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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: unknown_analysis on January 03, 2011, 06:18:27 AM

Title: Chromate-Dichromate Equations?
Post by: unknown_analysis on January 03, 2011, 06:18:27 AM

This is fairly basic but... :-\

I was asked to write the chemical equation (not the net-ionic equation) of the following:

a. K₂CrO₄ + HCl  ::equil:: ?
b. K₂CrO₄ + NaOH ::equil:: ?
c. K₂Cr₂O₇ + HCl  ::equil:: ?
d. K₂Cr₂O₇ + NaOH  ::equil:: ?

And can anyone explain to me why chromate turns orange in acidic medium, while dichromate turns yellow in basic medium?

Thanks for the replies! :D

Title: Re: Chromate-Dichromate Equations?
Post by: Borek on January 03, 2011, 08:01:39 AM

There is an equilibrium between chromate & dichromate, depending on pH.

Title: Re: Chromate-Dichromate Equations?
Post by: AWK on January 03, 2011, 08:45:33 AM

Both: chromate and dichromate react with an excess of HCl in redox reaction

Title: Re: Chromate-Dichromate Equations?
Post by: Borek on January 03, 2011, 09:58:06 AM

In general - yes, but i doubt that's what the problem is aiming at.

Probably lousy selection of acid.

Title: Re: Chromate-Dichromate Equations?
Post by: monica on January 03, 2011, 01:20:13 PM

2Cr2O7 2-  + h20   ::equil:: 2CrO4 -  + 2 H+
orange                              yellow

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion(orange colour). As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left and when NaOH is added the eq. is shifted toward right according to LE Chatlier principle and the orange colour changes to yellow.

Title: Re: Chromate-Dichromate Equations?
Post by: AWK on January 04, 2011, 01:24:24 AM

Quote from: monica on January 03, 2011, 01:20:13 PM

2Cr2O7 2-  + h20   ::equil:: 2CrO4 -  + 2 H+
orange                              yellow

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion(orange colour). As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left and when NaOH is added the eq. is shifted toward right according to LE Chatlier principle and the orange colour changes to yellow.

Chromate cannot exist in the presence of H⁺

The correct reaction is:
Cr₂O₇^2- + 2OH^- = 2CrO₄^2- + H₂O

Title: Re: Chromate-Dichromate Equations?
Post by: monica on January 04, 2011, 10:09:28 AM

@awk
 thanks for correction!

Title: Re: Chromate-Dichromate Equations?
Post by: unknown_analysis on January 05, 2011, 12:05:23 PM

One question though, why do you say that chromate cannot exist with H⁺?  ???

Thanks for the replies! :D

Title: Re: Chromate-Dichromate Equations?
Post by: Borek on January 05, 2011, 03:06:05 PM

What he means is that H⁺ shifts the equilibrium to dichromate. As presence of water implies presence of both H⁺ and OH^-, stating that chromate can't exist in the presence of H⁺ is far too fetched, and both reactions (one with H⁺/H₂O and one with OH^-/H₂O) are equivalent - although obviously equilibrium constant will be different.

Title: Re: Chromate-Dichromate Equations?
Post by: unknown_analysis on January 08, 2011, 10:23:37 AM

I got it. Thank you very much! :D