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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: Hunt on December 23, 2007, 12:27:57 PM

Title: Effect of ethanol on the shape of crystals
Post by: Hunt on December 23, 2007, 12:27:57 PM
During the synthesis of tetraammine copper (II) sulphate monohydrate crystals,  we proceeded in two distinct methods. The 1st was the usual involving the addition of ammonia, and then ethyl alcohol to ppt the crystals. We filtered the soln and all that. Eventually we observed small crystals of the copound. In the 2nd method ethyl alcohol was added to the test tube but slowly on the surface of the solution. Two phases are observed , the alcohol phase is on top. We left the soln for a week and then observed that the shape of the tetraammine copper (II) sulphate crystals were larger.

I cant seem to understand why. How is alcohol diffusion related to the shape of the crystals formed?

Thanks
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 23, 2007, 01:41:04 PM
Shape or size?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 23, 2007, 02:40:16 PM
Oh I meant size not shape. The size of the crystals is greater. honestly, I havent seen them yet. But our professor said they'd be larger , so I'm wondering ...

Also why do synthetic chemists care so much about the shape of crystals ? What does the shape of crystals , be it in organic or inorganic synthesis , mean ? 
Title: Re: Effect of ethanol on the shape of crystals
Post by: Mitch on December 23, 2007, 02:51:35 PM
Usually anything allowed to crystallize for longer will make larger crystals.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 23, 2007, 03:10:19 PM
That seems to be the case Mitch. In organic lab , our professor also told us the same thing.  So it's related to time ... any idea why/how ?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 23, 2007, 03:27:22 PM
Why the crystal that grows longer is larger is rather obvious.

The most difficult part of growing crystals is starting crystallisation. Once the crystal exist, it grows easily, but nucleation often has high activation energy. Think how it realates to your situation and to the speed at which ethanol concentration at the very bottom of the test tube grows.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 23, 2007, 06:54:10 PM
Directly related to what Borek said...think of how the makeup of the solution changes when the ethyl alcohol is added, it would help you to visualize the crystal forming in both situations and what helps or hinders the crystallization process. It is very similar to how temperature change affects the size of the crystals, faster cooling->smaller crystals, in fact the relation is tied in with the delta G of each solution.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 23, 2007, 07:59:10 PM
Yah thanks guys , I cant say I grasped the full idea but perhaps this might do for now. I can guess that when alcohol diffuses slowly, there's a higher probability that the reaction occurs on the surface of the crystals and hence they're bigger in size. Is this plausible? When alcohol is added directly , there's random distribution of alcohol particles and the precipitation occurs with less probability of a crystal "build-up", which leads to the formation of smaller crystals.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 23, 2007, 09:05:15 PM
You have the idea but I think you have it backwards, as the concentration of alcohol increases it slows the formation of the crystals because the tetraammine copper (II) sulphate molecules are further apart and need to travel further in order to randomly run into the forming crystal.  While ethanol conc is low crystals form faster, fast crystal formation lends itself to many small crystals, the slower it crystallizes the larger the forming crystals can become. It is directly related to the TdS term in the free energy equation.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 23, 2007, 09:31:07 PM

I dont follow.

Quote
as the concentration of alcohol increases it slows the formation of the crystals because the tetraammine copper (II) sulphate molecules are further apart and need to travel further in order to randomly run into the forming crystal.

??

If that's the case, how did I end up with larger crystals when the alcohol conc was larger ? Your argument doesnt seem to be right. 

Alcohol is added to change the solubility of the solution , so that precipitation of the crystals does occur. When the alcohol conc is large , precipitation occurs rapidly and the crystals are small in size. 

If you wish to explain how the rate of formation of crystals and thermodynamics are related , the plz by all means do but write full equations so I would understand.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 23, 2007, 11:12:10 PM
My apologies...I wrote my last response in haste. If the alcohol conc. is high, entropy in the solution is increased resulting in a larger |dG| than the solution where the alcohol conc. is low.

If we assume that the formation of crystals is an elementary process(no intermediates) then the rate of crystallization is ~Keq. using the relation between Keq and dG, it works out that Ea in the kinetics equation is equal to (-)dG.  In the solution with a high concentration of alcohol it's dG is greater than the low concentration. The greater the (-)dG in this situation, the faster the formation of crystals. The lower the (-)dG(low alcohol conc.) the slower the rate of crystal formation is, therefore larger crystals can be made. let me know if I clarified that well enough for you. Again I apologize for the hasty initial response. I didn't mean to confound you, or myself for that matter...
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 24, 2007, 03:14:34 AM
Think about it in microscopic terms. When the ethanol concentration grows, solubility becomes lower and your salt 'wants' to get out from the solution. It can either create new crystals (high Ea) or attach itself to the ones already existing. If you add lots of ethanol and mix fast, solution becomes oversaturated fast and there is no problem with overcoming Ea - many crystals are formed, then there is no salt in the solution to let them gro. When the ethanol concentration grows slowly, only some crystals are formed (no huge oversaturation) then when the ethanol concentration rises these crystals grow, not allowing for oversaturation and new crystals formation.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 24, 2007, 07:02:00 PM
Ok I have a clearer image of what's going on in the solution. I searched for nucleation too , very interesting stuff. There's one last thing concerning this experiment. After synthezing the crystals , we took a small amount and dissolved them in distilled water and then added hydrochloric acid. When the conc of HCl is large , precpitation occurs.
I think the ionic reaction is :

[Cu(NH3)4]2+ + 2Cl- ---> [Cu(NH3)4]Cl2(s)

My lab partner thinks the ppt is CuCl2 , but this solid cannot form unless a coordination Cu-NH3 bond is broken. The 1st possibility seems more probable than the 2nd. I do not have any data to confirm this and I could not find any information about this reaction. Any help is appreciated.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 24, 2007, 07:26:00 PM
What happens to NH3 in low pH?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 24, 2007, 09:15:13 PM
Good pt , at low pH , Ammonium predominates. I completely neglected the hydronium ions. I guess then it's CuCl2 ppt.

But what if NaCl is added, would the ppt be  [Cu(NH3)4]Cl2(s)  ?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 24, 2007, 09:49:17 PM
How is the state of the equilibrium affected after adding NaCl?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Mitch on December 25, 2007, 03:39:27 AM
It'll push solutes out of the water phase.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 25, 2007, 02:16:50 PM
Mitch I did not get your point.

How to know whether a complex molecule is soluble in water or not ? For binary salts, there are certain solubility rules that serve for qualitative purposes. Is there anything like that for coordination compounds?

For clarity my question should have been,

Does precipitation occur at all between Cl- and [Cu(NH3)4]2+ ?
Quote
But what if NaCl is added, would the ppt be  [Cu(NH3)4]Cl2(s)  ?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Mitch on December 25, 2007, 02:25:39 PM
in general, salt pushes other solutes out of water. I have no clue whether your complex is soluble in water, I'm just stating a common principle of adding salt to solutions.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 25, 2007, 10:56:37 PM
I wonder if that complex with Cl(-) would even exist. If that complex does exist I'm sure that there is a Ksp associated with it. You should do a literature to see if it does exist. I say this because Cu's electron configuration with the four NH3 coordinated to it has a favorable half full D-shell. Cl has such a high electronegativity that it would form an ionic type bond to the complex. But as we know ionic Cl bonds usually dissociate in aqueous solutions.  So I don't think the Cl(-) complex would actually exist as a ppt. You would definitely need to do some search through literature. My guess is that adding NaCl would precipitate CuCl2 but the amount would be affected by the pH of the solution, which would become basic.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 26, 2007, 07:20:25 AM
Copper (II) chloride solubility is in the range of 45 g (mass of anhydrous salt) per 100 g of H2O (solution density well over 1.2). You may convert that to Kso, although saturated solution has a high ionic strength (around 16), so take the number obtained with a grain of salt. That's not a sparringly soluble salt, thats why Kso is not listed in tables. What was amount and concentration of HCl added?

My guess is that adding NaCl would precipitate CuCl2 but the amount would be affected by the pH of the solution, which would become basic.

Please elaborate - why do you expect pH to rise after NaCl addition?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 26, 2007, 12:25:00 PM
I suspect a pH rise because some of the released NH3 ligands would surely become NH4+ which would increase the [OH-].
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 26, 2007, 02:08:48 PM
That'll be possible only if Cl- will either remove NH3 from the complex (with stablility constants of Cu2+/Cl- complexes around 1 it aint gona happen) or if the CuCl2 precipitates - not very likely with so high solubility.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Rabn on December 26, 2007, 02:48:52 PM
Thanks for making me double check myself Borek. I mistakenly looked at the Ksp of Copper(I) Chloride. Which in effect made me contradict myself. I'm curious about these stability constants you mentioned. I haven't run into those in my studies(I haven't taken any advanced inorganic/organic courses yet).  I'm going to do a search for some info, if you know of any good sites describing those would you mind posting a link?
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 26, 2007, 03:36:35 PM
I have several books with different aqueous equilibrium constants collections; however, these are mostly published in Eastern Europe back in seventies/eighties so they will be mostly likely out of reach for you.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 28, 2007, 05:07:53 AM
Mitch , nevermind
Borek , the experiment was qualitative.

Quote from: Borek
What was amount and concentration of HCl added?
Title: Re: Effect of ethanol on the shape of crystals
Post by: AWK on December 28, 2007, 06:08:22 AM
Stability constants for copper(II) ammonia system are given in Journal of Chemical Education 82(3) 408-14 (2005)
A.R. Johnson, T.M McQuinn, K.T. Rodolfa
Title: Re: Effect of ethanol on the shape of crystals
Post by: Borek on December 28, 2007, 06:44:12 AM
Borek , the experiment was qualitative.

Still, there is completely different picture between adding 1mL of 1M HCl and 5mL of 35% HCl... I am not asking about exact numbers, rather about orders of magnitude.
Title: Re: Effect of ethanol on the shape of crystals
Post by: Hunt on December 28, 2007, 07:49:28 PM
It wasn't more than 1 ml of HCl in a test tube, its concentration might have been 1 or 2 M. I cant really remember.

Is there any possibility that hydrogen gas was emitted during the process?
Title: Re: Effect of ethanol on the shape of crystals
Post by: AWK on December 31, 2007, 01:09:06 AM
Is there any possibility that hydrogen gas was emitted during the process?

Did you try writing down a balanced reaction?