LETTER pubs.acs.org/jchemeduc
Phosphorous and Arsenious Acids David Tudela* Departamento de Quimica Inorganica, Universidad Autonoma de Madrid, Madrid 28049, Spain ABSTRACT: An activity related to the acid strength of phosphorous and arsenious acids and the reason why they have different structures is suggested. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Inorganic Chemistry, Inquiry-Based/Discovery Learning, Acids/Bases, Descriptive Chemistry, Nonmetals
I
found fascinatingly simple and beautiful the demonstration on the red and white allotropes of phosphorus described by Golden et al. in a recent article in the Journal.1 Furthermore, the article includes some interesting chemistry on the element and its simplest oxides and oxoacids. In relation to phosphorous acid, I would like to suggest an activity that could increase the students’ interest in phosphorus chemistry and improve related learning outcomes. The problem-based learning activity is related to the different structures of phosphorous and arsenious acids,2,3 the relationship between acid strength and structure for oxoacids,26 and a possible explanation for the different structures of both compounds. Although the relation structure pKa for phosphorous acid has been discussed,2,6 the comparison with arsenious acid, which one would expect to be quite similar, and the explanation of the different structures in terms of bond-energy data, have additional pedagogical interest. The activity begins with the question: Why is phosphorous acid (pK1 = 1.8) a much stronger acid than arsenious acid (pK1 = 9.2)? When looking for the acid strength of oxoacids, the students find the relationship between the number of nonprotonated oxygen atoms in the formula EOn(OH)m and the first acid dissociation constant.26 Indeed, there is an approximate empirical equation, known as Bell’s rule5 or Pauling’s rule,6 that relates pK1 to the number of oxo groups, n:2,5,6 pK1 ≈8 5n
enthalpy change for the gas phase reaction 2: ð2Þ
ΔH ¼ DðE—OÞ þ DðO—HÞ DðEdOÞ DðE—HÞ
ð3Þ
With the bond energy data, D, found in the literature,9 ΔH = 72 kJ mol1 for phosphorous acid and +124 kJ mol1 for arsenious acid, thus, helping in understanding the different structures of both compounds.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Golden, M. L.; Person, E. C.; Bejar, M.; Golden, D. R.; Powell, J. M. J. Chem. Educ. 2010, 87, 296–298. (2) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997. (3) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (4) Rayner-Canham, G.; Overton, T. Descriptive Inorganic Chemistry, 4th ed.; Freeman: New York, 2006. (5) Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 3rd ed.; Pearson Education: Harlow, U.K., 2008. (6) Atkins, P.; Overton, T.; Rourke, J.; Weller, M.; Armstrong, F. Shriver and Atkins’ Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, 2010. (7) Testemale, D.; Hazemann, J. L.; Pokrovski, G. S.; Joly, Y.; Roux, J.; Argoud, R.; Geaymond, O. J. Chem. Phys. 2004, 121, 8973–8982. (8) Jenkins, H. D. B.; Tudela, D. J. Chem. Educ. 2003, 80, 1482–1487. (9) Kildahl, N. K. J. Chem. Educ. 1995, 72, 423–424.
ð1Þ
According to eq 1, although phosphorous and arsenious acids have the same empirical formula, H3EO3 (E = P, As), they have different structures. While arsenious acid (pK1 = 9.2) has no oxo groups and it is, therefore, a hydroxoacid with a pyramidal As(OH)3 structure, phosphorous acid (pK1 = 1.8) has one oxo group, and therefore, it must have a distorted tetrahedral structure, PH(O)(OH)2, with one PH, one PdO, and two POH bonds. As a result, phosphorous acid is diprotic. It should be indicated that, although As(OH)3 does exist in aqueous solutions,7 it has never been isolated in the solid state because, on crystallization, it yields As2O3.2,5 A further question is why phosphorous and arsenious acids have different structures. Bond energy data are useful to rationalize the chemical behavior and stability of molecules containing covalent bonds, thus explaining many aspects of nonmetal chemistry.8 In this case, students are asked to calculate the approximate Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
EðOHÞ3 f EHðOÞðOHÞ2
Published: September 02, 2011 1456
dx.doi.org/10.1021/ed200135a | J. Chem. Educ. 2011, 88, 1456–1456
