MadSci Network: Chemistry |
How is it that K2Cr2O7 is an equilibrium mixture containig CrO4 ions???
I recently completed a lab with the applications of Le Chatelier's principle. I don't really see how a water solution of potassium dichromate could actually be an equilibrium mixture containing potassium chromate. It makes no sense to me! Any oxyacid can form an anhydride; for example, polyphosphates (found in the backbone of DNA) are anhydrides of phosphoric acid: H3PO4 ® H4P2O7 + H2O When polyphosphoric acid is dissolved in water, it reverts to phosphoric acid:HO(HPO3)nH + (n-1)H2O ® n H3PO4 In the same way, "dichromic acid" (which isn't isolated) is an anhydride of chromic acid:2 H2CrO4 ® H2Cr2O7 + H2O Sodium dichromate is an anhydride, the sodium salt of "dichromic acid." Dissolving sodium dichromate in water produces two chromate ions, two sodium ions and two hydronium ions:Na2Cr2O7 + H2O ® 2 Na+ + 2 H+ + 2 CrO4- but normally we write it this way:Na2Cr2O7 + H2O ® Na2CrO4 + H2CrO4
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