7/29/2019 JCE-2011-88-1456

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Published: September 02, 2011

Copyrightr 2011 American Chemical Society andDivision of Chemical Education, Inc. 1456 dx.doi.org/10.1021/ed200135a|  J. Chem. Educ. 2011, 88, 1456–1456

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 diff erentstructures 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 onthe 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 phosphor-ous acid, I would like to suggest an activity that could increasethe students’ interest in phosphorus chemistry and improverelated learning outcomes. The problem-based learning activ-ity is related to the diff erent structures of phosphorous andarsenious acids,2,3 the relationship between acid strength andstructure for oxoacids,2À6 and a possible explanation for thediff erent structures of both compounds. Although the relationstructure p K a for phosphorous acid has been discussed,2,6 thecomparison with arsenious acid, which one would expect to be quite similar, and the explanation of the diff erent structuresin terms of bond-energy data, have additional pedagogicalinterest.

The activity begins with the question: Why is phosphorous acid 

(p K 1 = 1.8) a much stronger acid than arsenious acid (p K 1 = 9.2)? When looking for the acid strength of oxoacids, the studentsfindthe relationship between the number of nonprotonated oxygenatoms in the formula EO

n(OH)

mand the first acid dissociation

constant.2À6 Indeed, there is an approximate empirical equation,known as Bell’s rule5 or Pauling’s rule,6 that relates p K 1 to thenumber of oxo groups, n:2,5,6

p K 1≈8 À 5n ð1Þ

 According to eq 1, although phosphorous and arsenious acidshave the same empirical formula, H3EO3 (E = P, As), they havediff erent structures. While arsenious acid (p K 1 = 9.2) has no

oxo groups and it is, therefore, a hydroxoacid with a pyramidal As(OH)3 structure, phosphorous acid (p K 1 = 1.8) has one oxogroup, and therefore, it must have a distorted tetrahedral struc-ture, PH(O)(OH)2 , with one PÀH, one PdO, and twoPÀOH bonds. As a result, phosphorous acid is diprotic. It should be indicated that, although As(OH)3 does exist in aqueoussolutions,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 acidshave diff erent structures. Bond energy data are useful to ratio-nalize the chemical behavior and stabilityof molecules containingcovalent bonds, thus explaining many aspects of nonmetal chem-istry.8 In this case, students areaskedto calculate theapproximate

enthalpy change for the gas phase reaction 2:

EðOHÞ3 f EHðOÞðOHÞ2 ð2Þ

ΔH  ¼ DðE—OÞ þ DðO—HÞ À DðEdOÞ À DðE—HÞ ð3Þ

 With the bond energy data, D , found in the literature,9

ΔH = À72 kJ molÀ1 for phosphorous acid and +124 kJ molÀ1

for arsenious acid, thus, helping in understanding the diff erentstructures of both compounds.

’AUTHOR INFORMATION

Corresponding Author

*E-mail: david.tudela@uam.es.

’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 , 2nded.; 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.