Arlo D. Harris and Lee H. Kalbus California State College San Bernardino, California 92407
I
I
~echn~osition of Copper(l) Sulfate Pentahydrate A sequential gravimetric analysis
Thermal dehydration of copper(I1) sulfate pentahydrate is used commonly to demonstrate gravimetric analysis (1,2, 3).Ease of water loss and a marked color change during the process are reasons for this. However, experimental difficulties arise since little or no control of the heat source (usually an open flame) is maintained. Results obtained are often poor due to some decomposition of the anhydrous salt to copper(I1) oxide. Improvements described here are use of a controlled temperature environment and a quantitative study of the decomposition reaction to a thermally stable oxide. The number of molecules of water per formula unit of a hydrate is variable and in some cases may not even he an integer. However, the formula in no way distinguishes how these are attached to the parent compound. They may he held by (a) coordinate covalent bonds, (h) hydrogen bonds, or (c) uniform or random arrangement in a crystal lattice. In any given hydrate they need not he held in the same way. Copper(I1) sulfate pentahydrate has four water molecules hound covalently through oxygen to copper. These form a square planar configuration with CuO distances of 2.0 A. The fifth water molecule is attached via hydrogen honding to sulfate ions and to the water molecules attached to copper (see the figure). As such it is held in the crystallattice in auniform manner (4.5). Gentle heating of CuS01.5 H z 0 causes loss of the lattice water and one water attached to copper yielding CuS043 H20 (6). One sulfate and three water molecules f o m a square plane with CuO distances of 1.96 The irregular octahedron is completed by two oxygen atoms from two other sulfate ions a t 2.34 .&and 2.45 These coordination polyhedra are held together hy hydrogen bonding (7,s). On controlled heating, CuS045 H z 0 effloresces to an isolable monohydrate containing one water attached. to the copper while the other five positions of a distorted octahedron are taken u p by oxygens from sulfate ions(9.10). Intermediates containing 2 and 4 waters are also reported (11); however, these do not interfere with the experiment described here. Stronger heating effects total dehydration (12, 13) to anhydrous copper(11) sulfate. This has a much more distorted structure with two oxygeus a t 1.89 two a t 2.00 and two a t 2.37 (14, 15). Ignition degrades the product to copper(11) oxide which has the crystal lattice of a giant molecule. Each copper atom is surrounded by four oxygens in a square plane with CuO distances of 1.95 .& (16).
A.
A.
A
A,
A
The Experiment The purpose of the experiment is fourfold: (a) to determine the value for X in the starting material, (b) to determine the formula of the intermediate hydrate, (c) to determinethe formula ofthedegradation product, and (dl to write all balanced equations. Far the dehydration steps, the only information given is the formula CUSOIX H20and that at 4W°C all the water of hydration will be lost. For the degradation step, the student is told that both CuS and CuO are black.
Materials CuS04.5 H20: Baker AR 5-1843 Fine Crystal Crucible, procelain: Cwrs 25006-0 Spot plate, porcelain: Coors 550-00 Balance: Torhal Model ET-1 Oven: National Appliance Model 5510-6
The environment of the waters of hydmTion in drab
copper(l1)sulfate pentahy-
Furnace: Lindherg Heavy Duty, Control Model 59344, Ignition Chamber Model 51442 Procedure (3Parts) I. A 2.5 g sample of CuSOc5 HzO,weighed to the nearest milligram, is placed into a previously cleaned, oven dried, and weighed crucible. The crucible is placed in an oven at 140°C.Aporcelain spot plate may be used to support up to four crucibles in the oven. After 1 hr the crucible is removed and placed on a second porcelain spot plate at room temperature. The porcelain plate method dissipates heat very quickly. After 5 min cooling, the crucible is reweighed. A second 1-hr heating period may be used to verify completion of this step. Weight loss at this point correspondsto 4 waters of hydration. The residue is pale blue compared to the initial deep blue color. 11. Next, the crueihle is placed on a spot plate in afurnace at 4W°C for 1 hr. After cooline as described ahove. the erueihle is reweiehed. A second 1-ir heatine oeriod ma; heused forverifi anhydrous copper(I1)sulfate, with no evidence of copper(I1) oxide. 111. Finally, the crucible is placed on a spot plate in a furnace at 1000'C for 1 hr. After cooling as described above, it is reweighed. A second 1-hr ignition period may be used to verify complete degradation. Weight loss in this step corresponds to sulfur trioxide. The black residue is copper(I1)oxide. Results and Calculations Data including standard deviation and range for 12 samples run simultaneouslyare as follows: (a) average sample weight 2.574g; (b) weight loss at 140°C,0.741 f 0.009 g, range 0.721-0.752 g; (c) weight loss at 400°C,0.186 &,0.003g, range 0.179-0.192 g; (d):residue at 1000°C, 0.812 & 0.013 g, range 0.794-0.834 g. Using these data the required calculations and equations determined by students are as follows.
2.574 g 1.647 g 159.6 g/mole 0'927
18.02 glmole
1.647 g
0.927 g
= 0.01032 mole CuS04
= 0,05144 mole Hz0
Volum 56. Number 6, June 1979 1 417
0.01032 - 1 mole CUSO* Mole ratio: 0.01032
:.
-0'05144 - 4.98 mole H 2 0 0.01032 CuS0a:HzO = 1:5, X = 5 and the formula is CuSOl. 5 H s 0
cuS01.X
llODC Hz0 A cuS01.Y A
-
(1)
+
H20 Z H20
-
1.833 e 0.741- -e 2.574 e 1.833 . a - 1.647 . a = 0.186 . a H -s 0 far Y. 0'18' = 0.01032 mole Hz0 18.02 glmole
is followed visually by observing color changes during the decomposition. Most importantly, these refinements demonstrate some of the more subtle aspects of crystal chern~stry.For example, students discover that all 5 waters of hydration in CuSOu5 H 2 0 are neither thermally ,t,ucturally equivalent, Several adaptations of this experiment are possible. For a single 3-hr laboratory, the two separate dehydration steps may be completed. If asecond 3-hr laboratory period is available, the additional degradation step makes a concise experimental package. A combination experiment is also possible using a stoichiometric study of copper sulfate pentahydrate previously published in this Journal (17). determines water "~ eravimetricallv. The . . Our ex~eriment - - ~ experiment by Silber determines copper and sulfate, thereby obtaining the amount of water by difference. These may be combined to produce an experimental package covering several quantitative techniques utilizing this interesting pentahydrate salt. If combined, the total time would he from five to six 3-hr laboratory periods. d~
~~~~~~~~~
6.01032 = 1.0 mole HzO 0.01032
Mole ratio:
.: CUSOLH~O= 1:l. Y = 1and Z = 4.
(2)
The balanced equations for (1) and (2) are:
~
+
CUSOCH~O ~ C U S O *HzO CuS01.5 Hz0
(1)
5 - CuSOcH20 + 4 HzO 4
-
For the degradation step: CuSOd 1.647 g
1WO'C
black residue + lost material 0.812 g
-
0.835 g
(3)
the equation far degradation is: CuSO4
.
lWO0C 4
CuO
+ SOa
(3)
Discussion yields data accurate enough todem. This improved onstrate sequential gravimetric analysis. Advantaw gained by using a controlled temperature environment are (a) isolating a stable monohydrate intermediate and (h) preserving the purity of the an. hydrous salt' The added aspect of degradation to oxide makes i t more interesting and instructive. Progress of the reaction
418 1 Jwmal of Chemical Education
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Acknowledgment T h e a u t h o r s wish t o t h a n k t h e m e m b e r s of t h e Chemistry D e p a r t m e n t for reviewing the manuscript, a n d i n particular Professor R a l p h Petrucci whose critical c o m m e n t s were invaluable to t h e success of t h i s project. Literature Cited i l l H e . G. G.and Kask. U.. "Exoerirnental General Chemistrv." Bamesand Noble.Ine..
Dickinnon Publishing Cn.. Ine. 1967,p. 41. 141 Beeuers, C. A. and I.ipson, H., Pme. Roy. Soc.. l46A. 570 l1934l. 15) Bac0n.G. F..and Curry, N. A.,Pror. Roy. Sac, 266A.95, 1962. 161 Galimherti.L.,Boil. Sci. Foc. Chim. ind.. Bologns.,272.1940. 171 Zahrohrky,R. and Baur, W. H.,Noturuias. 52,388,1965. is1 Zahrohrky, R. and Baur. W. H..Acto Cryst., 824,508 1968. 191 Marin, Y.and Duva1.C.. Anal. Chim Aeto.. 6.47 ,952. i101 Duva1.C.. Anal. Chim. Arfo.. 16.223.1967. ("1 Taylor. T I. and Klug. H. P . ; J ~ h r mPhys ,4.601.1936. 1121 Kohler. K.and Zaske,P..Z. an or^. und A l l ~ e mChm.. . 7,331.19M. 1131 pannetier, G., G U ~ ~ S and ~ , JManoli. , J. M.. RUII soc. them pmnc., 2832. ISM. 1141 Rae. B. R.,Aeta Cryst., 14.321,1961. 1151 Almdov~,I..Fra2er.B.C.,Hunt.J. J.,Fox.D.E,andBrown,P.H.Phys.Rsu., MA, 153, 1965. 1161 Tunnell, G..Pomjsk,E.. end Ksanda, C.J. Z Krisl.. W.120.1935. (171 Silber. H.B., J.CHEM. EDUC., 43,586,1972.
