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Topic: Solution enthalpy of partially hydrated salt  (Read 464 times)

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Offline darpyu

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Solution enthalpy of partially hydrated salt
« on: August 31, 2020, 07:07:06 AM »
I am investigating the relationship between the water of crystallization and the solution enthalpy of a salt for a project in school.

I am taking a hydrated salt Na2CO3 10H2O and heating it for a small amount of time (2 min) and this results in a partially hydrated salt. (Say it forms Na2CO3 7 H2O. I can find out how much mass was lost and find the mols of water lost accordingly)

I can find out the enthalpy of solution this partially hydrated salt experimentally, but I need a theoretical value to compare it to. I know I can find the solution enthalpy of an anhydrous salt by using lattice enthalpy + hydration enthalpy. But I don't think it will work for a partially hydrated salt. Any suggestions for finding the solution enthalpy of partially hydrated salt?

Offline sjb

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Re: Solution enthalpy of partially hydrated salt
« Reply #1 on: August 31, 2020, 09:19:54 AM »
One way to do this is to measure the heat change when you dissolve in a certain volume of solution, and compare it to things like the decahydrate or anhyhdrous form to the same volume.

Offline darpyu

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Re: Solution enthalpy of partially hydrated salt
« Reply #2 on: August 31, 2020, 09:31:16 AM »
I'll try that. But is there any way I can predict the heat change before the experiment to have a theoretical value to compare to?

Offline Corribus

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Re: Solution enthalpy of partially hydrated salt
« Reply #3 on: August 31, 2020, 09:40:16 AM »
You can look for tabulated heat of formation values in the literature.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: Solution enthalpy of partially hydrated salt
« Reply #4 on: September 01, 2020, 07:52:29 AM »
[...] a theoretical value [...]

I don't believe the heat of formation of the varied hydrates can be predicted. I depends on every detail of the crystal arrangement. Worse, the heat of dissolution is small as compared with the heat of formation of the compounds, hence its relative uncertainty is huge.

I wouldn't even assume a linear relationship from the anhydrous solid, a solid with few H2O, with 10 H2O, and a solution. Most salts show a finite set of combinations with crystallisation H2O, which tells that geometrical details are all-important. This would be an interesting observation to make and to comment from your experiment. Or even: start from 10 H2O, use scales to evaporate 1 H2O at a time as a mean value (the solid will be a mix of varied crystals), measure each time the heat of dissolution, and attribute breaks in the curve to favoured numbers of crystallisation water.

Software would be far too inaccurate here.

The only "prediction" I imagine is to find experimental values for each hydration number. Passing numerically between the heats of formation and dissolution isn't a big deal then. I'd prefer measured heats of dissolution, because uncertainties on the heat of formation make a huge relative difference on the heat of dissolution.

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