The Barium Hydroxide-Ammonium Thiocyanate Reaction
Arlo D. H a r r i s Cahfornta State College San Bernardlno. CA 92407
A titrimetric continuous variations experiment T h e method of continuous variations is a well known auantitative techniaue for t h e establishment of stoichiumetric relationships ( I ) . ~ < ; l o r i m e thas r ~ long been used to determine t h e formulae of intenselv colored species in solution (2-4). Precipitation techniques a r e successful for insoluble species (5).Our first-year laboratory sequence includes a gravimetric method involving silver chromate ( 6 ) .T h e experiment described herein utilizes acid-base titrimetry to s t u d y t h e stoichiometry of a novel solid s t a t e reection. Barium hydroxide 8-hydrate a n d a m m o n i u m thiocyanate react in the solid s t a t e , spontaneously, endothermically a n d a t room t e m p e r a t u r e according to t h e following equation: Ba(OH)?. 8 HzO + 2 N H 8 C N
-
Ba(SCNh
+ 2 NHlOH + 8 Hz0
The a m m o n i u m hydroxide produced u n d e r these conditions decomposes to a m m o n i a gas a n d water. 2 NH40H
-
2 NHl
+ 2 HzO
F r o m t h e s e t w o equations t h e following molar equivalences a r e readily obtained. 2 NHIOH = 2 NHB 2 NHaSCN = 2 NH40H
T h i s l a s t relationshio is used as a n indirect measure of t h e reaction stoichiometry. T h e ammonia produced is t i t r a t e d with hvdrochloric acid a s in a classical Kieldahl m e t h o d for nitrogen (7). P r i o r to the actual continuous variations experiment, two s h o r t qualitative experiments a r e performed. T h e s e serve t o give t h e s t u d e n t s o m e understanding of t h e development of a n experiment. Understanding why a n experiment works a n d why i t is d o n e in a specific way is often a s i m p o r t a n t a s t h e experiment itself. Materials:
4% BO& acid solution Rramocresol green indicator Tube, gas dispersion with fritted disc; Corning 39525 Poros. M., O.D. 35 mm, dia. 20 mm Apparatus as shown in Figure 1.
The Experiment The experiment is divided into four parts. The first two are qualitative in nature. The third is the actual continuous variations part, while the fourth is an acid-base titration. (1) As a qualitative exercise 2.0 g of NHdSCN and 4.0 g of Ba(OH)2.8H 2 0 arevigornusly shaken together in a 125 ml Erlenmeyer flask. The student records all observations, citing them as evidence that a reaction has occurred and, if possible, identifying any prod,,rts. > i t . 2 p l r n ~ m n 1 c l0 ) ic snmples < Ithe ~ L I rr., sufficient. Welght variations for Ba(OH)r.8 Hz0 are between 0.75 g and 2.75 g. The corresponding variations for NH&N are between 2.25 g and 0.25 g. Values outside these ranges produce amounts of ammonia too small for usable titration data. Increment changes of 0.25 g are sufficiently separated to yield good data. Prior to placing both reactants intn the sidearm test tube, theentire system should be assembled and ready. The gas absorption receiving vessel should contain 100 ml of 4% boric acid solution. A slow hut steady stream of compressed air (3-4 bubbles per second) should be coming through the gas inlet tube. The water bath should be hot so as not to delay the experiment. The order of addition of the solid reactants is insignificant. No solvent is necessary since one product of the reaction is water and the mixture will liauefv a t the temoerature of the boiline water bath. Ilr.,r ..:rr, uPon'xiAitim 01 l r s t l ; rrnctsnti to thc .idtam tc.1 t u l ~ c ~t~d the r w ~ t i , ml t e ~ i wat once. 'I'hr .yrttn, 111114 I ~ c l < ~itntw~iatvl\, to prevent the escape of any ammonia. The reaction is allowed to proceed for about 25 min during which time the water bath is kept b d i n g a n d the stream of compressed air is continued. The flow of air is adjusted carefully to allow far total absorption of ammonia. As the reaction proceeds, some of the water distills out with the ammonia and the remaining mixture becomes a semi-solid mass. The flow of air is then increased slightly to keep the mixture well stirred and to get the last traces of ammonia out of it. When ammonia absorption into the boric acid solution is complete, thegas dmpersion tuhe is removed while the air flow continues. This assures that any solution whieh may be trapped in the tuhe is removed. The boric acid solution containing the ammonia is then ready for titration with 0.3 M hydrochloric acid solution. Before another reaction can he done, the system must be thoroughly cleaned and dried to remove all traces of reactants and products. (4) The titration portion of the experiment is performed using standard technioues. The ammonia from each reaction whieh has heen hromocresol green indicator
Figure 1. Apparatus far continuous variations experiment
Results Data from a typical experiment are presented in the table. From these a plot of volume of HCI versus g Ba(OH)z.8 Hz0 is obtained and shown in Figure 2. A perpendicular dropped to the X-axis from the paint where the two straight lines cross shows that 2.04 gBa(OHIz8 H 2 0 produced a maximum amount of ammonia. A similar plot could Volume 56, Number 7, July 1979 1 477
Exoerlmental Data Ba(OH)2.8HzO Grams mMales
NHISCN Grams mMoles
mMoles NU3 Theoretical
Discussion Volume HCI in ml Theo.
Exp.
The experimental purpose is a study of the barium hydraxideammonium thiocyanate reaction; however, the learning objectives are many and varied. The qualitative studies in parts (1) and (2) demonstrate the development of an experiment which is performed quantitatively in parts (3) and (4). In part (I), students observe the rare phenomenon of two solids reacting spontaneously to produce a semi-solid slurry. So often students write equations where water is one of the products. Here, students actually observe water being ~rodueed.It is also observed that the resultant-slurry is cold to the &ch and liberates ammonia. From these observations students readily understand why a hot water bath is used and the need for a steady stream of air to carry aver the ammonia to the boric aeid absorbing solution. In part (2). students observe that NHaSCN remains a solid up t o the boiling point of water. However, Ba(OH)z.8 H 2 0 melts a t 78' C releasmg the waters of hydration. This is actually observed when the experiment is performed as described in part C. This, coupled with observations in part A, is useful in writing the equation for the reaction. in part (31, the continuous variations portion, students use the concepts of mass, moles, and stoichiometry. Techniques learned here are weighing, construction of a simple apparatus, and gas collection by absorption. The simplicity of the experiment is such that even beginning students can manage quite well. In part (4), the student is introduced to aeid-base titrimetry. The hydrochloric aeid titration of ammonia also teaches the student an indirect method far determining a reaction stoichiometry. Actually, the0.3 A4 HClsolution need not be standardized. As long as the same solution is used for e v e n titration. there will be no effect other than movine the eraoh uo ";down with resoect to the X-axis. Also. the the titration reaction. Variations As a class project, pairs of students are assigned a set mass ratio for part (3). Eaeh person in the pair does identical experiments and compares the results. Results from part (4) for all pairs in the class are collected and graphed. This can be done in a 3-hr lab period. Eaeh pair of students in the class submits a joint lah report including a graph of all data from the class as well as the information from their own experiments. Included are the three equations shown previ-
......., "ld"
Figure 2. Plot of volume of HCI versus weight of Ba(OH),.8H20. have been made using the mass values for NH4SCN. From these data, the student calculates the molar ratio as follows:
Acknowledgment
-"04 - 0.00647 moles Ba(0H)zX HzO 315.5
(1)
-0'96 - 0.0126 moles NHISCN 7fi 1 2
(2)
000647 - 1 and 0.0126 -= 1.95 (3) 0.00647 0.00647 Hence. the molar ratio of Ba(OHh.8 t o N H S C N is 12. These . .- HnO " values are then used to balance the equation for the reaction being studied. The student is then able t o write three equations:
+ 2 NHdSCN Ba(SCN)e + 2 NH40H + 8 HzO 2 NH3 + 2 H 2 0
-- -
Reaction: Ba(OHh8 H 2 0 Distillation: 2 NHIOH Titration: 2 NH3 2 HCI
+
2 NH&I
478 / Journal of Chemical Education
For a n inrl~ud~ral appro;x h. ~ , n e s t u d m is r rirm th~ent~rerrpprimenr and i i e x p ~ r t e d1 u dl, it alme. F w the range i n d ~ ~ n t ethere d, arc eirht wmi 10 Ire dcterminrd nhivh will r r q w r P t w I h w r lah periods when all parts of the experiment are included. 'l'hr mthor ~ i ~ h 10 e cthnnk Pmtrsslr 1.w Knlhus m d Profesic.r Rnlph Prrrurr> h r thrlr encuuraccment ;and helpful suggcsr~on~ during the development of this experiment. Literature Cited (1) Waser,J.. Quantitative Chemistry. rev. edition, W. A. Benjamin,
351.
New York, 1964,p
35s. 131 Ramette.R. W.. J. CHEM.EDUC.40.71 (19631. 14) Reference 13).p. 262. IS) Lippincott,W. T., Meek, D.W., and Verhaek, F. H.,"Erperimental General Chemistry." 2nd Edition. W. B. Saundem Co., Philsdeiphia, 1974,p. 53. (6) "Geneml Chemistry Lahorafnry Manua1"rev. edition. Department of Chemistry,CalL iforma State College, San Bernardino. 1918, p. 36. (71 Foulk,C. W., Mnyer. H. V., and Mac Nevin. W. M., "QuantitativeChemical Analyail." McGrsw~HillBookCo. he., New York, 19SZ.p. 343. (2) Reference ( I ) . p.
