 0.686814 -0.183034 -0.068 323.752 8996.537 344.680 344.748*

11 0.744450 -0.24 06 7 0 -0.090 178.443 7518.430 304.511 304.601

13 0.692056 -0.188276 -0.070 161.635 8195.455 301.870 301.940

14 0.645499 -0.141719 -0.053 201.753 9640.158 305.734 305.787

ave = 305.367 ± 1.880

15

Table 4

Heat of Solution of NaCl(c) in

H^O and in As^O^Caq)

Exp. Solvent M E %

Calib I Calib II Calib I Reaction Calib II

g J'Q" 1 J*Q-1 Q Q Q

1 H2° 2.0698 20971.524 20975.709 0.353104 0.457848 0.558960

2 HO 1.7026 20958.109 20960.749 0.444269 0.545944 0.644838

2a «2° 1 . 5449 20993.917 21012.807 0.337339 0.705236 0.526779

3 As

2 O^(aq) 2.3348 20892.048 20926.757 0.323676 0.430666 0.534306

3a

1

CM <3 CO 0

3

(aq) 1.5521 20931.993 20978.465 0.333020 0.703396 0.523275

Exp. AR E

/ Q Added heat Q Q x M.W.

c s corr M

Q j*r

r

1 J J J KJ. iriSl " 1

1 -0.007313 II 20965.598 -153.441 0.0 -153.441 -4.333

I 20981.998 -153.321 0.0 -153.321 -4.329

2 -0.005462 II 20950.860 -114.529 0.0 -114.529 -3.931

I 20968.277 -114.434 0.0 -114.434 -3.928

2a 0.161384 21030.653 3394.011 3502.981 -108.970 -4.122

3 -0.005007 II 20917.958 -104.736 0.0 -104.736 -2.622

I 20900.009 -104.646 0.0 -104.646 -2.619

3a 0.163216 21009.986 3431.371 3499.466 -68.095 -2.564

ave = H.0 = -4.129

As

2

0

3

(aq) =- -2.602

16

Table 5

Normalities of NaOH and Coefficients

for Equations 5 and 6

Compound Exp. Normality X y m n

AS

2

O

2

(octa.) 1 1.057 21.734 19.734 52.382 57.741

2 1.057 26.671 24.671 52.382 56.669

3 1.057 26.959 24.959 52.382 56.620

4 1.057 24.524 22.524 52.382 57.078

AS

2

O

3

(monoclinic) 1 1.057 32.165 30.165 52.382 55.888

2 1.057 33.348 31.348 52.382 55.756 ^S 2^3 (v^- treous ) 1 1.057 25.665 23.665 52.382 56.851

2 1.057 25.437 23.437 52.382 56.895

3 1.057 23.668 21.668 52.382 57.263

AsC1

3

(£) 2 1.057 49.967 45.967 52.382 56.984

3 1.057 26.451 22.451 52.382 61.804

4 1.057 32.678 28.678 52.382 59.758

5 1.0 60.982 56.982 55.371 59.293

6 1.0 27.419 23.419 55.371 64.914

7 1.0 67.356 63.356 55.371 58.898

8 1.0 120.693 116.693 55.371 57.286

9 1.038 32.660 28.660 53.339 60.853

10 1.038 34.852 30.852 53.339 60.319

17

Table 6

Analysis of AsCl^ Hydrolysis Products

Exp. Mass of

Sample

Solution

Aliquot

Iodine

Solution o Normality

Iodine Soln.

A S

(obs .

)

As

+3

(cal.) Ratio

(obs /cal.

)

g cm^ cm g g

1 2.56649 20 33.425 0.01604 1.00420 1.06070 0.94673

2 1.72883 20 23.150 0.01604 0.69550 0.71451 0.97339

3 3.26580 20 43.950 0.01604 1.32040 1.34972 0.97828

4 2.64335 20 35.567 0.01604 1.06855 1.09247 0.97810

5 1.34298 20 18.425 0.01604 0.55355 0.55504 0.99732

6 2.98697 20 41.383 0.01604 1.24330 1.23448 1.00714

7 1.21587 20 14.725 0.01826 0.50360 0.50251 1.00217

8 0.67854 40 17.225 0.01766 0.28488 0.28043 1.01587

9 2.59940 50 48.233 0.02936 1.06098 1.07431 0.98759

10 2.43583 50 45.817 0.02936 1.00784 1.00670 1.00113

All product solutions

3

were diluted to 1000 cm before the aliquot

samples were taken.

18
•

•
Experiments 1 , 3, 6, 7 , 12, and 15 of the AsCl^ formation

were not calculated for the following reasons:

#1 - the final temperature greatly exceeded the jacket

temperature

#3 - some of the liquid product was visibly carried into

the gaseous product receivers

#6 and 7 - chlorine leak into the system gave spurious

temperature rise

#12 and 15 - analysis unobtainable

Experiments 2, 5, and 10 were omitted from the final

considerations for obvious reasons.

All of the hydrolysis experiments except the first AsClg

hydrolysis were considered.

The energy equivalent, Eg, of the system is calculated from

the electrical calibration data and is assigned to the mid-point

of the calibration temperature rise. The change of energy equivalent

with temperature has been determined experimentally. The Eg(corr ) reported is the measured E„ adjusted to the mid-point of the reaction

u temperature rise.

In the Eq. 2 experiments some of the product left the system

in a gaseous state. Corrections to the calculated heats were

made on the basis of AH (AsCl0 ) = 43932 J/g or 10.5 Kcal/mol. [13],

vap 3

These corrections appear in Table 3 as "Cor^".

Evaluation of the hydrolysis data was based on the following

equations

19

5. As

2

0

3

(c) + x(Na0H*mH

2 0) - [2NaAs(>2

4- y(Na0H*nH

2 0) ] + H2

0 (ji)

6. AsCl^

(

j£) + x Na0H*mH

2

0) - [NaAs0

2

4- y(Na0H*nH

2

0) 4- 3NaCl] + 2^0(1)

7. 3NaCl(c) + [NaAs0

2 + y (Na OH*nH2 0) ] - [NaAs0

2 + y(Na0H*nH2 0) + 3NaCl]

The volume of alkali used in the solution experiments was always

3 450 cm . The difference in sample size and some slight variations in

alkali strength caused the coefficients x, y, m, and n in equations 5-7

to differ from experiment to experiment. (See Table 5). By use of

appropriate dilution data from Parker [14] all experiments were reduced

to a common basis wherein x=21.734 and m=52.382. A typical set of

such calculations is shown here for the 4th As

2

0

3

(c^ octa . ) experiments

As ? 0 + 24.524(NaOH*52.382HoO) -* J [2NaAs0

2

4- 22.524(NaOH*57.078H

2

O)] + H 0(X)

2.790[ (NaOH*52.382H2 0) -> (NaOH*57. 0781^0) ZSH=0.0368 Kj/mol]

As„0 4- 21.734 (NaOH*52 .382H2 0) -

2 J [2NaAs0

2

4- 19 . 734 (NaOH*57 .078^0) ] 4- H

2

0(£)

19 . 734 [ Na GH*5 7 . 74 1H2 0) -

(NaOH*57.078H

2

O) AH=-0.003 KJ/mol]

As 0 4* 21.734(NaOH*52.382H2 0) -

J [2NaAs0

2

4- 19.734(NaOH*57.741H

2 0) ] 4- 1^0(4)

AH=-43 . 959 KJ/mol

40.103

-44.063

-0.059

-44.003 KJ/mol

Experiments were made on the solution of NaCl(c) in both H

2

0

and ^s 2^3 + Na 0H. The water experiments were made as a check of the

system and procedure. The values obtained were compared with the

published value of the heat of solution of NaCl.

Both of the above reactions are endothermic, so some experiments

were run with the addition of measured electrical heat during the

reaction period. The data are given in Table IV. In the "a"

experiments the two calibrations were run consecutively prior to

20
'
the reaction, in the others the calibrations bracketed the reaction.

From these data we calculate:

3NaCl (c) + [NaAsO + 17 . 734 (NaOH*57 . 741H2 0) ] -

[NaAs0

2

+ 177734 (NaOH*57. 74 1H

2

0) + 3NaCl] ZNH-l-7.806 KJ

The heat of reaction for Eq. 7 was determined as +7.806 KJ. (See

Table 4)

.

Combining Eqs. 5, 6, and 7 leads to the following:

8. 2AsC 13

(4) + 21.734 (Na OH*52 . 382H 0) - As 0 (c,octa.) + 6NaCl(c)

+ 3H

2 0(jO + 15.734(NaOH*57.741H2

0)

Z J

AH=-55 1.035 ± 0.570 KJ/mol (-131.712 ± 0.136 kcal/mol)

Our experiments give

15 = -305.367 KJ/mol

(-72.984 kcal/mol). Inserting this value into Eqn. 8 along with

the accepted values of AHf for NaCl(c), ILO(i'), and the NaOH

solutions, we get for As 0 (c,octa.) £Hf_ pft = -658.416 ± 1.96 KJ/mol

1 5 V * (-157 . 365 ± 0.468 kcal/mol)

21
* i.-: ' ' 19
REFERENCES

[1] R. Schumann, J. Am. Chem. Soc . 4jj, 1444 (1924).

[2] H. J. Kirschning and K. Plieth, Z. anorg. u. allgem. Chem. 280 , 346 (1955).

[3] C. T. Anderson, J. Am. Chem. Soc. _52, 2296 (1930).

[4] J. Thomsen, Thermochemische untersuchungen, Vols. I, II, III, and

IV, (J. A. Barth, Leipzig, 1882-1886).

[5] I. N. Stranski, K. Plieth, and I. Zoll, Z. Elektrochem. 62, 366 (1958).

[6] J. H. Schulman and W. C. Schumb, J. Am. Chem. Soc. J35, 878 (1943).

[7] D. D. Wagman, W. H. Evans, V. B. Parker, I. Halow, S. M. Bailey,

and R. H. Schumm, NBS Technical Note 270-3 (1968).

[8] A. E. Beezer, C. T. Mortimer, and E. G. Tyler, J. Chem. Soc.,

4471 (1965).

[9] A. E. Beezer and C. T. Mortimer, J. Chem. Soc., 330 (1966).

[10] S. S. Chang and A. B. Bestul, J. Chem. Phys . J55, 933 (1971).

[11] W. H. Johnson, A. A. Gilliland, and E. J. Prosen, J. Res. NBS

6 3 A, 161 (1959).

[12] P. Morries, Comprehensive Analytical Chemistry, Ic. Classical

Analysis p 237.

[13] V. B. Parker, NBS, private communication.

[14] V. B. Parker, NSRDS-NBS2 (1965).

22
I

i

ii