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REDUCTION OF OXIDES BY HYDROGEN A Quantitative Experiment for General Chemistry Laboratory WILLIAM L. MASTERTON and JOSEPH J. DEMO, Jr. University of Connecticut, Storrs
ONEof the classic:tl experiments in the general chemistry laboratory is the reduction of metaloxides t o the free metals by hydrogen. The experiment is performed quantitatively a t this university, the oxides being issued as unknowns. From the weight of the oxide sample and the weight of metal remaining after reduction, the gram equivalent weight of the metal is calculated and reported. There appears to be little information availablein the literature as to oxides suitable for quantitative reduction by hydrogen. Bray and Latimer' give directions for the reduction of CuO. Selwood2discusses the formation of volatile CuCl on reduction of CuO samples with hydrogen containing small amounts of HCI. Kash3 indicates that CuO and NiO are satisfactory unknowns but reports that Fez03often gives inaccurate results because of reoxidation of the iron formed. None of these sources gives any quantitative data as t o the accuracy t o be expected. With an experiment involving large classes it is desirable t o use at least five unknowns. For this reason, a study has been made of the reduction of several oxides in addition to those mentioned above. Six oxides (CuO, NiO, COO, Sn02, Co304 and Cu20) have been found to give satisfactory results. A procedure has been worked out which is applicable to the quantitative reduction of all of these. Student results are reported for these unknowns. THE STUDENT EXPERIMENT
The apparatus used is easily constructed from equipment available in large quantities in the general chemistry laboratory. The hydrogen is generated by allowing dilute H1S04 to drop from a thistle tube into a 250-1111. Erlenmeyer flask containing about 10 g. of zinc covered with water. The reaction is often slow to start; a few ml. of concentrated CuS04solution can be used as a catalyst. The hydrogen passes from the generating flask through a 6-in. CaCL drying tube. This is connected to a glass delivery tube which leads to within 2 inches of the end of a 25 X 150-mm. Pyrex test tube containing the sample. This tube is clamped in a horizontal oosition so that it can be heated a t the BRAY,W. C., AND W. M., LATIMER,"A C0~18ein General Chemistrv." 3rd ed.. The Macmillan Co.. New Yark., 1947., Assienment 5, 35-7. ' SELWOOD, P.,J. CHEM. EDUC.,19, 375 (1942). a NASH, JANE,J. CHEM. EDUC., 21,46 (1944).
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sample end. The hydrogen leaves the sample tube through a short piece of glass tubing bent up at a right angle and drawn out to a tip about 1mrn. in diameter. The sample tube referred to above is weighed to *.001 g. on an analytical balance. I t is most convenient t o suspend the tube from the balance a m by means of a loop of copper wire passing under the lip of the tube. The sample, weighing about 1 g., is then placed in the tube. Any solid adhering to the upper part of the tube is carefully removed. The tube is reweighed and the weight of the sample calculated. The apparatus is then assembled; the student is cautioned not to allow the glass.delivery tube to come in contact with the sample. A laborat,ory instructor inspects the apparatus, checking t o make sure that there are no obstructions or loose connections in the system. After approval, the generating flask is wrapped with a towel as a precautionary measure and the dilute HzSO4 added. After hydrogen has passed through the apparatus for several minutes, the gas issuing from the exit tube is collected in a small test tube. The inverted test tube is transferred to a burner kept a t a safe distance from the apparatus. When the collected gas burns quietly, it is judged that all of the air has been flushed out of the system. The issuing gas is then ignited with the tube of burning hydrogen and the heating of the sample is begun. The student is instructed to heat gently a t first and then strongly for at least 15 minutes. The tube is allowed t o cool in hydrogen, removed and heated near the lip to drive off condensed water. After coolmg to room temperature, the tube is weighed and the weight of metal calculated. The weight of oxygen is obtained by difference; the gram equivalent weight of the metal is then calculated and reported. STUDENT RESULTS
Results for six oxides are given in Table 1; data for Fez03,which proved to be an unsatisfactory unknown, are included for comparison. All results are based on 100 or more reported samples except Coa04(27 samples). and CuzO (20 samples). One other oxide, SnO, was tested and found to give unsatisfactory results on the basis of 20 samples. Many of the larger errors are due to mistakes in weighings or in calculations; this is one of the first quantitative experiments performed by the students. A major source of error appears t o be failure to heat the sample a t a sufficiently high temperature for a long JOURNAL OF CHEMICAL EDUCATION
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TABLE 1 Summary of Student Results
Ozide COO SnOa NiO CuO COJO, Cu10 FexOr
Percentage of results within the indicated range of the thewetieal gram equivalent weight +I% 13% *5% *lo% *SO% *SO%
14 8 8 6 7 5 2
28 15 13 15 10 4
35 20 18 19 19 15 10
48 33 24 31 33 35 15
62 51 48 49 52 50 22
78 57 63 57 63 60 30
summarized in Table 2. The apparatus used was essentially the same as that used by the students. A dropping funnel was substituted for the thistle tube and weighiugs were carried out to *.0001 g. All reductions except that of FezOawere carried out with a Bunsen burner. Each sample was tested for complete reduction by reheating in hydrogen for a five minute period. At least three samples of each oxide were analyzed. Oxides Ouantitativelv Reduced bv Hvdroaen
enough time. Many students assume that the reduction is complete when the sample changes color, despite instructions to the contrary. I t is perhaps significant that COO, which does not show a pronounced color change on reduction, gives the best results. FepOa, which requires prolonged heating to avoid reoxidation on exposure t o air, gives poor results. Slightly over half of the students reported results more than 50% greater than the theoretical for the gram equivalent weight of iron. The poor results with SnO can be attributed t o deposition of some of the finely divided oxide on the delivery tube. Students receiving samples of COO, SnO,, CosOl, FelOa, and SnO, which require relatively high temperatures for reduction, were issued Meker burners; the other oxides were heated with Bunsen burners. Several students were unable t o keep their hydrogen &me burning steadily. The principal cause of this difficulty is the generation of hydrogen a t an irregular rate. This can be corrected by using a dropping funnel rather than a thistle tube to admit the dilute sulfuric acid. Other causes include improper design of the exit tube tip and condensation of water in the tube lead'mg from the generating flask t o the calcium chloride tube. It is desirable t o use a long drying tube t o make sure that the hydrogen is dry when it enters the sample tube. Mr:iter vapor in the hydrogen will of conrsr hare an undesirnl)lr effect on the position of the reduction rquilibrium. It is t o be noted that with the procedure and unknowns used the number of explosions is extremely small. The few that do occur result when impatient students light the hydrogen with a match contrary to instructions. This may result in blowing the stopper out of the sample tube. Each student i s repired to wear safety glasses during the experiment. SELECTION OF SUITABLE OXIDES
An oxide to be used as an unknown should be quantitatively reduced to the metal under the conditions of the experiment. It should not give a volatile product which will condense in the delivery tube or escape from the apparatus altogether. This consideration eliminates such oxides as HgO, CdO and A s 2 0 3 . For use with large classes, the oxide should be relatively easy to obtain in a satisfactorily pure state.4 With these criteria in mind, the following oxides were investigated: COO, SnOz, NiO, CuO, Cos04, Cu20, FezOa,SnO, Ag,O, PbO, Pb02, Pba04,Mn02,and Fe304. Calculations show that the free energy change for the reduction of each of these oxides is negative at the operating temperatures (500'-700°C.). The results obtained with eight of these which proved satisfactory are VOLUME 35, NO. 5, MAY, 1958
Ozide
Equivalent weigh1 Calc. Obs. A". d m
COO SnO, NiO CuO CoaO, Cu*O Fe,O."
29.47 29.68 29.35 31.77 22.10 63.54 18.62
29.42 29.88 29.25 31.82 22.07 63.04 18.62
*.03 1.05 1.04 *.09 +05 *25 +.05
Snob
59.35
58.35
*.20
Heatinn time mqu&ed (minutes)
15 15 l(t15 5 15 5 15 (Meker burner) 15
Shorter heating periods or lower temperatures gave samples which reoxidieed on exposure t o air, as shown by the evolution of heat and, in extreme cases, a. color change from black to red. Even when the samples were heated as indicated, a small evolution of heat could be detected an exposure to air. The SnO samples were in the form of a finely divided dust which tended to deposit on the delivery tube. When the delivery tube was shortened so that i t projected only about halfway into the sample tube, the equivalent weight was found to be 58.75.
Four of these oxides, N ~ OCuO, , Fe20a,and SnO, are available commercially in satisfactorily pure form (reagent grade) to be used directly. Reagent grade SnOa when used directly gave results which were about 27& high; when washed with dilute nitric acid and ignited, the value given in Table 2 was obtained. The best grade of Cu20 available is only 96%-97% pure. A sample of Mallinckrodt CuzO of this purity was extracted with ether, washed with distilled water, alcohol and ether, and dried in a vacuum desiccator. The product remained impure; on reduction a copper mirror was formed in the test tube and the flame took on a green color (see footnote 2 ) . It would appear that the impurities present compensated for each other to give a surprisingly accurate value for the equivalent weight of copper. The Co304used was prepared from Cocoa (reagent grade, washed several times with distilled water) by heating with a Meker burner in an open crucible for about two hours. Samples of COO were prepared similarly except that the CoCOa was heated in an atmosphere of carbon dioxide or nitrogen. None of the other oxides tested gave satisfactory results. Manganese dioxide even on heating in hydrogen with a Meker burner for long periods of time gave incomplete reduction with small amounts of metal produced. Samples of Fes04on heating in hydrogen for an
' It is t o be noted that a sample of NiO could he contaminated with as much as 50% of COO and yet, because of the similar equivalent weights of nickel and cobalt, give a value for the equivalent weight of nickel of 29.41 as compared to the theoretical value of 29.35. On the other hand, if the sample contained only 1% of volatile matter it would give an equivalent weight of 28.02. The presence of volatile impurities becomes espeeialIy important with metals of high equivalent weight. O.lyo water in Cu20 lowers the caloulated equivalent weight of copper by 0.9%.
hour with a Meker burner lost about 85% of their oxygen. Samples of PbO and Pb304formed metallic lead but gave high and widely varying values for the equivalent weight. The test tubes used invariably cracked on cooling. From the appearance of the tubes it was
evident that some substance, presumably PbO, had entered the glass. Samples of PbOl and Ag,O evolved oxygen rapidly even when heated slowly with hydrogen at a low temperature. Consequently, explosions frequently occurred during their reduction.
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