MadSci Network: Chemistry
Query:

Re: How is it that K2Cr2O7 is an equilibrium mixture containig CrO4 ions???

Date: Mon Feb 12 13:38:20 2001
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 981499915.Ch
Message:

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

Dan Berger
Bluffton College
http://www.bluffton.edu/~bergerd



Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-2001. All rights reserved.