Reversible reactions
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How can you reverse the reaction of bismuth(III) chloride and water? I don't understand why, when you keep reversing the reaction in a cycle, it gets progressively harder?
Ulex writes …
This reaction is used to demonstrate Le Chatelier's principle. A concentrated solution of bismuth trichloride is transparent. When it is diluted with a little water a precipitate of BiOCl appears and the solution becomes acidic due to the production of hydrochloric acid.
BiCl3(aq) + H2O(l) <=> BiOCl(s) + 2HCl(aq)
The reaction is reversed when concentrated hydrochloric acid is added. When water is added, the precipitate again appears.
It takes quite a lot of concentrated acid to make the precipitate disappear again because there is now quite a lot of water present which dilutes the acid and reduces its effect. This is why it gets progressively harder to make the reaction oscillate.
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updated: 25 August 2003
