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Offline jkb2021

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Hydrates Question
« on: March 06, 2021, 04:48:05 PM »
I am doing a Lab and one of the questions asks for two reasons why you wouldn't want to use heat to find the percent water in a hydrated crystal. I'm completely at a loss. I've searched 2 chemistry books and scoured the internet and all I see is using heat.

Working my way through it, I thought:

Maybe some are combustible at low temp?

Maybe some have a low boiling point and become gaseous?

Maybe some of them become highly caustic when the water is heated? It wouldn't eat through the beaker/dish would it?

I see some mention that over heating can break them down into other compounds, such as CuSO4 into CuO, which is decomposition? In terms of thermodynamics, I know appreciate that heat could supply the activation energy required for different products to the anhydrous compound we are after. So there is one reason.

Since we are looking at % water specifically, I don't think I can consider things that only release HCl when dehydrated? do some release other compounds in edition to H2O?

Please note, I would prefer being pointed in the right direction or resources than straight up answers, but will take what I can get.

Offline AWK

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Re: Hydrates Question
« Reply #1 on: March 06, 2021, 05:13:28 PM »
Dehydration of the salt is a salt-specific procedure. There are salts that decompose during heating and for them, the dehydration process is sometimes very complicated.
Copper sulfate or calcium chloride, which are most often "tortured" by students by heating to about 300°C to constant weight. And that's all. For such exercises, there are always instructions with a description of the procedures and possible additional literature.
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Offline jkb2021

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Re: Hydrates Question
« Reply #2 on: March 06, 2021, 06:23:31 PM »
Thank you for your reply.

So then the decomposition is correct.

The only other thing that I have been able to find is that something like aluminum chloride trihydride cannot be recovered with heat and instead gives off HCl (g) instead, but this is still decomposition is it not?

I think my two answers are that some will decompose into into alternate products under heat that affect the amount of H2O evaporated and others will not form/produce water vapor when heated.

The instructor only gives background very specific to the experiment being preformed and gives no references or additional literature. They expect you to be able to find the information on your own and refuses to point you in the right direction if you are having trouble finding anything to do with the lab questions. They will only help with calculations.

Twice I have been asked questions that I only knew the answers to because I dealt with/learned them in organic chemistry (I'm refreshing credits for gen chem), but drove myself crazy trying to confirm the answers.

Offline AWK

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Re: Hydrates Question
« Reply #3 on: March 07, 2021, 02:49:06 AM »
Anhydrous AlCl3 can only be obtained by chemically removing water by reaction with SOCl2 or by direct reaction of aluminum metal with chlorine. This was discussed many times in the forum - look for e.g. anhydrous ZnCl2
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Offline jkb2021

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Re: Hydrates Question
« Reply #4 on: March 07, 2021, 07:34:13 AM »
Another reason may be that some hydrates are gases at room temperature and standard pressure, such as Methane. So then I wouldn't be able to recover the water through heating. I assume a different method would need to be used to recover the water.

But the question is specifically about hydrated crystals. :(

« Last Edit: March 07, 2021, 08:45:14 AM by jkb2021 »

Offline AWK

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Re: Hydrates Question
« Reply #5 on: March 07, 2021, 09:04:06 AM »
Gas hydrates are a completely different story.
That is why they are called clathrates.
Methane clathrate becomes solid only under high pressure, e.g. 4°C - 50 atm.
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Offline jkb2021

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Re: Hydrates Question
« Reply #6 on: March 07, 2021, 11:22:46 AM »
Thanks for trying. It is appreciated; but I'm done. I don't have time to spend more than 8 hours on a single lab question. There is just nothing I can do without proper access to background information. I have biochem to focus on, which is far more important for Medicine. 

So if decomposition is the only answer, why is the instructor asking for two reasons?

I can't form conclusions without more information. You can't build a wall without a foundation (background and definitions) and tools (resources/addition related readings).

As a previous tutor and biology lab TA, I would never ask a question where I didn't ensure they had access to all the information. I would make sure they had the background, examples, and a source for additional literature.

I would ask questions that made the students think about the information I gave; expecting them to form conclusions based on what has been taught.


Offline Borek

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Re: Hydrates Question
« Reply #7 on: March 07, 2021, 03:06:51 PM »
So if decomposition is the only answer, why is the instructor asking for two reasons?

Perhaps what they mean is that heating to dry something out is not guaranteed to give a correct answer, as different water molecules are attached with different strengths. For some salts depending on the drying temperature you can find a series of relatively stable hydrates - but you will not know if all water was lost without additional composition tests.
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