Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: DQD on November 09, 2011, 06:30:06 AM

Title: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 09, 2011, 06:30:06 AM

I prepare Chrome Alum from K₂Cr₂O₇, pure H₂SO₄ and Ethanol 95^o
And the question is
What is the Ethanol's effect in process ?

Which substance can Inhibiti Hydrogen peroxide while storing ?
And its mechanism ?

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 09, 2011, 07:59:20 AM

What you make is not a chrome alum. Chromic acid if anything.

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 09, 2011, 10:19:18 AM

It's impossible, solution after reacting has dark green color
and I have let it for 1-2 week(s), it appear crystal ???
I do this experiment from my text book

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 09, 2011, 12:15:36 PM

Hm, perhaps it makes sense - it guarantees correct Cr:K ratio. Interesting.

Think: why does the solution become green?

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 09, 2011, 07:19:42 PM

i wonder
May ethanol cause ?

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 10, 2011, 02:38:52 AM

Yes, it is because of the ethanol presence, but it is not ethanol that becomes green.

What is green in this solution? Initially the solution should be red/brown. What do you know about dichromate, especially in low pH?

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 10, 2011, 07:14:13 AM

Green in this solution, I guess mixture of Cr₂(SO₄)₃ and K₂SO₄ cause this color
not only Cr₂(SO₄)₃, which has pink color

And in low pH, I dont be sure to know about dichromate :)
Plz explain this problem

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 10, 2011, 11:11:16 AM

If memory serves me well Cr(III) in solutions is green as long as there are no chlorides.

I am not going to give you a direct answer. First paragraph of the wikipedia page on dichromate contains an important hint.

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 10, 2011, 08:52:10 PM

Um
So in low pH, It means in acid solution the unstable blue peroxo complex Chromium(VI) oxide peroxide, CrO(O₂)₂, is formed
CrO(O₂)₂ is a substance which I have never learn it  ::)

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 11, 2011, 05:28:23 AM

Another hint: redox process.

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: DQD on November 12, 2011, 11:40:53 PM

I think i dont have any anwser :D
btw, I hear my friend explain about it
due to the strong acid hydrolysis of dichromate, the ethanol presence is inhibiti this hydrolysis
According you, Does this thing right ?

Title: Re: Preparing Chrome Alum and Inhibiti Hydrogen peroxide
Post by: Borek on November 13, 2011, 06:09:37 AM

It is not about hydrolysis. It is about reducing Cr(VI) to Cr(III).

I already told you almost everything, start to follow hints and learn what a redox process is.