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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: stephen6119 on December 02, 2007, 05:46:17 PM

Title: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: stephen6119 on December 02, 2007, 05:46:17 PM: This is a question in my lab that I am unsure about...

Although Ag₂CrO₄ is insoluble in water, it is soluble in dilute HNO₃. Explain, using chemical equations...

Can anyone help me out?

Thanks,
Stephen
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Borek on December 02, 2007, 05:59:00 PM: H₂CrO₄ is strong, or weak acid?
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: stephen6119 on December 02, 2007, 06:04:43 PM: Quote from: Borek on December 02, 2007, 05:59:00 PM
H₂CrO₄ is strong, or weak acid?

H₂CrO₄ is a weak acid.
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Padfoot on December 02, 2007, 06:47:09 PM: In solution, there is an equilibruim between chromate and dichromate:

2CrO₄^2- + 2H⁺ <---> Cr₂O₇^2- + H2O
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Borek on December 02, 2007, 07:07:13 PM: Quote from: stephen6119 on December 02, 2007, 06:04:43 PM
Quote from: Borek on December 02, 2007, 05:59:00 PM
H₂CrO₄ is strong, or weak acid?

H₂CrO₄ is a weak acid.

What happens to weak acids in acidic solutions?
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Padfoot on December 04, 2007, 08:40:59 PM: Since the original poster isn't coming back, I suppose I'll have to post. I was wondering how H2CrO4 came into it. Can you (or someone else) explain it. My initial thought was that the equilibruim I wrote would be pushed to the right and this would cause more to dissolve. Is there a better reason?

Thanks :)
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Borek on December 05, 2007, 03:24:27 AM: In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO₄^2-) from the solution :)
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: AWK on December 05, 2007, 06:47:00 AM: H2CrO4 is not so weak pKa ~ -1. But the driving force for solubility is of course the removing of CrO₄^2- anoions by the equilibrium 2CrO₄^2- + 2H₃O⁺ <=> Cr₂O₇^2- + H₂O with huge K
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Borek on December 05, 2007, 07:25:11 AM: Quote from: AWK on December 05, 2007, 06:47:00 AM
H2CrO4 is not so weak pKa ~ -1

First dissociation step is of no use here.

My source tells pKa1 = 0.74 and pKa2 = 6.49. At pH 4 HCrO₄^- dominates solution. Reality is more complex, as this protonation is a first step to dichromate synthesis, but pKa2 alone is enough to explain silver chromate solubility in diluted acid.
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: AWK on December 05, 2007, 08:28:17 AM: Quote
At pH 4 HCrO4- dominates solution.
Well, silver chromate is sligtly soluble in acetic acid.
Title: Re: Why is Silver Chromate soluble in Nitric Acid but not water?
Post by: Padfoot on December 05, 2007, 05:19:50 PM: Quote from: Borek on December 05, 2007, 03:24:27 AM
In low pH chromate is protonated, pushing dissolution equilibrium to the right. Nothing fancy, you are just removing one of the products (CrO₄^2-) from the solution :)
oh OK. Thanks Borek for your clarification :)
From AWK's post I guess both are pretty good factors though, so we were all right!

Quote from: AWK on December 05, 2007, 06:47:00 AM
2CrO₄^2- + 2H₃O⁺ <=> Cr₂O₇^2- + H₂O
AWK you should have quoted me! ;)