Content uploaded by Vladimir Petrusevski
Author content
All content in this area was uploaded by Vladimir Petrusevski
Content may be subject to copyright.
233
Õèìèÿ, ãîä. XIX, êí. 3 (2010) Chemistry, Vol. 19, Iss. 3 (2010)
l Ó÷åáíè îïèòè è äåìîíñòðàöèè l
l Teaching Chemical Experiment l
REACTION OF ALUMINIUM WITH DILUTED
NITRIC ACID CONTAINING DISSOLVED SODIUM
CHLORIDE: ON THE NATURE OF THE
GASEOUS PRODUCTS
Vladimir M. PETRUŠEVSKI, Miha BUKLESKI,
Marina STOJANOVSKA
Ss Cyril & Methodius University, Skopje, Republic of Macedonia
Abstract. Metallic aluminium was found not to react with either concentrated or
diluted nitric acid. Providing the diluted acid contains dissolved sodium chloride and
traces of copper(II) cations, a vigorous reaction occurs. The product is basically ni-
trous oxide (possibly containing some elemental hydrogen and nitrogen gases), and
was identified by its IR spectrum.
Keywords: nitrous oxide, IR spectrum of, nitric acide, aluminium
Introduction
It is really unusual that some elementary chemistry of such well-known metals
as aluminium is either missing from the chemistry textbooks, or is simply wrong. The
reactions of aluminium with concentrated and diluted acids may serve as a good ex-
ample for the above assertion. In many of the standard textbooks of general/inorganic
chemistry [1-6] one finds that aluminium does not react with concentrated nitric acid
and this is indeed true. However, regarding other strong acids one finds quite a num-
234
ber of various statements. Some of the above textbooks [1-3] were rather old, and for
that reason their reliability was initially considered as being questionable. The infor-
mation found in the newer ones [4-6] are not consistent. Thus, according to Glinka [4]
and Brady [5], aluminium reacts with diluted acids displacing hydrogen, while Green-
wood & Earnshaw [6] say that the protective oxide cover prevents any reaction with
diluted acids. On the other hand, the experiments show [7] that the only reaction that
occurs in real time with diluted acids at room temperature is the reaction with HCl(aq),
giving rise to hydrogen gas and aqueous solution of aluminium chloride. Therefore, the
simple chemical reaction based on the equation
2Al(s) + 6H+(aq) = 3H2(g) + 2Al3+(aq)
seems not to be operative for strong acids other than HCl! That being so (and the
results of experiments say very clearly it is so, indeed), one is faced with a widely
spread preconception about the negligible role of the spectator ions in reactions of
hydrogen displacement, at least for reactions including aluminium. This problem will
be addressed in more detail elsewhere.1) At this point it may be interesting to say that,
although an old one, Mellor's textbook [1] gives information that seem to be the most
accurate and the most relevant ones.
In the course of the reinvestigation of the reactions of aluminium with strong
inorganic acids (concentrated and diluted), one fact soon became obvious: aluminium
is readily dissolved in practically all strong acids, providing the solution contains chlo-
ride anions (obviously acting as a catalyst). Traces of Cu2+ ions further catalyze the
process. The product thus obtained is practically pure hydrogen, in all cases except
when one deals with nitric acid. The product of the reaction with diluted nitric acid is,
again, a gas (depending on the quantity of the added Cu2+ ions the reaction may be
somewhat vigorous). In all instances but one, where a minor puff occurred, the prod-
uct gas was nonflammable, thus suggesting that hydrogen is far from being the princi-
pal component present. One of us (MB), being in the process of completing his B.Sc.
thesis in the field of vibrational spectroscopy, proposed to analyze the product using
FT-IR spectrometry, thus demonstrating the advantages and elegance of the method
used.
Experimental
The spectra were acquired using the Perkin Elmer System 2000 FT-IR interfer-
ometer in the 3000-500 cm-1 region. A Perkin Elmer gas cell was used (cell
length ~ 10 cm, CsBr cell windows) for the measurements. For measuring the back-
ground spectrum, the cell was filled with air. After that, the cell was filled with the
product gas (or gaseous mixture) diluted with air and the sample spectrum was re-
corded. Low resolution spectra (resolution of 4 cm 1) perfectly serve the purpose.
235
16 scans were accumulated for both the background and the sample spectra. GRAMS
32 program package2) was used for spectra manipulation.
Concentrated nitric acid was diluted to 1 : 2 with distilled water (volume ratios of
acid and water). 10 mL of the diluted acid were taken and ~ 1 g of NaCl. After the
salt was dissolved a grain of aluminium was added together with a drop of copper(II)
chloride solution with w(CuCl2) = 0.05. After the evolution becomes vigorous enough,
the gas was collected in the cell (precautions were taken for preventing microscopic
droplets of the liquid to enter the cell). Another sample with several drops of copper(II)
chloride was also prepared (this reacts more vigorously with the aluminium). The
product was again collected and its spectrum was analyzed.
Results and Discussion
The spectrum of the product (presumably a gaseous mixture) is given in Figs. 1
and 2. The wavenumbers of the three strongest bands (í3, 2223; í1, 1285 and í2,
589 cm-1) are in excellent agreement with those reported for nitrous oxide [8]. The
band at ≈ 2563 cm-1 can be safely attributed to the 2í1 mode. Further check and
comparison of the spectrum with the spectrum of N2O taken from a metal cylinder
shows beyond any doubt that the polyatomic molecules present belong to N2O (ni-
trous oxide). The presence of H2 or N2 cannot be precluded in this way (being homo-
nuclear diatomics, they are IR inactive).
The generation of N2O could be explained by the following reactions. First, due
to dissociation of the electrolytes, the solution behaves as a diluted mixture of HCl(aq)
and HNO3(aq). Aluminium then reacts with the diluted hydrochloric acid (actually,
with the hydrated hydrogen ions) giving atomic hydrogen
Al(s) + 3HCl(aq) = AlCl3(aq) + 3H(aq)
A part of the atomic hydrogen inevitably combines into molecular hydrogen, but
the rest reduces the diluted nitric acid, according to
8H(aq) + 2HNO3(aq) = 5H2O(l) + N2O(g)
The product obtained from the reaction mixture containing several drops of
copper(II) chloride (Fig. 2) contains N2O as the major component, as well as traces of
other gases (like NO2, evidenced by the band at 1617 cm-1, again in excellent agree-
ment with the literature value [9]).
236
Fig. 1. FT-IR spectrum of the product of the reaction between Al and diluted
HNO3 containing one drop of CuCl2(aq). The spectrum matches the spectrum
of N2O (the sharp peak at ≈ 667 cm-1 is due to CO2 uncompensations)
Fig. 2. FT-IR spectrum of the product of the reaction between Al and diluted
HNO3 containing several drops of CuCl2(aq). The spectrum matches the spectrum
of N2O (the peak at ≈ 1617 cm-1 is due to traces of NO2, and the nature
of that at ≈ 1800 cm-1 is open to discussion)
The appearance of the NO2 impurities, in the presence of higher concentration
of Cu2+ ions in the solution, might be explained by displacement of copper with alu-
minium 2Al(s) + 3Cu2+(aq) = 3Cu(s) + 2Al3+(aq)
and subsequent reaction of the elemental copper with the diluted HNO3
3Cu(s) + 8HNO3(aq) = 2NO(g) + 3Cu(NO3)2(aq) + 4H2O(l)
followed by immediate oxidation of the NO gas with atmospheric oxygen
2NO(g) + O2(g) = 2NO2(g)
The nature of the very weak vibrational band centered at ≈ 1800 cm-1 remains
open to discussion.
237
The principal product of the reaction, N2O, is also one of the products in the
reaction of zinc with diluted nitric acid (together with NO, NO2 and perhaps N2). It
might be appropriate to recall that both zinc and aluminium are above hydrogen in the
electrochemical series of elements.3) However, there is no reaction at all between
aluminium and diluted nitric acid (most probably due to the protective covering of
aluminium oxide [6]), unless the solution contains chloride anions. One could specu-
late that in the presence of chlorides, the aluminium oxide film dissolves via formation
of aluminium oxide chloride as an intermediate, which is more soluble in acids than
Al2O3. Once the film dissolves, the chloride ions (present in the solution) prevent its
reformation and aluminium dissolves vigorously. This explanation although logical, is a
pure speculation until more direct evidence is offered.
Finally, let us mention that the conclusions about the exact composition of the
gaseous mixture would be definitive if the analysis was complemented using a mass
spectrometer. Regrettably, we did not have one at our disposal.
Conclusion
For some (not completely clear) reasons, the protective Al2O3 covering cannot
survive in diluted nitric acid (and in other oxo-acids, as well) containing chloride ions.
Aluminium reacts with the hydrogen ions giving atomic hydrogen. The latter partly
combines into hydrogen gas, and the rest reacts with the HNO3 and reduces it to
N2O. If only traces of Cu2+ ions (acting as a catalyst) are present, the only oxide of
nitrogen that forms is N2O. This is, to the best of our knowledge, rather unique behav-
ior (in practically all other cases where metals react with nitric acid of medium con-
centration, a gaseous mixture of various NxOy is obtained).
NOTES
1. Petruševski, V.M. & M. Stoianovska, in preparation.
2. GRAMS/32, Spectral NotebaseTM, Version 4.10 Level 1, 1991-1996, Galactic
Industries Corporation.
3. http://en.wikipedia.org/wiki/Table_of_Standard_electrode_potentials
REFERENCES
1. Parkes, G.D. (Ed.). Mellor's Modern Inorganic Chemistry. Green and
Co., London, 1961.
2. Wiberg, E. A Textbook of Inorganic Chemistry (translated into Croatian).
Školska knjiga, Zagreb, 1967.
3. Nekrasov, B.V. General Chemistry (translated into Serbian). Nauèna knjiga,
Beograd, 1971.
4. Glinka, N.L. General Chemistry, Vol. 2. Mir, Moskwa, 1986.
238
5. Brady, J.E. General Chemistry: Principles and Structure. Wiley, New
York, 1990.
6. Greenwood, N.N., A. Earnshaw. Chemistry of the Elements. Butterworth-
Heinemann, Oxford, 1998.
7. Najdoski, M., V.M. Petruševski. The Experiment in the Teaching of Chem-
istry, Vol. 2. Magor, Skopje, 2002, pp. 120-122.
8. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordina-
tion Compounds. Wiley, New York, 1986.
9. Arakawa, E.T., A.H. Nielsen. Infrared Spectra and Molecular Constants of
N14O2 and N15O2. J. Molecular Spectroscopy 2, 413-427 (1958).
ÐÅÀÊÖÈß ÍÀ ÀËÓÌÈÍÈÉ
Ñ ÐÀÇÐÅÄÅÍÀ ÀÇÎÒÍÀ ÊÈÑÅËÈÍÀ,
ÑÚÄÚÐÆÀÙÀ ÍÀÒÐÈÅÂ ÕËÎÐÈÄ
Ðåçþìå. Ìåòàëíèÿò àëóìèíèé áóðíî ðåàãèðà ñ ðàçðåäåíà àçîòíà
êèñåëèíà, êîãàòî êèñåëèíàòà ñúäúðæà ðàçòâîðåí íàòðèåâ õëîðèä è
ñëåäè îò ìåäíè(II) êàòèîíè. Ïðîäóêòèòå íà ðåàêöèÿòà (N2O, ñëåäè îò
äðóãè àçîòíè îêñèäè è ìîæå áè ãàçîîáðàçíè âîäîðîä è àçîò), ñà èäåí-
òèôèöèðàíè ÷ðåç òåõíèòå èíôðà÷åðâåíè ñïåêòðè.
* Professor V.M. Petruševski (corresponding author)
Department of Chemistry, Faculty of Natural Science and Mathematics,
Ss Cyril and Methodius University,
Arhimedova, 5,
1001 Skopje, Republic of Macedonia
E-Mail: vladop@pmf.ukim.mk
