[Hunt]

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

[Borek]

Shape or size?

[Hunt]

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 ? 

[Mitch]

Usually anything allowed to crystallize for longer will make larger crystals.

[Hunt]

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 ?

[Borek]

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.

[Rabn]

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.

[Hunt]

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.

[Rabn]

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.

[Hunt]

I dont follow.

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.

[Rabn]

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...

[Borek]

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.

[Hunt]

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.

[Borek]

What happens to NH₃ in low pH?

[Hunt]

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)  ?