H. A. N e i d i g and R. T. Y i n g l i n g
I
Lebanon Valley College Annville, Pennsylvania
.....,.
mfhhies El..
This investigation introduces the student to several important areas of thermochemistry, including enthalpies of neutralization, enthalpies of dissolution, enthalpies of formation, and Hess' Law. The investigation of the enthalpy of formation of solid ammonium chloride has beeu previously described.' The procedure used to collect primary data for calculating the eut,halpy of formation of solid ammonium chloride can be used with equal effectiveness to collect data for calculating the enthalpies of formation of solid ammonium nitrate and solid ammonium sulfate. The laboratory work for each of the three systems is divided into two parts. I n the first part of the investigation the enthalpy of neutralization: NH8(l.50 M ) + HX (1.50 M ) NH4X(aq)
-
is determined where H X (1.60 M) is either hydrochloric acid, riitric acid, or sulfuric acid, and NH4X(aq) is the respective ammonium salt. Stoichiometric quantities of 1.50 ill solutions of ammonia and acid are placed in separate containers. Time-temperature data are collected for the two reactant solutions every 30 seconds for a 5-min pre-mixing time interval, using a thermomneter graduated in 0.1" units. The reactants are mixed in a Thermokup which serves as a calorimeter, and timetemperature data are collected for the reahon mixture every 30 sec for a 10-min posemixing time interval. A graph of temperature versus time is plotted, and the AT is determined by the extrapolation method. The heat transfer (Q) and the enthalpy of neutralization are calculated. Three determinations of the enthalpy of neutralization are made by each student. The second part of the investigation involves a determination of the enthalpy of dissolution: NH,X(s)
-
NH,X(aq)
The studenls are asked to calculate the mass of NHX(s) thaL must be added to a given quantity of distilled water in order to prepare an aqueous solution of NHIX with the same coneentration as in the reaction mixture obtained in the first part of the investigation. Timetemperature data are collected for the distilled wat,er during a 5-min premixing time interval. At the time of mixing, the calculated mass of NHX(s) is added to the distilled water while stirring; time-temperature data are continually collected for a 10-min post-mixing L~ ~ id^, l~~,,vestigating ~ ~ chemical h ~ ~ j IXvision, McGraw-Hill Book Co., he., New York, 1963, p. 22-1.
474 / Journal o f Chemical Educofion
of Formation of Solid Salts
time interval. The AT is obtained by extrapolation from a graph of temperature versus time, and the heat transfer (Q) and the enthalpy of dissolution (AHD)are calculated. Three determinations of the enthalpy of dissolution are made by each student. Comparison o f Average Values of the Enthalpy Changes Calculated b y the Students with Values Obtained from the Literature
EnthBlpy,Of
- 11.90
neutra~lra- +0.66
- 12.88
tion
-12.04
+072
- 12.89
-14.90 10.59
-15.16
.(kea~/mo~e .... N"l)
+382
f3.75
+587
Enthalpy of dissolution
*0.10
XHaX) Enthalpy of
.. .
-19.28
.. .
- 19.28
of 1.50 iM .UH~(ked/ mole NIIs) Enthalov of
.. .
-39.47
. ..
-49.23
(kohl/mole
forrnatlon
f6.01 2~0.23
Calculated from data obtained from "Seleoted values of Chemical Tl~ermodynamiel'ronertiea," Ciroular 500, National Rureau of S t m d a d r . Wasbingfon. 1961.
The enthalpies of ueutralization and the enthalpies of dissolution are used in conjunction with literature values (shown in the table) for the enthalpy of formation of 1.50 [J ammonia so~utiolland the el,thalpies of formation of 130 M solutions of each of the acids to calculate the enthalpies of formation of solid ammonium chloride, solid ammonium nitrate, and solid ammonium sulfat,e using Hess' Law. The average enthalpies obtained for the three systems by the 70 students enrolled in the first-year 'hemistry course at Lebanon Valley College are given in the table. The limits given vith the average dat.a are the standard deviations calculated from the st,udent data which remained after huge errors had been eliminated using the standard deviation data rejectio~~ procedure with a 9.5% confidence level. The similarities and differences between the values of the enthalpy of neutralization of ammonia obtained for the three ~yst~ems can be discussed, as well as the data obtained ~ for each of the three systems considered separately.
