In the Classroom
applications and analogies
Why Is Bismuth Subchloride Soluble in Acid? Damon Diemente Trinity School, 101 West 91st Street, New York, NY 10024
In this article the author shares his approach for stimulating thought about a reaction system by posing questions, performing two simple demonstrations, and conducting a discussion based on observation and prior knowledge. JEH
Sometimes a question that appears to call for a routine answer turns out upon reflection to be more subtle than first realized. Consider for example the aqueous solubility behavior of bismuth subchloride, BiOCl. This compound is insoluble in water, but can be made to dissolve by the addition of a little 6 M HCl (1, 2). Why is bismuth subchloride soluble in acid? What equations can we write to describe the solution process? These apparently simple questions do not have easy answers. Demonstration A Add 0.1 g of solid bismuth subchloride to 10 mL of water in a test tube. A milky suspension forms with no evidence of dissolving. Slowly and with stirring add drops of 6-M HCl. The solid rapidly dissolves to form a colorless solution. How can we explain this?
Discussion The chemical behavior here seems simple enough. It is reasonable to assume that BiOCl reacts directly with H3 O+ to give a soluble product. If this is so, we need to construct a balanced equation with BiOCl and H3O + as reactants. The simplest such equation is + { + BiOCl(s) + H3O → ← BiO + Cl + H3O +
(2)
or a protonation mechanism: + BiOCl(s) + H3O+ → ← Bi(OH)Cl + H2O
(3a)
{ 2+ Bi(OH)Cl + → ← BiOH + Cl
(3b)
3+ BiOH2+ + H3O+ → ← Bi + 2 H2O
(3c)
These are reasonable guesses; they are after all related to well-known proton-transfer reactions in aqueous solution: { O2{ + H3O+ → ← OH + H2O
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OH {+ H3O+ → ← 2 H2O Now, if we restrict our ideas to direct reactions between BiOCl and hydronium ion, we may believe we are on the right track with reactions like 2 and 3. But deeper insight is available if we attend carefully to the behavior of another salt, bismuth trichloride, as it mixes with water. Demonstration B Add 0.1 g of bismuth trichloride to 10 mL of water and stir. A small amount of the material may dissolve, but chiefly an earthy, white, insoluble substance forms. What has happened here?
Discussion When small amounts of BiCl3 are added to water, the salt first enters the solution with ionization in the normal fashion: { 3+ BiCl 3(s) + xs H 2O → ← Bi + 3 Cl
BiCl2+
(4)
BiCl4{
(Other species such as and are also present in the solution, but these are ignored in this treatment.) Then as further solid bismuth trichloride is added to concentrate the solution, the effects of hydrolysis (2, 4) become noticeable. Bismuth(III) ion reacts with water to produce BiO+, then this combines with chloride ion to give a precipitate of BiOCl. A partial mechanism is
(1)
But this cannot be right. BiO + may be an ion to expect in aqueous solution (3), but the H3 O+ ions in this reaction simply cancel, and the mere presence of hydronium should not cause the BiOCl molecule to ionize. Clearly, something more complicated is happening. Perhaps we can invoke neutralization reactions: { 3+ BiOCl(s) + 2 H3O+ → ← Bi + Cl + 3 H2O
{ O2{ + H2O → ← 2 OH
+ Bi3+ + 2 H2O → ← BiOH2+ + H3O
(5a)
+ + BiOH2+ + H2O → ← BiO + H3O
(5b)
BiO+ + Cl{ → ← BiOCl(s)
(5c)
The overall reaction for the hydrolysis is the sum of the three preceding reactions: + Bi3+ + 3 H2O + Cl{ → ← BiOCl(s) + 2 H3O
(6)
Reaction 6, or the series of reactions composing it, adequately explains why addition of HCl causes BiOCl to dissolve. Initially, high concentrations of bismuth(III) favor the hydrolysis and make the solution acidic. Then, increased concentration of hydronium ion shifts the equilibrium, and reaction 6 reverses. So the solubility of bismuth subchloride in acid need not be principally the consequence of a direct reaction between solid BiOCl and hydronium ion in solution (reactions 1, 2, and 3). Indeed, it is not required to assume that any direct reaction happens at all. The solubility is just as likely to be the result of shifting equilibriums (reactions 5 and 6). Our simple question does indeed call for a more complicated answer than we might at first have realized. Further investigation will have to be done to settle the question.
Journal of Chemical Education • Vol. 74 No. 4 April 1997
In the Classroom Disposal The Flinn catalogue (5), Flinn method #26a, recommends disposal in a landfill. Collect wastes as dry solids, pack them in sturdy cardboard boxes, seal, and closed with heavy tape. NOTE: Bismuth subchloride and bismuth trichloride are expensive. Teachers may prefer to prepare salts for these demonstrations from bismuth oxide, Bi2 O3. Pour about 50 mL of 6 M HCl over 3–5 g of Bi2 O3 in a 250-mL beaker. Stir and gently warm until the yellow powder is completely dissolved, adding more acid a few milliliters at a time as needed. Keep the solution warm (60 °C is good) under a fume hood until the liquid and excess HCl have evaporated. Perform the evaporation in a tall beaker, or with constant slow stirring as protection against spattering. Gentle heating will probably have to be continued overnight; excessive heat must be avoided to prevent possible partial volatilisation of the product. The dry residue is a rough, rocky product, chiefly BiCl3, that can be ground to powder
in a small mortar for use in demonstration B. The product is deliquescent and must be kept in a tightly closed bottle. To convert a portion of this crystalline product into the earthy subchloride, mix a gram of two with 50 mL of water, collect the resultant white precipitate by filtration, wash to remove HCl, and dry. This material can be used in demonstration A. Literature Cited 1. The Merck Index, 7th ed.; Stecher, P. G., ed.; Merck & Co.: Rahway, NJ, 1960; p 155. 2. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Interscience: New York, 1966; p 495. 3. Sidgwick, N. V. The Chemical Elements and Their Compounds; Oxford University: Oxford, 1962; Vol. 1, p 796. 4. Sidgwick, N. V. The Chemical Elements and Their Compounds; Oxford University: Oxford, 1962; Vol. 1, p 794. 5. The Flinn Chemical Catalogue Reference Manual is available from Flinn Scientific Inc., P.O. Box 219, 131 Flinn Street, Batavia, IL 60510.
Vol. 74 No. 4 April 1997 • Journal of Chemical Education
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